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Tidal breathing flow-volume loops in bronchiolitis in infancy: the effect of albuterol [ISRCTN47364493]
Critical Care volume 6, Article number: 160 (2002)
To evaluate the effect of nebulized albuterol on tidal breathing flow-volume loops in infants with bronchiolitis due to respiratory syncytial virus.
A randomized, double-blind, control study.
Pediatric unit in a community teaching hospital.
Twenty infants younger than 1 year of age (mean age, 5.8 ± 2.8 months) with a first episode of wheezing due to respiratory syncytial virus bronchiolitis.
Chloral hydrate (50 mg/kg) was administered orally for sedation. One dose each of nebulized albuterol (0.15 mg/kg in 3 ml saline) and saline (3 ml) were given at 6 hour intervals in a random order.
Tidal breathing flow-volume loops were obtained before and after each aerosol treatment with a Neonatal/Pediatric Pulmonary Testing System (Model 2600; Sensor Medics, Anaheim, CA, USA). At the same time, the fraction of tidal volume exhaled at peak tidal expiratory flow (PTEF) to total tidal volume (VPTEF/VE), and the fraction of exhaled time at PTEF to total expiratory time (tPTEF/tE) were measured. The PTEF, the tidal expiratory flows at 10%, 25%, and 50% of the remaining tidal volume (TEF10, TEF25, and TEF50), and the wheeze score were also determined.
There were no significant changes in VPTEF/VE and tPTEF/tE after albuterol or saline treatment. PTEF increased significantly both after albuterol and saline treatments but the difference between the two treatments was not significant (P = 0.6). Both TEF10 and the ratio of the tidal expiratory flow at 25% of the remaining tidal volume to PTEF (25/PT) decreased significantly (P < 0.05) after administration of albuterol. All other investigated variables were not significantly affected by aerosol administration.
Nebulized albuterol in infants with mild bronchiolitis due to respiratory syncytial virus did not improve VPTEF/VE and tPTEF/tE but did decrease TEF10 and 25/PT.
The use of inhaled bronchodilators to treat children with bronchiolitis remains controversial [1,2,3,4]. The controversy may be the result of differences in study populations, in choice of bronchodilators, or in measured outcome variables between trials. One common confounding variable of past studies has been the inclusion of children with a history of recurrent wheezing.
The measurement of pulmonary function during infancy is difficult because of the lack of subject cooperation. It has been shown that analysis of flow-volume loops at tidal breathing is useful in evaluating lung function in infancy [5,6,7]. Morris and Lane  described a rapid rise of tidal expiratory flow to a maximum value in adult patients with airflow obstruction. In their study, the ratio of the time to PTEF to total expiratory time (tPTEF/tE) and the ratio of the tidal volume at PTEF to total exhaled volume (VPTEF/VE) were correlated with conventional measurements of airway obstruction .
Studies have also used tidal breathing flow-volume (TBFV) loops to evaluate airway function in healthy newborns and infants with and without wheezing [5,6,7,9,10,11,12,13]. Cutrera et al.  reported a good correlation between tidal expiratory flow patterns and conventional spirometric measurements in school-age children. Analysis of tidal breathing was used to predict wheezing illness during infancy [9,10,13]. Respiratory function in infants after respiratory syncytial virus (RSV)-proven bronchiolitis showed a lower tPTEF/tE ratio compared with that of age-matched healthy infants .
Several groups have studied pulmonary function in infants with bronchiolitis, with varying results [1,2,3]. The recent literature also suggests either a lack of efficacy or a very small efficacy for albuterol administration in the management of bronchiolitis [16,17]. The purpose of the present study is to evaluate the effect of nebulized albuterol administration on TBFV loops and wheeze scores in infants with bronchiolitis due to RSV.
Materials and methods
Selection of patients
The study was conducted in the pediatric unit of a community teaching hospital. Patients admitted to the pediatric unit were eligible for the study if they met the following criteria: age younger than 1 year; first episode of wheezing; clinical features of bronchiolitis (rhinorrhea, tachypnea, and wheezing and/or rales); and nasopharyngeal secretions positive for RSV as determined by enzyme immunoassay. Preterm infants and infants with underlying cardiopulmonary disease, bronchopulmonary dysplasia, previous history of wheezing, or those needing admission to the pediatric intensive care unit were excluded from the study. The study was approved by the hospital's Institutional Review Board. Informed, written consent was obtained from all parents before enrollment of patients.
