Squid magnetometer diagnosis of experimental small bowel ischemia

Abstract

The magnetic field associated with action currents in the small bowel of five N m Zealand white rabbits was measured using a SQUID magnetometer before and &r the occlusion of the mesenteric artery. Results indiate a correlation between a decrease in the magnetic slow wave basic electrical rhythm (BER) frequency of the small bowel and decreved blood flow. INTRODUCTION Mesenteric ischemia is a devastating illness whose mortality remains at 9096, despite dl improvements in surgical care within the last several decades. The reasons for the continued high mortality of this disuse is that early diagnosis is almost impossible. More than half the patients with mesenteric ischemia have the diagnosis made during post-mortem examination and the ochers at the time of lapuotomy[l]. Previous studies show we can record buic electrical rhythm (BER) and spiking activity of small intestine in vitro(2) and in vivo[3]. The study was undertaken to a m i n e electrid activity of the small intestine during cxperimcnul small bowel ischemia. METHODS Five New ZeaLnd white rabbits were anesthetized with K c m i n e and Acepromahe. laparotomy was performed and a I O un segment of small bowel was exteriorized and placed in a non-magnetic reoording chamber containing Kreb's solution at 38°C. The animal and the recording chamber were placed in dose proximity t~ the Superconducting Wantum Interference Device (SQUID) magnetometer to record slow wave activity of the small intestine for 15 minutes prior to and 1 hour &r the injection of a thrombin clot into the mesenteric artery. RESULTS Results are expressed as meanSEM. Student's T-test was used br sratistid significance. Blood pressure, p u k rate, and arterial oxygenation remained c m s t a n t at * 5 % during the experiment. Inrestinal blood flow, measured by doppler flow merer, dccrcased 95% within five minutes of injection of a thrombin clot. SQUID measurcmena of the small intestine showed a significant decrease in slow wave frequency (15.5k0.3 to 8.9e0.2 cydcs/min), a decrease in propagation velocity (3.5k0.2 to 1.9k0.3 d s e c ) , and an 83% increase in BER wavelength (4.2*0.1 to 7.7k0.2 sec). These signifiunt (p<0.05) changes in slow wave activity occurred 30 minutes after the injection of a thrombin clot. DISCUSSION Messentic ischemia is a d i m e largely diagnosed at the time 1 V fipn I : IntuiinaI magnetic I& MUCI nwr&/" a non-iwhonir jrjnnnm of an a n e d r d d mbbig A m P M U plorud v c r y ~ time. of laparotomy. Other techniques of diagnosis of intestinal infarction arc darivcly insensitive only identifying the patient with ischemia in a pre-morbid condition[4]. Procedures such as doppler ultrasound[5] are unable to identify the mesenteric vessels when there is overlying gas. as in the majority of cases. Magnetic Resonance Imaging (MRI) has been used to identify the mesenteric vasculature, however current MRI technology docp not d o w the fine resolution required for identification of mesenteric vessels nor would it be helpful for non-ocdusivc mesenteric ischemia. Even if MRI identifies thrombosis of die major mesenteric veins it docs not allow identifiation of the physiological state of the bowel which is the important factor 0-78031377-1/93 $3.00 01993 IEEE 1521 in decisions regarding surgid intervcntion[b]. Because of relatively strong biomagnetic signals that we were abk to obtain from our small animals studies. we felt that measurement of human small bowel biomagnctic slow wave activity would be possible. MiuoSQUlD is a high resolution SQUID magnetometer developed by the Living State Physics Croup at Vanderbilt University. This device has an array of 4 pick-up coils arranged at the corners of a 4.4 mm 4uye and is meant for high resolution biomagnetic measurements. Although miuPSQUID is designed for high-resolution recording measurements of small animals, we have measured the gastric and small intestinal slow waves non-invasively in three human voluntecrs[7]. After demonstrating the feasibility of measuring the frequency and wavelength of the small intestinal BER noninvvively using SQUID technology, we hypothesized that magnetometer measurement of human intestinal slow wave activity could be used to non-invasively diagnose mesenteric ischemia These studies show that early in the course of mesenteric ischemia, small bowcl electrid activity is altered. This is in stark contrast to many of the previous diagnostic studies that demonstrated changes in serum levels of lactate dehydrogenase. transaminase, alkaline phosphatase. or phosphate to diagnose bowel infarction. Although elevated, they arc only elevated late in the course of the disease and arc non-specific findings of shock[4,8]. The changes shown in this study are significant within 30 minutes of the occlusion. Moreover, they are specific to the small intestine. Cumnt technology and cutaneous elccuicd recording of small bowel aaivity m limited to evaluation of BER frequency. Moreover, since the cutaneous electrodes record the entire small intestine and abdominal contents, stronger signals within the small intestine could effectively hide any lower frqucncy signals from ischemic segments of small intestine. Multi-array SQUIDS could thcoretiully negate this drawback of C U K ~ ~ C O ~ S electrical recordings by being able to mord magnetic field activity associated with the gastrointesintd tract. We believe with the appropriate computer recording and software, the clectriul rhythm of the sromach, colon, and small intestine can be ascertained in human subjects. Hypothermia will cause a significant &crew in slow wave frequency and therehre it was extremely important to control body temperature during the recording period. This was cantrolled within *5% during the experimental protocol thus eliminating this variable as a uuse of the electrical changes. The study also excluded possible damage to the extrinsic innervation of the bowel by injection of thrombin dot i n to the mesenteric arterial supply as opposed to ligation of the mesenteric vessels. The animal was hemodynamidly stable during the experimental protocol. suggesting that the findings of dccmsed slow wave frequency and increase in BER wavelength are specific to mesenteric ischemia. Studies to evaluate reperfusion, non-occlusive mesenteric ischemia, and mesenteric venous thrombosis are being underclken to evaluate further the changes that occur in the BER of the small intestine. Moreover, a study to examine when changes in BER activity reflect irreversible changes within h e bowel will be important to allow clinicians to eventually utilizc this technology in the diagnosis of irreversible bowel ischemia and the nccd for operative intervention. CONCLUSION We believe that advances in SQUID technology and further studies of mesenteric ischemia may allow non-invasive diagnosis of human mesenteric ischemia Decrease in BER frequency and increases in BER wavelength occur early in the course of experimental mesenteric ischemia and appear U) be specific to mesenteric ischemia Since the physiological state of the intestine (infarction vs. viable) dictates the nccd for operative intervention SQUID recordings may ultimately provide the sensitive data on the s w c of the intestine that is necessary for d i n i d decision making. REEERENCES [ I ] C.W. Gupta, D.G. C i h o u r , and C.W. Imrie, "Acute superior mesenteric ischaemia." Bnhjh J o u d of Surpy, [2] D.J. 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