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CONTENTS
TRENDS IN THE USE...
EQUIPMENT AND TECHNIQUES FOR...
NONINVASIVE VENTILATION TO...
NONINVASIVE VENTILATION FOR...
Evidence for Efficacy
MECHANISMS OF ACTION OF...
PRACTICAL APPLICATION OF NPPV
ADVERSE EFFECTS AND...
SUMMARY AND CONCLUSIONS
REFERENCES
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Trends in the Use of Noninvasive Ventilation
The Age of Body Ventilators
Proliferation of Invasive Positive Pressure Ventilation
Reemergence of Noninvasive Ventilation
Why the Interest in Noninvasive Ventilation?
Equipment and Techniques for Noninvasive Ventilation
Noninvasive Positive Pressure Ventilation
Ventilators for NPPV
Negative Pressure Ventilation
Abdominal Displacement Ventilators
Other Modes of Nonivasive Ventilatory Assistance
Techniques to Assist Cough
Noninvasive Ventilation to Treat Respiratory Failure
Noninvasive Ventilation in the Acute Care Setting
Evidence for Efficacy
Obstructive Diseases
Acute Cardiogenic Pulmonary Edema
Community-acquired Pneumonia
Hypoxemic Respiratory Failure
Other Applications of NPPV in Acute Respiratory Failure
Do-Not-Intubate Patients
Postoperative Patients
Pediatric Applications
Facilitation of Weaning and Extubation
Time Demands on Medical Personnel
Selection of Patients for NPPV in the Acute Setting
Noninvasive Ventilation for Chronic Respiratory Failure
Evidence for Efficacy
Noninvasive Positive Pressure Ventilation
Selection of Patients with Chronic Respiratory Failure to Receive Noninvasive Ventilation
Mechanisms of Action of Noninvasive Ventilation
Chronic Respiratory Failure
Practical Application of NPPV
Initiation
Selection of a Ventilator and Ventilator Mode
Interface Selection
Initial Ventilator Settings
Adjuncts to Noninvasive Ventilation
Role of the Clinician
Monitoring
Adaptation
Adverse Effects and Complications of Noninvasive Ventilation
Summary and Conclusions
Noninvasive ventilation refers to the delivery of mechanical ventilation to the lungs using techniques that do not require an endotracheal airway. During the first half of the 20th century, negative pressure types of noninvasive ventilation were the main means of providing mechanical ventilatory assistance outside of the anesthesia suite. By the 1960s, however, invasive (i.e. via an endotracheal tube) positive pressure ventilation superceded negative pressure ventilation, primarily because of better airway protection. The past decade has seen a resurgence in the use of noninvasive ventilation, largely because of the development of nasal ventilation, which has the potential of providing ventilatory assistance with greater convenience, comfort, safety, and less cost than invasive ventilation. The following will explore these trends in the use of noninvasive ventilation and then provide a current perspective on applications in patients with acute and chronic respiratory failure. The discussion will consider the rationale for use, currently available techniques and equipment, evidence for efficacy, selection of appropriate patients, and general guidelines for application, monitoring, and avoidance of complications.
This review is based on an evaluation of the literature using a multimethod approach. A computerized MEDLINE search from 1966 through June 2000 was undertaken using search terms including mechanical ventilation, intermittent positive pressure ventilation, negative pressure ventilation, respiratory insufficiency/failure, lung disease/obstructive, and lung disease/ restrictive. Bibliographies of articles were also searched for relevant articles. Review articles and consensus statements were also examined and recommendations synthesized into general guidelines.
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TRENDS IN THE USE OF NONINVASIVE VENTILATION |
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CONTENTS
TRENDS IN THE USE...
EQUIPMENT AND TECHNIQUES FOR...
NONINVASIVE VENTILATION TO...
NONINVASIVE VENTILATION FOR...
Evidence for Efficacy
MECHANISMS OF ACTION OF...
PRACTICAL APPLICATION OF NPPV
ADVERSE EFFECTS AND...
SUMMARY AND CONCLUSIONS
REFERENCES
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The Age of Body Ventilators
The earliest noninvasive ventilators were the "body" ventilators, so-called because they assist ventilation by applying negative or positive pressure to various regions of the body. The earliest description of a body ventilator was that of a tank-type negative pressure device by the Scottish physician John Dalziel in 1838 (1). It consisted of an airtight box in which the patient was seated with the head protruding through a neck seal. A manually powered bellows generated the negative pressure. Numerous other prototypes of this device were described during the 1800s (2), but they did not achieve widespread use until the 1900s when electricity became readily available and a large demand was created by the polio epidemics (3). The first electrically powered body ventilator used widely was the iron lung, developed in 1928 by Philip Drinker, a Boston engineer (4). It consisted of a one ton metal cylinder encasing the patient who lay supine on a mattress with his or her head protruding through an air-tight rubber neck seal. In 1931, J. H. Emerson, of Cambridge, Massachusetts, constructed a simpler, quieter, lighter and less expensive version of the iron lung that could be manually operated in the event of power failure. Drinker unsuccessfully sued Emerson for patent infringement, and the Emerson lung became the predominant version for ventilator support of patients with respiratory paralysis caused by poliomyelitis, with thousands manufactured between 1930 and 1960 (5).
The bulk and lack of portability of early tank ventilators stimulated the development of more portable negative pressure devices, including the chest cuirass or "shell" ventilator and raincoat (or wrap) ventilator (3). The first cuirass, developed in 1876 by Ignez von Hauke of Austria (2), consisted of an iron shell covering the anterior part of the thorax, with an air-filled rubber edge that created a tight seal. In 1927, R. Eisenmenger patented the first mechanical chest shell, the Biomotor, which was used from 1935 onward to treat respiratory paralysis (6). The first mass-produced chest shells, the Fairchild-Huxley chest respirator and the Monaghan Portable Respirator, were introduced in 1949. Shortly thereafter, the prototype wrap-style ventilator, the Tunnicliffe breathing jacket, was described (7). These devices saw widespread use for chronic support of polio patients with respiratory paralysis.
The polio epidemics stimulated the development of other approaches to noninvasive ventilation, including the rocking bed and the intermittent abdominal pressure ventilator, commonly referred to as the "pneumobelt." The technique of rocking to assist ventilation was described by F. C. Eve in 1932 (8) and was subsequently used by the British Navy until the early 1960s as a technique to resuscitate drowning victims (9). Wright introduced the first rocking bed during the late 1940s as a modification of a bed designed to improve circulation (10). This device became popular during the 1950s as a means of facilitating weaning from the iron lung. Some postpolio patients subsequently used it for chronic ventilatory support, sometimes for decades (11).
Sir William Bragg, a Nobel-Prize-winning physicist, invented the "pneumobelt" during the 1930s. He fashioned a pneumatic belt from a rubber football bladder for a friend with muscular dystrophy. The bladder was strapped around the abdomen and lower thorax and was inflated by a small air pump, compressing the abdominal viscera during expiration. Robert Paul improved the belt, and it was subsequently called the Bragg-Paul Pulsator. The device gained popularity during the late 1950s because of enhanced convenience and portability for chronic ventilator users, just as the polio epidemics were drawing to a close (12).
Proliferation of Invasive Positive Pressure Ventilation
Prior to 1960, invasive positive pressure ventilation was used mainly for administration of anesthesia. Although tracheostomy tubes were often placed to manage airway secretions in patients with bulbar polio, ventilatory support was still usually provided by iron lungs. A turning point occurred during a large outbreak of polio in Copenhagen, Denmark, in 1952 (13). The few available negative pressure ventilators were overwhelmed by the hundreds of afflicted patients. A massive effort was mobilized to provide round-the-clock ventilation to these patients using invasive positive pressure resuscitators borrowed from anesthesia suites and powered manually by medical students, nurses, and other volunteers. Survival rates using positive pressure ventilation were much better than those seen during use of negative pressure ventilation early during the epidemic, an improvement thought to be related to better airway protection from aspiration of secretions (5).
