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Food for Thought: Pre-anaesthetic Fasting

Pre-anaesthetic fasting


The practice of fasting patients prior to anaesthesia was instigated in human patients in 1946 when it was considered beneficial to give “nil by mouth” after midnight before the anaesthetic procedure1.  This was extrapolated to the veterinary patient and has been questioned little until recently. 


Inhalation of gastric contents can result in mechanical obstruction to respiration and chemical inflammation.  In the dog regurgitation/vomiting can be a major cause of oesophagitis with the potential for subsequent development of oesophageal strictures. However, the incidence may be underestimated as the episodes tend to be “silent” and frequently go un-noticed.1 

Gastro-oesophageal reflux (GOR) in dogs is influenced by: age; type of surgery; volume of gastric contents at the time of anaesthesia; acidity of gastric contents; premedicant selection; induction/maintenance drugs; abdominal surgery1.  Dorsal recumbency and the duration of pre-operative fasting can also affect GOR2.  Pregnancy, obesity (both of which increase abdominal pressure) and high ASA scores may also influence GOR1


Recent studies on the duration of fasting in dogs have reported conflicting results: 

Savvas et al (2016) suggest a light meal (1/2 the daily rate of a canned food) 3 hours before anaesthesia can reduce GOR when compared to administering the same amount/type of food 10 hours before3.  This may be due to the volume of gastric contents at the time of anaesthetic induction not being significantly increased by feeding a light meal 3 hours before anaesthesia4

A recent paper by Viskjer et al (2017) demonstrated that a half-ration given 3 hours prior to anaesthesia was associated with 61% GOR and 48% regurgitation compared to fasting for 18 hours which resulted in 43% GOR and 11% regurgitation.  The authors state: “Consumption of a light meal 3 hours prior to anaesthesia was associated with significantly greater odds of reflux and regurgitation compared with overnight food withholding.” 

The current recommendation for adult patients by Bradbrook (2017) is to fast overnight unless the patient presents for urgent/emergency treatment.  However, fasting for greater than 10 hours can increase the acidity of stomach contents with the potential for oesophageal injury should regurgitation occur.4 A 6-8 hour fast is now considered sufficient.7

In the emergency situation attention should be directed towards minimising the risk of aspiration of regurgitated or refluxed gastric contents before the airway is secure, and also during recovery.6 

For paediatric patients the recommendation is a shorter period of fasting primarily due to the risk of hypoglycaemia, and due care should be taken in these patients during the anaesthetic process.6 Jolliffe (2011) suggests fasting of juveniles for 3-4 hours prior to anaesthesia and Bednarski (2015) advises 1-2 hours. 


Rabbits are obligate nose-breathers and cannot vomit although they can regurgitate. They are hind-gut digesters, with digestible fibre being fermented in the caecum to produce volatile fatty acids (VFAs) which provide approximately 40% of the maintenance energy requirements of the rabbit.  Their high metabolic rate requires a constant intake of food and this continual supply of ingested fibrous material also assists in the maintenance of gut motility.

Rabbits should not be starved prior to general anaesthesia and hay should be available at all times.  Pellet/concentrate & fresh food items may be removed approximately 1 hour prior to induction to help minimise the presence of residual food particles in the oropharynx, and to decrease stomach volume in order to reduce pressure on the diaphragm.10 If assisted feeding is being performed the last feed should be approximately 30 minutes prior to premedication.11 

Once a premedicant/sedative has been administered all food items, including hay, should be removed so the rabbit does not become sedated with food still present in the oropharynx. Water bowls should also be removed at this time. 

Gastrointestinal support via prokinetics may be of value to the rabbit patient.  Ranitidine acts on the stomach, metaclopramide is a foregut prokinetic, and cisapride is a hindgut and general intestine prokinetic. All three may be administered orally and metoclopramide subcutaneously. Ranitidine can also be administered intravenously.12,13

Article by
Dr. Karen Heskin

Veterinary Technical Manager, Jurox UK

Originally published: Thursday, 25th October 2018


  1. Ratopoulos D & Savvas I (2004). Preoperative fasting: “Nil per Os After Midnight” – Time to Change?  Proceedings of the WSAVA World Congress.
  2. Gelatos AD et al (1995). Gastro-oesophageal reflux during anaesthesia in the dog: the effect of preoperative fasting and premedication. Vet Rec.137(19): 479-83
  3. Savvas I, Ratopoulos D & Rallis T (2016). A “light meal” three hours preoperatively decreases the incidence of gastro-oesophageal reflux in dogs. J Am Anim Hosp Assoc. 52(6): 357-363
  4. Savvas I, Rallis T & Ratopoulos D (2009). The effect of pre-anaesthetic fasting time and type of food on gastric content volume and acidity in dogs. Veterinary Anaesthesia & Analgesia. 36: 539-546
  5. Viskjer S & Sjostrom (2017). Effect of the duration of food withholding prior to anaesthesia on gastro-oesophageal reflux and regurgitation in healthy dogs undergoing elective orthopaedic surgery.  Am J Vet Res. 78(2): 144-150
  6. Bradbrook C (June 2017).  Ask the expert. BSAVA Companion: 4-6
  7. Duke-Novakovski T, de Vries M & Setmour C (2016). BSAVA Manual of Canine and Feline Anaesthesia and Analgesia.
  8. Jolliffe C (2011) Approaches to anaesthesia protocols. Vet Times 41(42):12-18.
  9. Bednarski RM (2015) In: Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, editors. Lumb & Jones' Veterinary Anesthesia and Analgesia. 5th ed: 819-826
  10. Harcourt-Brown FM (2007). Gastric dilation and intestinal obstruction in 76 rabbits. Vet Rec 161 (12): 409-414
  11. Meredith A & Lord B (2014).  BSAVA Manual of Rabbit Medicine. 1st Edition.
  12. Hedley J, Saunders R, Tittle D, Miles S, Potter J, Stapleton N, Wren T, Gardener C, Edis A, Bertrand H (2017). Considerations for anaesthetising rabbits – Expert panel guidelines. Jurox UK.
  13. Longley L (2009). Risky rabbits: safe protocols and successful anaesthetic recovery. Vet Times Sept 21

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