Background
Blood Flow Restriction (BFR) is a technique whereby a specialized tourniquet cuff is applied to a limb and inflated with air, thereby restricting arterial blood flow, and occluding venous outflow distal to the cuff. The blood flow restriction technique is combined with or without low intensity exercise of the limb, to elicit physiological adaptations which can positively impact muscle size, strength, and endurance. To generate muscle and physiological adaptations, traditional exercise must be performed at high loads and intensities, around 60% – 70% of a person’s 1 repetition maximum (1RM) [1]. With BFR, exercise can be performed at substantially lower loads (20% – 30% 1RM) and at lower intensities while generating similar muscular and physiological adaptations seen at higher loads [2].
The concept of BFR training to improve muscle mass has been around for decades, originating in its primitive use in Japan in the 1990s. Preliminary implementation of BFR training consisted of high intensity exercise focused on muscle adaptations and physiology, utilizing tight, narrow bands to restrict or occlude blood flow during training. However, this basic concept lacks controls necessary to ensure user safety and reliability of the effectiveness of the training. The gold standard for modern BFR technology evolved from surgical tourniquet innovation led by the inventor of the automatic pneumatic surgical tourniquet, Dr. Jim McEwen and his development partner Michael Jameson, in collaboration with researchers within the US military.
Over the last 20 years BFR has been researched to understand its mechanisms, methods of implementation, and benefits of use. It has been shown that controlled use of blood flow restriction therapy is very beneficial for clinical applications. For load compromised populations (e.g., elderly, ACL injury, osteoarthritis, etc.), it is typically not safe nor attainable to complete the necessary high intensity training required to promote muscle adaptations while protecting the joint/injury. Therefore, these clinical populations typically experience some muscle atrophy and loss of strength while recovering. In addition to protecting the joint, rehabilitation protocols now need to work to recover the lost muscle mass and strength. Traditional physical therapy and rehabilitation protocols are therefore lengthy and require patience and compliance to return to regular function.
The clinical application of blood flow restriction therapy (BFRT) accelerates the recovery process in load compromised patients [3]. BFRT can be therefore be implemented earlier than traditional therapy as substantially lower loads and lower intensities can be used and tolerated by the patients while still gaining similar physiological adaptations to traditional exercise [4].
Evolution of surgical grade tourniquets for safe and effective BFR Therapy
Primitive early applications of blood flow restriction training were performed at uncontrolled pressures, with tight, narrow cuffs applied to prevent blood flow. These early BFR devices mirror the surgical/emergency tourniquets used for centuries to control extremity bleeding, where preservation of life over limb was the priority. Historically, the evolution of surgical and clinical tourniquets began with a simple stick and belt style tourniquet, and evolved with wraps and inflatable bands [5]. However, with these early methods it was impossible to safely control and monitor the applied pressures. The complications, injuries, and safety issues with these early devices are well documented in clinical literature [5-7]. Bruising, pinching, muscle pain, and nerve injury (from temporary to paralysis) were all common risks with such early methods. The reports of serious patient injuries from surgical tourniquet use is what motivated Dr. McEwen to invent the automatic, pneumatic, microprocessor controlled tourniquet in 1979. Dr. McEwen’s invention remains the gold standard technology for modern tourniquet systems.
The modern surgical tourniquet is pneumatically powered, and automatically controlled with the use of a microprocessor. This allows for precise, accurate and reliable pneumatic monitoring and autoregulation, automatic monitoring and alarm conditions, and monitoring of the total inflation time [6]. Dr. McEwen and Michael Jameson continued to innovate to improve the safety and effectiveness of tourniquet devices. They developed a proprietary technique to measure a patient’s Limb Occlusion Pressure, where LOP is defined as the minimum pressure required in a specific instance to occlude blood flow distal to the cuff [8]. The ability to determine a patient’s LOP thereby allows personalized tourniquet pressures to be applied to safely and effectively occlude blood flow to the surgical site while mitigating tourniquet related risks [9].
The evolution of surgical tourniquet technology for improved safety and effective is important to understand, such that the lessons learned can be applied to blood flow restriction training to preserve patient’s safety and improve the effectiveness of the training. Decades of BFR research revealed that BFRT is the most effective and most safely applied at pressures causing arterial restriction rather than at full occlusion. Restrictive pressure should be applied in the range of 40% – 80% of the individual’s Limb Occlusion Pressure. Therefore, in order to achieve safe and effective use in the clinical population, BFR devices must be able to accurately determine the required pressure needed to restrict the patient’s blood flow, be able to accurately and reliably regulate around that pressure, and to monitor for proper function of the device and alarm if any issues occur [10].