Eligible patients were randomly assigned to receive either 0.15 mg/kg albuterol in 3 ml saline (group A) or 3 ml saline without albuterol (group B) via a nebulizer with an air flow of 6–8 l/min. The same patients were crossed-over to receive the alternate saline or albuterol treatment 6 hours after the first aerosol administration. The 6 hour interval before crossover measurements was used to exclude any carryover effect of nebulized albuterol or saline. The timing of postaerosol measurements was based on several previously published reports [2,4,18,19]. Observers were unable to distinguish between the two nebulizer solutions by any characteristics, and both nebulized solutions were dispensed by the pharmacy in identical syringes. Block randomization was also performed by the pharmacy department and the records were concealed until the end of the study.
Chloral hydrate (50 mg/kg, orally) was administered 30 min before the measurements were taken. An aerosol (either albuterol or saline) treatment was given following baseline measurements of heart rate, respiratory rate, wheeze score, arterial oxygen saturation by pulse oximetry, and pulmonary function. The same measurements were also repeated 15 min after the aerosol treatment. The entire procedure was repeated in 6 hours, with the second aerosol (either saline or albuterol) treatment. None of the patients received any other bronchodilators within 6 hours of the first aerosol treatment or between the first and second aerosol administration. In those infants who were on supplemental oxygen, the amount of supplemental oxygen was kept constant during the study period. Corticosteroids were not administered to any patients in the study.
Pulmonary function tests
All pulmonary function tests were performed by two experienced respiratory therapists. A SensorMedics 2600 Pediatric Pulmonary cart (SensorMedics Corp., Anaheim, CA, USA) was used to obtain TBFV loops. The TBFV loops were obtained and analyzed by a standard method described previously . A close-fitting face mask (Model VR-3100; Ventlab Corporation, Mocksville, NC, USA) with an air-inflated cuff was used to ensure that no air leaks occurred. Four representative tidal flow-volume loops were stored for analysis. Each loop was chosen from a series of breaths during established tidal breathing. A minimum of 16 loops was collected to select the four stored loops. The loops were selected from tidal breaths, with as stable a volume and shape as possible. The means from these four loops were used as the results for each child.
Several aspects of pulmonary function were chosen as outcome measures. These included the ratio of expiratory time to reach PTEF to the total expiratory time as a primary outcome measure, the tidal volume (VT), PTEF, TEF10, TEF25 and TEF50, and 25/PT as other pulmonary function tests of interest. Heart rate, respiratory rate, oxygen saturation, and wheeze score (0 = no wheezing; 1 = end-expiratory wheezing only; 2 = wheezing during entire expiration ± inspiratory phase, audible with stethoscope only; 3 = expiratory and inspiratory wheezing audible without stethoscope)  were also measured.
To estimate the sample size, we analyzed pulmonary function studies of 14 infants with a recent history of wheezing illness that were performed in our pulmonary function laboratory. We calculated the intrasubject coefficient of variation for tPTEF/tE as 20–25%. A 40% difference (twice the coefficient of variation) was considered a clinically important improvement. Assuming an α level of 0.05 (two-tailed) and 90% power, a sample size of 17–20 patients per group was predicted. All values are presented as mean ± standard deviation.
Nineteen measurements were made all together in each of the two categories (saline treatment and albuterol treatment). The values for each investigated variable were compared with baseline measurements (19 measurements before nebulization with saline and 19 measurements before nebulization with albuterol) by repeated-measures analysis of variance followed by Bonferroni correction for multiple comparisons. These comparisons included: baseline before saline treatment versus baseline before albuterol treatment; saline treatment versus albuterol treatment; baseline before saline treatment versus saline treatment; and baseline before albuterol treatment versus albuterol treatment. The wheeze scores in various categories were compared with a non-parametric Friedman test followed by Dunn's test. Improvement after saline treatment and albuterol treatment from their respective baseline values were compared with a paired t-test. P < 0.05 was considered significant.
