Guy Lester

The intestinal tract of the horse is a large and complex structure that is often affected with clinical problems associated with alterations in motility. Although factors such as diet, intestinal parasites and general anaesthesia are thought to predispose to many of these clinical problems, very little is known about the effect of these factors on intestinal motility. Also, a more thorough understanding is required about the effects on motility of the various drugs that may either predispose to or are used in the treatment of these clinical problems. The measurement of myoelectrical activity (MEA) from electrodes chronically implanted onto the serosal surface can be used to monitor intestinal motility. The manual analysis of myoelectrical recordings is, however, laborious and subject to error, particularly in records obtained from the large intestine. This study was undertaken primarily to develop and validate a computer-based system to collect and analyse MEA. Ileal, caecal and colonic MEA was recorded from 4 conscious, healthy horses using silver-silver chloride electrodes. Computer programs were developed to process the high frequency component of the digitized data. The frequency and duration of spike bursts, an integrated burst value, and the direction and velocity of propagative MEA were calculated. There was a high correlation (r > 0.95) between the results derived from the computer analysis and those from the manual analysis of chart recordings. The second aim of the study was to apply the system in an investigation of the effects on intestinal MEA of changes in dietary concentrate and roughage, the larval stages of the parasite, Strongylus vulgaris, general anaesthesia, and several drugs used in association with the treatment of equine colic. The effects of 3 diets that contained different proportions of roughage and concentrate were examined in 3 horses. It was found that increasing the dietary roughage:concentrate ratio significantly increased the caecal and colonic spike burst frequencies. There was also a concomitant decrease in the passage time of a marker. Intestinal MEA was monitored for 56 days in 4 parasite-naïve weanling foals that were each given 1000 S.vulgaris larvae. A biphasic increase in the large intestinal spike burst frequency was observed. This coincided with the periods of assumed intestinal mucosal penetration by the larvae and larval migration to the cranial rnesenteric artery. None of the foals showed signs of colic despite the presence of extensive arteritis at necropsy. In an anaesthetic trial, 4 horses were each given 3 different general anaesthetics and an intravenous saline control. Irrespective of the anaesthetic regime, spike burst frequency was significantly reduced in all areas monitored for the duration of anaesthesia. It was found that MEA returned to preanaesthetic levels soon after recovery. All anaesthetics appeared to reset the small and large intestinal migrating myoelectrical complexes (MMC). In drug trials, the effects of spasmolytics, non-steroidal anti-inflammatory drugs (NSAIDs) and narcotics were studied in groups of 4 horses. The NSAIDs had no significant effect on either small or large intestinal spike burst frequency or propagation, but prematurely initiated the excitatory phases of the small and large intestinal MMCs. Atropine and hyoscine reduced small and large intestinal spike burst frequency for a prolonged period of time. Morphine and pethidine both inhibited large intestinal spike burst activity. Pre-treatment with the narcotic antagonist, naloxone, suppressed the inhibitory effects of morphine. Morphine, pethidine and naloxone prematurely induced the intense phase of the small intestinal MMC when administered after an apparent refractory period. It was concluded that the computer-based system developed in the present study for recording and analyzing equine intestinal MEA was accurate and reliable. This was further demonstrated by applying the system in the investigation of factors implicated in motility dysfunction in the horse.