Inhaled CO 2 and CO 2 Retention during Heavy Exercise Underwater

Underwater breathing apparatus increases resistive work of breathing. That and other loads from breathing underwater may alter responses to inspired CO 2 and exercise, and lead to CO 2 retention. Minute ventilation (V’) and end tidal CO 2 partial pressure (P ET CO 2 ), 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 PCO 2 (P s CO 2 ) 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 CO 2 , V’ decreased from dry to wet (89 to 70 L/min). Mean V’ increased linearly with P s CO 2 [slope 3.3 (L/min)/kPa] but so did its variance, increasing 6‐fold from P s CO 2 =0 to 2 kPa. Mean P ET CO 2 also increased linearly from 5.4 kPa (41 Torr) (slope 0.66 kPa/kPa), indicating that V’ was inadequate for the CO 2 load. RPE scores at end exercise with no CO 2 showed mean and median 16.5, interquartile distance (IQ) 1.5. With P s CO 2 =1 kPa, the distribution was irregular, median 16, IQ 3. With P s CO 2 =2kPa, the distribution was almost uniform from 14 to 20, median 17, IQ 4. Most divers increased V’ somewhat with increasing P s CO 2 , but mean P ET CO 2 was 46 and 51 Torr with P s CO 2 of 1 and 2 kPa, respectively. For 3 and 8 divers with P s CO 2 of 1 and 2 kPa, respectively, P ET CO 2 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 CO 2 underwater leads to substantial and potentially hazardous CO 2 retention. Support: NAVSEA Deep Submergence Biomedical Program.