Functional residual capacity in anesthetized children: Normal values and values in children with cardiac anomalies

Abstract

To assess the increase in functional residual capacity (FRC) with growth, FRC was measured after induction of anesthesia in two groups of children. One group consisted of 74 children, 0.1-11.2 yr of age, without signs of cardiorespiratory disease (referred to here as 'normal' children), and the other of 21 children, 0.2-6.9 yr of age, with cardiac malformations. Anesthesia was maintained with halothane in the normal children and with fentanyl, droperidol, and nitrous oxide in the children with cardiac anomalies. All patients were paralyzed, their tracheas intubated, and their lungs mechanically ventilated. FRC was measured with an automated tracer gas washout technique. In 70 patients the measurements were performed in duplicate with a mean coefficient of variation of 2.0%. FRC correlated significantly with height, weight, and age in both groups. Multiple regression analysis for both groups considered together indicated no significant improvement when factors for the sex of the child or for the presence of cardiac anomalies were incorporated into the model. In normal children the simple linear and nonlinear regression equations for FRC (in milliliters) versus height (in centimeters) were: FRC = -529 + 9.48 x height, r = 0.96; and FRC = 0.00175 x height2.66, r = 0.97, respectively. The corresponding equations for FRC (in milliliters) versus weight (in kilograms) were: FRC = -92 + 29.9 x weight, r = 0.93; and FRC = 9.51 x weight1.31, r = 0.95. The ratio of FRC to body weight was lower in normal infants (n = 21) than in normal children above 1 yr of age (n = 53); the values (mean ± SD) were 17 ± 4 and 24 ± 6 ml/kg, respectively (P < 0.001). It is concluded that FRC in anesthetized children whose tracheas are intubated can be predicted from height, weight, or age; that the ratio of FRC to body weight was lower in infants than in older children; and that FRC was not affected by the presence of cardiac anomalies.