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UBC Theses and Dissertations

Estimation of limb occlusion pressure for surgical tourniquets based on the measurement of arterial pulse wave transit time Marko, Alexei John


While the use of pneumatic surgical tourniquet systems to maintain a bloodless surgical field in limb surgeries has become commonplace, their use may occasionally result in serious complications such as nerve injury and there is evidence that some injury occurs as a result of each usage. It is commonly accepted that these complications are a result of the pressure applied by the tourniquet cuff to the underlying tissue. The use of an adaptive surgical tourniquet system which adapts to maintain the minimum tourniquet pressure required for surgery may allow the use of tourniquet pressures that are on average much lower than could be achieved using a conventional tourniquet system set at a constant pressure and this lower pressure may result in reduced injury and thus improved safety. Although many variables affect the minimum tourniquet pressure required to prevent the flow of blood underneath the tourniquet cuff, intraoperative changes in that minimum tourniquet pressure are primarily a function of intraoperative changes in blood pressure. Prior to work described in this thesis, no clinically acceptable means had been developed for estimating changes in required tourniquet pressure due to changes in patient blood pressure at sufficiently frequent intervals to facilitate the operation of an adaptive surgical tourniquet system. In the research described in this thesis, the measurement of "pulse wave transit time" (PWTT), defined as the time required for the arterial pulse wave to propagate along a fixed arterial path, was investigated as a novel technique to allow the continuous estimation of "limb occlusion pressure" (LOP) for the control of adaptive surgical tourniquet systems. An experimental system was designed and built which measures PWTT using the output from an ECG monitor and an arterial pulse sensor. Initially, data was collected using the measurement system from 16 patients undergoing orthopedic surgical procedures. Based on analysis of this data it was concluded that in relatively short procedures performed on nonelderly subjects there existed a strong correlation between PWTT and LOP, and therefore it was concluded that measured PWTT provided an accurate method of continuously estimating LOP in these cases. However, in lengthy cases involving elderly patients, a poor correlation was found between PWTT and LOP. It was concluded that this was the result of the relative non-compliance of the arteries of elderly subjects and the administration of a wide variety of drugs affecting arterial compliance. Based on these findings, it was determined that in order to achieve a clinically acceptable level of reliability, a system which continuously estimates LOP based on the measurement of PWTT must integrate periodic estimates from a conventional blood pressure measurement device. These periodic estimates are required in order to develop a patient-specific model relating LOP to PWTT, and to identify subjects for whom PWTT is poorly correlated with LOP. An improved system was then proposed based on the integration of the PWTT measurement system with a patient monitor commonly used in operating rooms to provide blood pressure estimates periodically. Applying the algorithm proposed for the improved system retrospectively to the clinical data obtained from the initial study of 16 surgical patients yielded a 9 percent reduction in the time-averaged tourniquet pressure and a 27 percent average decrease in pressure during the periods of adaptive operation. A follow-up study on 8 additional surgical patients yielded a 12 percent estimated decrease in time-averaged tourniquet pressure and a 25 percent reduction in tourniquet pressure during periods of adaptive operation.

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