Carotid body chemosensitivity at 1.6 ATA breathing air versus 100% oxygen.

Hyperoxia reduces the ventilatory response to hypercapnia by suppressing carotid body (CB) activation. This effect may contribute to CO retention during underwater diving due to the high arterial O content associated with hyperbaria. We tested the hypothesis that CB chemosensitivity to hypercapnia and hypoxia is attenuated during hyperbaria. Ten subjects completed two, 4-h dry dives at 1.6 atmosphere absolute (ATA) breathing either 21% O (Air) or 100% O (100% O). CB chemosensitivity was assessed using brief hypercapnic ventilatory response ([Formula: see text]) and hypoxic ventilatory response ([Formula: see text]) tests predive, 75 and 155 min into the dives, and 15 and 55 min postdive. End-tidal CO pressure increased during the dive at 75 and 155 min [Air: +9 (SD 4) mmHg and +8 (SD 4) mmHg versus 100% O: +6 (SD 4) mmHg and +5 (SD 3) mmHg; all < 0.01] and was higher while breathing Air ( < 0.01). [Formula: see text] was unchanged during the dive ( = 0.73) and was not different between conditions ( = 0.47). However, [Formula: see text] was attenuated from predive during the dive at 155 min breathing Air [-0.035 (SD 0.037) L·min·mmHg; = 0.02] and at both time points while breathing 100% O [-0.035 (SD 0.052) L·min·mmHg and -0.034 (SD 0.064) L·min·mmHg; = 0.02 and = 0.02, respectively]. These data indicate that the CB chemoreceptors do not appear to contribute to CO retention in hyperbaria. We demonstrate that carotid body chemosensitivity to brief exposures of hypercapnia was unchanged during a 4-h dive in a dry hyperbaric chamber at 1.6 ATA regardless of breathing gas condition [i.e., air (21% O) versus 100% oxygen]. Therefore, it appears that an attenuation of carotid body chemosensitivity to hypercapnia does not contribute to CO retention in hyperbaria.