Dynamics of breathing in infants

Abstract

Passive compliance (C) was measured in 10 [human] infants at 10-90 min after birth and in 10 infants at a few days of life by recording mouth pressure after airways occlusions at end-inspiration. From the slope of the expiratory flow-volume curve, the passive time constant (.tau.) and resistance (R = .tau./C) were also computed. Examination of the changes of C with time and of the expiratory flow-volume curves indicates that the end-expiratory volume is maintained above functional residual capacity at both ages, but significantly more so at a few days (7.6 ml) than at 10-90 min (3.5 ml). The passive time constant (.tau. = C .cntdot. R) is shorter at the early age due to the smaller C. The active compliance (C'') and resistance (R'') values were estimated from the pressure generated by the infant when the airways are occluded at end-expiration. The active time constant of the respiratory system (.tau.'' = C'' .cntdot. R'') is less than .tau. due to a smaller active compliance, particularly at a few days. Active resistance is similar to R. Active stiffening of the respiratory system provides more stability of the infant''s respiratory system and a more ready volume response for any given change in pressure; its price is a higher work of breathing. At optimal breathing rates, the active work is 127% (10-90 min) to 183% (a few days) higher than that computed from the passive values. Inspiratory flow wave tends to be squared at both ages minimizing the energy losses due to friction.