Effects of CO2 insensitivity and respiratory pattern on respiration in divers

In a study of respiratory function under hyperbaric conditions one diver (TM) was found to have an extremely low ventilatory response to exercise with a postinspiratory pause typical of certain "carbon dioxide retaining divers." The respiratory function of diver TM is compared with that of four other divers having a normal ventilatory response. In exercise at 4 ATA hypoventilation and hypercapnia were potentiated to a greater extent in TM than in the other divers. Diver TM maintained a ventilation 25%-50% lower than that of the other divers and whereas their end-tidal Pco2 remained within reasonable limits (Pco2 less than or equal to 55 mmHg), that of TM rose to levels considered hazardous (Pco2 less than or equal to 76 mmHg). Results suggest that when a diver exhibits a postinspiratory pause in the breathing cycle, mixing of alveolar and dead space gas takes place. As a result, physiological dead space calculated according to the Bohr formula is unusually small. As alveolar Pco2 will rise during postinspiratory pause, mean arterial Pco2 may be lower than end-tidal Pco2. Such a respiratory pattern has a greater ventilatory efficiency than normal and may afford the diver some protection, albeit incomplete, from hypercapnia.