Respiratory oscillation mechanics in infants with bronchiolitis during mechanical ventilation

The aim of the study was to describe the pattern of respiratory oscillation mechanics and responses to positive end‐expiratory pressure (PEEP) in bronchiolitis. Six infants were studied during the course of mechanical ventilation. A 20 Hz sinusoidal pressure variation was applied at the endotracheal tube where flow was measured with a pneumotachograph. Resistance and reactance obtained from the complex pressure‐flow ratio were separated during inspiration (Rrs,i; Xrs,i) and expiration (Rrs,e; Xrs,e), and the differences between Rrs,i and Rrs,e (ΔRrs) and Xrs,i and Xrs,e (ΔXrs) were calculated. The data were corrected for the mechanical characteristics of the endotracheal tube. The measurements were repeated while PEEP was varied between 0 and 8 hPa. Two infants were found to have normal Rrs and near‐zero Xrs and both parameters exhibited little change within the respiratory cycle or with varying PEEP. Four infants had high Rrs at zero PEEP. In two, Rrs,i was markedly elevated (108.5 and 85.2 hPa · s/L, respectively), and Xrs,i was markedly negative (−25.0 and −22.5 hPa · s/L, respectively) at zero PEEP, while ΔRrs and ΔXrs were small. Rrs,i and the absolute value of Xrs,i decreased with increasing PEEP. This pattern of oscillation mechanics was consistent with low lung volumes and atelectasis, being reversed by increasing PEEP. In the remaining two subjects, Rrs,i was moderately elevated (57.8 and 53.6 hPa · s/L, respectively) and Xrs,i moderately negative (−12.5 and −7.7 hPa · s/L, respectively) at zero PEEP. ΔRrs (−59.8 and −56.5 hPa · s/L, respectively) and Δrs (28.1 and 48.7 hPa · s/L, respectively) were large, but were dramatically reduced by increasing PEEP. These patterns were consistent with expiratory airflow limitation. Measurements of respiratory impedance are, therefore, informative in regard to the pathophysiological mechanisms occurring in bronchiolitis during mechanical ventilation, and they may be helpful in setting the level and assessing the effect of PEEP. Pediatr. Pulmonol. 1998; 25:18–31. © 1998 Wiley‐Liss, Inc.