Partly as a consequence of this experience, there was a gradual transition to invasive positive pressure ventilation that was accelerated during the 1960s by the development of intensive care units and the introduction of simple to operate, relatively inexpensive positive pressure ventilators. Administration of positive pressure ventilation via translaryngeal endotracheal tubes became standard practice for the support of patients with acute respiratory failure.
Reemergence of Noninvasive Ventilation
Although rarely used in the United States after the 1960s for patients with acute respiratory failure, body ventilators continued to be used for patients with chronic respiratory failure through the mid-1980s, particularly for those with neuromuscular disease or kyphoscoliosis (14, 15). However, because of a number of disadvantages relative to noninvasive positive pressure ventilation, including patient discomfort, restrictions on positioning, problems with correct fitting, time-consuming application, lack of portability, and a tendency to potentiate obstructive sleep apneas, body ventilators have seen diminishing use since the mid-1980s (16).
The noninvasive application of positive pressure dates back to the 1930s, when the pioneering studies of Alvan Barach demonstrated that continuous positive airway pressure could be useful in the treatment of acute pulmonary edema (17). First described in 1947 (18), intermittent positive pressure breathing (IPPB) administered via a mouthpiece was used widely until the early 1980s in acute care hospitals in the United States. Although it was used mainly as a means of delivering aerosolized bronchodilators to patients with chronic obstructive pulmonary disease (COPD) and asthma, several studies evaluated this modality as a means of noninvasive ventilatory support. Fraimow and colleagues (19) observed that IPPB reversed the increase in PaCO2 occurring in patients with emphysema receiving oxygen. On the other hand, two studies in which IPPB was administered to patients with COPD at home for periods of 22 mo and 4 yr, respectively, found no benefit in FEV1 or arterial blood gases and reduced survival in comparison with nonrandomized control subjects (20, 21). After publication of these studies and the randomized, prospective trial sponsored by the National Institutes of Health that showed no benefit of IPPB over nebulizer treatments in patients with COPD (22), use of IPPB declined drastically. Perhaps the main reason for the failure of IPPB as a means of ventilatory support, though, was that it was used primarily to deliver aerosol treatments for only 10 to 15 min three or four times daily; too brief to substantially assist breathing.
Noninvasive positive pressure ventilation (NPPV), administered nocturnally and as needed during the daytime, was used successfully to treat patients with neuromuscular disease at centers such as the Goldwater Rehabilitation Center in New York dating back to the early 1960s (23). However, these centers used mainly mouthpiece interfaces that failed to gain wide acceptance elsewhere. Face masks were also available, but these likewise failed to gain wide acceptance for the chronic administration of noninvasive ventilation, largely because of poor patient tolerance. The signal change that led to the recent proliferation of noninvasive ventilation came in the early 1980s with the introduction of the nasal continuous positive airway pressure (CPAP) mask for the treatment of obstructive sleep apnea (24). Rideau and colleagues (25) of France proposed in 1984 that such masks should be used with positive pressure ventilators to achieve nocturnal respiratory muscle rest in patients with Duchenne muscular dystrophy (DMD), so that disease progression could thereby be slowed. Soon thereafter, the success of nocturnal nasal ventilation was reported in ameliorating gas exchange disturbances and symptoms in patients with chronic respiratory failure caused by a variety of neuromuscular diseases and chest wall deformities (26). Subsequent studies have confirmed the favorable findings of these earlier studies, establishing an important role for NPPV in the management of chronic respiratory failure. More recent work has begun defining the role in the management of patients with acute respiratory failure. Subsequent sections will critically review this recent work and provide guidelines on current indications for NPPV in both acute and chronic settings.
Why the Interest in Noninvasive Ventilation?
A major driving force behind the increasing use of noninvasive ventilation has been the desire to avoid the complications of invasive ventilation. Although invasive mechanical ventilation is highly effective and reliable in supporting alveolar ventilation, endotracheal intubation carries well-known risks of complications that have been described elsewhere in detail (29). These fall into three main categories: complications directly related to the process of intubation and mechanical ventilation, those caused by the loss of airway defense mechanisms, and those that occur after removal of the endotracheal tube.
The first category includes aspiration of gastric contents, trauma to the teeth, hypopharynx, esophagus, larynx, and trachea, arrhythmias, hypotension, and barotrauma that may occur during placement of a translaryngeal tube (30). With tracheostomy placement, risks include hemorrhage, stomal infection, intubation of a false lumen, mediastinitis, and acute injury to the trachea and surrounding structures, including the esophagus and blood vessels (32). In the second category, endotracheal tubes provide a direct conduit to the lower airways for microorganisms and other foreign materials, permitting chronic bacterial colonization, inflammation, and impairment of airway ciliary function. These factors facilitate the occurrence of nosocomial pneumonia, seen in as much as 21% of mechanically ventilated intensive care unit (ICU) patients (33), and sinusitis, that occurs in 5 to 25% of nasally intubated patients, related to blockade of the sinus ostia and accumulation of infected secretions in the paranasal sinuses (34). The chronic aspiration and irritation associated with endotracheal intubation also necessitate endotracheal suctioning that further irritates lower airway mucosa, causing discomfort, further inflammation, edema, and increased mucus production. In the third category, hoarseness, sore throat, cough, sputum production, hemoptysis, upper airway obstruction caused by vocal cord dysfunction or laryngeal swelling, and tracheal stenosis may follow extubation (35).
From the point of view of the patient, perhaps the most troubling aspects of translaryngeal intubation are tube-associated discomfort and the compromised ability to eat and communicate that contributes to feelings of powerlessness, isolation, and anxiety (36). This may increase the need for sedation, delaying weaning, adding to the costs of care, and potentiating the risks of further complications. Placement of a tracheostomy does little to simplify care. Sophisticated equipment, including suctioning paraphernalia and a high level of technical expertise among caregivers, is required, adding substantially to costs (37). In addition, tracheostomies lead to upper airway colonization with gram-negative bacteria, increasing the risk of pneumonias (35). Further, long-term tracheostomies are complicated by tracheomalacia, endotracheal granulation tissue formation, and tracheal stenoses that sometimes contribute to airway obstruction, chronic pain, and tracheoesophageal or even tracheoarterial fistulas (35). These considerations and potential complications may limit the options for chronic care placement and may even preclude home discharge in patients with limited personnel and financial resources who would otherwise be candidates for home placement.
By averting airway intubation, noninvasive ventilation has the potential of avoiding these problems if candidates are carefully selected using established guidelines which will be discussed in detail in later sections. In contrast to invasive ventilation, noninvasive ventilation leaves the upper airway intact, preserves airway defense mechanisms, and allows patients to eat, drink, verbalize, and expectorate secretions. Several recent studies indicate that NPPV reduces infectious complications of mechanical ventilation, including nosocomial pneumonia and sinusitis (38). Noninvasive ventilation may enhance comfort, convenience, and portability at no greater (41) or even less cost than endotracheal intubation (37). Furthermore, noninvasive ventilation may be administered outside of the intensive care setting, as long as adequate nursing and respiratory therapy support can be provided, allowing caregivers to more rationally utilize acute-care beds, and it greatly simplifies care for patients with chronic respiratory failure in the home.
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EQUIPMENT AND TECHNIQUES FOR NONINVASIVE VENTILATION |
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CONTENTS
TRENDS IN THE USE...
EQUIPMENT AND TECHNIQUES FOR...
NONINVASIVE VENTILATION TO...
NONINVASIVE VENTILATION FOR...
Evidence for Efficacy
MECHANISMS OF ACTION OF...
PRACTICAL APPLICATION OF NPPV
ADVERSE EFFECTS AND...
SUMMARY AND CONCLUSIONS
REFERENCES
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The following will describe equipment currently available for administration of noninvasive ventilation including NPPV and body ventilators. Mechanisms of action and principles of application of body ventilators will also be considered. Because of its current popularity, NPPV will be emphasized and its mechanisms of action and applications will be discussed in more detail later. CPAP as opposed to intermittent positive pressure ventilation may be used for some forms of respiratory failure, so equipment required for the noninvasive administration of CPAP will also be discussed. Finally, brief descriptions of other ventilatory aids and cough-enhancing techniques will also be included.
