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UBC Theses and Dissertations

The use of impedance plethysmography to predict the onset of blood flow beneath a tourniquet cuff McConnell, Gordon


Pneumatic tourniquets are in common use in hospitals to stop the flow of blood to a limb or digit and provide a bloodless surgical site. The risk of injury to the tissue beneath the tourniquet cuff from the high pressure in the cuff could be minimized with a tourniquet system capable of maintaining the cuff pressure at the occlusion pressure, which is defined to be the minimum pressure that will stop the flow of blood past the cuff. In the work described in this thesis, a novel impedance-based method was developed to estimate the occlusion pressure while blood flow is arrested by a tourniquet cuff. This should facilitate the development of actual practical tourniquet systems capable of safely and reliably maintaining the tourniquet cuff pressure near the occlusion pressure for the duration of a surgical procedure. The impedance of the tissue beneath the cuff was measured with a specially designed impedance plethysmograph. The electrode configurations used with the plethysmograph were evaluated with a computer model of the limb. A relationship was found between the impedance of the underlying tissue and the cuff pressure relative to the occlusion pressure that could be used to predict the nearness of the onset of blood flow. Eighteen subjects were used in the trials to establish the relationship. The occlusion pressure was typically 150 mmHg and the standard error of estimation was 16 mmHg. To establish the relationship, the tissue pressure profile along the arm beneath the cuff was measured with a special transducer and controlled by varying the pressures in the bladders of a dual-bladder tourniquet cuff used to arrest blood flow. An algorithm based on the relationship was developed to control the bladder pressures according to the impedance of the underlying tissue. The feasibility of a tourniquet system that uses impedance plethysmography to keep the cuff pressure as close as possible to the occlusion pressure while still preventing blood flow was demonstrated with a single subject by controlling the pressures in a dual-bladder tourniquet cuff placed about his arm using the algorithm. Sources of interference and artifact encountered in the study are discussed, and techniques for removing the artifact presented. The major contributions of the research in this thesis were as follows: 1) the development of a computer model for predicting the performance of electrode configurations; 2) the development, design, implementation, and evaluation of a novel impedance plethysmograph; 3) the measurement of the actual tissue pressure profile along the surface of the arm beneath various occlusive cuffs; 4) the discovery of a quantitative relationship between the pressure in a tourniquet cuff and the impedance of the underlying tissue which for the first time enables the prediction of the occlusion pressure while the tourniquet cuff pressure is above the occlusion pressure; 5) the identification and prioritization of the sources of artifact and interference together with the development of approaches for detecting and handling them; and 6) the establishment of the feasibility of controlling the pressure in a tourniquet cuff by incorporating the above into a practical adaptive tourniquet system.

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