Feeding and anaesthesia

Historically, fasting times of up to 14 hours have been recommended before general anaesthesia in horses.¹ However, it is important to look at what lies behind those recommendations and ask if they are based on evidence or opinion. Hypotension and hypoxaemia are frequently encountered in anaesthetised horses. The theoretical benefits of fasting are reduced gastrointestinal contents which may enhance lung function (reduced pressure on the diaphragm, less negative impact on Functional Residual Capacity and less hypoventilation) and cardiovascular function (decreased pressure on major abdominal vessels) when horses are in dorsal recumbency but there is little evidence to support this. Dobromylskyj and colleagues reported no statistically significant differences in arterial oxygen tension (PaO2) during anesthesia in horses that had been kept off concentrate feed for an average of 16.6 hours and off hay for 10.6 hours compared to those that had access to concentrate and hay until 3.8 and 0.8 hours respectively before anaesthesia.²

In donkeys, a 5-hour fast showed no advantage over no fasting on arterial blood gas variables.³

Horses are browsers and may become stressed when fasted resulting in reduced gastrointestinal motility. After 12 hours of fasting, the intensity of gastrointestinal sounds and motility were significantly decreased (by up to 90%) in adult horses.⁴ Fasting decreases myoelectric activity in the equine colon and therefore reduces contractile activity.5 Decreased gastrointestinal motility is a risk factor of colic. Post-anaesthetic colic is an adverse event that results in increased morbidity, cost, hospitalisation time and mortality. It has been reported to occur in between 2.8 and 12% of horses that undergo general anaesthesia, and, in these studies, most horses were fasted for a minimum of 6-12 hours, as part of their pre-operative preparation. Senior and others reported a 5.2% prevalence of post anaesthetic colic in a multi-centre study.⁶ In their study there was a trend for non-fasted horses to be more likely to develop colic after anaesthesia but the centre where the procedure was performed, and the type of surgery were also linked to the likelihood of post-anaesthetic colic occurring. The reasons for anaesthesia may play a role in the development of post-anaesthetic colic as only 1.5% horses undergoing magnetic resonance imaging developed colic compared to 7.1% of horses undergoing non-abdominal surgery.⁷ Surgery and pain stimulate the sympathetic nervous system which results in decreased gastrointestinal motility but drugs given to horses undergoing surgery (e.g., antibiotics) may also play a role.⁷ Bailey and others studied the incidence of post-anaesthetic colic in non-fasted horses that underwent elective anaesthesia for non-abdominal procedures.⁸ In their study population the incidence of post-operative colic was 2.5% and they proposed that providing food may help maintain normal gastrointestinal motility and decrease the risk of post-anaesthetic colic.8 Another adverse effect of fasting in horses is that their water intake will also decrease and reduced water intake is a risk factor for colic.⁹ The role of stress should be considered in discussions because the use of muzzles (which most horses dislike) to prevent ingestion of solid feed was associated with more cases of post-operative colic than just “not feeding” horses before anaesthesia.¹⁰

An exception to allowing access to food before surgery is when the horse is scheduled for a laparoscopic procedure.¹¹ An empty or minimally distended gastrointestinal system allows for better visibility and less risk of perforation. A so-called laparoscopic diet should produce little gas and have a short transit time. Long,stem-rich hay is recommended in the days before surgery, with a fasting period of 24-26 hours.¹¹

The literature does not support the idea that pre-anaesthetic fasting improves cardiopulmonary function in horses. The impact of fasting versus feeding before anesthesia on developing post-anaesthetic colic has less clear as many studies included multiple confounding factors. However, perusal of multiple studies along with clinical experience suggests that prolonged fasting is detrimental in horses. Access to hay (but restriction of grain) and allowing normal browsing activity may help maintain gastrointestinal motility and decrease post-anaesthetic colic and for most procedures this is now an acceptable strategy. Preparation for laparoscopic surgery is different and food restriction is advised. With respect to post-anaesthetic colic many other factors such as adequate pain control and decreasing stress must also be addressed.