VENTILATORY RESPONSE OF PREPUBERTAL BOYS AND ADULTS TO CARBON-DIOXIDE AT REST AND DURING EXERCISE

The aim of this study was to determine whether the greater ventilation in children at rest and during exercise is related to a greater CO2 ventilatory response. The CO2 ventilatory response was measured in nine prepubertal boys [10.3 years (SD 0.1)] and in 10 adults [24.9 years (SD 0.8)] at rest and during moderate exercise (VCO2 = 20 ml . kg-1 . min-1) using the CO2-rebreathing method. Three criteria were measured in all subjects to assess the ventilatory response to CO2: the CO2 sensitivity threshold (Th), which was defined as the value of end tital PCO2 (PETCO2) where the ventilation increased above its steady-state level; the reactivity slope expressed per unit of body mass (S(BM)), which was the slope of the linear relation between minute ventilation (V(E)) and PETCO2 above Th; and the slope of the relationship between the quotient of tidal volume (V(T)) and inspiration time (t(I)) and PETCO2 (V(T).t(I)-1.PETCO2(-1) values above Th. The V(E), V(T), breathing frequency (f(R)), oxygen uptake (VO2), and CO2 production (VCO2) were also measured before the CO2-rebreathing test. The following results were obtained. First, children had greater ventilation per unit body weight than adults at rest (P<0.001) and during exercise (P<0.01). Second, at rest, only V(T).t(I)-1.PETCO2(-1) was greater in children than in adults (P<0.001). Third, during exercise, children had a higher S(BM) (P<0.02) and V(T).t(I)-1.PETCO2(-1) (P<0.001) while Th was lower (P<0.02). Finally, no correlation was found between V(E)/PCO2 and Th while a significant correlation existed between V(E)/VCO2 and S(BM) (adults, r=0.79, P<0.01; children, r=0.73, P<0.05). We conclude that children have, mainly during exercise, a greater sensitivity of the respiratory centres than adult. This greater CO2 sensitivity could partly explain their higher ventilation during exercise, though greater CO2 production probably plays a role at rest.