UNDER PRESSURE ... BFR AND HEMODYNAMICS

The application of Blood Flow Restriction (BFR) is nearly a century old. Some of the earliest papers date back to the late 1930s. Much of the research on this intervention, and a large portion of the questions we receive center around safety. In previous blogs we addressed concerns around clots and dove more specifically into the implications for individuals with thrombophilia. In this installment, and as Kyle alluded to in the previous blog, we’re going to continue the safety discussion by addressing the effects of BFR on hemodynamics. The primary questions have been:

• How does the addition of BFR alter the hemodynamic responses of low load resistance exercise?
• How does this information influence the way we perform BFR?

BP / HR RESPONSES TO RESISTANCE EXERCISE

Changes in blood pressure and heart rate with exercise are well documented and are known to occur at relatively low intensity (20-50% MVC). (Lind and McNicol 1967) MacDougall et al showed that the increases in blood pressure and heart rate can be extreme when performing exercise at high intensities. When individuals performed bilateral leg press at 80-100% of their 1 rep max, blood pressure was measured as high as 480/350 mmHg and heart rate was as high as 170 beats per minute. (MacDougall et al. 1985) Even with a small muscle group in a single arm bicep curl, they measured BP as high as 255/190 mmHg. The authors postulated that the significant increases in pressure and heart rate occurred as a result of mechanical compression of the vasculature, valsalva or increased intraabdominal pressure, and a pressor response to maintain cardiac output. (MacDougall et al. 1985) The changes tended to be greatest with exercise that required multiple reps to achieve failure as opposed to a single max effort lift. Thus, the load, duration, and amount of muscle involved (Legs > Arms) all have an influence on the cardiovascular response.

DURATION OF PRESSURE

As it pertains to BFR exercise, the work of Alam and Smirk in the late 1930s provides a seminal understanding of how mechanical pressure applied to a limb’s vasculature influences the hemodynamic response (Alam and Smirk 1937) . Alam and Smirk designed a series of experiments to investigate the exercise pressor reflex. In one of their experiments they applied blood pressure cuffs to the thighs and inflated to “well above systolic blood pressure” while performing a soleus exercise. They noted that systolic and diastolic blood pressure increased while performing the exercise. This was demonstrated in the UE as well via similar methods and the use of a gripping task. Attempting to apply mechanical pressure to the vasculature in a different manner they actually had individuals plunge their arms into a vat of mercury to induce vasoconstriction while doing a forearm exercise (good luck getting this through IRB). The variables in these experiments were then manipulated in such a way that they discovered BP did not return to baseline until the mechanical pressure was released despite the cessation of exercise. Additionally, after 3-4 minutes of sustained pressure and no exercise, they identified a gradual rise in blood pressure. These findings lead them to suggest that the method of vascular compression doesn’t seem to matter, merely the restriction of blood flow into the limb would cause an increase in blood pressure. 

As we move forward in the BFR literature, Takano in 2005 and Ozaki in 2010 added support for the work of Alam and Smirk showing that the addition of BFR increased HR and BP more than low intensity work matched exercise. (Takano et al. 2005; Ozaki et al. 2010) The increases in HR and BP in these studies were statistically significant compared to the control group, but modest in comparison to the numbers seen in the MacDougall paper. One potential reason for these findings is that when BFR is added to resistance exercise the time needed to achieve high levels of fatigue is substantially reduced. This reduces the duration pressure would be applied to the exercising limb, thus giving further support to Alam and Smirk’s work. Providing further support to the influence duration of pressure application has can be found in Neto's work from 2016. They showed that deflating during the rest period (intermittent BFR) limited the increase in double product (a measure of heart rate times systolic pressure). (Neto et al. 2016)

AMOUNT OF PRESSURE

One of the primary reasons we recommend using the lowest pressure possible with BFR exercise is the pressure applied with BFR really seems to matter when it comes to the hemodynamic response. Brandner et al compared 130% of systolic pressure (intermittently) to 80% of systolic pressure (continuously) while performing a bicep curl exercise with 20% of a 1 rep max. They showed that the changes in blood pressure, heart rate, and mean arterial pressure are greater even when BFR is applied intermittently at a higher pressure. (Brandner et al. 2015) The high pressure group pretty closely resembled high intensity exercise. Eliciting that kind of a response may not be a concern for a young, healthy individual, but it would at minimum be cause for concern for those with comorbidities.

REGULATION OF PRESSURE

Additionally, Hughes et al showed that regulation of pressure throughout the exercise influences the hemodynamic response. In a study comparing the pressure actually being applied to the limb across multiple different tourniquet systems, they found that the two cuffs in their study which lacked the ability to rapidly adjust pressure during exercise significantly augmented mean arterial pressure. (Hughes et al. 2018) Conversely, the automatic personalized tourniquet was able to maintain consistent pressure to the limb throughout the exercise and did not cause a significant elevation in mean arterial pressure.

