Carbon Dioxide Toxicity at Great Depths in the Sea
If the diving gear is properly designed and functions properly, the diver has no problem due to carbon dioxide toxicity because depth alone does not increase the carbon dioxide partial pressure in the alveoli. This is true because depth does not increase the rate of carbon dioxide production in the body, and as long as the diver continues to breathe a normal tidal volume and expires the carbon dioxide as it is formed, alveo-lar carbon dioxide pressure will be maintained at a normal value.
In certain types of diving gear, however, such as the diving helmet and some types of rebreathing appara-tuses, carbon dioxide can build up in the dead space air of the apparatus and be rebreathed by the diver. Up to an alveolar carbon dioxide pressure (PCO2) of about 80 mm Hg, twice that in normal alveoli, the diver usually tolerates this buildup by increasing the minute respiratory volume a maximum of 8- to 11-fold to compensate for the increased carbon dioxide. Beyond 80-mm Hg alveolar PCO2, the situation becomes intolerable, and eventually the respiratory center begins to be depressed, rather than excited, because of the negative tissue metabolic effects of high PCO2. The diver’s respiration then begins to fail rather than to compensate. In addition, the diver develops severe respiratory acidosis, and varying degrees of lethargy, narcosis, and finally even anesthesia.
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