As BFR research continues to evolve, it is also important to consider the technology and methodology which these studies apply. Inconsistent implementation or lack of awareness of applied pressures can skew results, which has led to conflicting conclusions of many studies over the years and has made it difficult to optimize protocols and methods.
Delfi Medical Innovations Inc is a medical device company which has over 30 years of experience in the development and manufacturing of surgical tourniquet devices. In collaboration with the Center for Intrepid at San Antonio Military Medical Center led by Johnny Owens, BFR research initiatives for the clinical applications of BFR, its mechanisms, and protocols were implemented by adapting Delfi’s surgical tourniquet systems to apply personalized blood flow restriction. Delfi combined the proven proprietary limb occlusion pressure measurement technology used in surgery with its modern personalized surgical tourniquet systems to develop the first surgical grade Personalized Tourniquet System for Blood Flow Restriction (PTS for BRR) introduced to the market in 2015. The PTS for BFR includes all the safety features evolved and develop for surgical applications, combined with the ability to measure Limb Occlusion Pressure, in order to determine personalized pressures necessary to accurately, reliably, safely and effectively restrict blood flow during BFR training.
Technical requirements for restrictive pressures
As BFR training has gained in popularity, other devices have become available on the market. However not all devices are made equal, particularly as it relates to a device’s ability to enable both safe and effective use. It is essential to perform blood flow restriction therapy with a surgical grade tourniquet with the ability to personalize restriction pressures based upon a measured limb occlusion pressure to ensure safe and effective use, Figure 1.
In addition to the capability of available devices to determine and set restrictive pressures, it is equally important that these devices are clinically validated to demonstrate their ability to accurately maintain and regulate around the set pressure. Further, if the available devices present features for personalizing the set pressure, those features should be clinically validated to demonstrate their reliability and effectiveness.
Figure 1: Modern surgical-grade tourniquet instrument and cuff adapted for BFR rehabilitation, showing elements that provide improved safety, accuracy and reliability to consistently achieve optimal patient outcomes.
A review of BFR rehabilitation literature shows that inconsistencies exist in methodology, equipment and in levels of restriction pressure used. For example, Jessee et al. [11] summarized fifteen recently published BFR studies in the upper body and cuff pressures ranged widely. Some studies used a pressure applied with a tourniquet cuff at a level set as a percentage of personalized Limb Occlusion Pressure (LOP), other studies used a fixed cuff pressure applied with cuffs having a variety of sizes and shapes, and a few studies set pressure based on systolic blood pressure using old formulas that have been proven inaccurate, unreliable and largely discontinued in surgical tourniquet settings [8, 12-13]. These inconsistencies in methodology and equipment have made it difficult to apply a safe and consistent BFR stimulus to patients, they prevent a controlled comparison of different BFR protocols, and thus they limit the identification and delivery of optimal patient outcomes.
Limb occlusion pressure (LOP): the basis for personalized restriction pressures
As described above, the ability to determine a patient’s Limb Occlusion Pressure (LOP) in order to set a personalized restrictive pressure for the BFRT protocol is essential for both safety and efficacy. LOP can be defined as the minimum pressure required, at a specific time in a specific tourniquet cuff applied to a specific patient’s limb at a specific location, to stop the flow of arterial blood into the limb distal to the cuff [6, 10]. LOP is affected by variables including the patient’s limb characteristics; characteristics of the selected tourniquet cuff, including shape, width, length, presence or absence of circumferential bladder and internal stiffener; the technique of application of the cuff to the limb; physiologic characteristics of the patient including blood pressure and limb temperature; and other clinical factors (for example, the extent of any elevation of the limb during LOP measurement and the extent of any limb movement during measurement) [8].
A restriction pressure level set for each individual patient, based on a percentage of LOP measured at rest, and applied using a surgical-grade tourniquet cuff, enables those individual patients to receive a consistent BFR stimulus compared to other methods of setting the restriction pressure level [13]. Loenneke et al. [12, 13] demonstrated that setting BFR pressure as a function of blood pressure or at a fixed pressure does not provide a consistent stimulus across patients because these methods of setting pressure neglect important factors that affect LOP, including limb circumference and cuff width. This confirms what has been well established in the surgical tourniquet literature on LOP [8]. Fatela et al. [14] analyzed the effect of relative BFR pressure on the acute neuromuscular response to BFR resistance exercise and showed that muscular activation and neuromuscular fatigue varies as a function of relative blood flow restriction. Consequently, Fatela et al. [14] concluded that it is crucial to determine individual levels of vascular restriction, by quantifying the resting LOP, before engaging in BFR exercise and rehabilitation.
