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.

The main reason for 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.

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.

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.

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Wikipedia