Ventilatory responses to hypercapnia in divers and non-divers: effects of posture and immersion.
Level 3 - non-randomized controlled study
Non-randomized comparative physiological study with parallel cohorts
PubMed 11820330 · doi:10.1007/s004210100518
What was done
Sixteen healthy volunteers (8 trained divers and 8 non-divers) were tested under three conditions: seated in air, seated in water, and prone in water. Functional residual capacity (FRC) was measured using helium dilution. Respiratory drive and sensitivity to hypercapnia were assessed via Read's hyperoxic hypercapnia rebreathing method, measuring ventilation, mean inspiratory flow, and occlusion pressure across all conditions.
What was found
FRC decreased significantly across all subjects when seated in water (30.8% to 34.8%) and prone in water (20.3% to 20.9%) compared to seated in air (P < 0.0001), with no difference between groups. The slopes of linear regression for CO2 sensitivity demonstrated significant differences between divers and non-divers for ventilation (P < 0.0001) and mean inspiratory flow (P < 0.05). No significant differences were found for occlusion pressure or across the different immersion postures.
Why it matters
This study shows that trained divers possess blunted chemosensitivity to CO2, which lowers respiratory drive at the breath-hold breaking point, whereas immersion-induced lung volume reductions do not alter respiratory drive.
Limits
The sample size was very small (n = 16 total; 8 per group). The cross-sectional, comparative design cannot determine whether lower CO2 sensitivity is a training adaptation or a self-selection trait. Demographic details such as age, sex, and exact diving experience were not reported in the abstract.
Cited by
- partial The brainstem's ventilatory sensitivity and tolerance to elevated carbon dioxide levels can be trained and adapted through intentional breath-holding exercises.