A recurring theme in these columns is veterinarians are often forced to develop clinical practices without robust research evidence to guide us. As a result, we tend to develop protocols based on weak or limited research evidence, expert opinion, personal experience, or simply habit. When I train new veterinarians, I often find they have an excellent memory for how they are ‘supposed’ to treat specific medical problems. However, they rarely have any idea where the practices they have committed to memory come from, or what level of evidence they are based on. We are trained to absorb the views and practices of our teachers and mentors rather than question them. Fortunately (or, unfortunately, depending on your perspective), this column is here to encourage you to challenge what you have learned. This installment’s effort to undermine common habits involves pre-anesthetic fasting.
It is common in veterinary medicine to withhold food for some period of time prior to sedation or general anesthesia in dogs and cats. There is great variation, however, in specific fasting recommendations and practices.1-3
A common practice for healthy adult animals is no food after midnight the night before the procedure, often resulting in a 12- to 18-hour or longer fast. However, expert recommendations can be found for pre-anesthetic fasts lasting three to four hours,3 four to six hours,1 and at least six hours4 for healthy adult dogs and cats. Fasting recommendations also vary based on breed, age, size, specific health conditions, and other factors. Such inconsistency in recommendations and common practices is indicative of a lack of clear and definitive evidence to support one specific protocol.
The intent of pre-anesthetic fasting is to reduce anesthetic complications, specifically gastroesophageal reflux (GER), esophagitis, and aspiration pneumonia. It has been reported 17 to 50 per cent of dogs experience GER under general anesthesia, so these are potentially common, and serious, complications.2,5 In theory, fasting would reduce the volume of stomach contents, which would, in turn, reduce the risk of reflux, regurgitation, and aspiration. However, as usual, reality turns out to be more complicated than such a seemingly simple and straightforward theory can predict.
For one thing, the duration of fasting does not consistently affect the volume of stomach contents.6 Gastric emptying is affected by the specific composition of food ingested, including the protein, carbohydrate, and fat content.7 The form of the food is also important: dry food takes longer to process and expel from the stomach than canned food, which takes longer than liquids, such as milk.6
Breed and conformation, medications, and many other factors also influence gastric emptying. Further, even when all food is gone from the stomach, gastric secretions can accumulate, and these play a significant role in the risk of GER and esophagitis. The simplistic notion that patients should be fasted long enough “so their stomachs are empty” is not an accurate or reliable basis for specific fasting protocols.
Another variable relevant to the risk of GER and esophagitis is gastric and esophageal pH. Reflux of more acidic contents is more likely to cause clinically relevant esophageal injury. So far, several studies investigating various fasting intervals have agreed that longer fasting times reduces gastric pH, which potentially increases the risk of esophageal injury if GER occurs.2,5,7
Of course, as a clinician, my main concern is, “What should I do?” Knowing the optimal protocol for every pet would be ideal. I realize, though, a universal protocol is unlikely to exist, since risk factors are varied and interact in complex ways. A more realistic goal is to have a general guideline for appropriate fasting times and a set of relevant factors I can consider to adapt this for individual patients. It is possible to achieve this based on the existing research literature, though the evidence so far is not strong enough nor consistent enough to justify great confidence or rigid adherence to any specific protocol.
Several research studies have assessed the relationship between fasting and GER. One retrospective study of 240 dogs identified a greater incidence of regurgitation and lower gastric pH with longer fasting times.5
Several prospective studies have compared the incidence of GER and the pH of gastric contents between dogs fed three hours and 10 hours before general anesthesia. The first of these fed dogs half of their calculated daily energy requirements in the form of canned food three hours or 10 hours before induction. None of the dogs in the three-hour fasting group experienced GER, whereas 27 per cent of the dogs in the 10-hour fasting group did. Gastric pH was also significantly lower with the longer fast.2
Another study by the same group randomized 120 dogs to three- and 10-hour fasts, as well as several different foods, including dry food, various canned foods with different nutrient profiles, and cow’s milk. Once again, gastric pH was lower and rates of GER were higher in the 10-hour group (20 per cent GER) compared with the three-hour group (five per cent GER).6
However, a subsequent study by a different group attempting to replicate this comparison found somewhat different results. Gastric pH was still significantly lower in the dogs fasted 12 to 18 hours before surgery compared with those fed half their daily calorie requirement in canned food three hours before induction. However, the incidence of GER was actually higher in the three-hour group (61 per cent GER) than in the 12- to 18-hour group (43.9 per cent GER).7
The cause and significance of this inconsistency in the research evidence isn’t clear. The studies involved different foods with different macronutrient profiles, different anesthetic protocols, different fasting times, and other differences in the design and conduct of the studies, which could have led to the contrasting findings.