PUTTING THIS INTO PRACTICE...

From the information provided above, we understand that exercise with or without blood flow restriction will cause some changes in BP and HR. This may not matter for the average gym goer, but in a patient care setting, minimization of risk while applying adequate stimulus is crucial . There are a number of variables which influence the hemodynamic response while performing blood flow restriction that we can control for such as exercise intensity , pressure used, the time under pressure, and the exercise effort or exertion. 

Great recommendations for load and pressure can be found in the article formerly known as the Position Stand. (Patterson et al. 2019) For patients who have hypertension or other cardiovascular concerns, we recommend you explore exercise without BFR first to understand their tolerance and establish a baseline. If you decide to incorporate BFR, it may be prudent to initiate BFR with a lower percentage of their 1RM and a lower pressure than you might use otherwise. Work from Hughes, and some unpublished work from Kacin, indicates that the ability to rapidly adjust pressure changes created by the contracting muscle may also be important (Hughes et al. 2018; Kacin et al. ). The spikes in pressure that occur from this potentially influence the exercise pressor reflex via the intensity of pressure applied to the vasculature. Lastly, deflating during the rest period can be another strategy you deploy to limit the hemodynamic response. This reduces the amount of time that pressure is applied to the vasculature and potentially reduces the overall hemodynamic response. 

BFR is a safe and effective intervention when applied in an objective way. Patients, however, often present with comorbidities. This requires the clinician have a deeper understanding of how the intervention they choose will, and can be manipulated to, influence factors that are relevant to their comorbidities. So take an extra moment to consider all the variables you can control and apply them in a strategic fashion. Your patient deserves that!

1. Lind AR, McNicol GW. Circulatory responses to sustained hand-grip contractions performed during other exercise, both rhythmic and static. J Physiol. 1967;192(3):595-607. 
2. MacDougall JD, Tuxen D, Sale DG, Moroz JR, Sutton JR. Arterial blood pressure response to heavy resistance exercise. J Appl Physiol. 1985;58(3):785-790. 
3. Alam M, Smirk FH. Observations in man upon a blood pressure raising reflex arising from the voluntary muscles. J Physiol. 1937;89(4):372-383. 
4. Takano H, Morita T, Iida H, et al. Hemodynamic and hormonal responses to a short-term low-intensity resistance exercise with the reduction of muscle blood flow. Eur J Appl Physiol. 2005;95(1):65-73. 
5. Ozaki H, Brechue WF, Sakamaki M, et al. Metabolic and cardiovascular responses to upright cycle exercise with leg blood flow reduction. J Sports Sci Med. 2010;9(2):224-230. 
6. Neto GR, Novaes JS, Dias I, Brown A, Vianna J, Cirilo-Sousa MS. Effects of resistance training with blood flow restriction on haemodynamics: a systematic review. Clin Physiol Funct Imaging. 2017;37(6):567-574. 
7. Brandner CR, Kidgell DJ, Warmington SA. Unilateral bicep curl hemodynamics: Low-pressure continuous vs high-pressure intermittent blood flow restriction. Scand J Med Sci Sports. 2015;25(6):770-777. 
8. Hughes L, Rosenblatt B, Gissane C, Paton B, Patterson SD. Interface pressure, perceptual and mean arterial pressure responses to different blood flow restriction systems. Scand J Med Sci Sports. Published online April 6, 2018. doi:10.1111/sms.13092 
9. Counts BR, Dankel SJ, Barnett BE, et al. Influence of relative blood flow restriction pressure on muscle activation and muscle adaptation. Muscle Nerve. 2016;53(3):438-445. 
10. Ilett M, Rantalainen T, Keske M, May A, Warmington S. The Effects of Restriction Pressures on the Acute Responses to Blood Flow Restriction Exercise. Front Physiol. 2019;10:1018. 
11. Kacin, A., Rosenblatt, B., Žargi, T. G., & Biswas, A. (n.d.). SAFETY CONSIDERATIONS WITH BLOOD FLOW RESTRICTED RESISTANCE TRAINING. 
12. Neto GR, Novaes JS, Salerno VP, et al. Acute Effects of Resistance Exercise With Continuous and Intermittent Blood Flow Restriction on Hemodynamic Measurements and Perceived Exertion. Percept Mot Skills. Published online November 11, 2016. doi:10.1177/0031512516677900 
13. Neto GR, da Silva JCG, Freitas L, et al. Effects of strength training with continuous or intermittent blood flow restriction on the hypertrophy, muscular strength and endurance of men. Acta Scientiarum Health Sciences. 2019;41:e42273.