Effect of Sleep Stages on Measurements of Passive Respiratory Mechanics in Infants With Bronchiolitis Barbara Pratl, MD, Bernhard Steinbrugger, MD, Elisabeth Weinhandl, RT, and Maximilian S. Zach, MD* Summary. The measurement of passive respiratory mechanics by the single-breath occlusion technique is one of the more frequently used tests of infant lung function. Measurements are routinely done under chloral hydrate sedation, and a possible influence of sleep stages on these measurements has not been evaluated so far. We combined the assessment of passive respi- ratory mechanics with sleep stage monitoring in 44 infants and toddlers with mild to moderately severe bronchiolitis. In 31 infants, only nonrapid eye movement (NREM) sleep was recorded. In 13 patients who showed both NREM and rapid eye movement (REM) sleep, compliance of the respiratory system was significantly lower during REM than NREM sleep (73.2 ± 19.7 vs. 81.2 ± 21.3 mL/kPa, P = 0.0007), while resistance remained essentially unchanged. This finding was explained by an unchanged airway opening pressure in combination with a significantly de- creased extrapolated volume. As tidal volume did not change from NREM to REM, this indicates reduced dynamic elevation of lung volume during REM sleep and thus supports previous ob- servations of decreased lung volume in this sleep stage. From a practical perspective, these findings argue for the monitoring of sleep stage during measurements of passive respiratory mechanics, thereby increasing the complexity of these measurements significantly. Pediatr Pulmonol. 1999; 27:273–277. © 1999 Wiley-Liss, Inc. Key words: passive respiratory mechanics; infant lung function; sleep stage; compliance of the respiratory system; lung volume. INTRODUCTION Throughout the last two decades, infant lung function testing has made considerable methodological progress and refinement, and might now be approaching a stage of broad clinical application. One of the more frequently used techniques is the relatively easy assessment of pas- sive respiratory mechanics by the single-breath occlusion technique. 1 The obtained measurements of compliance and resistance are indicative of the mechanics of the entire respiratory system, i.e., the combination of lung and chest wall. These measurements are routinely made under chloral hydrate-induced sleep. 2 This raises the question of whether sleep stages could have an influence on the ob- tained measurements; so far, this has not been evaluated. As sleep architecture has been shown to exert complex influences on chest wall stability, inspiratory effort, and dynamic elevation of lung volume, 3 such an influence of sleep stages on respiratory mechanics does not appear unlikely. To investigate this question, we combined the assess- ment of passive respiratory mechanics with sleep stage monitoring in infants and toddlers with bronchiolitis, and compared measurements of compliance and resistance of the respiratory system in rapid eye movement (REM) and nonrapid eye movement (NREM) sleep. PATIENTS AND METHODS Sixty-two infants and toddlers with mild to moderately severe viral bronchiolitis, referred for routine lung func- tion testing between the third and the sixth day of their acute disease, were recruited for the study. The study was approved by the ethics committee of the local medical faculty. Written informed consent was obtained from the parents, and one or both were present during the inves- tigation. Measurements were made between 11 AM and 1 PM. Bronchodilator medication was withheld for a minimum of 6 h before measurements. Before lung function test- ing, each infant fasted for 4 h; then the infant was sedated Respiratory and Allergic Disease Division, Pediatric Department, Uni- versity of Graz, Graz, Austria. Grant sponsor: Jubila ¨umsfonds der O ¨ sterreichischen Nationalbank; Grant number: 5032. *Correspondence to: Maximilian S. Zach, M.D., Professor of Pe- diatrics, Universita ¨t-Klinik fu ¨r Kinder- und Jugendheilkunde Graz, Klinische Abteilung fu ¨r Pulmonologie/Allergologie, Auenbrug- gerplatz 30, A-8036 Graz, Austria. E-mail: maximilian.zach@ kfunigraz.ac.at Received 25 June 1998; Accepted 16 December 1998. Pediatric Pulmonology 27:273–277 (1999) © 1999 Wiley-Liss, Inc.