VOLUMETRIC CAPNOGRAPHY IN CHILDREN - INFLUENCE OF GROWTH ON THE ALVEOLAR PLATEAU SLOPE

Background: Lung growth in children is associated with dramatic increa ses in the number and surface area of alveolated airways. Modelling st udies have shown the slope of the alveolar plateau (phase III) is sens itive to the total cross-sectional area of these airways. Therefore, t he influence of age and body size on the phase III slope of the volume tric capnogram was investigated. Methods: Phase III slope (alveolar dc (CO2)/dv) and airway deadspace (VDaw) were derived from repeated singl e-breath carbon dioxide expirograms collected on 44 healthy mechanical ly ventilated children (aged 5 months-18 yr) undergoing minor surgery. Ventilatory support was standardized (V-T = 8.5 and 12.5 ml/kg, f = 8 -15 breaths/min, inspiratory time = 1 s, end-tidal partial pressure of carbon dioxide = 30-45 mmHg), and measurements were recorded by compu terized integration of output from a heated pneumotachometer and mains tream infrared carbon dioxide analyzer inserted between the endotrache al tube and anesthesia circuit. Experimental data were compared to sim ulated breath data generated from a numeric pediatric lung model. Resu lts: An increased VDaw, a smaller VDaw/V-T) and flatter phase III slop e were found at the larger tidal volume (P < 0.01). Strong relationshi ps were seen at V-T = 12.5 ml/kg between airway deadspace and age (R(2 ) = 0.77), weight (R(2) = 0.93), height (R(2) = 0.78), and body surfac e area (R(2) = 0.89). The normalized phase III slopes of infants were markedly steeper than that of adolescents and were reduced at both tid al volumes with increasing age, weight, height, and body surface area. Phase In slopes and VDaw generated from modelled carbon dioxide washo ut simulations closely matched the experimental data collected in chil dren. Conclusions: Morphometric increases in the alveolated airway cro ss-section with lung growth is associated with a decrease of the phase III slope, During adolescence, normalized phase III slopes approximat e those of healthy adults, The change in slope with lung growth may re flect a decrease in diffusional resistance for carbon dioxide transpor t within the alveolated airway resulting in diminished acinar carbon d ioxide gradients.