Unsurprisingly, there is far less evidence regarding appropriate fasting times in cats. One study compared blood glucose, blood gases, standard cardiopulmonary monitoring parameters, and anesthetic recovery times in cats fasted for eight, 12, and 18 hours and found no differences.8 However, GER and related anesthetic complications were not assessed, and the 2018 American Association of Feline Practitioners (AAFP) states, “there are no data in cats” and recommends a three to four hour fast “at the clinician’s discretion.”3
There are a number of variables that can reasonably be considered in making recommendations about fasting. The American Animal Hospital Association (AAHA) anesthesia guidelines recommend a four- to six-hour fast for healthy adult dogs, shorter fasts for dogs that are small (< 2kg), young (< 2 weeks), or diabetic, and longer fasts for dogs with known risk of regurgitation based on breed or individual history (Figure 1, page 25). These are reasonable guidelines based on a mixture of “clinical experience and experimental evidence.”1
Guidelines in humans are typically more specific and based on better evidence, though there is still not absolute agreement. Generally, recommended fasting times are shorter in humans than in veterinary patients, and the best evidence suggests fluids and foods that are not high in fats can be given within two to six hours before anesthesia without any increase in risk of complications9-11 (Figure 2).
Based on the limited available evidence, it is at least clear prolonged fasting is neither necessary nor beneficial for dogs and cats. Gastric pH is lower with prolonged fasting, and most studies suggest the risk of GER and regurgitation is lower in dogs fasted for only a few hours rather than 10 to 18 hours. Withholding of liquids is likely unnecessary.
There are many factors influencing the risk of GER and aspiration as well as the appropriateness of specific fasting periods, including age, size, breed, medications employed, and medical conditions. Within the general bounds of the information available in the research literature, veterinarians must still consider the specific risk factors in each case and make a reasoned judgment about appropriate fasting times. Hopefully, as more research information is developed, there will be a stronger foundation for this judgment.
Brennen McKenzie, MA, MSc., VMD, cVMA, discovered evidence-based veterinary medicine after attending the University of Pennsylvania School of Veterinary Medicine and working as a small animal general practice veterinarian. He has served as president of the Evidence-Based Veterinary Medicine Association and reaches out to the public through his SkeptVet blog, the Science-Based Medicine blog, and more. He is certified in medical acupuncture for veterinarians. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News Canada.
1 Bednarski R. AAHA Anesthesia Guidelines for Dogs and Cats*. J Am Anim Hosp Assoc. 2011;47:377-385. doi:10.5326/JAAHA-MS-5846
2 Savas I, Raptopoulos D. Incidence of gastro-oesophageal reflux during anaesthesia, following two different fasting times in dogs. Vet Anaesth Analg. 2000;27(1):59-60. doi:10.1046/j.1467-2995.2000.00008-11.x
3 Robertson SA, Gogolski SM, Pascoe P, Shafford HL, Sager J, Griffenhagen GM. AAFP Feline Anesthesia Guidelines. J Feline Med Surg. 2018;20(7):602-634. doi:10.1177/1098612X18781391
4 Warne L, Bauquier S, Pengelly J, Neck D, Swinney G. STANDARDS OF CARE Anaesthesia guidelines for dogs and cats. Aust Vet J. 2018;96(11):413-427. doi:10.1111/avj.12762
5 Galatos AD, Raptopoulos D. Gastro-oesophageal reflux during anaesthesia in the dog: the effect of preoperative fasting and premedication. Vet Rec. 1995;137(19):479-483. doi:10.1136/vr.137.19.479
6 Savvas I, Raptopoulos D, Rallis T. A "Light Meal" Three Hours Preoperatively Decreases the Incidence of Gastro-Esophageal Reflux in Dogs. J Am Anim Hosp Assoc. 2016;52(6):357-363. doi:10.5326/JAAHA-MS-6399
7 Viskjer S, Sjöström L. Effect of the duration of food withholding prior to anesthesia on gastroesophageal reflux and regurgitation in healthy dogs undergoing elective orthopedic surgery. Am J Vet Res. 2017;78(2):144-150. doi:10.2460/ajvr.78.2.144
8 Gering AP, Nunes N, Oliveira MCC, Horr M, Lopes PCF, Tormena AA. Different fasting periods in tiletamine-zolezepam-anethetized cats: Glycemia, recovery, blood-gas and cardiorrespiratory parameters. Arq Bras Med Veterinária e Zootec. 2013;65(6):1685-1693. doi:10.1590/S0102-09352013000600015
9 American Society of Anesthesiologists. Practice Guidelines for Preoperative Fasting and the Use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration: Application to Healthy Patients Undergoing Elective Procedures. Anesthesiology. 2017;126(3):376-393. doi:10.1097/ALN.0000000000001452
10 Brady MC, Kinn S, Ness V, O’Rourke K, Randhawa N, Stuart P. Preoperative fasting for preventing perioperative complications in children. Cochrane Database Syst Rev. 2009;(4). doi:10.1002/14651858.CD005285.pub2
11 Brady MC, Kinn S, Stuart P, Ness V. Preoperative fasting for adults to prevent perioperative complications. Cochrane Database Syst Rev. 2003;(4). doi:10.1002/14651858.CD004423