Limb Occlusion Pressure (LOP)
What is Tourniquet “Limb Occlusion Pressure” (LOP)?
Many studies and reviews published in the clinical literature have shown that higher tourniquet pressures and higher pressure gradients are associated with higher risks of tourniquet-related injuries, e.g. [1-7]. Studies have also shown that lower tourniquet pressures are associated with lower complications and pain, e.g. [8-9]. Therefore, when a tourniquet is used in surgery, the best practice is to use the lowest tourniquet pressure that is safely possible and to use a tourniquet cuff that is designed to produce the lowest pressure gradients.
Limb Occlusion Pressure (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. LOP is therefore personalized to each individual patient and each individual surgical procedure. Setting the tourniquet pressure on the basis of LOP minimizes the pressure and related pressure gradients applied by a cuff to an underlying limb, which helps to minimize the risk of tourniquet-related injuries.
Pressure gradient beneath a tourniquet cuff
Pressure gradient may be defined as the variation in pressure applied by a pressurized tourniquet cuff to the underlying limb across the width of the cuff. It is affected by the tourniquet pressure level and by the design and width of the pressurized cuff.
Figure 1 presents the pressure profiles across the widths of 3 tourniquet cuffs at tourniquet pressure levels required to achieve arterial occlusion in the limb. As shown in Figure 1, the pressure gradient is the slope of the curve on each graph. Pressure gradient (slope) increases as the applied pressure increases (y-axis) or as the cuff width decreases (x-axis). For example, a wide pneumatic tourniquet cuff (Cuff A) requires a lower tourniquet pressure level to achieve arterial occlusion in the limb (250 mmHg), and distributes the applied pressure over a wider cuff width (10 cm), resulting in lower pressure gradients being applied to the underlying soft tissues (up to 50 mmHg/cm). In comparison, narrow, non-pneumatic tourniquet cuffs (Cuffs B and C) require substantially higher tourniquet pressure levels to achieve arterial occlusion in the limb (up to 450 mmHg), and the applied pressure is distributed over a shorter cuff width (2 cm), resulting in much higher pressure gradients being applied to the underlying soft tissues (450 mmHg/cm). As noted above, higher tourniquet pressures and higher pressure gradients are associated with higher risks of tourniquet-related injuries [1-7].
The primary mechanism of tourniquet-related nerve injury is the application of a high pressure gradient along the length of the nerve underlying a tourniquet cuff [1, 5, 6, 10, 11]. A high pressure gradient results in an axial force being applied along the nerve, effectively stretching, invaginating, and buckling the myelin sheath which surrounds the nerves and displacing the nodes of Ranvier, thus injuring the nerve cells underlying the cuff [5, 6]. The location of such tourniquet-related nerve injuries is typically beneath the cuff location, and often near the distal edge of the cuff location, where the applied pressure gradient was greatest .
The hazard of high pressure gradients is mitigated in some modern tourniquet systems by (1) use of personalized tourniquet pressures, optimized to automatically identify the lowest tourniquet pressure level needed for individual patients, and (2) use of personalized tourniquet cuffs, designed to match the limb shape and optimally distribute cuff pressure from one cuff edge to the other, and circumferentially, thereby producing the lowest pressure gradients beneath the cuff when used in conjunction with personalized tourniquet pressures [2, 7].
Setting tourniquet pressure based on LOP
The best practice for setting tourniquet pressure is to set a personalized pressure that is based on LOP. The currently established guideline for setting tourniquet pressure based on LOP is that an additional safety margin of pressure is added to a measured LOP, to account for physiologic variations and other changes that may be anticipated to occur normally over the duration of a surgical procedure . A safety margin of 40-100 mmHg above the LOP has been suggested in the literature [12, 13, 14]. Typically the safety margin is lower for lower LOP levels and higher for higher LOP levels. A higher margin may be selected if a considerable amount of manipulation of the limb and/or large blood pressure rises are expected to occur during the procedure. One cuff pressure setting method that has been used successfully in clinical studies is LOP + 40 mmHg for LOP levels less than 130 mmHg, LOP + 60 mmHg for LOP levels between 131 – 190 mmHg, and LOP + 80 mmHg for LOP levels greater than 190 mmHg [13, 14]. This method of setting personalized pressures is commonly used in commercial systems that can measure LOP. In future, as tourniquet-related research continues, it may be possible to safety establish a personalized safety margin that is substantially lower than currently accepted levels.