There are three primary benefits of using personalized restriction pressures based on a relative percentage of LOP, determined automatically on a resting patient by a surgical-grade tourniquet instrument, and applied safely and consistently by a surgical-grade tourniquet cuff. First, the use of such tourniquet instruments and cuffs are based on decades of experience in surgical settings, and assures the safe, accurate, and reliable application of pressure to a patient’s limb [8]. Setting and regulating the pressure as a predetermined percentage of the LOP can help avoid adverse events that may result from inadvertently applying pressures that result in complete arterial occlusion [11]. Second, the application of a consistent level of restriction pressure limits variability in BFR intensity for individual patients, since muscular activation, as well as neuromuscular fatigue, varies as a function of relative BFR intensity [14]. Muscular contractions and extensions cause changes in limb circumference, which causes changes to the applied pressure within the cuff. Accurate autoregulation of the cuff pressure is crucial to maintain consistent level of restriction pressure throughout dynamic exercises. Third, accurately applying a consistent level of restriction pressure enables the outcomes and results of a full range of BFR studies to be compared on a meaningful basis so that optimal protocols can be identified and applied [15, 16].
For any device which claims the ability to determine personalized restriction pressures, it is important to verify that the device has been clinical validated for its accuracy and reliability.
Surgical grade autoregulation
Differences in equipment and methodology have led to inconsistent restrictions of blood flow. This makes it difficult to make meaningful comparisons of research studies and identification of optimal therapy protocols and outcomes.
In order to have safe, effective, and consistent BFR pressure stimuli, a BFR system should have the ability to accurately and reliably autoregulate around its set pressure. To maintain the same level of safety and effectiveness developed for surgical tourniquets, surgical grade autoregulation is defined as “automatic and rapid self-regulation of cuff pressure to within +/- 15mmHg of the target pressure, within one second, in the presence of transient pressure changes associated with exercise” [17]. This defined specification for autoregulation ensures that the BFR system can quickly and accurately adapt to the expected pressure changes that are associated with exercise, for example muscle contractions and extensions.
BFR systems which cannot effectively autoregulate risk applying too high or too low pressures throughout the BFR exercise protocol. Lower pressure than intended can cause the BFR protocol to be ineffective and can increase the risk of venous congestion and limb swelling [18]. Higher pressure than intended can pose substantial safety risks to the patient including nerve and muscle damage. In addition, higher pressures than intended could possibly fully occlude arterial blood flow, further reducing safety and effectiveness of the BFR protocol [18].
In addition to being able to maintain surgical grade autoregulation, a BFR system should be able to display the real time pressure and automatically alert the user if the autoregulation specification is not being met. It is important that the user can be confident that the set pressure is safely being achieved. Clinicians, therapists and researchers who are required to document treatments should be documenting the true and accurate values of applied BFR stimuli.
Limitations of other approaches to determine restriction pressures
In early studies the most commonly used method for determining LOP to set restriction pressure was based on the use of Doppler ultrasound by a specifically trained clinician. However, this method is time-consuming, requires additional equipment, and the accuracy of LOP measurement is highly dependent on the specific training and experience of the measuring clinician. Alternatively, Jessee et al. [11] developed equations to predict a patient’s LOP, taking into account some of the determinants of LOP investigated in their study, but these equations do not account for all variables known to affect LOP [8] and their application may be too complex and time-consuming for routine clinical use. Additionally, some low-cost, non-tourniquet cuffs [16] and other devices such as elastic knee wraps [19] have been proposed for BFR rehabilitation, but their effectiveness is unproven and they present safety hazards, Figure 3. This is because they do not have the ability to automatically take into account each patient’s LOP when setting the restriction pressure level for individual patients and they apply unknown and inconsistent pressures to a patient’s limb that can be much higher or much lower than the intended restriction pressure [7, 8]. Further low-cost, non-tourniquet cuffs lack important safety features proven in surgical-grade tourniquet instruments and cuffs such as safe limits on pressures and protocols, accurate surgical grade pressure autoregulation and low pressure levels and gradients beneath cuffs [8].