Noninvasive Positive Pressure Ventilation
Positive pressure ventilators, whether invasive or noninvasive, assist ventilation by delivering pressurized gas to the airways, increasing transpulmonary pressure, and inflating the lungs. Exhalation then occurs by means of elastic recoil of the lungs and any active force exerted by the expiratory muscles. The major difference between invasive and NPPV is that with the latter, gas is delivered to the airway via a mask or "interface" rather than via an invasive conduit. The open breathing circuit of NPPV permits air leaks around the mask or through the mouth, rendering the success of NPPV critically dependent on ventilator systems designed to deal effectively with air leaks and to optimize patient comfort and acceptance.
Interfaces for the delivery of NPPV or CPAP. Interfaces are devices that connect ventilator tubing to the face, facilitating the entry of pressurized gas into the upper airway during NPPV. Currently available interfaces include nasal and oronasal masks and mouthpieces.
Nasal masks. The nasal mask is widely used for administration of CPAP or NPPV, particularly for chronic applications. The standard nasal mask is a triangular or cone-shaped clear plastic device that fits over the nose and utilizes a soft cuff to form an air seal over the skin (Figure 1). Nasal masks are available from many manufacturers in multiple sizes (pediatric and adult small, medium, large, wide, and narrow, and so on) and shapes, largely because of the demand for such devices in the treatment of obstructive sleep apnea. The standard nasal mask exerts pressure over the bridge of the nose in order to achieve an adequate air seal, often causing skin irritation and redness, and occasionally ulceration. Various modifications are available to minimize this complication such as use of forehead spacers or the addition of a thin plastic flap that permits air sealing with less mask pressure on the nose. Recently, several manufacturers have introduced nasal masks with gel seals that may enhance comfort. In addition, newer "mini-masks" have been developed that minimize the bulk of the mask, reducing feelings of claustrophobia and permitting patients to wear glasses while the ventilator is in use. For occasional patients who have difficulty tolerating commercially available masks, custom-molded, individualized masks that can be made to conform to unique facial contours are also available.
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Straps that hold the mask in place are also important for patient comfort, and many types of strap assemblies are available. Most manufacturers provide straps that are designed for use with a particular mask. Straps that attach at two or as many as five points on the mask have been used, depending on the interface. More points of attachment add to stability. Strap systems with Velcro fasteners are popular, and elastic caps that help to keep the straps from tangling or sliding have been well received by patients.
An alternative type of nasal interface, nasal "pillows" or "seals," consist of soft rubber or silicone pledgets that are inserted directly into the nostrils (Figure 1). Because they exert no pressure over the bridge of the nose, nasal pillows are useful in patients who develop redness or ulceration on the nasal bridge while using standard nasal masks. Also, some patients, particularly those with claustrophobia, prefer nasal pillows because they seem less bulky than standard nasal masks.
Oronasal masks. Oronasal or full-face masks cover both the nose and the mouth (Figure 1). They have been used mainly on patients with acute respiratory failure but may also be useful for chronic applications. Oronasal masks have been used in approximately half of the studies evaluating NPPV for acute respiratory failure. During chronic use, patients may object to having both the nose and the mouth covered, and asphyxiation may be a concern in patients who are unable to remove the mask in the event of ventilator malfunction or power failure. Furthermore, interference with speech, eating, and expectoration, the likelihood of claustrophobic reactions, and the theoretical risks of aspiration and rebreathing are greater with oronasal than with nasal masks. On the other hand, oronasal masks may be preferred for patients with copious air leaking through the mouth during nasal mask ventilation. Also, recent improvements in oronasal masks, such as more comfortable seals, improved air-sealing capabilities, and incorporation of quick-release straps and antiasphyxia valves to prevent rebreathing in the event of ventilator failure, have increased acceptability of these interfaces for chronic applications. One recently commercially available interface that holds promise is the "total" face mask (42), which resembles a hockey goalie's mask. Made of clear plastic, it uses a soft cuff that seals around the perimeter of the face, avoiding direct pressure on facial structures.
The efficacy of nasal and oronasal masks has recently been compared in a controlled trial of 26 patients with stable hypercapnia caused by COPD or restrictive thoracic disease. The nasal mask was better tolerated than either nasal pillow or an oronasal mask but was less effective at lowering PaCO2, perhaps because of greater air leaking (43). This supports the commonly held belief that in the acute setting, oronasal masks are preferable to nasal masks, because dyspneic patients are mouth breathers, predisposing to greater air leakage and reduced effectiveness during nasal mask ventilation. However, efficacy of the mask types has been similar when compared among published reports that have used one mask or the other. Also, a recent preliminary report from a controlled trial comparing nasal and oronasal masks found that PaCO2 and respiratory rate fell at equal rates when the masks were used for patients with acute respiratory distress (44).
Mouthpieces. Mouthpieces held in place by lipseals have been used since the 1960s to provide NPPV for as long as 24 h a day to patients with chronic respiratory failure (Figure 1) (45). The mouthpiece has the advantages of being simple and inexpensive. Custom-fitted mouthpieces, that may increase comfort and efficacy are also available at some centers (23). Success using these devices has been reported in a large number of patients with neuromuscular disease, some with little or no vital capacity (46). During the daytime, patients receive ventilatory assistance via a mouthpiece attached to their wheelchair controls or held by a gooseneck clamp. During sleep, some patients use strapless custom mouthpieces, and others use strapped-on lipseals. Nasal air leaking may compromise efficacy, but this can be managed by increasing ventilator tidal volume or occluding the nostrils with cotton pledgets or noseclips. The use of mouthpieces has allowed some tetraplegic patients to be successfully converted from tracheostomies to NPPV (47).
Ventilators for NPPV
Delivery of CPAP. Although not a true ventilator mode because it does not actively assist inspiration, CPAP is used for certain forms of acute respiratory failure. By delivering a constant pressure during both inspiration and expiration, CPAP increases functional residual capacity and opens collapsed or underventilated alveoli, thus decreasing right to left intrapulmonary shunt and improving oxygenation. The increase in functional residual capacity may also improve lung compliance, decreasing the work of breathing (48). In addition, by lowering left ventricular transmural pressure, CPAP may reduce afterload and increase cardiac output (49, 50), making it an attractive modality for therapy of acute pulmonary edema. Further, by counterbalancing the inspiratory threshold load imposed by intrinsic positive end-expiratory pressure (PEEPi), CPAP may reduce the work of breathing in patients with COPD (51, 52). A few uncontrolled trials have observed improved vital signs and gas exchange in patients with acute exacerbations of COPD treated with CPAP alone (53), suggesting that this modality may offer benefit to these patients.
Pressures commonly used to deliver CPAP to patients with acute respiratory distress range from 5 to 12.5 cm H2O. Such pressures can be applied using a wide variety of devices including CPAP valves connected to a compressed gas source, small portable units used for home therapy of obstructive sleep apnea, and ventilators designed for use in critical care units (critical care ventilators). Depending on the critical care ventilator selected, CPAP may be administered using "demand," "flow-by," or "continuous flow" techniques, with imposed work differing slightly between them (56). The main considerations for selection of an appropriate device include provision of an adequate air flow rate so that a continuous positive pressure is maintained, even in patients with acute respiratory failure, who may breathe at high flow rates, and the perceived need for alarms, convenience, or portability. Although not appropriate for the acute care setting where high flow rates may be needed, simple, small, inexpensive portable units are usually adequate for home applications.
Pressure-limited ventilators. Pressure-limited modes are available on most ventilators designed for use on intubated patients in critical care units. Most such "critical care ventilators" provide pressure support ventilation (PSV) that delivers a preset inspiratory pressure to assist spontaneous breathing efforts and has attained popularity in recent years as a weaning mode (57). Many also offer pressure control ventilation (PCV) that delivers time-cycled preset inspiratory and expiratory pressures with adjustable inpiratory:expiratory ratios at a controlled rate. Most such modes also permit patient-triggering with selection of a backup rate. Nomenclature for these modes varies between manufacturers, causing confusion. For the pressure support mode, some ventilators require selection of a pressure support level that is the amount of inspiratory assistance added to the preset expiratory pressure and is not affected by adjustments in PEEP. Others require selection of peak inspiratory and expiratory positive airway pressures (IPAP and EPAP), the difference between the two determining the level of pressure support. It is important to recall that with the latter configuration, alterations in EPAP without parallel changes in IPAP will alter the pressure support level.
