Tourniquet Safety and Design: How can hazardously high tourniquet pressure gradients be minimized?
It is well established in the medical literature that the probability of tourniquet-related injuries increases as tourniquet pressure increases, and as the pressure gradients near the edges of tourniquet cuffs increase, eg [1-7]. Unnecessarily high tourniquet pressure gradients represent a serious and recognized hazard associated with unnecessarily high probabilities of patient injuries. Hazardously high pressure gradients can be minimized by the design of improved pneumatic tourniquet cuffs and by design of improved pneumatic tourniquet instruments.
Design improvements in some new tourniquet instruments and cuffs
Some tourniquet cuffs allow the automatic measurement of Limb Occlusion Pressure (LOP) . It is well established in the medical literature that the optimal guideline for setting the pressure of a constant-pressure tourniquet is based on LOP . 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. The use of LOP to set tourniquet pressure for individual patients is an important factor that allows tourniquet pressure gradients to be reduced, eg [10-14].
Tourniquet pressure gradients can also be reduced by improved design of tourniquet cuffs so that the pressure beneath the cuff decreases smoothly and gradually near the cuff edges and yet is sufficient near the center of the cuff to stop underlying arterial bloodflow at low tourniquet pressures. Three key improvements in the design of some types of tourniquet cuffs have facilitated this:
(1) the advent of wider, low-pressure tourniquet cuffs, and especially variable-contour low-pressure tourniquet cuffs to match tapered limb shapes;
(2) the incorporation of new materials into certain new cuffs having cylindrical shapes and contour shapes, to reduce cuff thicknesses, to reduce pressure variations underlying the cuff circumferentially around the limb, and to achieve desired pressure gradients from proximal to distal cuff edges; and
(3) advances in the design of stiffeners overlying inflatable bladders in certain cylindrical and variable contour cuffs, improving their effectiveness in selectively controlling bladder pressurization inwardly toward the encircled limb.
Examples of improved tourniquet cuff safety arising from improved tourniquet cuff designs and technologies are given in [15-22] below.
Tourniquet safety and pressure gradients
An excellent and comprehensive review of the published medical literature on how and why high pressure gradients lead to tourniquet-induced nerve injuries, including electron micrographs of injured nerves, is given in included in reference  below (“Current concepts: surgical tourniquets in orthopaedics”). The three key conclusions in that review are:
(1) Higher levels of tourniquet pressure and higher pressure gradients beneath tourniquet cuffs are associated with a higher risk of nerve-related injury.
(2) Measurement of Limb Occlusion Pressure (LOP) can help to minimize tourniquet pressure levels and pressure gradients for individual patients and individual surgical procedures.
(3) Selective use of pneumatic, wider, and contoured tourniquet cuffs reduces tourniquet pressure levels and the applied pressure gradients.
The history and pathogenesis of tourniquet-related nerve injuries is well summarized in  as excerpted below:
“The risk of tourniquet-related nerve injury remains a particular concern. In an early study, before the introduction of automatic tourniquet systems and before the routine use of lower tourniquet pressures, electromyographic evidence of peripheral nerve injury was found in a high percentage of limbs after tourniquet use. In prospective randomized studies conducted in the 1980s, when mechanical tourniquets and higher tourniquet pressures were in common use, there was evidence of denervation in 71% (seventeen) of twenty-four patients after lower-extremity tourniquet use and in 77% (twenty-four) of thirty-one patients after upper-extremity tourniquet use. The prevalence of electromyographic abnormalities was reported to increase with tourniquet time, and evidence of denervation typically lasted from two to six months. Electromyographic abnormalities correlated with impaired postoperative function and delayed recovery, suggesting that tourniquet-induced neuropathy played a causal role in impaired rehabilitation.
“On the basis of a questionnaire survey in Norway, the incidence of neurological complications associated with tourniquet use was estimated to be one per 6155 applications to the upper limb and one per 3752 applications to the lower limb. Other estimates have varied, and it has been suggested that the actual incidence of so-called tourniquet paresis may be underreported . Such nerve injuries range from a mild transient loss of function to permanent, irreversible damage and are a potential source of litigation. To minimize risk and potential litigation, an understanding of both the mechanism of injuries and possible preventive measures is important. Ochoa et al. showed that most cases of nerve damage were limited to the portion of the nerve beneath and near the edges of the cuff. They found that compressive neurapraxia rather than ischemic neuropathy or muscle damage was the underlying cause of tourniquet paralysis and demonstrated that compression of the large myelinated fibers involves a displacement of the node of Ranvier from its usual position under the Schwann-cell junction. This was accompanied by stretching of the paranodal myelin on one side of the node and invagination of the paranodal myelin on the other. The nodal axolemma was sometimes identified as far as 300 mm from its original position under the Schwann-cell junction, causing partial or complete rupture of the stretched paranodal myelin (Fig. 3). The nodal displacement was maximal under the edges of the cuff, wherethe applied pressure gradient was greatest. There was relative or complete sparing under the center of the cuff, and the direction of displacement was away from the cuff toward the uncompressed tissue.”
