increase the tone of the lower oesophageal sphincter and reduce gastro-oesophageal reflux , but they also increase stomach acidity [ 7 ] .
Alpha-2 agonists , including medetomidine and dexmedetomidine , relax the lower oesophageal sphincter and can increase gastro-esophageal reflux and vomiting [ 10 ] . Gut transit time is increased , as the gut secretions and blood flow are reduced [ 4 ] . Insulin secretions are also reduced , so these drugs can cause hyperglycaemia [ 11 ] .
Methadone can reduce gastric motility and cause hypersalivation in dogs , as well as lip licking in both cats and dogs ; either of these signs can be mistaken for a sign of nausea [ 10 ] . Opioids , including morphine [ 10 ] , given in the absence of pain , cause emesis , reduce gastric secretions , increase sphincter tone and reduce peristalsis [ 4 ] . Dopamine antagonists prevent nausea and vomiting , reduce dyspepsia in diabetic patients , and inhibit oesophageal and gastric peristalsis . However , they can increase gut emptying when given alone , and have been shown to have side effects on the central nervous system in human studies [ 12 ] .
Propofol used in anaesthetic induction reduces oesophageal tone , but also has anti-inflammatory and antioxidant effects [ 4 ] . Giving preoperative omeprazole , or potentially metoclopramide , has been advocated by some studies to reduce gastro-oesophageal reflux , although metoclopramide should , in theory , have less impact due to its antiemetic action [ 13 , 14 ] .
The efficacy of mirtazapine in dogs is anecdotal . It has appetite-regulatory and antiemetic effects in cats , but can cause hyperactivity , agitation , vomiting , ataxia , tremors , hypersalivation , mydriasis , tachypnoea , tachycardia and lethargy after oral administration . Transdermal application can be considered , although it can cause erythema , flaking , crusting or pruritus . Use with serotonergic drugs or tramadol should be avoided due to interactions . Additional considerations should be given to serotonergic medications , as serotonin has varying effects on different areas of the gut [ 15 ] .
Capromorelin initiates the sensation of hunger and stimulates feeding behaviour . It has side effects of hypersalivation , gastrointestinal upset and abdominal discomfort in dogs . It should be avoided in cats , particularly those with compromised cardiovascular systems , as it causes hypersalivation , gastrointestinal upset , behaviour changes , and transient decreases in heart rate and blood pressure [ 9 ] .
Glucocorticoids and megestrol acetate can result in polyphagia , but long-term use may be unsuitable for anything other than palliative care , due to side effects of insulin resistance , polydipsia and behaviour changes [ 9 ] .
Metoclopramide can be used to counteract some of the gastric motility-reducing effects of other drugs , as it has an upper gastrointestinal tract stimulant property , as well as being antiemetic . It is contraindicated in cases of suspected perforation or obstruction [ 6 ] .
Deciding when and how to feed
Whether it is related to anaesthesia , surgery or organ compromise , prolonged starvation of the gastrointestinal tract results in gut muscle atrophy , mucosal integrity loss in the intestinal tissues , bacterial translocation , bacteraemia and possibly septicaemia [ 16 ] .
Chronic conditions such as diabetes mellitus ( DM ) and chronic kidney disease ( CKD ) have their own influences on medication metabolism , and on healing and recovery post anaesthesia , so these patients should be stabilised or supported appropriately . For example , patients with CKD affected by uraemia will have a longer healing time , while patients with DM have an increased risk of infection [ 17 ] . If possible , both conditions should be stabilised or the affected patients provided with intravenous fluid therapy for a minimum of 48 hours prior to surgery . A post-surgery drop in metabolic blood flow and metabolic rate leads to poor perfusion , and is the reason postoperative feeding is important , as this rate increases at 25 hours to 14 days postoperatively , resulting in a higher nutritional requirement [ 18 ] .
Parenteral ( intravenous ) feeding should be used only in cases where the gastrointestinal tract needs complete rest , or the patient is unable or unlikely to fulfil its nutritional requirement orally , as its unnecessary use increases morbidity and mortality rates [ 19 ] .
Changing recommendations
Routine pre-anaesthetic starvation recommendations used to advise starving patients for 8 – 12 hours . Now , in cases of small , geriatric or juvenile patients , it is recommended that starvation times be reduced to 4 – 6 hours prior to surgery , as these patients are at increased risk of hypoglycaemia caused by depleted liver glycogen reserves . A 2015 study by Castro et al . [ 20 ] identified that the gastric emptying time of cats fed wet food differed considerably from that of cats fed dry food . They concluded that it is possible to reduce the preoperative starvation time to 4 hours in cats fed wet food ( Figure 1 ) and this would be especially beneficial for elderly cats . Consideration should also be given when admitting patients in the morning for an afternoon procedure , or if procedures are delayed [ 21 ] .
Gastro-oesophageal reflux in dogs and cats is well documented . It occurs in approximately 17 – 50 % of anaesthetic cases . To reduce this likelihood , a starvation period of 6 – 8 hours has been recommended , as longer starvation increases stomach acidity and the risk of oesophagitis [ 22 ] . Other studies show that a further reduction in starvation time to 2 – 4 hours is the most beneficial in eliminating gastroesophageal reflux [ 13 , 23 ] ; this is particularly relevant to brachycephalic breeds , which are prone to reflux [ 14 ] . In cases where reflux does occur , it is vital to clear the airway using suction before extubation [ 24 ] .
40 Veterinary Nursing Journal