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

Carotid body chemosensitivity at 1.6 ATA breathing air versus 100% oxygen. J Appl Physiol 129: 247–256, 2020. First published June 25, 2020; doi:10.1152/japplphysiol.00275.2020.—Hyperoxia reduces the ventilatory response to hypercapnia by suppressing carotid body (CB) activation. This effect may contribute to CO₂ retention during underwater diving due to the high arterial O₂ 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% O₂ (Air) or 100% O₂ (100% O₂). CB chemosensitivity was assessed using brief hypercapnic ventilatory response (CB_CO2) and hypoxic ventilatory response (CB_O2) tests predive, 75 and 155 min into the dives, and 15 and 55 min postdive. End-tidal CO₂ pressure increased during the dive at 75 and 155 min [Air: +9 (SD 4) mmHg and +8 (SD 4) mmHg versus 100% O₂: +6 (SD 4) mmHg and +5 (SD 3) mmHg; all P < 0.01] and was higher while breathing Air (P < 0.01). CB_CO2 was unchanged during the dive (P = 0.73) and was not different between conditions (P = 0.47). However, CB_O2 was attenuated from predive during the dive at 155 min breathing Air [0.035 (SD 0.037) L·min·mmHg¹; P = 0.02] and at both time points while breathing 100% O₂ [0.035 (SD 0.052) L·min·mmHg 1 and 0.034 (SD 0.064) L·min·mmHg ¹; P = 0.02 and P = 0.02, respectively]. These data indicate that the CB chemoreceptors do not appear to contribute to CO₂ retention in hyperbaria.