Twenty patients were enrolled in the study during the study period. Analysis of one patient's flow-volume loops was consistent with grunting and the subject was excluded from the study. All 19 infants in our study had a cough (3.6 ± 2.8 days). Eighteen infants had symptoms of upper respiratory infection (2.94 ± 0.9 days), wheezing (2.3 ± 1.7 days), and breathing difficulties (1.7 ± 0.9 days). Sixteen infants had difficulties in feeding (1.6 ± 0.8 days), and nine of the 19 infants had fever (1.5 ± 0.7 days). A family history of wheezing or atopy was present in nine infants.
Exposure to passive smoking and household pets were present in 12 and eight infants, respectively. One child presented with apnea, and four infants had otitis media at the time of admission. The duration of hospital stay in our study group was 3.9 ± 1.1 days. Five infants received oxygen supplementation and eight infants needed intravenous fluids for more than 1 day. All infants were positive for RSV and none were premature or had bronchopulmonary dysplasia.
All baseline demographic and clinical variables were evenly distributed between the two groups except for gender distribution (Table 1). No significant changes were observed in heart rate, respiratory rate, oxygen saturation, and wheeze score after aerosol treatments (Table 2). The PTEF increased both after albuterol and saline treatments but the difference between the two treatments was not significant. Both TEF10 and 25/PT significantly decreased after albuterol administration (P < 0.05). There were no significant changes in VPTEF/VEand tPTEF/tE either with albuterol or with saline (Table 3).
Pulmonary function has been assessed in infants with bronchiolitis in several published reports. In these reports, the effects of bronchodilators on pulmonary function tests in children with bronchiolitis have been contradictory. Several other studies have demonstrated either no change in pulmonary function [3,21] or worsening of pulmonary function following bronchodilator administration [22,23]. The low tPTEF/tE value found in our study may be indicative of bronchial narrowing, but it failed to improve after albuterol administration. Several of the previous studies were not randomized and included infants with recurrent wheezing. We used a randomized, double-blind study, including only infants with bronchiolitis due to RSV and excluding infants with recurrent wheezing, bronchopulmonary dysplasia, and prematurity.
Relationship between tidal flow indices and airflow obstruction
Morris and Lane  demonstrated that, in older children and adults, VPTEF/VE and tPTEF/tE values from TBFV loops correlate well with specific airway conductance, with forced expiratory volume in the first second, and with the ratio of forced expiratory volume in the first second to the forced vital capacity. They also showed that the values of VPTEF/VE and tPTEF/tE were significantly lower in subjects with obstructive airway disease.
In children younger than 2 years of age with bronchopulmonary obstruction, tPTEF/tE and VPTEF/VEvalues were lower than in the controls and improved after salbutamol administration . The magnitude of change in tPTEF/tE was significantly correlated with the concentration of serum eosinophilic cationic protein (an inflammatory marker) . The same investigators showed, in a different study, that VPTEF/VE, tPTEF/tE and 25/PT ratios were significantly lower in asthmatic children and improved significantly after salbutamol when compared with controls . In another study in school-age children, however, the VPTEF/VE value was not significantly different in asymptomatic asthmatics when compared with age-matched healthy controls . In addition, forced vital capacity, peak expiratory flow (by spirometry), and forced expiratory flow at 75% of forced vital capacity (FEF75%) were similar in both groups. Only FEF50% and FEF25% discriminated asymptomatic asthmatics from the healthy controls.
The tidal flow index, tPTEF/tE, measured during the first 3 months of life, has been shown to be predictive of subsequent wheezing in boys during the first 3 years [9,10]. Alder et al. , however, concluded in their study that tPTEF/tE is only weakly associated with the development of lower respiratory tract illness during the first year of life, and the ratio is less precise and an epidemiologically less useful measure than maximum expiratory flow at functional residual capacity. Clarke et al.  were also not able to detect any difference in tPTEF/tE between healthy infants who did and did not develop lower respiratory illness.
In older infants (aged 6–14 months) who suffered from obstructive airway disease, Banovcin et al.  reported significant correlation between maximum expiratory flow at functional residual capacity corrected for lung volume and VPTEF/VE and tPTEF/tE. They also showed a weak correlation between VPTEF/VE and airway resistance, reported as percentage predicted . Banovcin et al. concluded that VPTEF/VE and tPTEF/tEcorrelate better with measures of peripheral airway obstruction than with airway resistance reflecting mainly central airway patency. Dezateaux et al.  compared tPTEF/tE with airway function measured by plethysmography. They found a weak but significant association between tPTEF/tE and specific airway conductance in infants aged older than 3 months, irrespective of their previous wheezing status.
