Work of Breathing and Effects of Noninvasive Ventilatory Assistance |
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ABSTRACT |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Breathing pattern, gas exchange, and respiratory effort were assessed in five awake children with chronic stridor caused by laryngomalacia during spontaneous breathing (SB) and noninvasive mechanical ventilation (NIMV). During SB, the youngest children were able to maintain normal gas exchange at the expense of an increased work of breathing as assessed by calculated diaphragmatic pressure-time product (PTPdi), whereas the opposite was
observed in the older children. NIMV increased tidal volume, from
8.77 ± 2.04 ml/kg during SB to 11.67 ± 2.52 ml/kg during NIMV,
p = 0.04, and decreased respiratory rate, from 24.4 ± 5.6 breaths/ min during SB to 16.6 ± 0.9 breaths/min during NIMV, p = 0.04. NIMV unloaded the respiratory muscles as reflected by the significant reduction in PTPdi, from a mean value of 541.0 ± 196.6 cm
H2O · s · min
1 during SB to 214.8 ± 116.0 cm H2O · s · min1 during NIMV, p = 0.04. Therefore, NIMV successfully relieves the additional load imposed on the respiratory muscles. Long-term home
NIMV was provided to a total of 12 children with laryngomalacia (including these five) and was associated with clinical improvement in sleep and growth.
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INTRODUCTION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Keywords: stridor; laryngomalacia; diaphragmatic pressure-time product; noninvasive mechanical ventilation
Laryngomalacia is an anomaly of the larynx with laxity of the pharyngeal tissues causing the epiglottis, arythenoids, and aryepiglottic folds to involute and partially obstruct breathing during inspiration. Laryngomalacia accounts for more than 75% of cases of congenital stridor and is the most common cause of symptomatic partial upper airway obstruction in infants (1, 2). The majority of infants with laryngomalacia do well with resolution of stridor during the first 2 yr of life (3). However, potentially serious complications, including airway obstruction and sudden death (4, 5), pulmonary hypertension and cor pulmonale (5, 6), failure to thrive (5, 7), and possibly intellectual impairment can develop (8). In some severe cases of laryngomalacia, even surgery (for example, endoscopic resection of the aryepiglottic folds or epiglottoplasty) can fail to relieve upper airway obstruction (5, 7). When respiratory problems persist after surgery, a tracheostomy may be required. However, tracheostomy is associated with a significant morbidity and impairs normal development and, particularly, language development (9, 10).
Noninvasive mechanical ventilation (NIMV) has been shown to reduce the work of breathing, and in particular pressure support (PS), associated with positive end-expiratory pressure (PEEP), has been recognized as an efficient treatment of upper airway obstruction associated with alveolar hypoventilation (11). Our hypothesis is that NIMV may be proposed as an effective treatment for laryngomalacia, which might be an alternative to a tracheostomy.
To confirm our hypothesis, the aim of the study was, first, to analyze the breathing pattern and the respiratory effort in children with stridor caused by severe laryngomalacia; second, to evaluate the immediate physiologic value of NIMV in unloading the respiratory muscles in these children; and third, to investigate the long-term clinical effects of intermittent NIMV.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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The study was approved by our institutional board, and written informed consent was obtained from all parents. Criteria for enrollment were severe laryngomalacia with symptoms of upper airway obstruction and nocturnal hypoventilation. (Methods are detailed in the online supplement.)
The first part of the study evaluated the gas exchange, breathing pattern, and respiratory effort in five patients (Group 1) during spontaneous breathing (SB) and NIMV. The second part of the study evaluated the clinical long-term follow up of these five patients and seven additional patients (Group 2).
Two conditions were analyzed and compared; SB and NIMV (PS ventilation) delivered through a well-fitting nasal mask. Custom-made masks were used in infants (dead space < 5 ml) and commercial masks in older children. For NIMV, the initial PS level was set at 6 cm H2O and the initial positive PEEP at its lowest level, i.e., 2 cm H2O. PS and PEEP were titrated alternatively and progressively by 1 to 2 cm H2O steps, as has been used successfully by other groups (12, 13). The final pressure settings were the highest values tolerated by the patients. Therefore, target swings for esophageal pressure (Pes) and transdiaphragmatic pressure (Pdi) were the lowest value that we could obtain according to the tolerance of the patients. Other parameters were set in line with consensus guidelines (14).