What distinguishes PSV from other currently available ventilator modes is the ability to vary inspiratory time breath by breath, permitting close matching with the patient's spontaneous breathing pattern. A sensitive patient-initiated trigger signals the delivery of inspiratory pressure support, and a reduction in inspiratory flow causes the ventilator to cycle into expiration. In this way, PSV allows the patient to control not only breathing rate but also inspiratory duration. As shown in patients undergoing weaning from invasive mechanical ventilation (58), PSV offers the potential of excellent patient-ventilator synchrony, reduced diaphragmatic work, and improved patient comfort. However, PSV may also contribute to patient-ventilator asynchrony, particularly in patients with COPD. High levels of pressure support and the resulting large tidal volumes may contribute to inadequate inspiratory efforts on subsequent breaths, leading to failure to trigger (59). Also, brief rapid inhalations that may be seen in patients with COPD may not permit adequate time for the PSV mode to cycle into expiration, so that the patient's expiratory effort begins while the ventilator is still delivering inspiratory pressure (60). The patient must exert expiratory force to cycle the ventilator, and this may contribute to breathing discomfort. During noninvasive ventilation, these forms of asynchrony are exacerbated in the presence of air leaks.
Although noninvasive PSV is often administered using standard critical care ventilators, portable devices that deliver pressure-limited ventilation (Figure 2) have also seen increasing use for both acute and chronic applications. These devices, sometimes referred to as "bilevel" devices because they cycle between two different positive pressures, are lighter (5 to 10 kg) and more compact (< 1 ft3) than critical care ventilators, offering greater portability at lower expense (61). Some offer not only a spontaneously triggered pressure support mode but also pressure-limited, time-cycled, and assist modes. Some also offer adjustable trigger sensitivities (62), "rise time" (the time required to reach peak pressure), and inspiratory duration, all features that may enhance patient-ventilator synchrony and comfort. Further, the performance characteristics of these ventilators compare favorably with those of critical care ventilators (63).
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On the other hand, unlike the critical care ventilators, the bilevel devices have limited pressure-generating capabilities (20 to 35 cm H2O, depending on the ventilator) and most lack oxygen blenders or sophisticated alarm or battery backup systems. Therefore, they are not currently recommended for patients who require high oxygen concentrations or inflation pressures, or are dependent on continuous mechanical ventilation unless appropriate alarm and monitoring systems can be added. Recently, however, new versions of bilevel ventilators have been introduced that have more sophisticated alarm and monitoring capabilities, graphic displays, and oxygen blenders and are quite suitable for use in the acute care setting.
Because of their portability, convenience, and low cost, the bilevel devices have proven ideal for home use in patients with chronic respiratory failure requiring only nocturnal ventilatory assistance. In addition, unlike volume-limited ventilators, they are able to vary and sustain inspiratory airflow to compensate for air leaks, thereby potentially providing better support of gas exchange during leaking (64). However, in addition to limited alarm capabilities, other concerns have been raised about bilevel ventilators. Because they use a single tube with a passive exhalation valve, rebreathing may occur (65). This concern is discussed further in the section on applications of NPPV.
Volume-limited ventilators. Most critical care ventilators offer both pressure- and volume-limited modes, either of which can be used for administration of noninvasive ventilation. If volume-limited ventilation is desired for chronic applications, portable volume-limited ventilators (Figure 2) are usually chosen because of their greater convenience and lower cost. These are applied just as for invasive ventilation, using standard tubing and exhalation valves, with oxygen supplementation and humidification as necessary. Compared with the portable pressure-limited ventilators described above, the volume-limited portable ventilators are more expensive and heavier. However, they also have more sophisticated alarm systems, the capability to generate higher positive pressures, and built-in backup batteries that power the ventilator for at least a few hours in the event of power failure. These ventilators are usually set in the assist/control mode to allow for spontaneous patient triggering, and backup rate is usually set at slightly below the spontaneous patient breathing rate. The only important difference relative to invasive ventilation is that tidal volume is usually set higher (10 to 15 ml/kg) to compensate for air leaking. Currently available volume-limited ventilators are well suited for patients in need of continuous ventilatory support or those with severe chest wall deformity or obesity who need high inflation pressures.
Newer noninvasive ventilator modes. Because patient comfort and compliance with the therapy are so critical to the success of noninvasive ventilation, newer modes that are capable of closely mirroring the patient's desired breathing pattern are of great interest. One such new ventilator mode is proportional assist ventilation (PAV), which targets patient effort rather than pressure or volume (66). By instantaneously tracking patient inspiratory flow and its integral (volume) using an in-line pneumotachograph, this mode has the capability of responding rapidly to the patient's ventilatory effort. By adjusting the gain on the flow and volume signals, the operator is able to select the proportion of breathing work that is to be assisted. This ventilator mode is not yet commercially available in the United States, but preliminary reports on noninvasive applications are promising (67, 68). Ventilators designed specifically for the administration of noninvasive ventilation are being introduced by a number of ventilator manufacturers. These offer a variety of pressure-limited modes and include proportional assist ventilation or similar modes except in the United States, where the Food and Drug Administration has yet to approve these latter modes.
Negative Pressure Ventilation
Although negative pressure ventilators are used much less often now than they were in the past, knowledge of their characteristics and applications is useful because they may be used for patients who fail to adapt to NPPV. Negative pressure ventilators work by intermittently applying a subatmospheric pressure to the chest wall and abdomen, increasing transpulmonary pressure and causing atmospheric pressure at the mouth to inflate the lungs. Expiration occurs passively by elastic recoil of the lungs and chest wall as pressure within the device rises to atmospheric levels.
The efficiency of negative pressure ventilation (tidal volume generated for a given negative pressure) is determined by the compliance of the chest wall and abdomen and the surface area over which the negative pressure is applied. The tank ventilator is the most efficient by virtue of its application of negative pressure over the entire chest wall and abdomen. The cuirass is least efficient, because it applies negative pressure only to a portion of the anterior chest and abdomen (69). Although the wrap ventilator is usually more efficient than the shell ventilator, collapse of the jacket onto the upper chest wall and lower abdomen during use may compromise its efficiency. Problems with air leaking may also reduce efficiency of the wrap and chest shell ventilators and, to a lesser extent, the iron lung, which only has to seal around the neck.
The tank ventilator is reliable and relatively comfortable, but it is bulky (3 m long) and heavy (300 kg), virtually precluding portability. It is also intolerable to claustrophobic patients and interferes with nursing care, although it does have portholes on the sides to facilitate access. A more portable fiberglas tank ventilator is available, but it weighs approximately 50 kg and requires two persons for portage. The chest shell and wrap are lightweight, but the negative pressure generators necessary to power them still weigh 15 to 30 kg. Also, the tank and wrap ventilators restrict patients to the supine position, often inducing musculoskeletal back and shoulder pain. The chest shell may be used in the sitting position, but it can induce discomfort and pressure sores at points of skin contact, particularly if fit is suboptimal. Patients with chest wall deformities can be managed with custom-fit cuirasses, but efficiency of these may be poor.
These limitations of negative pressure ventilation interfere with patient tolerance, but most can be overcome with fitting adjustments or nonsteroidal anti-inflammatory drugs (16). However, the tendency for negative pressure ventilators to induce obstructive sleep apnea, even in normal subjects (70), may affect safety. Obstructive apneas associated with severe oxygen desaturations occur commonly during negative pressure ventilation in patients with restrictive thoracic disorders and may necessitate a switch to positive pressure ventilation (71). This problem is related to the lack of preinspiratory contraction of pharyngeal muscles that prevents collapse of upper airway structures during a normal patient-initiated breath (74). Traditional negative pressure ventilators, lacking patient-triggered modes, render the upper airway susceptible to collapse during ventilator-triggered breaths that are out of synchrony with the patient's spontaneous breaths. It remains to be seen whether newer patient-triggered negative pressure ventilators, such as the NEV-100 or Emerson NPV, will alleviate this problem.
