The need for defining tolerance limits to CO2 for long exposures is becoming increasingly important with the development of saturation diving, the use of submersibles and extended submarine patrols (see Chapter 67).
Marked adaptation to inspired CO2 levels between 0.5 per cent and 4 per cent has been demonstrated. This adaptation is characterized by an increased tidal volume, a lower respiratory rate and a reduction in the ventilatory response to hypercapnia produced by exercise.
Biochemically, there is a reversal of the initial increase in hydrogen ion concentration, a rise in the plasma bicarbonate and a fall in the plasma chloride, i.e. mild compensated respiratory acidosis. There is a slight rise in PaCO2. These latter changes are almost complete in 3 to 5 days’ exposure, although there is a significant reduction in the ventilatory response in the first 24 hours. There is also a rise in serum calcium and other mineral changes.
While at rest, the average diver can tolerate a surface equivalent of up to 4 per cent inspired CO2 (a PICO2 of 30 mm Hg), without incapacitating physiological changes. During exercise, alveolar ventilation does not increase sufficiently to prevent a significant degree of CO2 retention as shown by an elevation of PaCO2. This loss of the ventilatory response to CO2 (of the order of 20 per cent in submariners) may also be of great significance in the saturation diver, particularly during exercise.