If possible, LOP measurement should be made while the blood pressure is stabilized at a level expected during surgery. Depending on the anesthetic technique, the LOP measurement may therefore occur before or after induction of anesthesia. Blood pressure at the time of LOP measurement should be noted.
Measurement of LOP
Surgical staff can measure LOP manually by palpation or Doppler ultrasound, or automatically using a distal photoplethysmographic sensor or a dual-purpose tourniquet cuff.
Manual measurement of LOP using Doppler ultrasound
One technique for manual measurement of LOP based on monitoring arterial pulsations as an indicator of arterial blood flow is as follows: tourniquet cuff pressure is increased by an operator slowly from zero while monitoring arterial pulsations in the limb distal to the cuff until the pulsations can no longer be detected; the lowest tourniquet cuff pressure at which the pulsations can no longer be detected can be defined as the ascending LOP. A second manual technique is that an operator can slowly decrease tourniquet cuff pressure while monitoring to detect the appearance of arterial pulsations distal to the cuff; the highest pressure at which arterial pulsations are detected can be defined as the descending LOP. The accuracy of such manual measurements of LOP is highly dependent on the sensitivity, precision and noise immunity of the technique for detecting and monitoring arterial pulsations, on the time taken to slowly increase and decrease cuff pressure, and on operator skill, technique and consistency. Under the best circumstances considerable elapsed time is required on the part of a skilled, experienced and consistent operator, using a sensitive and precise technique for detecting and monitoring pulsations as an indicator of distal blood flow, to accurately measure LOP by manual means.
Automatic measurement of LOP using plethysmography
An automated plethysmographic system built into the tourniquet instrument that measures LOP in about thirty seconds at the beginning of an operative procedure has been developed and is widely used in surgical settings [15, 16]. The automated system incorporates a plethysmographic probe placed on a digit of the patient’s operative limb (See Figure 2). The probe detects the presence of arterial pulsations in the limb distal to the tourniquet cuff as an indicator of arterial blood flow past the cuff and into the distal limb. By automatically inflating the tourniquet cuff in predetermined steps having consistently defined durations, the minimum cuff pressure at which arterial blood flow past the cuff is stopped can be accurately determined. This cuff pressure is the patient’s personalized limb occlusion pressure (LOP). Upon the determination of LOP, the plethysmographic probe is removed.
A study by Younger et al.  demonstrated that use of the automated plethysmographic system and the wide contoured cuff reduced average pressure by 33% to 42% from typical pressures. The study also found that systolic blood pressure was not correlated well to LOP, and should not be used to set the cuff pressure.
A randomized controlled study of 164 patients by Olivecrona et al.  demonstrated that the measurement of LOP with the automated plethysmographic system provides a significant reduction in tourniquet pressure and more individual cuff pressures when used with wide contour cuffs. Although the study method demonstrated no difference in postoperative pain between the LOP and control groups, the authors noted that patients with cuff pressures below 225 mmHg had fewer postoperative complications.
In another study, McEwen et al.  used the automated plethysmographic system to measure LOP on the lower legs of adult volunteers. The study showed that, based on the volunteer results, LOP plus a safety margin of 40, 60, 80 mmHg (for LOP < 130, 131-190, or 190+ respectively) with a standard width cylindrical cuff had an average cuff pressure of 223 mmHg (range 170-299 mmHg, SD 36), which is 11% lower than typical practice and up to 80 mmHg (32%) lower on some patients. Using a wide, contoured cuff reduced the average cuff pressure further to 195 mmHg (range 160-280 mmHg, SD 33), which is 22% lower than typical practice and a reduction of up to 90 mmHg (36%).
