Effect of Cuff Pressure on Blood Flow During Blood Flow–Restricted Rest and Exercise

Recently, an interesting paper by Crossley et. al. was published in the Medicine & Science in Sports & Exercise Journal [1].

The study investigated the relationship between blood flow and cuff pressure at rest, and during plantar flexion exercise, by measuring blood flow (Doppler Ultrasound) through the superficial femoral artery in 23 adults [1]. Based on the results, Crossley et. al. concluded that, “Blood flow through the SFA exhibits a non-linear relationship with cuff pressure, such that cuff pressures in the range of 40-80% rAOP reduce blood flow to approximately the same degree. BFR interventions opting for lower (e.g. 40% AOP), more comfortable pressures will likely provide an ischemic stimulus comparable to that of higher (80% AOP), less-comfortable pressures” [1].

However, the methodology is fundamentally flawed, resulting in erroneous conclusions.

The study showed that ischemic stimulus (as estimated by using a Doppler probe to study blood flow in the superficial femoral artery) does not have a linear relationship with pressure in the non-tourniquet cuff employed. Specifically, the authors found almost no difference in blood flow between 30%-80% AOP/LOP. Therefore, the author suggested the use of lower pressures (40% AOP) for equivalent ischemic stimulus at lower stress and more comfort for the patient.

The limitations of this study emphasize the importance of measuring Limb Occlusion Pressure (LOP), which accurately indicates occlusion of all arterial vessels in the limb underlying a surgical-grade tourniquet cuff, rather than estimating limb occlusion by employing a Doppler ultrasound probe on one superficial artery and a non-tourniquet cuff.

Methodological problems are summarized below:

1. Consideration of only the superficial femoral artery

By applying non-uniform pressure over the superficial femoral artery, it can be readily demonstrated experimentally that blood flow in this artery can be restricted without similarly restricting blood flow in deeper arteries of the lower limb. This is done in practice, for example, to stop localized arterial hemorrhage. Therefore, 40% occlusion of one underlying superficial artery does not necessarily provide the same total ischemic stimulus as 40% of Limb Occlusion Pressure (LOP). LOP accurately indicates the minimum pressure at which all arterial vessels are occluded in the limb underlying a tourniquet cuff applying uniform pressure circumferentially.

2. General limitations of Doppler ultrasound method used in the study

A calibrated Doppler ultrasound system was used to measure mean blood velocity, and blood vessel diameter during end diastole. Volumetric blood flow in the superficial femoral artery was calculated. The study only measured the volumetric blood flow in the superficial femoral artery. It does not measure the total volumetric blood flow in the limb past the cuff, which is important for determining the total ischemic stimulus to the subject.

3. Specific limitations of Doppler method used in the study

Doppler ultrasound measures blood velocity not blood flow. Blood flow is estimated based on the average blood velocity, and the estimated diameter of the vessel taken at a single point of time in the cardiac cycle. Thus, there can be errors associated with the calculation of the blood flow. Sources of error possible in the study methodology include the following:

No repeated measurements: According to Gill [2], when using ultrasound to calculate measured blood flow there are considerable random errors, attributable primarily to errors in measuring the cross-sectional area and the angle of approach. Repeating the measurement of flow several times and averaging the results can reduce these random errors to an acceptable level. Repeated measurements were not reported in the study.

Vessel diameter measured during end diastole instead of systole: According to Blanco [3], the cross-sectional area A is calculated using the vessel diameter D which commonly changes along the cardiac cycle (systolic dimensions larger than diastolic) . Observing the formula for calculation of A, small errors in measurement of the D result in large errors in A (squared), and thus in volume flow calculation. The measurement of D must be performed at largest artery dimension (systole) and if possible exactly at the same site where the sample volume is placed. In practice, this is very difficult to do and was not done in this study.

4. Cuff limitations of the study

Depending on cuff design, as shown by Hughes [4], cuff pressure may be different or significantly different from the pressure actually applied to the limb (interface pressure). This is especially true during exercise.
Hokanson provides both blood pressure cuffs and ‘rapid version’ cuffs. The ‘rapid version’ cuffs are used as tourniquet cuffs or for venous reflux measurements. In the study, a Hokanson cuff (not described in the paper) was used with a Hokanson E20 Rapid Inflator. Unlike surgical-grade tourniquet cuffs, neither the BP or the ‘rapid version’ cuffs have stiffeners to assure uniform pressure distribution around the limb circumference. This means the indicated cuff pressure may be significantly different from the pressure actually applied to the limb around its circumference. If a Hokanson BP cuff was used, then the bladder would not encircle the entire limb circumferentially resulting in poor, non-uniform blood flow restriction and measurement results.
In contrast to the cuffs used in the study, surgical-grade tourniquet cuffs produce uniform pressure distribution around the circumference of the limb underlying the cuff, and the indicated cuff pressure accurately reflects the maximum pressure applied to the limb.

5. A conclusion based on unverified assumption  

In addition to the major limitations of the methodology which are summarized above, the authors use Figure 1, blood flow measured at rest, to make a conclusion on blood flow measured during exercise.  They assume their results of blood flow during rest can be transferred to blood flow during exercise.  However, Figure 4 shows that during exercise, they only examined blood flow at 40% of rAOP and 40% of eAOP.  The authors should have measured blood flow during exercise at 80% of rAOP and 80% of eAOP and plotted it on Figure 4.  Since they failed to do this, their conclusion that a 40% of AOP will provide a similar ischemic stimulus as 80% is unverified by appropriate experimental data.

Sources:

[1] Crossley KW, Porter DA, Ellsworth J, Caldwell T, Feland JB, Mitchell U, Johnson AW, Egget D, Gifford JR. Effect of Cuff Pressure on Blood Flow during Blood Flow-restricted Rest and Exercise. Medicine and science in sports and exercise. 2019 Sep 16.

[2] Gill RW. Measurement of blood flow by ultrasound: accuracy and sources of error. Ultrasound in medicine & biology. 1985 Jul 1;11(4):625-41..

[3] Blanco P. Volumetric blood flow measurement using Doppler ultrasound: concerns about the technique. Journal of ultrasound. 2015 Jun 1;18(2):201-4.

[4] Hughes L, Rosenblatt B, Gissane C, Paton B, Patterson SD. Interface pressure, perceptual, and mean arterial pressure responses to different blood flow restriction systems. Scandinavian journal of medicine & science in sports. 2018 Jul;28(7):1757-65.

2019-10-06T13:03:39-07:00