Describe Preoperative Fasting & Aspiration Prophylaxis in detail.

Rationale

Anaesthesia abolishes the protective airway reflexes, so any gastric contents that regurgitate may be aspirated into the lungs, causing a chemical pneumonitis (Mendelson’s syndrome) or airway obstruction. Preoperative fasting aims to ensure an empty stomach at induction, reducing the volume and acidity of gastric contents and hence the risk and severity of aspiration.

The ‘2–4–6 (–8)’ Rule

Standard elective fasting follows the ‘2–4–6’ rule: 2 hours for clear fluids, 4 hours for breast milk, and 6 hours for solids/formula/non-human milk (with 8 hours for fatty meals). Importantly, encouraging clear fluids up to 2 hours before surgery keeps patients comfortable and does not increase risk.

Patients at Increased Risk

Some patients have a ‘full stomach’ despite fasting or delayed gastric emptying: emergencies/trauma (pain, opioids), pregnancy (from the second trimester), gastro-oesophageal reflux/hiatus hernia, diabetes (gastroparesis), bowel obstruction, and raised intra-abdominal pressure/obesity. These patients need aspiration precautions.

Aspiration Prophylaxis

In at-risk patients, prophylaxis reduces the volume and acidity of gastric contents and secures the airway: pharmacological — an H₂-receptor antagonist (ranitidine) or proton-pump inhibitor the night before/morning of surgery, a prokinetic (metoclopramide), and a non-particulate antacid (sodium citrate) immediately before induction; and technique — a rapid sequence induction with cricoid pressure and a cuffed tracheal tube to protect the airway.

Physiology of Gastric Emptying & the ‘full Stomach’

Fasting works because the healthy stomach empties clear fluids within about two hours, whereas solids and fatty meals take much longer, so a timed fast leaves the stomach reliably empty for elective surgery. That assumption breaks down whenever gastric emptying is delayed or gastric volume is increased — by pain and opioids after trauma, by the mechanical and hormonal effects of pregnancy, by the autonomic neuropathy of diabetes, by bowel obstruction, or by raised intra-abdominal pressure — which is why such patients are managed as having a ‘full stomach’ regardless of how long they have fasted, and why fasting alone is never regarded as a guarantee of an empty stomach.

Balancing Fasting Against its Harms

While fasting reduces aspiration risk, excessive or poorly-managed fasting has its own harms — thirst, hunger, headache, irritability, dehydration and, in children and the frail elderly, hypoglycaemia and haemodynamic instability — and prolonged ‘nil by mouth’ from cancelled or delayed lists is a common, avoidable problem. Modern practice therefore actively encourages clear fluids up to two hours preoperatively and, in some settings, carbohydrate drinks, because these empty rapidly and improve patient comfort and metabolic state without increasing gastric volume at induction. The goal is a stomach that is empty of solids and low in residual fluid, not a needlessly dehydrated, miserable patient.

The 2-4-6-8 rule balances aspiration risk against dehydration.

• Anaesthesia abolishes airway reflexes → aspiration risk (Mendelson’s syndrome).
• Elective fasting ‘2–4–6’: clear fluids 2 h, breast milk 4 h, solids/formula 6 h (fatty 8 h).
• Clear fluids up to 2 h are encouraged — comfort without added risk.
• ‘Full stomach’: emergency, pregnancy, reflux, diabetes, obstruction, obesity.
• Prophylaxis: H₂ blocker/PPI + prokinetic + sodium citrate; RSI with cricoid pressure + cuffed tube.