External high frequency ventilation offers an alternative to
standard negative pressure ventilation (75, 76). Consisting of a
chest and abdominal cuirass connected to an oscillator, this device is capable of delivering pressures ranging from 
70 to +70 cm H2O at frequencies as great as 60/min for ventilation
and 999/min for secretion removal. I:E ratios can be from 6:1
to 1:6. The Food and Drug Administration in the United States
has approved this device for frequencies only as great as 1 Hz.
The chest wall oscillator has been proposed as a means of
enhancing secretion clearance by applying frequencies of 1 to 1.5 Hz and inverting the I:E ratio. Although the device has
been shown to augment minute volumes in normal subjects
(75) and patients with COPD (76), it has not been adequately
tested on patients with respiratory failure, and it should be
considered investigational for this application.
Abdominal Displacement Ventilators
The rocking bed (77) and pneumobelt (12) both rely on displacement of the abdominal viscera to assist diaphragm motion and, hence, ventilation. The rocking bed consists of a mattress on a motorized platform that rocks in an arc of approximately 40 degrees on a fulcrum at hip level. The patient lies supine with the head and knees raised slightly to prevent sliding. When the head rocks down, the abdominal viscera and diaphragm slide cephalad, assisting exhalation. As the head rocks up, the viscera and diaphragm slide caudad, assisting inhalation. The rocking rate is between 12 and 24/min, adjusted to optimize patient comfort and minute volume, as measured with a handheld spirometer or magnetometer. The chief advantages of the rocking bed are ease of operation, lack of encumbrances, and patient comfort, although bulkiness, noisiness, and lack of portability limit its appeal.
The pneumobelt consists of a corsetlike device that wraps around the patient's midsection and holds an inflatable rubber bladder firmly against the anterior abdomen (12). The rubber bladder is connected to a positive pressure ventilator that intermittently inflates the bladder. When the patient is sitting, bladder inflation compresses the abdominal contents, forcing the diaphragm upward and actively assisting exhalation. With bladder deflation, gravity returns the diaphragm to its original position, assisting inhalation. Raising bladder inflation pressure increases tidal volume; typical pressures are between 35 and 50 cm H2O. Desired minute volume can then be attained by adjusting ventilator rate, usually between 12 and 22/min. The pneumobelt is highly portable, easily hidden under clothing with the ventilator mounted on a wheelchair to facilitate mobility, and leaves the hands and face unencumbered. Because it requires gravity to pull the diaphragm down during bladder deflation, it is ineffective unless patients sit at angles of at least 30 degrees. Hence, nocturnal use is limited to patients who can learn to sleep while sitting (78, 79). However, it may be valuable as a daytime adjunct in appropriate patients who are using other forms of noninvasive ventilation nocturnally (80).
Both the rocking bed and pneumobelt are especially well-suited for use in patients with bilateral diaphragmatic paralysis because their main action is to assist diaphragm motion (81, 82). However, they are both relatively ineffective ventilators and are of limited value in patients with acute respiratory deteriorations. Furthermore, efficacy of both depends on abdominal and chest wall compliance, so that patients with severe kyphoscoliosis, excessive thinness, or obesity may not be adequately ventilated.
Other Modes of Noninvasive Ventilatory Assistance
Diaphragm pacing and glossopharyngeal breathing are ventilatory methods used in selected patients to increase independence from more cumbersome modes. Diaphragm pacing consists of a radio-frequency transmitter and antenna that signal a receiver and electrode that are surgically implanted, usually in the subclavicular area (83). The receiver and electrode stimulate the phrenic nerve, causing diaphragmatic contraction. Use of diaphragm pacing is limited to patients with central hypoventilation or high spinal cord lesions who have an intact diaphragm and phrenic nerve, or a phrenic nerve that can be repaired (84). However, recent advances in NPPV have virtually eliminated the need for diaphragm pacing in patients with central hypoventilation (85).
Diaphragm pacing has a number of limitations, including high cost and the tendency to produce upper airway obstruction by the same mechanism as negative pressure ventilators, necessitating continuation of tracheostomy in as much as 90% of users (85). In addition, there are no controlled studies that demonstrate long-term efficacy. On the other hand, diaphragm pacers are very easy to use, highly portable, and free patients from the need to be connected to positive pressure ventilators. Thus, some patients with high cord lesions still prefer diaphragm pacing to other types of ventilatory assistance. Its chief application at the present time is in children with high spinal cord lesions who have difficulty adapting to noninvasive forms of ventilation (86).
Glossopharyngeal or "frog" breathing utilizes intermittent motions of the tongue and pharyngeal muscles to inject (or gulp) air into the trachea (87). When gulping, the patient lowers and then raises the tongue against the palate in a pistonlike fashion, forcing air into the trachea. With practice, each gulp injects 50 to 150 ml of air in about 0.5 s. The patient then closes the glottis to prevent escape of air and rapidly repeats the gulping until a tidal volume of approximately 500 or 600 ml is achieved. The air is then exhaled, and the maneuver is repeated eight to 10 times per minute, so that a normal minute volume can be attained. The technique can be used instead of mechanical ventilation for periods of several hours, even in patients with severely weakened lower respiratory muscles. It can also be used to augment individual breaths in patients with low tidal volumes or to achieve inhaled volumes of 2 to 2.5 L to assist in coughing. The obvious advantage of the technique is that it requires no mechanical appliance. However, use is limited to patients who have intact upper airway musculature, more or less normal lungs, and who are capable of learning the technique. Good candidates include those with high spinal cord injuries, postpolio syndrome, and selected patients with other neuromuscular diseases (88).
Techniques to Assist Cough
The techniques to assist ventilation described above serve mainly as aids to inspiration. However, when weak expiratory muscles are combined with a markedly reduced vital capacity, as occurs in end-stage neuromuscular diseases, the cough mechanism is severely impaired. The inability to cough effectively is tolerable for patients who have minimal airway secretions and an intact swallowing mechanism, but an episode of acute bronchitis or aspiration of oral secretions can precipitate a life-threatening crisis. When this occurs, strategies to assist cough and expectoration may be lifesaving.
An effective cough depends on the ability to generate adequate expiratory airflow, estimated at > 160 L/min (89). Expiratory airflow is determined by lung and chest wall elasticity, airway conductance, and, at least at higher lung volumes, expiratory muscle force. By generating an adequate vital capacity (> 2.5 L) to take advantage of respiratory system elasticity, inspiratory muscle function also contributes to cough adequacy. In addition, an effective cough requires intact glottic function, so that explosive release of intrathoracic pressure can generate high peak expiratory cough flows (90). Considering that many patients with severe neuromuscular disease are too weak to take advantage of many of these mechanisms and have insufficient cough flows, techniques to assist cough should be applied.
The simplest maneuver to augment cough flow is manually assisted or "quad" coughing. This consists of firm, quick thrusts applied to the abdomen using the palms of the hands, timed to coincide with the patient's cough effort. The technique should be taught to caregivers of patients with severe respiratory muscle weakness with instructions to use it whenever the patient encounters difficulty expectorating secretions. With practice, the technique can be applied effectively and frequently, with minimal discomfort to the patient. Peak expiratory flows can be increased severalfold when manually assisted coughing is applied successfully (91). To minimize the risk of regurgitation and aspiration of gastric contents, the patient should be semiupright when manually assisted coughing is applied, and the technique should be used cautiously after meals.
Although manually assisted coughing may enhance expiratory force, it does not augment inspired volume. Patients with severely restricted volumes, therefore, may still achieve insufficient cough flows, even when assisted by skilled caregivers. To overcome this problem, the inhaled volume should be augmented (92). One approach is to "stack" breaths using glossopharyngeal breathing or volume-limited ventilation and then to cough using manual assistance. Another is to use a mechanical insufflator-exsufflator, a device that was developed during the polio epidemics to aid in airway secretion removal. This device delivers a positive inspiratory pressure of 30 to 40 cm H2O via a face mask and then rapidly switches to an equal negative pressure (91). The positive pressure assures delivery of an adequate tidal volume, and the negative pressure has the effect of simulating the rapid expiratory flows generated by a cough. Use of the insufflator-exsufflator has been combined with manually assisted coughing in an effort to further augment cough flows.
