Cardiac responses and adaptations to blood flow restriction exercise

Low-intensity endurance training with blood flow restriction (BFR) elicits greater improvements in maximal oxygen consumption (V̇o_2max) compared with volume-matched training. However, determinants of V̇o_2max, such as oxygen-carrying capacity and mitochondrial content, do not exhibit proportional improvement. Despite the hemodynamic alterations during BFR exercise that could stimulate remodeling, cardiac adaptations remain unexplored. We assessed cardiac function in athletes (11 M/5 F) at rest, during semirecumbent cycling with BFR, and at a matched work and heart rate (HR) using echocardiography. In an exploratory analysis, a subset of athletes (5 M/2 F) then completed 6 wk of low-intensity BFR walking three times per week, and echocardiograms were repeated. Compared with rest and unoccluded exercise, BFR increased arterial elastance (rest: 1.07 ± 0.32 mmHg/mL, BFR: 1.32 ± 0.33 mmHg/mL, work-match: 0.93 ± 0.25 mmHg/mL, HR-match: 0.97 ± 0.25 mmHg/mL; all P < 0.01) and altered left ventricular (LV) filling, with a greater proportion of filling achieved through atrial contraction (rest: 45 ± 8%, BFR: 56 ± 8%, work-match: 49 ± 6%, HR-match: 46 ± 7%; all P < 0.05) to maintain end-diastolic volume (rest: 163 ± 40 mL, BFR: 163 ± 40 mL vs. work-match: 167 ± 37 mL, HR-match: 168 ± 38 mL; P = 0.5). Concurrently, stroke volume was reduced (rest: 99 ± 26 mL, BFR: 95 ± 23 mL, work-match: 109 ± 27 mL, HR-match: 110 ± 25 mL; P < 0.05), and HR was elevated (rest: 54 ± 10 beats/min, BFR and HR-match: 87 ± 13 beats/min vs. work-match: 74 ± 11 beats/min; P < 0.01) to maintain cardiac output (rest: 5.1 ± 1.2 L/min, BFR: 7.5 ± 1.0 L/min, work-match: 8.0 ± 1.2 L/min, HR-match: 9.1 ± 1.7 L/min; P < 0.0001). BFR training did not affect LV mass index (Pre: 121 ± 18 g/m², Post: 123 ± 11 g/m²; P = 0.5), nor LV function at rest or during unoccluded exercise. However, posttraining, stroke volume during BFR exercise was increased (Pre: 101 ± 25 mL, Post: 113 ± 23 mL; P = 0.03), suggesting adaptation of the cardiac response to this specific stress. This highlights how the heart supports oxygen delivery during BFR exercise and provides insight into how cardiac adaptations may contribute to BFR training-associated improvements in V̇o_2max.NEW & NOTEWORTHY Blood-flow restriction exercise uniquely challenges the heart through an increase in afterload, which necessitates altered diastolic filling patterns and increased heart rate to compensate for reduced stroke volume. Repeated exposure to low-intensity endurance-type blood-flow restriction exercise through training improves the hearts' ability to support stroke volume, specifically when performing blood-flow restriction exercise, but not low-intensity free-flow exercise.