4. PREOPERATIVE FASTING

All patients must fast, if possible, before surgery.

Physiology

With the onset of anaesthesia, protective airway reflexes are diminished and patients are at risk of regurgitation and inhaling (aspirating) their stomach contents.

The aim of fasting is to minimize the risk of aspiration. However the anaesthetist should also consider patient comfort in the preoperative period and minimise any potential significant physiological changes that may occur from prolonged fasting.

As gastric secretion is continuous at 6 ml/kg/h and 1 ml/kg/h of saliva is swallowed, the stomach is never truly empty. These volumes and the speed at which the stomach empties food and liquid will change with diseases, emotion, pain and hunger. It is important to remember that a patient who is in pain and/or sustained an injury soon after eating may still have a full stomach even with prolonged fasting, and should be treated as at risk of aspiration. This is common in children.

Preoperative Assessment

The preoperative assessment must try to identify those patients with an increased risk of aspiration. The anaesthetist should ask about a history of gastroesophageal reflux disease, dysphagia, gastrointestinal motility disorders, metabolic disorders (e.g. diabetes), obesity, pregnancy and drugs (e.g. morphine) that may increase the risk of regurgitation and pulmonary aspiration. The anaesthetist must be aware of surgical conditions such as intra-abdominal infective/inflammatory disorders (e.g. appendicitis) and obstructive disorders (e.g. bowel cancer) that will also increase the risk of regurgitation and aspiration. Finally the anaesthetist must consider the fasting time.

If the anaesthetist believes the patient to be at an increased risk of regurgitation and aspiration then they will need to alter their anaesthetic management (e.g. rapid sequence induction and intubation of the trachea).

The risk of aspiration can be reduced by fasting, emptying the stomach (nasogastric tube or causing vomiting), reducing stomach acidity (non-particulate antacid, histamine-2 receptor antagonists) and increasing the speed of emptying of the stomach (metoclopramide). Nasogastric tubes and inducing vomiting are unpleasant for the patient and are not routinely done. Nasogastric tubes may be appropriate for patients with an ileus.

Fasting time

The fasting times for clear fluids and solids are different. Solids are emptied from the stomach at a much slower rate than clear fluids. Aspiration of solids can cause obstruction of airways and potentially greater morbidity and mortality. There are also differences in stomach emptying between breast milk, cow’s milk and formula. Gastric emptying is much slower for formula compared with breast milk. It should be treated as a solid.

Recommendations for Fasting Times

For elective surgery

Preoperative fasting solids and non-human milk: 6 hours

Preoperative fasting infant formula: 6 hours

Preoperative fasting breast milk: 4 hours

Preoperative fasting clear fluids: 2 hours

All patients must be allowed to take most of their usual medications before surgery with 30 ml of water.

Recommendations for Drug Treatment

(There are many drugs that affect stomach emptying)

The routine preoperative use of gastrointestinal stimulants (e.g. metoclopramide) for reducing gastric volume in patients who are not at increased risk of aspiration is not recommended.

The routine preoperative use of histamine-2 receptor antagonists that block gastric acid secretion (e.g. cimetidine or ranitidine) in patients who are not at increased risk of aspiration is not recommended.

If antacids are given preoperatively to reduce gastric acidity, then only non-particulate antacids should be used.

These drugs should be used in patients who are at risk of aspiration.