Functional residual capacity (FRC) measurements by plethysmography and helium dilution in normal infants

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Abstract
Comparative measurements of functional residual capacity (FRC) made by plethysmography (FRCpleth) and by helium dilution (FRCHe) were obtained on 27 infants and young children without known pulmonary disease (14 males, 13 females; 4 weeks–26 months; mean age 32.2 weeks) while under chloral hydrate sedation. Clinical histories, clinical examinations, and pulmonary functions were normal for all members of the group. FRCpleth, whether measured near end expiration (EE) or near end inspiration (EI), and corrected to mean expiratory levels of at least 3 breathing cycles, was consistently and significantly greater than FRCHe. Comparative values for mean (± standard deviation) were FRCpleth EE, 182.0 (±79.7) mL and FRCpleth El, 171.8 (±77.4) mL vs. FRCHe 154 (±72.2) mL, P < 0.0001 and P < 0.005, respectively. Normalizing values by weight, FRCpleth EE was 23.8 mL/kg (±5.3) vs. FRChe 20.2 (±4.7) mL/kg, mean (+ standard deviation). The difference between FRCpleth and FRChe, expressed as FRCpleth – FRCHe/FRCpleth × 100, was 9% for occlusions at end inspiration and 16% for occlusions at end expiration. The following equations describe our FRC results in relation to length: In The difference between FRCpleth and FRCHe was more marked when occlusions were performed at end expiration than at end inspiration. We conclude that normal infants and young children, at least when studied supine and sedated, have a small but significant amount of airway closure.