Response.

Abstract

To the Editor-in-Chief, In their letter to the editor, McEwen et al. (1) question our results that indicate blood flow restriction (BFR) cuff pressure of 40% arterial occlusion pressure (AOP) may reduce blood flow the same amount as 80% AOP (2). Although their concerns may be important in other scenarios, as discussed below, most have little relevance to our results. First, McEwen et al. question the extent to which muscle blood flow was reduced because AOP was measured in a single artery. Indeed, when applying AOP, if multiple arteries supply the area of interest, it is advisable to measure AOP in all major applicable arteries. Nevertheless, the region of interest in our study, the calf, is almost exclusively perfused by arteries with origin in the superficial femoral artery (SFA) (3). The potential for collateral branches, not originating from the SFA, to compensate for an acutely occluded SFA is reportedly minimal (4). McEwen’s remedy of measuring limb occlusion pressure (LOP) provides no more assurance of total occlusion than AOP because it too relies on the observation of a pulse in a single artery (posterior tibial or doralis pedis artery) (5), which in contrast to the SFA provide no direct insight into the perfusion of typical muscles of interest in BFR. Ultimately, AOP has been used extensively in BFR training with promising results (6), whereas the assessment of LOP has primarily been in the operating room (5). Second, McEwen et al. suggest that random error associated with Doppler ultrasound may explain our observed data. Doppler ultrasound is a common technique that exhibits very good agreement with other measurements of blood flow (7), especially when multiple measurements are averaged over time, as in our study. To think that random error associated with the technique would consistently result in the same nonlinear relationship across multiple subjects in multiple studies (2,8) is illogical. Third, McEwen et al. question our results, suggesting that our very common (6,8) cuffing system (Hokanson rapid inflator cuffs) may not evenly distribute pressure throughout the limb. Because we verified that our cuffing system occluded the conduit artery of interest, any discussion of equal distribution of pressure, bladder size, and stiffeners is immaterial to our results and unjustified. Their final concern has merit. We observed similar rates of exercise flow at two different cuff pressures between approximately 40% and 50% AOP but did not assess exercise flow at 40% and 80% AOP (2). Exercise training studies suggest that 40% and 80% AOP elicit similar ischemic conditions during exercise, by revealing no difference in strength or mass gains when exercising at these different pressures (6), but direct comparisons of flow are lacking. Future studies should characterize the entire pressure–flow relationship during exercise, as we did during rest (2), to verify which pressures elicit similar flow rates during exercise. In conclusion, when AOP is applied and measured in the appropriate conduit arteries for a region of interest, 40% AOP appears to reduce blood flow to a similar degree as 80% AOP at rest. Studies are needed to completely characterize the pressure–flow relationship during exercise. Jayson R. Gifford Aaron Wayne Johnson Ulrike Mitchell J. Brent Feland Department of Exercise Sciences Brigham Young University Provo, UT