Pressure-Volume Relations of the Respiratory System in Healthy Children

Static pressure-volume (P-V) curves of the respiratory system were obtained 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 40 cm H2O, and during the subsequent deflation an interrupter placed in the airway tubing opened and closed every 0.16 s. Airway flow was integrated to obtain the volume decrement between consecutive flow interruptions. Airway pressure was measured during interruptions, and a curve relating pressure to lung volume was plotted, assuming the lung volume at zero pressure to equal functional residual capacity (FRC). FRC was measured using tracer gas washout. The maximum slope of the P-V curve (maximum compliance = Crsmax, ml/cm H2O) was closely related to length (in centimeters) of the child: Crsmax = 7.7 x 10(-4) x length2.38; r = 0.97. The pressure coinciding with Crsmax 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 body weight increased with age and was 52 +/- 13 ml/kg in infants and 87 +/- 11 mg/kg in older children. The FRC/TLC ratio was greater in infants (38 +/- 4%) than in older children (30 +/- 5%). The lung volume coinciding with Crsmax was nearly the same at all ages, when expressed as a percentage of TLC: 62 +/- 3%. Specific compliance of the respiratory system, that is, Crsmax/TLC, decreased with growth and was 0.044 +/- 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 respiratory system changed markedly with growth, especially during the first year of life, the lung volume (%TLC) at which maximum compliance occurred varied little.