Although no controlled trials have evaluated the efficacy of the cough insufflator-exsufflator, anecdotal evidence suggests that it enhances removal of secretions in patients with impaired cough (92, 93). It has been particularly useful in patients' homes to treat episodes of acute bronchitis, permitting avoidance of hospitalization (94). Other devices that aid expectoration such as the percussive ventilator and Hayek oscillator have theoretical advantages over some of the other techniques for assisting secretion removal (92). Their use of high frequency vibrations (as much as 10 to 15 Hertz) may facilitate mobilization of airway secretions. Unfortunately, even anecdotal evidence to support their use is lacking.
Clinicians caring for patients with severe cough impairment should be familiar with the various techniques available to assist expectoration. These are particularly important with noninvasive ventilation, because there is no direct access to the airway, and secretion retention is a frequent complication and common cause for failure. Although controlled data are lacking, these techniques appear to be helpful in maintaining airway patency in patients with cough impairment during use of noninvasive ventilation in both acute and chronic settings.
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NONINVASIVE VENTILATION TO TREAT RESPIRATORY FAILURE |
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TOP
CONTENTS
TRENDS IN THE USE...
EQUIPMENT AND TECHNIQUES FOR...
NONINVASIVE VENTILATION TO...
NONINVASIVE VENTILATION FOR...
Evidence for Efficacy
MECHANISMS OF ACTION OF...
PRACTICAL APPLICATION OF NPPV
ADVERSE EFFECTS AND...
SUMMARY AND CONCLUSIONS
REFERENCES
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Noninvasive Ventilation in the Acute Care Setting
Until recently, endotracheal intubation has been the preferred mode for the ventilatory management of acute respiratory failure. The recent increase in use of noninvasive ventilation in the acute care setting has been fueled by the desire to reduce complications of invasive ventilation and to improve resource utilization, as discussed previously. Noninvasive ventilation for acute respiratory failure has the potential of reducing hospital morbidity, facilitating the weaning process from mechanical ventilation, shortening length of hospitalization and thereby costs, and improving patient comfort. However, patients must be selected carefully because the risk of complications could be increased if noninvasive ventilation is used inappropriately. Evidence for efficacy, selection guidelines and concerns about time demands on medical personnel are discussed in detail below.
Evidence for Efficacy
Continuous positive airway pressure. Although not a form of mechanical ventilatory assistance per se, CPAP is commonly used for the therapy of certain forms of respiratory failure. The use of CPAP to treat acute pulmonary edema was first described in 1938 (17). In more recent years, four randomized prospective trials and one large prospective series (95) have demonstrated significant improvements in vital signs and gas exchange as well as drastic reductions in intubation rates attributable to the use of CPAP (10 to 12.5 cm H2O) administered via a face mask (Table 1). The evidence for CPAP's ability to improve oxygenation and avoid intubation in these studies is very strong, with average intubation rates dropping to 19% from 47% in control subjects. However, with the exception of ICU length of stay in one study (98), improvements in other outcome variables such as complication rates, lengths of hospital stays, or mortality have not been demonstrated.
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CPAP has also been tried in patients with various other causes of respiratory failure, both hypoxemic and hypercapnic. In a number of uncontrolled studies on mainly postoperative and trauma patients, application of CPAP by face mask was associated with an abrupt improvement in oxygenation and little need for intubation (100). However, entry criteria permitted inclusion of patients with mild to moderate respiratory distress, and, in the absence of controls, it is unclear that CPAP was more successful than oxygen supplementation alone would have been. CPAP has also been tried in patients with acute exacerbations of COPD (104, 105) and deteriorations of obstructive sleep apnea (106). In these studies, use of nasal CPAP (5 to 9.3 cm H2O) was associated with improvements in PaCO2 and PaO2, with few patients requiring intubation. In COPD, relatively low levels of CPAP (5 cm H2O) appear to be beneficial, perhaps by counterbalancing the effects of auto-PEEP (107). However, the lack of controls renders these studies inconclusive, and further studies comparing CPAP to conventional therapy or NPPV are needed.
Negative pressure ventilation. In recent years, a few reports of negative pressure ventilation used to treat acute respiratory failure have come from Spain and Italy. In patients with COPD exacerbations, Montserrat and colleagues (108) compared a 6-h period of wrap ventilation with a 6-h control period on consecutive days, observing lower PaCO2 values and improvements in oxygenation and dyspnea during ventilator use. In an uncontrolled study also using wrap ventilation for patients with COPD exacerbations, Sauret and colleagues (109) showed reductions in PaCO2 and improved oxygenation during ventilator use. Subsequently, Corrado and colleagues (110) reported a 16-yr experience using the tank ventilator to treat 2,011 patients, mainly with acute COPD exacerbations and some with restrictive thoracic disorders. Arterial blood gas values improved substantially during tank use (PaCO2 fell from 80 mm Hg on admission to 50 mm Hg on discharge) and hospital mortality rate was only 10%. Even patients who were initially in coma had a mortality rate of only 23%. However, the study was retrospective, and selection criteria were not given.
More recently, Corrado and colleagues (111) compared the outcomes of 26 patients with COPD and acute respiratory failure treated with negative pressure ventilation to those of 26 matched patients ventilated with invasive positive pressure ventilation, but the patients were treated in different units. The investigators found similar mortality rates (23 versus 27%, negative pressure versus invasive) and hospital lengths of stay (12 d for both), but negative pressure ventilation was used for only 16 h on average, whereas invasive mechanical ventilation was used for 96 h (p < 0.05). Thus, available evidence suggests that negative pressure ventilation can improve alveolar ventilation, tolerance of O2 supplementation, and the sensation of dyspnea in acutely ill patients with COPD. However, the lack of randomized controls or well-defined selection criteria among the available studies makes it difficult to draw firm conclusions or offer general recommendations.
Noninvasive positive pressure ventilation. The recent enthusiasm for treating acute respiratory failure with noninvasive ventilation has been directed at NPPV. The demonstration that NPPV reduces esophageal pressure swings and the diaphragmatic electromyogram (EMG) sum signal in patients with respiratory disease (112, 113) led investigators to hypothesize that NPPV would be useful for supporting ventilation in patients with acute respiratory decompensations who were at risk for respiratory muscle fatigue. After the signal studies of Meduri and colleagues (114), Brochard and colleagues (113), and Elliott and colleagues (115), numerous other uncontrolled studies examining this hypothesis were reported (116). These uncontrolled studies will not be examined in detail; rather, the following discussion will focus on randomized controlled trials while reviewing the available evidence on the efficacy of NPPV for various applications in the acute care setting.
Obstructive Diseases
Chronic obstructive pulmonary disease (COPD). Patients with exacerbations of COPD constitute the largest single diagnostic category among reported recipients of NPPV. Among the numerous uncontrolled studies, success rates in avoiding intubation have ranged from 58 to 93%. In an early study using historically matched control subjects, Brochard and colleagues (113) reported that only 1 of 13 patients with acute exacerbations of COPD treated with face mask NPPV required endotracheal intubation, compared with 11 of 13 control subjects. In addition, patients treated with NPPV were weaned from the ventilator faster and spent less time in the intensive care unit than did the control subjects. A larger, more recent historically controlled trial has yielded similar results (126). However, historically-matched control subjects may bias studies in favor of the treatment group (127), so these studies are unable to provide definitive evidence.
