What is Tourniquet “Limb Occlusion Pressure” (LOP)?

Many studies published in the medical literature have shown that the safest tourniquet pressure is the lowest pressure that will stop the flow of arterial blood past a specific cuff applied to a specific patient for the duration of that patient’s surgery. Such studies have shown that higher tourniquet pressures are associated with higher risks of tourniquet-related injuries to the patient. Therefore, when a tourniquet is used in surgery, surgical staff generally try to use the lowest tourniquet pressure that in their judgment is safely possible.

It is well established in the medical literature that the optimal guideline for setting the pressure of a constant-pressure tourniquet is based on Limb Occlusion Pressure (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 currently established guideline for setting tourniquet pressure based on LOP is that an additional safety margin of pressure is added to the measured LOP, to account for physiologic variations and other changes that may be anticipated to occur normally over the duration of a surgical procedure.

Surgical staff can measure LOP manually by detecting the presence of arterial pulsations in the limb distal to a tourniquet cuff as an indicator of arterial blood flow past the cuff and into the distal limb. Such arterial pulsations can be defined as the rhythmical dilation or throbbing of arteries in the limb distal to the cuff due to blood flow produced by regular contractions of the heart. Detecting blood flow thus can be done using palpation, Doppler ultrasound or photoplethysmography to measure arterial pulsations. 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 very dependent on the sensitivity, precision and noise immunity of the technique for detecting and monitoring arterial pulsations, 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.

Some advanced surgical tourniquet systems include means to measure LOP automatically. When LOP is measured, an additional pressure margin based on recommendations in published surgical literature is added to the automatically measured LOP to provide a “Recommended Tourniquet Pressure” (RTP), as a guideline to help the surgical staff select the lowest tourniquet pressure that will safely stop arterial blood flow for the duration of a surgical procedure. Such prior-art systems allow the surgical staff to select the RTP, based on LOP, as the tourniquet pressure for that patient or to select another pressure based on the physician’s discretion or the protocol at the institution where the surgery is being performed. The difference in pressure between the measured LOP and the tourniquet pressure selected for surgery, which may be the RTP, can be defined as the cuff pressure safety margin. Ideally the cuff pressure safety margin is selected to be greater than the magnitude of any increase in LOP normally expected during surgery due to changes caused by drugs used for anesthesia, the patient’s physiologic response to surgery and other variables. Change in blood pressure is one physiologic characteristic that varies during surgery and has been shown to affect the LOP during surgery, and therefore the cuff pressure safety margin during surgery. For example, an increase in the patient’s blood pressure will lead to an increase in LOP, with attendant decrease in the safety margin.

References for educational viewing only

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Reilly et al. "Survey of tourniquet use in podiatric surgery." Journal of Pediatric Orthopaedics, 29 (2009): 275-280. Younger A, McEwen JA, Inkpen K. "Wide contoured thigh cuffs and automated limb occlusion measurement allow lower tourniquet pressures." Clinical Orthopaedics and Related Research, 428 (2004): 286-93. McEwen, JA, et al.,. United States Patent Application No. 11/122,600, May 5, 2005, "Surgical tourniquet apparatus for measuring limb occlusion pressure."