Some aspects of gastro-oesophageal reflux in anaesthetized sheep

Abstract

Gastro-oesophageal reflux is a serious problem associated with ruminant anaesthesia. The results of the present investigation of this problem are summarised below.Management of animals prior to and during anaesthesia influenced the incidence of reflux. Depriving the sheep of food and water for 24 hours prior to induction of anaesthesia significantly reduced the incidence of reflux. The occurrence of reflux was also influenced by the positioning of the animal during anaesthesia. Dorsal recumbancy with the head tilted down was the most vulnerable position. In this position reflux occurred in 100% of cases as compared with right lateral recumbancy with head up position where the occurrence of reflux was only 12%.Intraruminal pressure built up during anaesthesia was monitored over a period of 60 minutes. Two planes of anaesthesia, light and deep, were maintained for two 30 minute periods in a single anaesthetic session. Intraruminal pressure build up in deep followed by light anaesthesia was significantly greater than that in light followed by deep anaesthesia. The maximum pressure gradient between the rumen and thoracic oesophagus and the occurrence of reflux were recorded and it was found that only 24% of refluxes were associated with the maximum pressure gradient. The intraruminal pressure build up in the starved sheep was significantly lower when compared with that of the unstarved sheep.The effects of intraruminal insufflation on the occurrence of reflux were investigated and it was found that the intraruminal pressure required to produce reflux was unusually high (about 40 mmHg) in comparison with pressures that can be built up during normal periods of anaesthesia lasting for 2-3 hours.The influence of halothane anaesthesia on the motor functions of the oesophagus and rumen was studied using manometric and electromyographic techniques in both light and deep planes of anaesthesia. The motility of these organs was frequently present in light anaesthesia while it was almost totally abolished in deep anaesthesia. The oesophageal pressure waves in light anaesthesia were always peristaltic in nature, the velocity being 25 cm/sec.A zone of high pressure was detected at the gastro-oesophageal junction in the anaesthetized sheep using balloon tip catheter and a pull through technique. The length of this zone was 2.9 cm. The resting LOS pressure measured by balloon tip catheter was significantly greater than that measured by open tip catheter. The resting LOS pressure was not significantly influenced by the depth of anaesthesia. The LOS usually contracted prior to rumen contraction in light anaesthesia. In deep anaesthesia, when the oesophageal and ruminal contractions were totally abolished, the LOS still showed rhythmic fluctuation in baseline pressure.The response of LOS to an increase in intraruminal pressure was studied. The LOS pressure was also increased with the increase of intraruminal pressure. The occurrence of reflux was associated with diminished LOS pressure.The influence of pentagastrin, atropine sulphate and propranolol hydrochloride on LOS pressure was studied. The LOS pressures were increased within one minute of pentagastrin injection while after atropine, the pressure was decreased within one minute. These changes in LOS pressure however were not statistically significant. Propranolol (after atropine) also had no significant influence on LOS pressure.The motor activities of the oesophagus, gastro-oesophageal junction and reticulorumen were studied by electromyography (EMG). These activities in the cervical oesophagus were characterised by individual spikes in light anaesthesia which were almost abolished in deep anaesthesia. The reticular and ruminal EMGs comprised of regular spike bursts, the reticular bursts usually preceded the ruminal ones. These activities were usually present in light anaesthesia while in deep anaesthesia they were totally abolished. The electromyographic activity of the gastro-oesophageal junction was characterised by continuous spike discharges which were present in both light and deep planes of anaesthesia.The influence of intravenous anaesthetic agents (pentobarbitone, thiopentone, chloral hydrate-magnesium sulphate and alphaxalone/ alphadolone) on the intraruminal pressure build up, oesophageal and ruminal motility and gastro-oesophageal reflux was studied. The largest intraruminal pressure build up was found in alphaxalone/ alphadolone anaesthesia and the smallest with thiopentone anaesthesia. The highest incidence of reflux was found with thiopentone (70%) and the lowest with alphaxalone/alphadolone (40%) anaesthesia.The barbiturates (pentobarbitone and thiopentone) caused total abolition of oesophageal and ruminal motor functions in both light and deep planes of anaesthesia. These activities, however, were frequently present in light chloral hydrate and alphaxalone/alphadolone anaesthesia. The direction of the oesophageal pressure waves in these studies was peristaltic.