Plus-Hex CLINICAL
ABSTRACT Pre-anaesthetic starvation protocols have been mainstays of routine surgeries and procedures to prevent vomiting and increased anaesthesia risks , while postoperative starvation has , in the past , been common for gastrointestinal surgery patients .
This literature review considers the current guidelines for appropriate starvation periods in the perioperative period and the effect of inadequate nutrition on healing , as well as obstacles to managing a patient ' s appropriate nutritional status .
Keywords anaesthetic , surgery , starvation period , reflux , vomiting , healing , nutrition
Introduction
Research into starving times for preoperative patients to reduce the risks associated with regurgitation and aspiration pneumonia has led to changes in the related advice for veterinary teams and animal owners .
This article looks at the entire perioperative period . It includes the effect of not feeding immediately postoperatively , and the ways in which approaches to nutrition around the anaesthetic period have changed and may continue to change in the future .
Defining the perioperative period
The perioperative period can be difficult to define . In human medicine , it is deemed to begin from the time that a surgery , diagnostic test or anaesthesia is planned , and to continue at least until the patient is discharged [ 1 ] . In veterinary medicine , this can vary depending on the nature of the procedure . For elective procedures it may start months before the day of surgery ; for emergency surgeries it may start immediately prior to induction . This means that the planning and preparation can be discussed in theory , but there will always be cases where a gold standard cannot be adhered to without endangering the patient by delaying urgent care .
The routine fasting of veterinary patients preoperatively was originally based on advice given in human medicine . Only in more recent years has specific veterinary research been undertaken in this area . While the preoperative starvation period is deemed necessary to limit the regurgitation of gastric contents , prolonged fasting can lead to hypoglycaemia , and new research suggests that it may actually exacerbate gastric reflux [ 2 ] . This knowledge has led to a change in advice in human medicine , with the new recommendation being that healthy adults are to be fasted of solids for 6 hours and liquids for 2 hours , and that no postoperative fasting is to be undertaken [ 3 ] .
Gastrointestinal effect of anaesthetic and sedative agents
Many of the drugs administered for anaesthetic or analgesic purposes affect the gastrointestinal system , which can influence the gut ' s ability to retain its motility during a period of starvation and stress . However , some drugs may be chosen because of their side effects – for example , butyrophenones such as azaperone are also excellent antiemetics in rabbits , guinea pigs , rats and mice [ 4 ] . Morphine and alpha-2 agonists are well known for causing vomiting . They may also be involved in occult gastro-oesophageal reflux , with an associated risk of aspiration pneumonitis . Species differences in physiology at the cardiac sphincter are an influencing factor . Opioids and anaesthetics can reduce the pressure difference and indicate a higher risk , which is compounded by delayed gastric emptying caused by the same drug classes . Risk is increased with buprenorphine ( more than morphine or pethidine ), in young patients and in cases of increased abdominal pressure , such as pregnancy , deep-chested breed conformation and pre-existing gastrointestinal disease . Alfaxalone has been reported to be antimuscarinic , while propofol and alfaxalone both increase gastrooesophageal reflux [ 4 ] .
Anticholinergic / antimuscarinic drugs , such as atropine , reduce contractility by reducing acetylcholine ' s effect on muscles , which can reduce gut transit . In humans they can reduce saliva production . In contrast , calcium-channel blockers act locally as an antispasmodic agent in the gut wall , which can cause nausea , dizziness , abdominal discomfort and increased blood pressure in humans [ 5 ] .
Gut contractility and motility are reduced by anticholinergics , but spasmolytics , alpha-2 agonists , calcium blockers and opiate μ partial or full agonists have a varied effect on the propulsion of material through the gut , particularly spasmolytics and calcium-channel blockers , which increase the peristalsis propulsion [ 6 , 7 ] . In addition to the muscle contractility , the effect on the gut sphincters must be considered ; buprenorphine is less spasmodic to the sphincter of Oddi , in particular [ 6 ] . Anticholinergics such as acepromazine can be used in this way to reduce contractility in cases such as irritable bowel syndrome . However , they should be combined with another agent to reduce the level of relaxation in the lower oesophageal sphincter , as seen with atropine [ 4 ] . Using this combination , the beneficial properties of acepromazine ' s anti-inflammatory and antioxidant effects and antiemetic side effects remain available [ 4 , 8 ] .
Benzodiazepines , such as diazepam , can increase the tone of the cardiac sphincter and reduce gastro-oesophageal reflux , while acting as an immediate appetite stimulant in cats [ 4 ] , although the contraindications for the oral administration of diazepam to cats must be considered [ 9 ] . Cholinergics can also
Volume 39 ( 1 ) • February 2024
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