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, full stomach and altered gastrointestinal mobility. Increased fasting times leads to decreased injury if aspiration occurs. 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.
Gastric conditions
In addition to fasting, antacids are administered the night before and then once again two hours prior to surgery. This is to increase the pH 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. Gastroparesis may occur and is due to metabolic causes, decreased gastric motility or pyloric obstruction. 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. Usually, gastroesophageal reflux may be associated with delayed gastric emptying of solids, but clear liquids are not affected. Raised intra-abdominal pressure 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.
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. However, since then, the American Society of Anesthesiologists, followed by the Association of Anaesthestists of Great Britain and Ireland, recommended new fasting guidelines for the minimum fast prior to surgery. 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. The following are the recommended guidelines for nil by mouth prior to surgery in healthy patients:
Age
Solids
Clear liquids
<6 months
4 hours
2 hours
6–36 months
6 hours
3 hours
>36 months
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.
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.
Public information
A 2016 systematic review found that the information on the internet often provided inaccurate and out-of-date recommendations on preoperative fasting.
