Sodium bicarbonate reduces ventilation without altering core temperature threshold or sensitivity of hyperthermia-induced hyperventilation in exercising humans

Abstract

Hyperthermia stimulates ventilation (hyperthermia-induced hyperventilation). In exercising humans, once the core temperature reaches ~37°C, minute ventilation (VE) increases linearly with rising core temperature, and the slope of the relation between VE and core temperature reflects the sensitivity of the response. We previously reported that sodium bicarbonate ingestion reduces VE during prolonged exercise in the heat without affecting the sensitivity of hyperthermia-induced hyperventilation. Here, we hypothesized that reductions in VE associated with sodium bicarbonate ingestion reflect elevation of the core temperature threshold for hyperthermia-induced hyperventilation. Thirteen healthy young males ingested sodium bicarbonate (0.3 g/kg body weight) (NaHCO3 trial) or sodium chloride (0.208 g/kg body weight) (NaCl trial), after which they performed a cycle exercise at 50% of peak oxygen uptake in the heat (35°C and 50% relative humidity) following a pre-cooling. The pre-cooling enabled detection of an esophageal temperature ( Tes: an index of core temperature) threshold for hyperthermia-induced hyperventilation. The Tes thresholds for increases in VE were similar between the two trials (P = 0.514). The slopes relating VE to Tes also did not differ between trials (P = 0.131). However, VE was lower in the NaHCO3 than the NaCl trial in the range of Tes = 36.8-38.4°C (P < 0.007, main effect of trial). These results suggest that sodium bicarbonate ingestion does not alter the core temperature threshold or sensitivity of hyperthermia-induced hyperventilation during prolonged exercise in the heat; instead, it downshifts the exercise hyperpnea.