Hypercapnic and Hypoxic Ventilatory Responses during Growth

Cross-sectional studies on hypercapnic and hypoxic ventilatory chemosensitivities were performed in 71 children ranging in age from 7 to 18yrs. The subjects were classified into 6 successive 2-year age groups. CO₂ ventilatory response was measured by rebreathing 5% CO₂ in O₂, a slight modification of the method originally proposed by Read. The results were evaluated when the CO₂-ventilation feedback control system was supposed to have attained the open-loop condition. Hypoxic ventilatory response was measured by the isocapnic progressive hypoxia test. To obtain good reproducibility in the ventilatory response, end-tidal P_CO2 was maintained at 5 mmHg higher than the resting condition throughout the test. Normalized ventilatory responses to CO₂ by body surface area (S/BSA) progressively decreased from the 7-8 through the 11-12 yr groups, and then tended to decrease further in a more gradual manner with increasing age. This trend was very similar to the normalized CO₂ output (V_CO2/BSA), but did not parallel so closely the normalized O₂ intake (V_CO2/BSA). When ventilatory and metabolic parameters were normalized by body weight (BW), or the lean body mass (LBM), qualitatively similar relationships between CO₂ sensitivities and metabolic parameters were also obtained. Contrary to the hypercapnic response, hypoxic ventilatory chemosensitivities were not significantly different among the 6 different age groups. We concluded that normalized hypercapnic chemosensitivity decreased during growth and corresponded well with decreased CO₂ output per unit body mass.