What determines the tPTEF/tE value? There is controversy in infants about the validity of tests to measure airway obstruction . Little is known about the determinants of tPTEF/tE in infancy. The relationship of tPTEF/tE to lung function may exist because tPTEF/tE quantifies the degree of active tidal slowing that occurs in response to respiratory system mechanics. In normal subjects, during expiration, the peak flow is reached approximately one-third of the way through the expiration. This is because of the slow cessation of inspiratory muscle activity at the end of inspiration. If the inspiratory muscle activity ceases abruptly at the end of inspiration, completely passive expiration results in early peaking of the flow due to unopposed recoil of the lung and the chest wall.
Both tPTEF and tE will influence the final value of the ratio tPTEF/tE. In a patient with airway obstruction, slow exhalation secondary to an increased expiration time constant will increase tE. In addition, active breaking is diminished, leading to decreased tPTEF. Both these mechanisms result in a low tPTEF/tE value in a patient with airway obstruction. This effect could be further increased in severe obstruction with active exhalation and no active breaking. In other words, tPTEF/tE may not be a direct indicator of airway caliber, but may reflect a neuromuscular response to respiratory mechanics in airway obstruction.
The behavior of VPTEF/VE mainly parallels that of tPTEF/tE. The same mechanisms responsible for determining tPTEF/tE may be operative in determining VPTEF/VE. The role of several measured tidal flows (TEF50, TEF25, TEF10) and ratios (25/PT) in the evaluation of pulmonary function is mostly unknown. In airway obstruction, tidal flows may become flow limited, especially towards the end of the tidal volume, because of dynamic compression . The rate of airflow is highly dependent on the lung volume at which the flow is measured. Without knowing the lung volumes at which the tidal flows are measured, the tidal flows have limited value in the evaluation of lung function.
Tidal breathing parameters and bronchiolitis
Low tPTEF/tE values in infants with bronchiolitis may result from an increased time constant (i.e. increased tE) and/or decreased active braking (i.e. reduced tPTEF). Both of these changes can happen because of airway obstruction. Theoretically, sedation may reduce tPTEF/tE by diminishing active braking. In fact, tPTEF/tE was found to be lower in awake compared with sleeping newborn infants . The mean value for tPTEF/tE (0.16 ± 0.09) in our study is lower than that previously reported values for healthy infants [10,15,27]. The probable explanation for the low values of tPTEF/tE in our study population is airway obstruction.
In our study, PTEFs have increased with the concomitant decrease in tidal flows near the end of exhalation (TEF10) and with the decrease in the 25/PT ratio. Peak flow during tidal breathing is submaximal; it can be increased with increased effort. Theoretically, it can also increase due to bronchodilation. We cannot determine whether bronchodilation or increased force of contraction of respiratory muscles resulting from aerosol administration is the cause of the increased PTEF in the present patients.
The later in expiration that the flow is measured, the more the measurement reflects the resistance of the very small airways . A significant decrease in tidal expiratory flows at the remaining 10% of tidal volume in the present study may suggest narrowing of the small airways after albuterol administration. The ratio 25/PT will be influenced by the values of TEF25 as well as PTEF. In the present study, PTEF significantly increased and TEF25 decreased (although not statistically significant) after albuterol administration. Both these changes have lead to significant decrease in the ratio of 25/PT. Increased effort during exhalation can theoretically lead to an increase in PTEF and, by dynamic compression of smaller airways, to a decrease in TEF25; this leads to a decrease in 25/PT. The resistance of small airways is believed to have a greater effect on flow at lower lung volumes . Although tidal volumes remained constant in the present study, we do not know whether the expiratory flows at the remaining 10% of tidal volume were measured at identical lung volumes before and after aerosol treatment.
The lack of improvement of tidal flow indices of airflow obstruction in the present study may be because of a true absence of bronchodilation in these patients, or because these indices are not sensitive enough to detect bronchodilation if one existed. As the index tPTEF/tE reflects the neuromuscular response of pulmonary mechanics, it may not be sensitive enough to detect small changes in the airway caliber. As we have not compared TBFV indices with conventional measures of airway obstruction, we cannot be certain whether we have missed bronchodilation, if one existed.
