Tolerance to central hypovolemia is greater following caffeinated coffee consumption in habituated users

We investigated the influence of caffeinated coffee consumption on cardiovascular responses and tolerance to central hypovolemia in individuals habituated to caffeine. Thirteen participants completed three trials, consuming caffeinated coffee, decaffeinated coffee or water before exposure to central hypovolemia via lower body negative pressure (LBNP) to pre syncope. Tolerance to central hypovolemia was quantified as cumulative stress index (CSI: LBNP level multiplied by time; mmHg × min). Prior to the consumption of caffeinated coffee, decaffeinated coffee, and water, heart rate (HR: 62 ± 10, 63 ± 9 and 61 ± 8 BPM, respectively), stroke volume (SV: 103 ± 23, 103 ± 17 and 102 ± 18 mL/beat, respectively), and total peripheral resistance (TPR: 14.2 ± 3.0, 14.0 ± 3.0, and 14.3 ± 2.7 mmHg/L/min, respectively), were not different between trials (all P > 0.05). Mean arterial pressure (MAP) increased following consumption of all drinks (Post Drink) (Caffeinated coffee: from 86 ± 8 to 97 ± 7; Decaffeinated coffee: from 88 ± 10 to 94 ± 7; and Water: from 87 ± 10 to 96 ± 6 mmHg; all P = 0.0001) but was not different between trials (P = 0.247). During LBNP, HR increased (P = 0.000) while SV decreased (P = 0.000) relative to post drink values and TPR as unchanged (P = 0.109). HR, SV, and TPR were not different between trials (all P > 0.05). MAP decreased at pre syncope in all trials (60 ± 5, 60 ± 7, and 61 ± 6 mmHg; P < 0.001). LBNP tolerance was greater following caffeinated coffee (914 ± 309 mmHg × min) relative to decaffeinated coffee and water (723 ± 336 and 769 ± 337 mmHg × min, respectively, both P < 0.05). Tolerance to central hypovolemia was greater following consumption of caffeinated coffee in habituated users

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

New FindingsWhat 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.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 similar to 38% (Steady State) or alternating between 10 and similar to 88% (Interval) of the maximal power output whilst wearing a hot water perfused suit until core temperatures increased similar to 1.4 degrees 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 degrees C and 38.3 +/- 0.2 degrees C, respectively, both P 0.05). In the Interval trial, heart rate was greater (122 +/- 12 beats min(-1)) prior to LBNP, relative to the Steady State trial (107 +/- 8 beats min(-1), 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