Ventilatory responses to hypercapnia in divers and non-divers: effects of posture and immersion

The aim of this study was to determine the effects on respiratory drive of two factors, one mechanical (lung volume) and one chemical (sensitivity to hypercapnia), that are involved in determining the breath-hold duration (BH D). Functional residual capacity was measured by helium dilution with the s ubject seated in air, seated in water and in the prone position in water. H yperoxic hypercapnia rebreathing (Read's method) was carried out under iden tical environmental conditions to assess the effects of CO2 pressure on res piratory centre output by measuring ventilation, mean inspiratory flow and occlusion pressure. Sixteen healthy volunteers were tested, 8 trained diver s and 8 non-divers. Functional residual capacity decreased for the postures seated in water (30.8%-34.8%) and for prone position in water (20.3%-20.9% ) when compared to the posture seated in air (P < 0.0001), all subjects poo led. No difference was found between groups. The slopes of the linear regre ssion. which characterised the sensitivity to CO2 and were determined with the rebreathing tests, revealed differences between the two populations (ve ntilation: P < 0.0001, mean inspiratory flow: P < 0.05). No difference was found for occlusion pressure or between the different postures. These resul ts confirmed a lower sensitivity to CO2 for trained divers. This adaptation was shown to decrease respiratory centre activity at the origin of the bre ath-hold breaking point. The immersion, did not influence respiratory drive , despite a decrease in lung volumes. The authors suggest that these findin gs may be explained by a specific apnoea training and a pronounced bradycar dia in immersion.