Inhaled CO2 and CO2 Retention during Heavy Exercise Underwater

Abstract

Underwater breathing apparatus increases resistive work of breathing. That and other loads from breathing underwater may alter responses to inspired CO2 and exercise, and lead to CO2 retention. Minute ventilation (V’) and end tidal CO2 partial pressure (PETCO2), one‐minute averages in late exercise, were measured in 16 men during dry and underwater endurance exercise at nominally 85% of divers' maximum oxygen uptake rates. Divers reported relative perceived exertion (RPE) scores. Underwater, supplied PCO2 (PsCO2) was 0, 1, or 2 kPa, and breathing resistance was symmetrical to give 1 kPa at 62.5 L/min, the minimal resistance of mask and large‐bore hoses, about 1/3 of the maximum acceptable. Oxygen partial pressure underwater was 135 kPa, and dry, 100 kPa. Without added CO2, V’ decreased from dry to wet (89 to 70 L/min). Mean V’ increased linearly with PsCO2 [slope 3.3 (L/min)/kPa] but so did its variance, increasing 6‐fold from PsCO2=0 to 2 kPa. Mean PETCO2 also increased linearly from 5.4 kPa (41 Torr) (slope 0.66 kPa/kPa), indicating that V’ was inadequate for the CO2 load. RPE scores at end exercise with no CO2 showed mean and median 16.5, interquartile distance (IQ) 1.5. With PsCO2=1 kPa, the distribution was irregular, median 16, IQ 3. With PsCO2=2kPa, the distribution was almost uniform from 14 to 20, median 17, IQ 4. Most divers increased V’ somewhat with increasing PsCO2, but mean PETCO2 was 46 and 51 Torr with PsCO2 of 1 and 2 kPa, respectively. For 3 and 8 divers with PsCO2 of 1 and 2 kPa, respectively, PETCO2 was greater than 51 Torr, the value above which Sayers et al (1987) found cognitive decrements. The combination of even minimal resistance, heavy exercise, and inhaled CO2 underwater leads to substantial and potentially hazardous CO2 retention. Support: NAVSEA Deep Submergence Biomedical Program.