© 2003 American Thoracic Society
Noninvasive Ventilation for Cardiogenic Pulmonary EdemaFroth and Bubbles?Department of Critical Care Medicine Flinders Medical Centre and School of Medicine Flinders University Adelaide, South Australia, Australia Acute cardiogenic pulmonary edema usually presents with the sudden onset of acute hypoxemic respiratory failure that requires rapid assessment and treatment. Although this may be secondary to sudden decompensation of chronic heart failure, myocardial ischemia is also common. Typically, patients are hypoxemic with increased work of breathing, acidemic because of both respiratory and metabolic factors (1), and hypertensive and tachycardic. Diastolic dysfunction is the major contributor to raised hydrostatic pressure and pulmonary edema (2). Despite standard medical therapy with oxygen, nitrates, and diuretics, ventilator assistance may be needed. Noninvasive ventilation is widely used in cardiogenic pulmonary edema (3, 4), usually resulting in rapid relief of dyspnea. Although this simple but dramatic clinical observation was made many years ago (5), there are many unanswered questions. In this issue of the Journal (pp. 1432–1437), Nava and coworkers (6) report important findings from a multicenter study that helps focus some of these issues. While the term noninvasive ventilation covers numerous methods of ventilator assistance without endotracheal intubation, it is most commonly applied as positive airway pressure using a mask as the interface. The simplest and most commonly used technique in cardiogenic pulmonary edema is continuous positive airway pressure (CPAP). There has been growing interest, however, in the use of modes where inspiratory effort is supported by a greater level of positive pressure (pressure support) interposed on top of positive end-expiratory pressure (PEEP), also termed bilevel positive airway pressure. Raised intrathoracic pressure increases functional residual capacity and oxygenation, improves lung mechanics, and reduces work of breathing (7, 8). There may also be beneficial cardiovascular effects resulting in reduced afterload and preload; clinical studies, necessarily performed some hours after onset, show reduced transmural ventricular filling pressures (7). As mask CPAP leads to more rapid physiologic improvement, consistently reduced intubation rate, and a tendency to reduced mortality (9), there are qualified recommendations for its use in cardiogenic pulmonary edema (10–12). Nava and colleagues (6) prospectively randomized 130 patients with cardiogenic pulmonary edema, receiving standard medical therapy, to either oxygen at ambient pressure or oxygen with mask pressure support plus PEEP. Consistent with previous studies, this led to a more rapid improvement in physiologic variables, such as oxygenation, arterial carbon dioxide tension, respiratory rate, dyspnea, blood pressure, and heart rate. Mortality and intubation rate, however, were unchanged. How do the new data add to our understanding and use of noninvasive ventilation? Compared with CPAP, where airway pressure is designed to be constant, pressure support plus PEEP augments tidal volume and inspiratory unloading. This may be advantageous in cardiogenic pulmonary edema, in which there already appears to be an inverse relationship between initial arterial pH and intubation risk reduction with CPAP (9). Consistent with these data, subgroup analysis by Nava and colleagues (6) found a reduced risk of intubation in patients who were hypercapnic at baseline (2/33 versus 9/31, p = 0.015), supporting their contention that future trials need to stratify for this factor. Although Masip and colleagues (13) had reported that noninvasive pressure support reduced intubation risk, but not mortality, Sharon and coworkers (14) found that intubation and myocardial infarction were increased when compared with high-dose nitrates. The 80% intubation rate in their noninvasive ventilation group, however, well exceeds that expected from previous studies, and may reflect the low inspired oxygen concentration and low airway pressures used. An earlier study (15) comparing mask CPAP with pressure support plus PEEP, with comparable inspired oxygen concentrations, also found a greater rate of myocardial infarction with pressure support, but this may have been due to small sample size and a trend toward fewer patients with chest pain at baseline in the CPAP group. Importantly, Nava and coworkers (6) found no increased risk of myocardial infarction with mask pressure support ventilation. With this important safety issue now clarified, it is appropriate for studies to compare mask CPAP with noninvasive pressure support ventilation, and for there to be adequate power to detect different intubation risks, and possibly mortality differences. As blinding of these modes is difficult, robust guidelines for intubation, protocolized standard medical therapy, titration, and weaning of airway pressure, equivalence of inspired oxygen concentration, and ease of delivering airway pressure will all be important. For example, the average expiratory and inspiratory airway pressures used by Nava and colleagues (6) were 6.1 and 14.5 cm H2O, respectively. Is this equivalent to 10 cm H2O CPAP, which has been commonly used and recommended? Rather than augment systolic function, standard medical therapy for cardiogenic pulmonary edema targets the consequences and cause of diastolic dysfunction. If a parallel (systole with inspiration and diastole with expiration) can be made with the approach to prescribing noninvasive ventilation, it may be worth reexamining the prescription of PEEP when pressure support is used. Where are these patients best treated? Nava and colleagues (6) managed their patients in the emergency department, with an average period of 11.4 hours noninvasive ventilation—consistent with previous studies. While emergency department presentation is common, other investigators have managed these patients in their intensive or coronary care units. Practical issues, including local hospital setup and staffing, usually combine to determine where noninvasive ventilation is performed, although there is also the influence of multiple stakeholders. It is also important to recognize that the workload in the first 6–8 hours may be greater than required for a conventionally managed intensive care patient (16). In acute hypoxemic respiratory failure, the ability to rapidly respond with intubation and cardiorespiratory support is crucial. In cardiogenic pulmonary edema, there are additional demands, such as the recognition and treatment of arrhythmias and myocardial ischemia. Consequently, patients with severe cardiogenic pulmonary edema requiring noninvasive ventilator support need to be triaged to a high acuity environment with an adequate nurse–patient ratio and 24 hour medical care. In addition to standard medical care and noninvasive ventilation, the possibility that this represents an acute coronary syndrome needs to be considered. Unlike Nava and coworkers, who excluded or intubated patients requiring thrombolysis, many centers would concurrently provide support with noninvasive ventilation while administering thrombolytics or proceeding to percutaneous revascularization. FOOTNOTES Conflict of Interest Statement: A.D.B. has no declared conflict of interest. REFERENCES
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