Nichtinvasive Messung von Darmischämie mit dem SQUID Magnetometer

Abstract

Introduction: Acute mesenteric ischemia is an abdominal catastrophe that carries high morbidity and mortality rates. Current diagnostic methods, however, lack sensitivity and specifity and do not provide information about the viability of the affected bowel. The basic electrical activity of the gastrointestinal tract is characterized by BER (basic electrical rhythm). The frequency of BER decreases early after induction of intestinal ischemia. SQUID (Superconducting QUantum Interference Device) magnetometers can non-invasively record the intestinal BER. SQUIDs measure magnetic fields that are created by the electrical activity of the gastrointestinal smooth muscle. Methods: A midline laparotomy was performed on 10 male adult New Zealand rabbits under general anesthesia (acepromazine, xylazine, ketamine). One segment of small bowel was lateralized to the right and one to the left side and sutured to the abdominal wall. The animal was positioned under the SQUID (GutSQUID I, one Channel, Conductive, San Diego,USA). After baseline recordings of 15 min, ischemia was induced in one of the lateralized bowel segments by ligation of its vascular supply. Alternate 2 min recordings were taken from the ischemic and the normal segment for 120 min. In a second experiment, 4 rabbits were measured for 15 min baseline and 45 min ischemia with GutSQUID II (19 Kanäle, Tristan, San Diego, USA). Here, the animal was kept in one position under the SQUID. Data from each channel was filtered and spectrally analyzed using an autoregressive technique. We produced false-color isoharmonic frequency (IHF) maps of magnetic field spectra in different frequency bands (only GutSQUID II). Two independent investigators blinded to the experimental preparation evaluated the results (mean ± SEM). Results: Both SQUID magnetometers recorded oscillatory magnetic fields with frequencies typical for BER in all animals. In experiment 1 (GutSQUID I), the baseline BER was 11.8 ± 0.9 cpm (cycles per min). After 120 min of ischemia, BER decreased to 7.8 ± 0.6 cpm but stayed unchanged in the not ligated segment (p < 0.001). In experiment 2, BER decreased from baseline 18.4 ± 1.25 cpm to 12.7 ± 0.75 cpm after 15 min and further to 10.1 ± 0.7 cpm after 1 hour. GutSQUID II allowed mapping of gastrointestinal activity and exact localization of the ischemic segment. The blinded investigators correctly identified healthy and ischemic magnetic recordings with a sensitivity of 94% and a specifity of 100%. Conclusion: Ischemia leads to characteristic changes in the bioelectrical activity of small bowel, which can be non-invasively detected by SQUID magnetometers. Multi-channel SQUIDs allow exact localization of ischemic bowel segments.