Influence of prior alkalosis or acidosis on physiological responses during passive hyperthermia

This study manipulated the blood pH of participants to be mildly acidic or basic prior to passive hyperthermia to examine its effects on thermoregulatory and chemo-regulatory responses, including ventilatory parameters, rectal temperature (T_re) and thermal perception. Twelve healthy males attended three experimental sessions in a double-blind, randomised, counterbalanced design. Passive hyperthermia was induced by hot water immersion at 40°C following the consumption of corn starch (placebo; PLA), ammonium chloride (AC) or sodium bicarbonate (SB). Compared with PLA, SB consumption increased blood pH and HCO₃ ^-, whilst AC decreased both blood pH and HCO₃ ^- (all P < 0.001). Specifically, minute ventilation was unchanged following SB (11.0 ± 3.7 L min^-1) but higher following AC (13.2 ± 5.3 L min^-1) compared with PLA (11.9 ± 5.1 L min^-1; P = 0.002). Changes in ventilatory equivalent for O₂ and CO₂ were greater in AC and lower in SB compared with PLA (P < 0.05). T_re increased similarly across all conditions (P = 0.089), whilst immersion times approached a difference (SB: 51.1 ± 10.2 min, AC: 52.9 ± 8.1 min, and PLA: 56.8 ± 6.8 min; P = 0.06). Thermal sensation was higher in AC compared with PLA and SB (all P < 0.001), with no difference between PLA and SB (P = 0.45). Thermal discomfort was not different between AC and SB (P = 0.66), both of which were higher than PLA (all P < 0.001). The magnitude and characteristics of ventilatory response during hyperthermia were influenced by prior alkalemia or acidosis, coinciding with differences in thermal perceptions.