The small sample size and variability of the tidal flow indices might have also contributed to the negative results of the present study. The power of the study was to detect 40% improvement in tPTEF/tE. We could have easily missed a much smaller response because of the sample size, although the clinical significance of such a response is unknown. We have only used a single dose of aerosol and measured the pulmonary function once after the aerosol administration. Theoretically, we may have missed a peak effect of bronchodilation. Although we have used a standard method of aerosol delivery and a standard dose, altered respiratory mechanics and small-airway disease in these infants with bronchiolitis may have lead to a decrease in the delivered dose of bronchodilator, resulting in a lack of response.
Clinical scores in bronchiolitis
In recent years there have been several reports of improvement of clinical scores such as oxygen saturation and respiratory distress scores, etc., after bronchodilator administration in infants with bronchiolitis [19,20]. The failure of clinical scores to improve in the present study may be because of the small sample size and the inclusion of patients with mild disease. However, the observed improvement in clinical scores in several previous studies may be due to mechanisms other than bronchodilation. Transient improvement in oxygenation may have occurred because of increased minute ventilation secondary to alteration in tidal breathing pattern following aerosol administration without bronchodi-lation. An improvement in respiratory distress scores may be explained by an increase in nasal and upper airway caliber, especially in the studies with epinephrine. In the present study group, there was no clinical improvement with a bronchodilator. The bronchodilators may increase oxygen consumption, precipitate paradoxical hypoxemia, and increase the cost of hospitalization.
This present study in infants with mild bronchiolitis due to RSV demonstrates that nebulized albuterol does not improve VPTEF/VE and tPTEF/tE, but can decrease TEF10 and 25/PT.
25/PT = the ratio of tidal expiratory flow at 25% of the remaining tidal volume to peak tidal expiratory flow
PTEF = peak tidal expiratory flow
RSV = respiratory syncytial virus
TBFV = tidal breathing flow volume
TEF10 = tidal expiratory flow at 10% of the remaining tidal volume
tidal expiratory flow at 25% of the remaining tidal volume
TEF50 = tidal expiratory flow at 50% of the remaining tidal volume
= the fraction of exhaled time to achieve peak tidal expiratory flow to total expiratory time
= the fraction of exhaled volume to achieve peak tidal expiratory flow to total expiratory tidal volume.
Rutter N, Milner AD, Hiller EJ: Effect of bronchodilators on respiratory resistance in infants and young children with bronchiolitis and wheezy bronchitis. Arch Dis Child 1975, 50: 719-722.
Soto ME, Sly PD, Uren E, Taussig LM, Landau LI: Bronchodilator response during acute viral bronchiolitis in infancy. Pediatr Pulmonol 1985, 1: 85-90.
Sly PD, Lanteri CJ, Raven JM: Do wheezy infants recovering from bronchiolitis respond to inhaled salbutamol? Pediatr Pulmonol 1991, 10: 36-39.
Alario AJ, Lewander WJ, Dennehy P, Seifer R, Mansell AL: The efficacy of nebulized metaproterenol in wheezing infants and young children. Am J Dis Child 1992, 146: 412-418.
Lødrup Carlsen KC, Carlsen KH: Lung function in awake healthy infants: the first five days of life. Eur Respir J 1993, 6: 1496-1500.
Banovcin P, Seidenberg J, von der Hardt H: Assessment of tidal breathing patterns for monitoring of bronchial obstruction in infants. Pediatr Res 1995, 38: 218-220.
Lødrup Carlsen KC, Halvorsen R, Ahlstedt S, Carlsen K-H: Eosinophil cationic protein and tidal flow volume loops in children 0–2 years of age. Eur Respir J 1995, 8: 1148-1154. 10.1183/09031936.95.08071148
Morris MJ, Lane DJ: Tidal expiratory flow patterns in airflow obstruction. Thorax 1981, 36: 135-142.
Martinez FD, Morgan WJ, Wright AL, Holberg CJ, Taussig LM: Diminished lung function as a predisposing factor for wheezing respiratory illness in infants. N Engl J Med 1988, 319: 1112-1117.
Martinez FD, Morgan WJ, Wright AL, Holberg C, Taussig LM: Initial airway function is a risk factor for recurrent wheezing respiratory illnesses during first three years of life. Am Rev Respir Dis 1991, 143: 312-316.
