Effects of elevated core temperature and normoxic 30% nitrous oxide on human ventilation during short duration, high intensity exercise

• We studied effects of hyperthermia and normoxic 30% nitrous oxide (30% N2O) on exercise ventilation.
• The results support a positive influence of hyperthermia on breathing in humans.
• Breathing with normoxic N2O consistently suppressed breathing during exercise, irrespective of elevations in core temperatures.
• Interactions of hyperthermia and this nitrogen narcosis suppressed breathing giving CO2 retention.
• This CO2 retention supports that divers in similar exercise conditions are at an increased risk of deep water black-out.

It was hypothesized that normoxic 30% nitrous oxide (N2O) would suppress and hyperthermia would increase exercise ventilation during short duration, high intensity exercise. Thirteen males (24.2 ± 0.8 y; mean ± SE), of normal physique (BMI, 23.8 ± 1.0 kg m−2), performed 4 separate 30 s Wingate tests on a cycle ergometer. Exercise ventilation and its components, as well as mean skin and esophageal temperature (TES), were assessed in 2 way experimental design with factors of Thermal State (Normothermia or Hyperthermia) and Gas Type (Air or 30% Normomoxic N2O). In the 2 hyperthermic tests TES was elevated to ∼38.5 °C in a 40 °C bath. The main results indicated a significant interaction (F = 7.14, P = 0.02) between Gas Type and Thermal state for the exercise-induced increase in ventilation (ΔV˙E). During both the normothermia and hyperthermia conditions with AIR breathing, the exercise ΔV˙E was ∼80 L min−1 and it was significantly decreased to 73.1 ± 24.1 L min−1 in the normothermia condition with N2O breathing relative to that of 92.0 ± 25.0 L min−1 in the hyperthermia condition with N2O breathing. In conclusion, normoxic N2O breathing suppressed high intensity exercise ventilation during normothermia relative to that during hyperthermia on account of decreases in the tidal volume and this led CO2 retention.