Some of the effects of tourniquet cuff design on tourniquet pressure gradients are also well summarized in reference  as excerpted below:
“The actual levels of pressure applied by a pneumatic tourniquet cuff to the underlying limb and soft tissues vary widely in comparison with the pneumatic inflation pressure within the tourniquet cuff. McLaren and Rorabeck measured the distribution of tissue pressures under pneumatic tourniquets in canine limbs. The peak pressure, which was 97% of the cuff inflation pressure, was in the subcutaneous tissue just proximal to the midposition along the tourniquet width. Tissue pressures decreased progressively as they became closer to the cuff edges, with a decrease of about 90% from the midpoint of the cuff width to the cuff edge. Pressures were lower in deeper tissues as well, but the decrease from the limb surface to the center was only about 2%. At the midpoint of the cuff width, surface tissue pressure was 95% of the cuff inflation pressure.
“Shaw and Murray also showed a decrease in tissue pressure with increasing depth, midway along the width of a cylindrical pneumatic tourniquet cuff on the lower extremities of human cadavers. They noted that the pressure measured in the soft tissue was consistently lower than the pneumatic pressure in the tourniquet cuff and that the level of tissue pressure varied inversely with the thigh circumference. All such studies suggest that higher tourniquet inflation pressures and higher applied pressure gradients on the limb surface correspond to higher pressures and higher pressure gradients in the underlying soft tissues. The distribution of perineural pressures under the cuff is described by a parabolic curve (Fig. 4), with peak levels at the midpoint of the cuff and much lower pressures at the proximal and distal edges. The difference between soft-tissue pressures at the cuff midpoint and those at the cuff edges increases at higher levels of cuff inflation, establishing a direct relationship between the level of the cuff inflation pressure and the pressure gradient in the underlying soft tissue.
“There is an inverse relationship between limb occlusion pressure and the ratio of the cuff width to the limb circumference. This relationship is shown in Figure 4, indicating that, for a given limb circumference, a narrower cuff requires a much higher tourniquet pressure to stop blood flow (higher limb occlusion pressure). This is associated with higher applied pressure gradients and a greater risk of neurological injury. Conversely, for the same limb circumference, a wider cuff requires a lower tourniquet pressure to stop blood flow. Additionally, a contoured tourniquet cuff occludes blood flow at a lower inflation pressure than does a straight (cylindrical) cuff of equivalent width24,25. This may be attributable to a better fit of the cuff to the limb and thus more efficient transmission of pressure to the underlying tissue. These facts have motivated the development and increasing use of wider, variable-contour cuffs that conform to a wide range of limb shapes, stopping blood flow at pressures that are lower than are necessary with narrower, cylindrical cuffs.”
(Excerpted From ) Fig. 4 Limb occlusion pressure (LOP) versus the ratio of tourniquet cuff width to limb circumference. For any given limb circumference, the tourniquet pressure required to stop arterial blood flow decreases inversely as the width of the tourniquet cuff increases. (Reproduced, with modification, from: Graham B, Breault MJ, McEwen JA, McGraw RW. Occlusion of arterial flow in the extremities at subsystolic pressures through the use of wide tourniquet cuffs. Clin Orthop Relat Res. 1993;286:257-61.)
(Excerpted From ) Fig 5. A comparison of applied pressures and pressure gradients typically produced by a modern pneumatic surgical tourniquet cuff (A); a nonpneumatic,non-elastic, belt-type military tourniquet designed for self-application on the battlefield (B); and a non-pneumatic ring made of elastic material, designed to be rolled from a distal location to a proximal location on a limb and to remain there for surgery, thereby combining exsanguination and tourniquet functions (C). Each tourniquet was selected and applied as recommended by the respective manufacturer to stop arterial blood flow in an upper limb. Higher levels of pressure and higher pressure gradients are associated with higher probabilities of patient injuries6.
References and Citations
3. Gilliatt R and Ochoa J. The cause of nerve damage in acute compression. Trans Am Neurol Ass 1974: 99: 71-4.
7. Noordin et al. “Surgical Tourniquets in Orthopaedics” Journal of Bone and Joint Surgery, 91 (2009): 2958-2967. http://www.jbjs.org/
8. McEwen JA et al. “Surgical tourniquet apparatus for measuring limb occlusion pressure.” US Pat No. 7,479,154, Jan 2009. http://patft.uspto.gov/
9. www.tourniquets.org/lop.php – McEwen JA, Educational website focused on surgical tourniquets, and related tourniquets for military and emergency applications, including tourniquet safety and usage.
11. Younger, SE et al. “Automated Cuff Occlusion Pressure Effect on Quality of Operative Fields in Foot and Ankle Surgery: A Randomized Prospective Study.” Foot and Ankle international, 32(3)(2011): 239-243.
12. Reilly et al. “Minimizing Tourniquet Pressure in Pediatric Anterior Cruciate Ligament Reconstructive Surgery. A Blinded, Prospective Randomized Controlled Trial.” Journal of Pediatric Orthopaedics, 29 (2009): 275-280.
19. McEwen JA et al. “Adaptive tourniquet cuff system.” US Pat No. 7,331,977, Feb 2008. http://patft.uspto.gov/
20. McEwen JA. “Occlusive cuff.” US Pat No. 5,312,431, May 1994. http://patft.uspto.gov/
21. McEwen JA et al. “Low-cost contour cuff for surgical tourniquet systems.” US Pat App No. 20100268267, Oct 2010. http://patft.uspto.gov/
22. McEwen JA et al. “Low-cost disposable tourniquet cuff.” US Pat App No. 20100004676, Jan 2010. http://patft.uspto.gov/