PRESSURE-VOLUME RELATIONS OF THE RESPIRATORY SYSTEM IN HEALTHY-CHILDREN

Static pressure-volume (P-V) curves of the respiratory system were obt ained in 48 healthy children (1 mo to 16 yr of age) during anesthesia and muscle paralysis. The lungs were inflated to a pressure of 25 to 4 0 cm H2O, and during the subsequent deflation an interrupter placed in the airway tubing opened and closed every 0.16 s. Airway flow was int egrated to obtain the volume decrement between consecutive flow interr uptions. Airway pressure was measured during interruptions, and a curv e relating pressure to lung volume was plotted, assuming the lung volu me at zero pressure to equal functional residual capacity (FRC). FRC w as measured using tracer gas washout. The maximum slope of the P-V cur ve (maximum compliance = C(rs)max, ml/cm H2O) was closely related to l ength (in centimeters) of the child: C(rs)max = 7.7 x 10(-4) x length( 2.38); r = 0.97. The pressure coinciding with C(rs)max was 6 +/- 1 cm H2O (mean +/- SD) in infants (1 to 6 mo of age) and 12 +/- 1 cm H2O in older children (> 1.5 yr of age). Total lung capacity (TLC) per kg bo dy weight increased with age and was 52 +/- 13 ml/kg in infants and 87 +/- 11 ml/kg in older children. The FRC/TLC ratio was greater in infa nts (38 +/- 4%) than in older children (30 +/- 5%). The lung volume co inciding with C(rs)max was nearly the same at all ages, when expressed as a percentage of TLC: 62 +/- 3%. Specific compliance of the respira tory system, that is, C(rs)max/TLC, decreased with growth and was 0.04 4 +/- 0.006 cm H2O-1 in infants and 0.035 +/- 0.004 cm H2O-1 in older children. It is concluded that although the P-V relations of the respi ratory system changed markedly with growth, especially during the firs t year of life, the lung volume (%TLC) at which maximum compliance occ urred varied little.