Data were recorded during the last 5 min after a stable period for at least 20 min. Nasal mask was not tolerated during the SB period by the two youngest infants who desaturated immediately. For this reason, recording of flow and determination of tidal volume (VT) and inspiratory time/duty cycle ratio (TI/Ttot) during SB were only made in the three oldest patients.
We measured respiratory flow (this was integrated to yield VT), airway pressure (Paw), SaO2, respiratory rate (RR), heart rate, and end-tidal carbon dioxide (PETCO2) directly from the mask. Pes and gastric pressures (Pga) were measured using catheter-mounted transducers (Gaeltec, Dunvegan, Isle of Skye, UK) (15) positioned using standard techniques (16). We measured the Pdi swings and the diaphragmatic pressure-time products (PTPdi) on the whole duty cycle as previously described (17, 18).
Long-term follow up was obtained in these five patients (Group 1) and seven additional patients (Group 2). The setting of the PS and PEEP levels in Group 2 were determined on a clinical basis, i.e., the disappearance of stridor, chest retractions, and snoring, as well as desaturations and hypercapnia during sleep (19, 20). Compliance to the treatment was systematically assessed at home at a monthly basis by means of a data logger that recorded date and time as well as the duration of the NIMV use. The effects of NIMV on nocturnal SaO2 and growth was assessed in all patients as well as the tolerance and duration of NIMV.
Statistical Analysis
Data are given as mean ± SD. Comparison between SB and NIMV were made using Wilcoxon's rank test. Correlation between the different variables was made by simple regression.
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Patients
The characteristics of the patients are presented in Table 1. Five infants had chest wall deformity and six had failure to thrive requiring nutritional support (Patients 1, 2, and 6 to 9). All the patients were naive to NIMV. The mean age of the patients was 32.9 ± 25.8 mo, with three patients being younger than 1 yr of age. The four youngest patients (Patients 1, 6, 7, and 8) had the most severe upper airway obstruction. Levels of PS ranged from 4 to 8 cm H2O and levels of PEEP from 4 to 10 cm H2O (Table 1).
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Gas Exchange and Breathing Pattern during SB and the Effect of NIMV
Gas exchange was severely impaired in Patient 5 and within normal range in the other patients (Figure 1). SaO2 increased from 94.9 ± 2.7% during SB to 96.9 ± 2.0% during NIMV, p = 0.07, and PETCO2 decreased from 41.0 ± 6.5 mm Hg during SB to 34.8 ± 6.2 mm Hg during NIMV, p = 0.04 (Figure 1).
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Respiratory rate decreased from 24.4 ± 5.6 breaths/min during SB to 16.6 ± 0.9 breaths/min during NIMV, p = 0.04 (Figure 2). In the three oldest patients, mean VT increased from 8.77 ± 2.04 ml/kg during SB to 11.67 ± 2.52 ml/kg during NIMV (Figure 2), and mean TI/Ttot decreased significantly during NIMV, with a value of 0.59 ± 0.25 during SB and 0.35 ± 0.04 during NIMV. There was a negative correlation between age and SaO2, r2 = 0.526, p < 0.0001, a positive correlation between age and PETCO2, r2 = 0.374, p = 0.009 (Figure 3) and a positive correlation between age and TI/Ttot, r2 = 0.570, p = 0.04.
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Respiratory Effort during SB and the Effect of NIMV
A tracing from Patient 3 during SB and NIMV is presented in Figure 4, which shows a decrease in Pes and Pdi swings during NIMV. In addition, Figure 5 shows the dynamic relationship between Pes and Pga during a representative cycle of SB and NIMV in Patient 3. It demonstrates, as in all patients of Group 1, the absence of an abnormal increase of Pga during expiration, i.e., during the increase of Pes during SB and/or NIMV. These results permit us to state that no patient expiratory muscle recruitment was present during SB and/or during NIMV.
