Preoperative fasting

Preoperative fasting is the practice of a patient abstaining from oral food and fluid intake for a time before an operation is performed. This is intended to prevent pulmonary aspiration of stomach contents during general anesthesia.[1] The latest guidelines do not support preoperative fasting, as there is no difference in residual gastric fluid volume, pH or gastric emptying rate following semi-solid meals or drinks, whether in obese or lean individuals.[2]

Pulmonary aspiration

The main hypothesized benefit of preoperative fasting is to prevent pulmonary aspiration of stomach contents while under the effects of general anesthesia. Aspiration of as little as 30–40 mL can be a significant cause of suffering and death during an operation and therefore fasting is performed to reduce the volume of stomach contents as much as possible. Several factors can predispose to aspiration of stomach contents including inadequate anesthesia, pregnancy, obesity, difficult airways, emergency surgery (since fasting time is reduced), full stomach and altered gastrointestinal mobility. Increased fasting times leads to decreased injury if aspiration occurs.[1]

The latest guidelines do not support preoperative fasting, as there is no difference in residual gastric fluid volume, pH or gastric emptying rate following semi-solid meals or drinks, whether in obese or lean individuals.[2][3]

Gastric conditions

In addition to fasting, antacids are administered the night before (or in the morning of an afternoon operation) and then once again two hours prior to surgery. This is to increase the pH (make more neutral) of the acid present in the stomach, helping to reduce the damage caused by pulmonary aspiration, should it occur. H2 receptor blockers should be used in high-risk situations and should be administered in the same timing intervals as antacids.[1]

Gastroparesis (delayed gastric emptying) may occur and is due to metabolic causes (e.g. poorly controlled diabetes mellitus), decreased gastric motility (e.g. due to head injury) or pyloric obstruction (e.g. pyloric stenosis). Delayed gastric emptying usually only affects the emptying of the stomach of high-cellulose foods such as vegetables. Gastric emptying of clear fluids such as water or black coffee is only affected in highly progressed delayed gastric emptying.[1]

Usually, gastroesophageal reflux (GERD) may be associated with delayed gastric emptying of solids, but clear liquids are not affected. Raised intra-abdominal pressure (e.g. in pregnancy or obesity) predisposes to regurgitation. Certain drugs such as opiates can cause marked delays in gastric emptying, as can trauma, which can be determined by certain indicators such as normal bowel sounds and patient hunger.[1]

Minimum fasting times

The minimum fasting times prior to surgery have long been debated. The first proposition came from British anesthetists stating that patients should be nil by mouth from midnight.[4] However, since then, the American Society of Anesthesiologists (ASA), followed by the Association of Anaesthestists of Great Britain and Ireland (AAGBI), recommended new fasting guidelines for the minimum fast prior to surgery.[1] This was based upon evidence by Canadian anesthesiologists who found that drinking clear fluids two hours prior to surgery decreased pulmonary aspiration compared to those nil by mouth since midnight.[4] The following are the recommended guidelines for nil by mouth prior to surgery in healthy patients:[5]

Age	Solids	Clear liquids
<6 months	4 hours	2 hours
6–36 months	6 hours	3 hours
>36 months (including adults)	6 hours	2 hours

When anaesthesia is required in an emergency, nasogastric aspiration is usually performed to reduce gastric contents and the risk of its pulmonary aspiration.[6]

Unrestricted clear fluids

Fasting guidelines often restrict the intake of any oral fluid after two to six hours preoperatively. However, it has been demonstrated in a large retrospective analysis in Torbay Hospital that unrestricted clear oral fluids right up until transfer to theatre could significantly reduce the incidence of postoperative nausea and vomiting without an increased risk in the adverse outcomes for which such conservative guidance exists.[7]

Public information

A 2016 systematic review found that the information on the internet often provided inaccurate and out-of-date recommendations on preoperative fasting.[8]

References

1. Allman, Keith G.; Iain H. Wilson (2006). Oxford Handbook of Anaesthesia (2nd ed.). Oxford University Press. ISBN 978-0-19-856609-0.
2. Thorell, A; MacCormick, AD; Awad, S; Reynolds, N; Roulin, D; Demartines, N; Vignaud, M; Alvarez, A; Singh, PM; Lobo, DN (September 2016). "Guidelines for Perioperative Care in Bariatric Surgery: Enhanced Recovery After Surgery (ERAS) Society Recommendations". World Journal of Surgery (Professional society guidelines). 40 (9): 2065–83. doi:10.1007/s00268-016-3492-3. PMID 26943657.
3. Crowley, Marianna (20 September 2019). "UpToDate". www.uptodate.com.
4. Maltby JR (April 2006). "Preoperative fasting guidelines" (PDF). Can J Surg. 49 (2): 138–9, author reply 139. PMC 3207537. PMID 16630428. Archived from the original (PDF) on 2015-12-23. Retrieved 2008-08-20.
5. Coté CJ (July 1999). "Preoperative preparation and premedication". Br J Anaesth. 83 (1): 16–28. doi:10.1093/bja/83.1.16. PMID 10616330. Retrieved 2008-08-20.
6. Legal review of need to place NG tube Archived 2012-02-16 at the Wayback Machine
7. McCracken, Graham C.; Montgomery, Jane (2017-11-06). "Postoperative nausea and vomiting after unrestricted clear fluids before day surgery: A retrospective analysis". European Journal of Anaesthesiology. 35 (5): 337–342. doi:10.1097/EJA.0000000000000760. ISSN 0265-0215. PMID 29232253.
8. Roughead, Taren; Sewell, Darreul; Ryerson, Christopher J.; Fisher, Jolene H.; Flexman, Alana M. (December 2016). "Internet-Based Resources Frequently Provide Inaccurate and Out-of-Date Recommendations on Preoperative Fasting". Anesthesia & Analgesia. 123 (6): 1463–1468. doi:10.1213/ANE.0000000000001590. PMID 27644057.