Hypohydration Increases Heart Rate Despite Preserved Hemodynamic Compensation During Blood Flow Restricted Exercise.

During blood flow restriction (BFR) exercise, blood becomes sequestered in the active limbs due to impeded venous return, contributing to central hypovolemia. Dehydration also reduces blood volume and can exacerbate the ability of the cardiovascular (CV) system to defend against CV stressors, such as exercise and orthostatic stress. As BFR becomes more widely used, its application in individuals who may be suboptimally hydrated underscores the need to understand how hydration status influences cardiovascular strain. We tested whether hypohydration affects the CV response to BFR exercise and its relationship to orthostatic tolerance. Seventeen participants (7 females) performed a lower-body negative pressure test to assess orthostatic tolerance, and a bilateral BFR walking (5 km/h at 5% incline) bout under two conditions, counterbalanced: with regular fluid consumption (Hydrated), or after 24-h of fluid restriction (Hypohydrated). Heart rate, beat-to-beat blood pressure, and thoracic impedance were continuously measured, from which stroke volume and cardiac output were estimated. Hypohydration reduced body mass (-2.3% ± 0.7%, p < 0.0001) and estimated blood volume (-4.1% ± 3.1%, p < 0.0001). Heart rate increased during BFR exercise in the hypohydrated state (124 ± 21 vs. 131 ± 23 bpm, p = 0.0016), but not during non-BFR exercise (102 ± 12 vs. 104 ± 13 bpm, p = 0.76). Stroke volume, cardiac output, and blood pressure responses were maintained during hypohydrated exercise (all p > 0.21). Orthostatic tolerance did not differ between conditions (354 ± 179 vs. 312 ± 128 mmHg•min, p = 0.17). The preservation of stroke volume, cardiac output, and orthostatic tolerance suggests that CV compensatory mechanisms during BFR exercise remain effective under mild hypohydration with the increased heart rate resulting from the manipulation of non-vascular fluid compartments.