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Pdi swing decreased from 20.7 ± 4.3 cm H2O during SB to
8.9 ± 4.3 cm H2O during NIMV, p = 0.04 (Figure 6). Similar
reductions were observed for PTPdi with mean PTPdi decreasing from 541.0 ± 196.6 cm H2O·s·min1 to 214.8 ± 116.0 cm
H2O·s·min1 during NIMV, p = 0.04 (Figure 6). In addition,
we observed an excellent relationship for the five patients between the decrease in Pes and Pdi swings and the disappearance of inspiratory dyspnea characterized by stridor, loud
breathing, chest deformity, sweats, and a low SaO2. There was
also a negative correlation between age and indices of respiratory effort, as reflected by 
Pdi, r2 = 0.461, p = 0.03, and
PTPdi, r2 = 0.730, p = 0.01 (Figure 7).
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Clinical Follow-up of the Patients
NIMV was associated with important clinical improvements in all the patients.
Patients of Group 1.
NIMV was started at the age of 8 mo
in Patient 1, when his weight was 5.4 kg (
3.5 SD) and height
63 cm (3 SD). He slept with his NIMV for a mean of 12 ± 2 h/d. A significant growth catch-up was observed during
NIMV, with a weight gain of 3.6 kg during the first 6 mo despite the discontinuation of his nasogastric feeding 3 wk after
the start of NIMV. At the age of 14 mo, he weighted 9 kg (1 SD)
and measured 73 cm (1.5 SD). One year later, a trial of discontinuation of NIMV because of an improvement of his sleep
under room air, was associated with a 1 kg weight loss in 3 wk,
which was regained after recommencing NIMV. This infant also had an important chest-wall deformity with xyphoid and
subcostal recession, which totally regressed 8 mo after the
start of NIMV.
Patients of Group 2. NIMV was well tolerated in all the patients. The chest wall deformity, which was present in three patients (Patients 6, 7, and 11) improved noticeably after a mean of 6 mo of NIMV. NIMV was also associated with significant weight gain in four patients (Patients 6 to 9).
In this group, SaO2 was checked during a whole night in the hospital before discharge, once the patient was totally adapted to the ventilator, which occurred generally within 2 wk after the start of NIMV. Mean nocturnal SaO2 improved significantly from 91.7 ± 2.3% before to 96.2 ± 2.0% during NIMV, p = 0.03. The nocturnal nadir SaO2 also improved significantly after the initiation of NIMV, with a mean nadir SaO2 of 74.7 ± 7.5% before NIMV to a nadir of 88.0 ± 2.5% while receiving NIMV. The percentage of night time spent with a SaO2 < 90% fell from 29.5 ± 19.6% before NIMV to 0.5 ± 0.8% while receiving NIMV, p = 0.03. Successful discontinuation of NIMV was possible in four patients after 6 to 30 mo. Patient 10, who has a picnodysostosis, is still dependent on NIMV after 52 mo. Compliance was systematically assessed at home in the two groups of patients on a monthly basis and was excellent, with a mean daily use of 10.2 ± 2.3 h/d in the patients performing a daytime nap and a mean daily use of 7.9 ± 1.9 h per night in the other patients.| |
DISCUSSION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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This study is the first to quantify the breathing effort in children with stridor caused by laryngomalacia and to demonstrate that NIMV can adequately unload the respiratory muscles and improve the clinical status of these children on a long-term follow up.
The Importance of the Ventilator Equipment
Different nasal masks and ventilators were used in this study. A limited number of commercial masks are available for infants. Furthermore, the volume of the mask is often too large, therefore limiting the benefit of NIMV because of the increase in dead space, especially in these young children. For this reason, we recommend custom-made masks, molded when the child sucks his or her pacifier, which favors simultaneous closure of the mouth during NIMV. The dead space of the custom-made masks did not exceed 5 ml. Concerning the ventilators, no specific comparison was made in this study. The most efficient ventilator and ventilatory mode, based on the lowest Pdi swings, as well as what was best tolerated by the patient, was adopted (Table 1).
