Influence of facial cooling on carotid body tonic activity and sensitivity

Facial cooling can increase ventilation and augment the hypoxic ventilatory response. Whole body cooling increases both carotid body tonic activity and sensitivity; however, whether isolated facial cooling induces similar carotid body hyperexcitability was unknown. We investigated whether facial cooling alters carotid body function by assessing tonic activity and hypoxic sensitivity. Fourteen healthy adults (11 M/3 F; age 26 ± 4 years) completed a counterbalanced, crossover study involving transient hyperoxia and poikilocapnic hypoxia (9.5% O₂) under thermoneutral (facial temperature: 34.2 ± 1.2°C) and facial cooling (19.4 ± 3.3°C) conditions. Carotid body tonic activity was inferred from the ventilatory suppression during transient hyperoxia. Sensitivity was assessed via the change in end-tidal CO₂ ( $P_{\mathrm{ETC}{O}_2}$ ) relative to oxygen saturation ( $S_{p{O}_2}$ ) during hypoxia. Facial cooling induced hyperventilation, evidenced by reduced $P_{\mathrm{ETC}{O}_2}$ (35 ± 8 vs. 41 ± 3 mmHg; P = 0.008), and elevated ventilatory equivalent for CO₂ production (28 ± 6 vs. 23 ± 2; P = 0.02). Carotid body tonic activity did not differ between facial cooling and thermoneutral conditions, but carotid body sensitivity was reduced during facial cooling (0.20 ± 0.14 vs. 0.28 ± 0.13 mmHg/%; P = 0.044). The reduction in $P_{\mathrm{ETC}{O}_2}$ experienced during facial cooling correlated with enhanced carotid body tonic activity (R² = 0.39, P = 0.022) and reduced sensitivity (R² = 0.33, P = 0.03). Collectively, facial cooling induces hyperventilation and the attendant hypocapnia reduces carotid body sensitivity. Although this hyperventilation is related to carotid body tonic activity, facial cooling likely produces a cold shock response that stimulates ventilation separately from the carotid body. These findings offer new insights on the interaction between stimuli relevant to outdoor activities in cold environments (e.g., snow shovelling, mountaineering, cold water swimming) and carotid body function.