Subsequently, five randomized controlled trials have been published (128) (Table 2) that lend support to the earlier observations of Brochard and colleagues (113). Bott and colleagues (128) randomized 60 patients with acute exacerbations of COPD to receive nasal NPPV or conventional therapy. Within the first hour of therapy, mean PaCO2 fell from 65 to 55 mm Hg, and dyspnea scores improved among treated patients, whereas no significant changes occurred among control subjects. In addition, mortality rate fell from 30% among control patients to 10% among NPPV-treated patients, although this reduction became statistically significant only after exclusion of four patients who were randomized to the NPPV group but never actually received it. Kramer and colleagues (129) randomized 31 patients with various etiologies for respiratory failure, 21 of whom had COPD, to receive NPPV or conventional therapy. Among patients with COPD who received NPPV in their study, respiratory rates and PaCO2 values fell more rapidly during the first hour of therapy than among control patients, and intubation rates were reduced to 9% compared with 67% in control patients. Hospital lengths of stay and mortality rates tended to be less among the COPD subgroup of patients treated with NPPV, but differences were not statistically significant, perhaps because the number of patients was small.
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Subsequently, a multicenter European trial randomized 85 patients with COPD to receive face mask PSV or conventional therapy (130). Respiratory rate but not PaCO2 fell significantly during the first hour, and intubation rates were lowered from 74% among control patients to 26% among NPPV patients. In addition, complication rates were reduced from 48 to 16%, mortality rates from 29 to 9%, and hospital lengths of stay from 35 to 23 d, respectively, among controls versus NPPV patients (all p < 0.05). Questions have been raised about the adequacy of standard therapy, the high complication and mortality rates and lengths of stay among control patients, and the generalizability of this study, considering that it was performed in ICUs and only 31% of patients admitted with COPD exacerbations were enrolled (133). On the other hand, its size and prospective randomized design are important strengths, and careful patient selection is clearly important to the success of NPPV.
An additional controlled trial compared the effects of NPPV with those of doxapram over 4 h in patients with acute exacerbations of COPD, finding that although doxapram transiently improved PaO2, it had no effect on PaCO2 (131). After three deaths in the doxapram group, the protocol was amended so that patients who deteriorated while receiving doxapram could be offered NPPV. This occurred in two patients, who were treated successfully and discharged home. NPPV was deemed to be more effective than doxapram, because it brought about sustained improvements in both PaCO2 and PaO2. Another randomized controlled trial on patients with COPD compared the efficacy of standard medical therapy with that of NPPV in 30 patients with acute hypercapnic respiratory failure caused by exacerbations, pneumonia, or congestive heart failure (132). Those randomized to NPPV had greater improvements in pH and respiratory rate within 6 h, a higher success rate (93 versus 60%, p < 0.05), and a shorter hospital length of stay (11.7 versus 14.6 d, p < 0.05) than control subjects.
The most recent and largest randomized controlled trial was performed on 236 patients with COPD exacerbations and pH values between 7.25 and 7.35 treated on general respiratory wards at 14 United Kingdom hospitals (134). Patients treated with NPPV, as opposed to control subjects, had a reduced need for intubation (15 versus 27%, p = 0.02), and more rapid improvements in pH and respiratory rate. The investigators noted that patients with a pH < 7.30 had a higher mortality rate than did those with a higher pH and suggested that a higher dependency unit might be preferable (if available) for this sicker subgroup. They also acknowledged that differences in ICU availability between countries might limit the generalizability of the results.
Among the numerous controlled and uncontrolled studies examining the efficacy of NPPV in acute respiratory failure due to COPD, only two have obtained negative results. Foglio and colleagues (135) used nasal NPPV to treat 49 consecutive patients with COPD and acute exacerbations. Twenty-four failed to tolerate the mask and served as the control group for the 25 who tolerated the mask. Blood gas determinations in both groups improved at similar rates, and no differences in outcome were apparent between the two groups. More recently, Barbe and colleagues (136) randomized 24 patients with acute COPD exacerbations to receive nasal NPPV or standard therapy. Patients in both groups had similar improvements in blood gas determinations and hospital lengths of stay, and no differences in breathing pattern or indices of respiratory muscle strength were apparent. Both Foglio and colleagues (135) and Barbe and colleagues (136) concluded that NPPV is ineffective in treating exacerbations of COPD.
However, both studies (135, 136) enrolled patients with average pH values (7.33 and 7.34, respectively) that were higher than those of patients in the favorable studies, and baseline PaCO2 in the Barbe study was lower (Table 2). This suggests that these patients had milder exacerbations than did those entered in most of the other studies. Furthermore, none of the control patients in the Barbe study required intubation, whereas almost three quarters of the control patients in the studies by Kramer and colleagues (129) and by Brochard and colleagues (130) were intubated. This supports the contention that patients in the two studies with negative findings were less acutely ill than those in the favorable studies and argues that NPPV should be reserved for patients with COPD who are at risk of requiring intubation. In fact, Foglio and her colleagues have subsequently reported favorable results with NPPV in a study on patients with COPD and more severe exacerbations (137). This latter study also used pressure- as opposed to volume-limited ventilation and full face masks instead of nasal masks, other factors that may have contributed to the more favorable results. Further studies have also found that survival rates are better and the need for rehospitalization less for patients treated with NPPV as opposed to those treated with conventional therapy, even for as long as a year after the acute episode (138, 139). Although these latter studies were not randomized so that less ill patients may have been treated noninvasively, it is also possible that NPPV avoids late complications of invasive ventilation such as sustained muscle weakness or swallowing dysfunction (140).
In summary, the available evidence establishes that NPPV improves vital signs and dyspnea scores and avoids intubation in patients with severe COPD exacerbations. Also, based on statistically significant differences or trends in the controlled studies, evidence suggests that NPPV reduces morbidity and mortality rates and intensive care unit or hospital lengths of stay. In a recent meta-analysis, Keenan and colleagues (141) concluded that the evidence from the combined controlled studies supports the use of NPPV in the therapy of COPD exacerbations. Although some investigators have questioned the strength of the evidence related to improvements in morbidity and mortality associated with NPPV use (133), these reported benefits pose ethical concerns if further confirmatory studies are to be performed. It should also be borne in mind that NPPV in the above studies was used to avoid intubation, but not to replace it. Thus, although NPPV may be viewed as the ventilatory therapy of first choice for selected patients with COPD (see section on patient selection), invasive ventilation remains the method of choice for COPD patients with contraindications to NPPV.
Asthma. No randomized controlled trials have been published on the use of NPPV to treat acute asthma. Most studies have included two or fewer patients with asthma, but among five patients with acute asthma included in a study of 158 patients with acute respiratory failure treated with face mask NPPV (average initial PaCO2 67 mm Hg), only one required intubation, and there were no mortalities (125). In a larger subsequent study (142), 17 patients with asthma and an average initial pH of 7.25 and PaCO2 of 65 mm Hg were treated with face mask PSV. Only two required intubation (for increasing PaCO2), average duration of ventilation was 16 h, and no complications occurred. The investigators concluded that NPPV appears to be highly effective in correcting gas exchange abnormalities and avoiding intubation in patients with acute severe asthma exacerbations. However, medical therapy alone may be highly effective (143), and in the absence of controls or well-defined selection criteria, no conclusions can be drawn regarding the relative effectiveness of NPPV versus conventional therapy in asthma exacerbations.
Cystic fibrosis. Hodson and colleagues (144) described the use of NPPV to treat patients with end-stage cystic fibrosis with FEV1 values ranging from 350 to 800 ml and severe acute on chronic CO2 retention (initial PaCO2 values ranging from 63 to 112 mm Hg). Six patients were supported for periods ranging from 3 to 36 d, four of whom survived until a heart-lung transplant could be performed. This study illustrates the potential utility of NPPV as a rescue therapy in supporting patients with acutely deteriorating cystic fibrosis and in providing a "bridge to transplantation," but given the lack of any controlled trials, the efficacy of this approach remains unproven.
Restrictive diseases. The use of NPPV in patients with chronic respiratory failure caused by restrictive thoracic diseases is well accepted. However, controlled studies on the management of acute respiratory failure in these patients have not been done, perhaps because they make up only a small portion of patients presenting with acute respiratory failure. In a study that used NPPV to treat all eligible patients admitted to an intensive care unit during a 2-yr period, only five of 158 patients had restrictive lung disease (125). On the other hand, some uncontrolled series have reported success using NPPV to alleviate gas exchange abnormalities and avoid intubation in neuromuscular disease (145) and kyphoscoliosis (146) patients with acute respiratory failure.
