CO2 retention with minimal symptoms but severe dysfunction during wet simulated dives to 6.8 atm abs

During wet dives in a hyperbaric chamber to 6.8 atm abs (690 kPa), air breathing subjects were experimentally exposed to external breathing resistance. Two of them were, unbeknownst to themselves, severely incapacitated. In the first incident the subject had been exercising for 25 min (end-tidal PCO2 60-65 mmHg, 7.3-8.0 kPa) when the breathing resistance was rapidly increased from low to very high (requiring pressure swings of 80 cmH2O, 8 kPa, peak to peak). He functioned normally (end-tidal PCO2 72 mmHg, 9.6 kPa) for about 100 s but 20 s later he was confused and irrational. After being extracted from the water (end-tidal PCO2 above 90 mmHg, 12 kPa), he lost consciousness for about 60 s. In the second incident the subject was exercising and breathing against a high resistance (pressure swings of 50-55 cmH2O, 5.0-5.6 kPa). His end-tidal PCO2 was high (65-68 mmHg, 8.7-9.3 kPa) throughout the exercise period, and after 24 min he reported mild dyspnea. A few seconds later he became confused. In other experiments both subjects voluntarily terminated experiments when the breathing resistance became overwhelming. These 2 subjects generally had high end-tidal PCO2 levels, but 1 other subject with end-tidal PCO2 levels in the same range never experienced any problems. These incidents indicate that severe hypercapnia does not necessarily correlate with dyspnea and that severe disturbances in mental function due to hypercapnia can develop suddenly when high breathing resistance is encountered in diving.