High-intensity interval exercise reduces tolerance to a simulated haemorrhagic challenge in heat-stressed individuals.

New Findings: What is the central question of this study? In heat-stressed individuals, does high-intensity interval exercise reduce tolerance to a simulated haemorrhagic challenge (lower body negative pressure, LBNP) relative to steady state exercise? What is the main finding and its importance? LBNP tolerance was lower in heat-stressed individuals following high-intensity interval exercise relative to steady state exercise. This was likely owing to the greater cardiovascular strain required to maintain arterial blood pressure prior to and early during LBNP following high-intensity interval exercise. These findings are of importance for individuals working in occupations in which combined heat stress and intense intermittent exercise are common and where the risk of haemorrhagic injury is increased.

Abstract: This study investigated whether tolerance to a simulated haemorrhagic challenge (lower body negative pressure, LBNP) was lower in heat-stressed individuals following high-intensity interval exercise relative to steady state exercise. Nine healthy participants completed two trials (Steady State and Interval). Participants cycled continuously at ∼38% (Steady State) or alternating between 10 and ∼88% (Interval) of the maximal power output whilst wearing a hot water perfused suit until core temperatures increased ∼1.4°C. Participants then underwent LBNP to pre-syncope. LBNP tolerance was quantified as cumulative stress index (CSI; mmHg min). Mean skin and core temperatures were elevated in both trials following exercise prior to LBNP (to 38.1 ± 0.6°C and 38.3 ± 0.2°C, respectively, both P < 0.001 relative to baseline) but not different between trials (both P > 0.05). In the Interval trial, heart rate was greater (122 ± 12 beats min ) prior to LBNP, relative to the Steady State trial (107 ± 8 beats min , P < 0.001) while mean arterial pressure was similarly reduced in both trials prior to LBNP (from baseline 89 ± 5 to 77 ± 7 mmHg; P = 0.001) and at pre-syncope (to 62 ± 9 mmHg, P < 0.001). CSI was lower in the Interval trial (280 ± 194 vs. 550 ± 234 mmHg min; P = 0.0085). In heat-stressed individuals, tolerance to a simulated haemorrhagic challenge is reduced following high-intensity interval exercise relative to steady state exercise.