Reily et. al.  conducted a study on pediatric patients aged 10 to 17 comparing the quality of surgical field using standard cuffs with a standard pressure of 300 mmHg to using wide contour cuffs with a pressure of LOP plus a safety margin of 50 or 100 mm Hg (depending on the level of the measured LOP). They found that the use of an automatic LOP measurement with the use of wide contour cuffs significantly reduced the mean tourniquet cuff pressure in pediatric patients from typical practice of 300 mmHg to 151 mmHg .
Automatic measurement of LOP using a dual-purpose tourniquet cuff
The best practice for setting tourniquet pressure is to set a personalized pressure that is based on LOP. However the routine use of personalized tourniquet settings based on LOP has been somewhat limited by practical difficulties of manual LOP determination using Doppler ultrasound, and because of limitations inherent in the current technique of automatic LOP measurement . These limitations include: a distal blood flow sensor is required; the sterile field may be affected; perioperative workflow and time may be impacted affected, and the success rate of LOP measurement is dependent on affecting measurement of low blood flow distal to the cuff such as cold digits or poor peripheral circulation.
A new technique for measuring LOP has been developed in an effort to facilitate routine use of personalized tourniquet pressures, using a tourniquet cuff with continuous pneumatic passageway surrounding the limb as a dual-purpose patient sensor and pneumatic effector, and no distal sensor. A study on 143 pre-surgical and post-surgical patients found that the new technique of LOP measurement has surgically acceptable accuracy that is comparable to LOP measurement by a Doppler ultrasound, and that the new technique is feasible for incorporation into improved personalized tourniquet systems . Further, many limitations of present techniques of LOP measurement are overcome with the new technique, for example: no distal blood flow sensor is required; the sterile field is unaffected; perioperative workflow and time are less affected as this technique allows measurement of the LOP while the limb is elevated and being prepared for surgery; and the success rate of LOP measurement should be substantially greater because the new technique is not dependent on variables affecting measurement of low blood flow distal to the cuff such as cold digits or poor peripheral circulation.
 McEwen J, Casey V. Measurement of hazardous pressure levels and gradients produced on human limbs by non-pneumatic tourniquets. In: Proceedings of the 32nd Conference of the Canadian Medical and Biological Engineering Society 2009. Calgary, Canada; 2009 May 20-22. p 1-4.
 Graham B, Breault MJ, McEwen JA, McGraw RW. Perineural pressures under the pneumatic tourniquet in the upper extremity. The Journal of Hand Surgery: British & European Volume. 1992 Jun 1;17(3):262-6.
 Olivecrona C, Ponzer S, Hamberg P, Blomfeldt R. Lower tourniquet cuff pressure reduces postoperative wound complications after total knee arthroplasty. J Bone Joint Surg Am. 2012 Dec 19;94(24):2216-21.
 Younger AS, McEwen JA, Inkpen K. Wide contoured thigh cuffs and automated limb occlusion measurement allow lower tourniquet pressures. Clinical orthopaedics and related research. 2004 Nov 1;428:286-93.
 Reilly CW, McEwen JA, Leveille L, Perdios A, Mulpuri K. Minimizing tourniquet pressure in pediatric anterior cruciate ligament reconstructive surgery: a blinded, prospective randomized controlled trial. Journal of Pediatric Orthopaedics. 2009 Apr 1;29(3):275-80.
 Masri BA, Day B, Younger AS, Jeyasurya J. Technique for Measuring Limb Occlusion Pressure that Facilitates Personalized Tourniquet Systems: A Randomized Trial. Journal of Medical and Biological Engineering. 2016 Oct 1;36(5):644-50.