Assessment of Peripheral and Central Hemodynamics During Leg Exercise With "Blood Flow Restriction"

Abstract

“Blood flow restriction” (“BFR”) is a type of exercise training where a pneumatic cuff is applied to the proximal portion of an exercising limb. In “BFR”, cuff inflation to 40% of an individual’s arterial occlusion pressure (AOP; cuff pressure which fully occludes limb arterial flow at rest) is assumed to reduce exercising muscle blood flow. However, no study has validated this assumption. Therefore, we sought to determine whether pneumatic cuff inflation at the proximal thigh impairs exercising leg blood flow and total cardiovascular circuit flow (cardiac output), and whether the blood flow impairment is dependent on the magnitude of the applied cuff pressure. Eighteen healthy young participants (9 Female) performed two-legged knee flexion/extension exercise at 25% peak work rate with bilateral pneumatic cuffs applied to the proximal thigh at 0% (CTL), 20% and 40% AOP. Leg blood flow (Doppler and echo ultrasound) and central cardiovascular responses (finger photoplethysmography) were measured during rest and exercise. Leg blood flow values were doubled to account for both exercising legs. Resting leg blood flow was lower in 40% vs. CTL (P<.001) and 20% AOP (P=.002). Resting cardiac output was higher in 20% AOP vs. CTL (P=.001) and 40% AOP (P=.005). During exercise, both 20% and 40% AOP reduced exercising leg blood flow compared to CTL (P<.001). The reduction in blood flow at 40% AOP was greater than at 20% AOP (P<.001). However, the magnitude of the leg blood flow restriction by 40% AOP was progressively reduced from 37.4% to 15.9% across the five-minute exercise bout due to an increase in calculated vascular conductance (P<.001). In early exercise, 40% AOP significantly reduced cardiac output compared to CTL (P<.001) and 20% AOP (P<.001). By steady-state exercise, there were no significant differences in cardiac output between cuff pressures (all P>.129). In conclusion, "BFR" at 20% and 40% AOP restricts limb blood flow but not total circuit flow during steady-state exercise. The blood flow impairment was significantly attenuated across the exercise bout due to compensatory vasodilation. This compensatory effect on the magnitude of “BFR” may have implications for the degree of muscle adaptations that occur following “BFR” training.