BACKGROUND AND OBJECTIVES:In gastrointestinal surgery related to intestinal ischemia, the surgeon has to assess the viability of intestines that have been exposed to ischemic injury. There are at present limited means by which tissue viability can be assessed. The standard clinical method is still visual inspection and palpation. This method is non-specific and unreliable, and requires a high level of clinical experience. Bioimpedance has been utilized to measure changes in electrical parameters during ischemia in tissues like the liver, skeletal muscle, and the heart. The physical changes on the cellular and structural levels after the onset of ischemia, results in time-variant changes in the electrical properties of the tissue. The aim of this thesis is to investigate the possibility of utilizing bioimpedance measurements to aid in intraoperative assessment of the viability of ischemic small intestinal tissue.
MATERIALS AND METHODS:We have accomplished 5 initial pilot studies in a porcine model of small intestinal ischemia, and 7 further studies on porcine models based on the initial findings. After induction of anesthesia, a warm ischemic model with full mesenteric occlusion in a small part of the jejunum was implemented. Measurements were conducted by placing electrodes on the serosa of the jejunum, applying a constant voltage, and measuring the resulting admittance. Several electrode setups were tested. We used the recognized and commonly used Solartron 1260/1294 impedance analyzer setup. As a control we used measurements on parts of the jejunum with full perfusion in the same porcine model.
RESULTS:The 2 electrode Silver-Silver chloride setup appeared the best of the tested setups for measuring small intestinal ischemia interoperably. The collected data from the porcine models show significant changes in electrical parameters of modulus, phase, and tan d as a function of ischemic time, compared to the control. A Matlab program with pilot algorithms has been developed that show 78,2% sensitivity, 94,2% specificity, 92,7% positive predictive value, and 82,0% negative predictive value in assessing if the porcine small intestine is ischemic or not, based upon analysis of the collected data. The program also show promising results in discerning the time duration of ischemic small intestine within the tested 6 hour time frame. Histological samples show ischemic changes to the small intestinal tissue that correlate with the measured time variable changes.
CONCLUSIONS:The time development of the electrical properties measured within a 6 hr period of ischemia is statistically significant, and can be correlated with the onset and duration of ischemia. These changes in electrical parameters can be utilized in programs to assess the ischemic status of porcine small intestine. Based on the results from the porcine models, it is not yet possible to assess the viability of the small intestine to the point of irreversible ischemic damage, as the 6 hour ischemic period of porcine model 2 (PM2) did not create a 100% certain ischemically irreversible damaged porcine small intestine. The animal model developed through these experiments seems suitable for further studies related to the determination of the viability of ischemic small intestine.