BackgroundLung growth in children is associated with dramatic increases in the number and surface area of alveolated airways. Modelling studies have shown the slope of the alveolar plateau (phase III) is sensitive to the total cross‐sectional area of these airways. Therefore, the influence of age and body size on the phase III slope of the volumetric capnogram was investigated.MethodsPhase III slope (alveolar dcCO2/dv) and airway deadspace (VDaw) were derived from repeated single‐breath carbon dioxide expirograms collected on 44 healthy mechanically ventilated children (aged 5 months‐18 yr) undergoing minor surgery. Ventilatory support was standardized (VT= 8.5 and 12.5 ml/kg, [florin] = 8–15 breaths/min, inspiratory time = 1 s, end‐tidal partial pressure of carbon dioxide = 30–45 mmHg), and measurements were recorded by computerized integration of output from a heated pneumotachometer and mainstream infrared carbon dioxide analyzer inserted between the endotracheal tube and anesthesia circuit. Experimental data were compared to simulated breath data generated from a numeric pediatric lung model.ResultsAn increased VDaw, a smaller VDaw/VT, and flatter phase III slope were found at the larger tidal volume (P < 0.01). Strong relationships were seen at VT= 12.5 ml/kg between airway deadspace and age (R2= 0.77), weight (R2= 0.93), height (R2= 0.78), and body surface area (R2= 0.89). The normalized phase III slopes of infants were markedly steeper than that of adolescents and were reduced at both tidal volumes with increasing age, weight, height, and body surface area. Phase III slopes and VDawgenerated from modelled carbon dioxide washout simulations closely matched the experimental data collected in children.ConclusionsMorphometric increases in the alveolated airway cross‐section with lung growth is associated with a decrease of the phase III slope. During adolescence, normalized phase III slopes approximate those of healthy adults. The change in slope with lung growth may reflect a decrease in diffusional resistance for carbon dioxide transport within the alveolated airway resulting in diminished acinar carbon dioxide gradients.