Clinical safety of blood flow-restricted training? A comprehensive review of altered muscle metaboreflex in cardiovascular disease during ischemic exercise

Abstract

Blood flow restriction training (BFRT) is an increasingly widespread method of exercise that involves imposed restriction of blood flow to the exercising muscle. Blood flow restriction is achieved by inflating a pneumatic pressure cuff (or a tourniquet) positioned proximal to the exercising muscle before, and during, the bout of exercise (i.e., ischemic exercise). Low-intensity BFRT with resistance training promotes comparable increases in muscle mass and strength observed during high-intensity exercise without blood flow restriction. BFRT has expanded into the clinical research setting as a potential therapeutic approach to treat functionally impaired individuals, such as the elderly, and patients with orthopedic and cardiovascular disease/conditions. However, questions regarding the safety of BFRT must be fully examined and addressed before the implementation of this exercise methodology in the clinical setting. In this respect, there is a general concern that BFRT may generate abnormal reflex-mediated cardiovascular responses. Indeed, the muscle metaboreflex is an ischemia-induced, sympathoexcitatory pressor reflex originating in skeletal muscle, and the present review synthesizes evidence that BFRT may elicit abnormal cardiovascular responses resulting from increased metaboreflex activation. Importantly, abnormal cardiovascular responses are more clearly evidenced in populations with increased cardiovascular risk (e.g., elderly and individuals with cardiovascular disease). The evidence provided in the present review draws into question the cardiovascular safety of BFRT, which clearly needs to be further investigated in future studies. This information will be paramount for the consideration of BFRT exercise implementation in clinical populations.

Keywords: Kaatsu training; autonomic nervous system; blood flow restriction training; cardiac rehabilitation; exercise pressor reflex; sympathetic nervous system.

Fig. 1.

Cardiovascular adjustments to exercise: the role of the autonomic nervous system. During exercise, feedforward signals from higher brain areas (i.e., central command) and feedback information arising from different somatic and visceral afferents (i.e., such as the arterial baroreflex and the exercise pressor reflex) convey sensory information to the central nervous system, which then coordinates the efferent response to the cardiovascular system. SNA, sympathetic nerve activity; PNA, parasympathetic nerve activity; RAS, renin–angiotensin system; Adr, adrenaline; Nor, noradrenaline.

Fig. 2.

Seminal experiments (Refs. and 65) with anesthetized cats showed that electrical stimulation of the ventral root of the spinal cord, or capsaicin injection into the hindlimbs (A), produced an increase in activity of group IV fibers, which was accompanied by increases in heart rate (HR) and mean arterial pressure (B). The black arrow in B illustrates the beginning of stimulation (either by capsaicin or an electrical stimulus).

Fig. 3.

Hypothetical time course of blood pressure responses occurring during resistance exercise with and without blood flow restriction (BFR). Absence of interruption of BFR during interset intervals may promote progressive increase in systolic (SBP) and diastolic (DBP) blood pressure throughout exercise in BFR exercise. S1–S4, sets 1 to 4; I1–I4, interset intervals 1 to 4.