Ventilatory response of prepubertal boys and adults to carbon dioxide at rest and during exercise

Abstract

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 (\(\dot V\)CO₂ = 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 titalPCO₂ (P_ETCO₂) where the ventilation increased above its steady-state level; the reactivity slope expressed per unit of body mass (SBM), which was the slope of the linear relation between minute ventilation (\(\dot V\)_E) andP_ETCO₂ above Th; and the slope of the relationship between the quotient of tidal volume (V_T) and inspiration time (t_I) andP_ETCO₂ (V_T ·t_I⁻¹ ·P_ETCO₂⁻¹) values above Th. The\(\dot V\)_E,V_T, breathing frequency (f_R), oxygen uptake (\(\dot V\)O₂), and CO₂ production (\(\dot V\)CO₂) 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, onlyV_T ·t_I⁻¹ ·P_ETCO₂⁻¹ was greater in children than in adults (P<0.001). Third, during exercise, children had a higher S_BM (P < 0.02) andV_T ·t_I⁻¹ ·P_ETCO₂⁻¹ (P<0.001) while Th was lower (P<0.02). Finally, no correlation was found between\(\dot V\)_E/\(\dot V\)CO₂ and Th while a significant correlation existed between\(\dot V\)_E/\(\dot V\)CO₂ 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.