The Influence of Age
Breathing pattern, gas exchange, and PTPdi were related to age. Normal gas exchange was preserved in the youngest children at the expense of an increased PTPdi, whereas the opposite was observed in older children. We speculate that the longer duration of the resistive load in these older children could have caused a "resetting" of the respiratory centers so that the penalty of a reduced oxygen cost of breathing was an increase in arterial CO2. Whether respiratory muscle fatigue could also be implicated in this response cannot be excluded, though the present data do not address this.
NIMV Can Adequately Decrease the Respiratory Effort
This study shows that NIMV can effectively unload the respiratory muscles in children with severe laryngomalacia. The maintenance of a continuous pressure by PEEP is the most important component of a ventilatory assistance in patients with an upper airway obstruction. This PEEP alleviates the airway obstruction, either by splinting the upper airway open, or by increasing functional residual capacity, which in turn reflexively dilates the pharynx (21, 22). The level of PEEP sufficient to relieve airway obstruction depends on the severity of the obstruction and the patient's state, i.e., sleep or wakefulness. In this study, the relatively low levels of PEEP, sufficient to adequately decrease Pes and Pdi during wakefulness, are probably insufficient during sleep. Most investigators agree that the recording of Pes represents the "gold standard" to adjust CPAP and bilevel PS titration (23, 24). However, because in the patients of Group 1, we observed an excellent relationship between the decrease in Pes and Pdi swings and the disappearance of the clinical signs of loud breathing, we decided to use a noninvasive method on the patients of Group 2 for the titration of PEEP and PS.
Long-term Beneficial Effect of NIMV
The tolerance of NIMV was excellent in all patients. This certainly contributed to the good compliance in these patients. The better comfort of this noninvasive technique, as compared with a tracheostomy, is likely to translate into a better quality of life, not only for the patients but also for their families.
We did not perform polysomnography in our population, but we clearly observed an improvement of nocturnal SaO2 in Group 2. In addition, all the families claimed a subjective improvement in the quality of sleep, daytime sleepiness, and quality of life of their child when NIMV was used. Abnormal respiratory effort during sleep has been shown to worsen sleep architecture (25). This makes us speculate that NIMV could improve the quality of sleep of our population by reducing the respiratory effort.
Moreover, we observed a decrease in clinical indices of malnutrition after initiation of NIMV. If the cost of breathing represents less than 5% of the total energy expenditure in normal subjects, it is well known that it can be markedly elevated in critically ill patients. This is also probably the case in our population, as reflected by the improvement of anthropometric indices during chronic NIMV, which suggests that the decrease of work of breathing during NIMV observed in a very short period (20 to 30 min) persists in a longer and nonexperimental period such as home NIMV.
In conclusion, our study documents for the first time the presence of an increase in the resistive load during wakefulness in infants and in children with severe stridor caused by laryngomalacia. This results in an increase in respiratory effort, even during wakefulness. This increase in respiratory effort was inversely related to age and resulted in abnormalities of breathing pattern and gas exchange. Although this evaluation was performed in a small group, the benefits of NIMV are of potential relevance in view of the systematic improvement of work of breathing, quality of life, and anthropometric parameters.
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Footnotes |
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Correspondence and requests for reprints should be addressed to Dr. Brigitte Fauroux, Service de Pneumologie Pédiatrique, Hôpital d'Enfants Armand Trousseau, 28 avenue du Docteur Arnold Netter, 75012 Paris, France. E-mail: brigitte. fauroux{at}trs.ap-hop-paris.fr
(Received in original form December 29, 2000 and accepted in revised form September 4, 2001).
This article has an online data supplement, which is accessible from this issue's table of contents online at www.atsjournals.org
Acknowledgments:
Supported by Breas Medical (Molndal, Sweden).
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References |
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