Lødrup KC, Mowinckel P, Carlsen KH: Lung function measurements in awake compared to sleeping newborn infants. Pediatr Pulmonol 1992, 12: 99-104.
Dezateux CA, Stocks J, Dundas I, Jackson EA, Fletcher ME: The relationship between t PTEF :t E and specific airway conductance in infancy. Pediatr Pulmonol 1994, 18: 299-307.
Adler A, Tager IB, Brown RW, Ngo L, Hanrahan JP: Relationship between an index of tidal flow and lower respiratory illness in the first year of life. Pediatr Pulmonol 1995, 20: 137-144.
Cutrera R, Filtchev SI, Merolla R, Willim G, Haluszka J, Ronchetti R: Analysis of expiratory pattern for monitoring bronchial obstruction in school-age children. Pediatr Pulmonol 1991, 10: 6-10.
Dezateaux C, Fletcher ME, Dundas I, Stocks J: Infant respiratory function after RSV-proven bronchiolitis. Am J Respir Crit Care Med 1997, 155: 1349-1355.
Dobson JV, Stephens-Groff SM, McMahon SR, Stemmler MM, Brallier SL, Bay C: The use of albuterol in hospitalized infants with bronchiolitis. Pediatrics 1998, 101: 361-368.
Kellner JD, Ohlsson A, Gadomski AM, Wang EEL: Efficacy of bronchodilator therapy in bronchiolitis: a meta-analysis. Arch Pediatr Adolesc Med 1996, 150: 1166-1172.
Lødrup Carlsen KC, Stenzler A, Carlsen K-H: Determinants of tidal flow volume loop indices in neonates and children with and without asthma. Pediatr Pulmonol 1997, 24: 391-396. 10.1002/(SICI)1099-0496(199712)24:6<391::AID-PPUL3>3.0.CO;2-K
Sanchez I, De Koster J, Powell RE, Wolstein R, Chernick V: Effect of racemic epinephrine and salbutamol on clinical score and pulmonary mechanics in infants with bronchiolitis. J Pediatr 1993, 122: 145-151.
Schuh S, Canny G, Reisman JJ, Kerem E, Bentur L, Petric M, Levison H: Nebulized albuterol in acute bronchiolitis. J Pediatr 1990, 117: 633-637.
Seidenberg J, Masters IB, Hudson I, Olinsky A, Phelan PD: Effect of ipatropium bromide on respiratory mechanics in infants with acute bronchiolitis. Aust Paediatr J 1987, 23: 169-172.
Prendiville A, Green S, Silverman M: Paradoxical response to nebulised salbutamol in wheezy infants, assessed by partial expiratory flow-volume curves. Thorax 1987, 42: 86-91.
Hughes DM, LeSouëf PN, Landau LI: Effects of salbutamol on respiratory mechanics in bronchiolitis. Pediatr Res 1987, 22: 83-86.
Carlsen K-H, Lødrup Carlsen KC: Tidal breathing analysis and response to salbutamol in awake young children with and without asthma. Eur Respir J 1994, 7: 2154-2159. 10.1183/09031936.94.07122154
Clarke JR, Aston H, Silverman M: Evaluation of a tidal expiratory flow index in healthy and diseased infants. Pediatr Pulmonol 1994, 17: 285-290.
Statement ATS-ERS: Respiratory mechanics in infants: physiologic evaluation in health and disease. Am Rev Respir Dis 1993, 147: 474-496.
Stocks J, Dezateux CA, Jackson EA, Hoo AF, Costeloe KL, Wade AM: Analysis of tidal breathing parameters in infancy: how variable is T PTEF :T E ? Am J Respir Crit Care Med 1994, 150: 1347-1354.
West JB: Ventilation. In Pulmonary Pathophysiology – The Essentials edn 4. Edited by West JB. Baltimore: Williams & Wilkins, 1992: 3-17
The authors gratefully acknowledge the assistance of Mark McMurty and Mary Ellen in data collection, and Dr Dan Torbati for statistical analysis.
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Cite this article
Totapally, B.R., Demerci, C., Zureikat, G. et al. Tidal breathing flow-volume loops in bronchiolitis in infancy: the effect of albuterol [ISRCTN47364493]. Crit Care 6, 160 (2002). https://doi.org/10.1186/cc1476
- pulmonary function tests
- respiratory syncytial virus
- tidal breathing flow-volume loops