Bach and colleagues (94) recently described a regimen for managing acute deteriorations in patients with chronic respiratory failure caused by neuromuscular disease. The patients receive 24-h noninvasive ventilation during the exacerbation. Pulse oximetry is monitored continuously and when oxygen saturation falls below 90%, secretion removal is aggressively assisted using manually assisted coughing and mechanical aids such as the cough insufflator-exsufflator until oxygen saturation returns to the 90% range. Although no controlled studies have established the efficacy of this approach, Bach and colleagues (94) reported that its use during acute exacerbations permitted management in the home, with a dramatic reduction in the need for hospitalization.
No information is available on NPPV therapy of acutely deteriorating restrictive lung diseases such as interstitial fibrosis. However, this application would not be recommended unless an acute reversible superimposed condition was thought to be responsible for the deterioration.
Acute Cardiogenic Pulmonary Edema
As discussed previously, CPAP has been shown to be effective in avoiding intubation in patients with acute pulmonary edema (Table 1). Considering that inspiratory assistance combined with expiratory pressure could reduce breathing work and alleviate respiratory distress more effectively than CPAP alone, patients with acute pulmonary edema have been included in a number of uncontrolled reports on the use of NPPV for acute respiratory failure. In their initial study, Meduri and colleagues (114) reported that one of two patients with acute pulmonary edema had an "excellent" response to NPPV. The same investigators have also described eight patients with acute pulmonary edema treated with face mask PSV, four of whom avoided intubation (125). Others (147) have reported small case series of patients with acute pulmonary edema successfully treated with NPPV.
More recently, two prospective but uncontrolled studies have evaluated face mask PSV administered to patients with acute pulmonary edema (148, 149). In the first (148), pulse oximetry, pH, and PaCO2 all improved within 30 minutes of initiation of NPPV in 29 patients, only one of whom required intubation. The second study (149) observed similar effects on gas exchange, but five of 26 patients required intubation, and successfully treated patients had higher PaCO2 values (54 versus 32 mm Hg) and lower creatine phosphokinases (176 versus 1,282 IU) (both p < 0.05) than failures. Further, four patients died in the first study and five in the second, three and four with myocardial infarctions, respectively. The investigators concluded that NPPV is a "highly effective technique," but an accompanying editorial advised caution when applying NPPV to patients with acute myocardial infarction (150). In addition, a retrospective survey of the emergency management of acute pulmonary edema (151) found that use of NPPV was associated with a 2-d shorter length of ICU stay than invasive ventilation.
In the only controlled trial yet published comparing CPAP with NPPV (using bilevel positive airway pressure), Mehta and colleagues (152) found that patients treated with NPPV had more rapid reductions in PaCO2 than did those in the CPAP group. However, the myocardial infarction rate was higher (71% in the NPPV group versus 31% in the CPAP group, p = 0.05), leading to premature termination of the study by the investigators. Rates of intubation, morbidity, and mortality were similar between the two groups. More patients in the NPPV groups than in the CPAP group had chest pain upon entry into the study (10 versus four, p = 0.06), raising concerns about the adequacy of patient randomization. The investigators concluded that most patients can be managed successfully with CPAP alone, but because it lowers PaCO2 more rapidly than CPAP, NPPV may have advantages in patients with CO2 retention on presentation. They also advised caution when using NPPV in patients with acute myocardial infarctions and further evaluation of the hemodynamic effects of NPPV. In a meta-analysis of studies on the noninvasive therapy of acute pulmonary edema, Pang and colleagues (153) came to the same conclusion regarding CPAP, but they considered the evidence on NPPV too scanty to support any conclusions. Thus, pending the publication of more studies comparing CPAP and NPPV, CPAP (10 to 12.5 cm H2O) should be considered the initial therapy of choice for acute pulmonary edema, with inspiratory pressure added in patients with hypercapnia or persisting dyspnea after initiation of CPAP.
Community-acquired pneumonia. Controlled trials examining the effect of NPPV in acute pneumonia have appeared only recently. Earlier large series included patients with pneumonia, but were unable to establish efficacy of NPPV (125). In fact, the presence of pneumonia has been associated with a poor outcome of NPPV in some studies (154). Recently, however, Confalonieri and colleagues (155) randomized 56 patients with severe community-acquired pneumonia to receive NPPV plus conventional therapy or conventional therapy alone. Patients treated with NPPV had reduced intubation rates (21 versus 50%, p < 0.03) and a shorter duration of ICU stay (1.8 versus 6 d, p < 0.04) than did control subjects, although hospital lengths of stay and hospital mortality rates were similar. In addition, a subgroup analysis revealed that significant benefits were attributable only to patients with underlying COPD, who also had lower 2-mo mortality rates if treated initially with NPPV (11 versus 63%, p = 0.05). Although these results are promising, routine use of NPPV for community-acquired pneumonia in patients without COPD cannot be advocated until more studies clarify selection criteria and demonstrate benefit in this subgroup.
Hypoxemic Respiratory Failure
Studies on the use of NPPV for patients with hypoxemic respiratory failure (defined as those with a PaO2/FIO2 ratio of < 200, respiratory rate > 35/min, and diagnoses including acute pneumonia, acute pulmonary edema, ARDS, and trauma) (125) have yielded conflicting results. In their original study, Meduri and colleagues (114) included four patients with hypoxemic respiratory failure, two with acute pulmonary edema, and two with acute pneumonia. All were treated successfully with NPPV. Subsequently, Wysocki and colleagues (156) found that seven of eight patients in their trial with PaCO2 values < 45 mm Hg failed NPPV, whereas seven of nine with initial PaCO2 values > 45 mm Hg were successfully treated. In a follow-up randomized trial on patients with a variety of causes for their acute respiratory failure (157), the same investigators found no benefit of NPPV over conventional therapy among all entered patients. When patients with a PaCO2 < 45 mm Hg (90% of whom required intubation) were excluded in a post hoc analysis, NPPV significantly reduced intubation rate, length of ICU stay, and ICU mortality among the remaining hypercapnic patients. The implication of these findings is that hypoxemic respiratory failure without CO2 retention responds poorly to NPPV.
However, more recent uncontrolled studies suggest that some patients with hypoxemic respiratory failure may respond favorably to NPPV. Patrick and colleagues (67) reported the successful use of noninvasively administered proportional assist ventilation in eight of 11 patients with de novo respiratory failure who were in need of immediate intubation. These eight patients, four of whom were severely hypoxemic without CO2 retention, had rapid improvements in dyspnea scores and avoided intubation while the cause of their respiratory failure was treated. In the larger series of Meduri and colleagues (125), 41 of 158 patients had hypoxemic respiratory failure. These patients had multiple causes for their respiratory failure including COPD, pneumonia, ARDS, pulmonary edema, and restrictive lung disease. Despite having average initial PaO2/ FIO2 ratios of 110 mm Hg, these hypoxemic patients treated with NPPV required intubation in only 34% of cases. In addition, mortality was 22% compared with a predicted mortality (using the APACHE II score) of 40%. Among patients with ARDS, Rocker and colleagues (158) reported that NPPV successfully avoided intubation in six of 12 episodes among 10 patients with an average initial PaO2/FIO2 of 102. Beltrame and colleagues (159) examined trauma patients and found rapid improvements in gas exchange and a 72% success rate in 46 patients with respiratory insufficiency treated with NPPV, although those with burns did poorly. Despite the generally promising results, the retrospective and uncontrolled design of the above studies limits any conclusions that can be drawn.
In a recent controlled trial of 64 patients with hypoxemic respiratory failure randomized to receive NPPV or intubation (38), only 31% of the NPPV-treated patients required intubation. Improvements in oxygenation were comparable in the two groups, and NPPV-treated patients had significantly fewer septic complications such as pneumonia or sinusitis (3% versus 31%). In addition, there was a trend toward decreased mortality and length of ICU stay (27 versus 45% and 9 versus 15 d, respectively) in NPPV-treated