Why is preoperative fasting necessary before surgery?

Why We Keep Fast Before Surgery

Preoperative fasting exists primarily to prevent pulmonary aspiration of gastric contents during anesthesia, which can cause aspiration pneumonia, respiratory failure, and death—though the actual risk is extremely low at 1 in 7,000 anesthetics, with mortality occurring in only 1 in 100,000 cases. 1

The Core Rationale

The fundamental concern is that general anesthesia impairs upper airway protective reflexes, creating a window where gastric contents can be regurgitated and aspirated into the lungs. 1 Historically, the two deaths attributed to aspiration in Mendelson's landmark study actually resulted from solid food pieces causing airway obstruction, not from acid aspiration pneumonitis. 1

The critical insight is that prolonged fasting does NOT provide additional protection and actually causes harm. 1

Evidence-Based Fasting Guidelines

For Clear Liquids

• Clear liquids may be consumed up to 2 hours before anesthesia in healthy patients undergoing elective procedures. 1, 2
• Clear liquids include water, fruit juice without pulp, carbonated beverages, clear tea, and black coffee. 3
• Clear fluids empty the stomach within 60-90 minutes, making prolonged fasting unnecessary. 3
• Meta-analysis demonstrates that 2-4 hour fasting for clear liquids results in smaller gastric volumes and higher gastric pH compared to prolonged fasting. 2

For Solid Foods

• Light meals require a minimum 6-hour fast before elective procedures. 1, 2
• Fatty or fried foods require a minimum 8-hour fast due to prolonged gastric emptying. 2, 3
• Extending solid-food fasting beyond 6-8 hours offers no additional benefit and may be harmful. 2

For Pediatric Patients

• Breast milk may be consumed up to 4 hours before elective procedures in neonates and infants. 2
• Infant formula requires a minimum 6-hour fast. 2
• There is good evidence to support a 1-hour fast for clear liquids in children, with no increase in risk of pulmonary aspiration. 1

The Harms of Prolonged Fasting

Excessive preoperative fasting causes significant physiological and metabolic detriment that worsens surgical outcomes. 1

Metabolic Consequences

• Prolonged fasting triggers a catabolic state with gluconeogenesis and increased organic stress response to trauma. 4
• It induces insulin resistance, which impairs postoperative recovery. 1
• Carbohydrate loading 2 hours before surgery attenuates insulin resistance and improves metabolic state. 1, 2

Patient Comfort and Safety

• Dehydration from prolonged fasting causes hemodynamic instability and impaired oxygen delivery. 1
• Patients experience unnecessary thirst, hunger, anxiety, and discomfort. 1, 5
• Risk of hypoglycemia, particularly in vulnerable populations. 1, 2

High-Risk Populations Requiring Modified Protocols

Patients Who Should NOT Fast Routinely

• Patients with diabetes mellitus are at high risk of severe hypoglycemia during extended fasting and should not fast without close medical supervision. 2
• Patients with Child class C cirrhosis should not fast due to risk of encephalopathy, ascites, and metabolic decompensation. 2

Patients Requiring Longer Fasting

• Patients with gastroesophageal reflux disease or known delayed gastric emptying require longer fasting periods than standard recommendations. 1, 2, 3
• Patients with obesity, diabetes, hiatal hernia, ileus, or bowel obstruction may need modified protocols. 1
• Emergency surgery patients require more conservative fasting protocols. 2

Common Pitfalls to Avoid

The most common error is applying "nil per os after midnight" orders indiscriminately, which results in 12-16 hours of unnecessary fasting. 4 This occurs due to:

• Poor communication as operating room schedules change. 6
• Deficient parent and patient education about the 2-hour clear liquid guideline. 6
• Outdated institutional policies that haven't adopted evidence-based guidelines. 7

Another critical mistake is assuming that longer fasting equals greater safety—the opposite is true. 1 Gastric acid secretion continues during fasting, so prolonged fasting does not reduce gastric acidity. 1

Medication Management During Fasting

• Oral medications may be taken with approximately 240 mL of water up to 2 hours before anesthesia without breaking the fast. 2
• Non-nourishing medications via parenteral routes, suppositories, and inhalers are allowed during fasting periods. 2

The Bottom Line for Clinical Practice

For healthy patients undergoing elective surgery: allow clear liquids until 2 hours before anesthesia, light meals until 6 hours before, and fatty meals until 8 hours before. 1, 2 This approach minimizes aspiration risk (which is already extremely low) while avoiding the proven harms of prolonged fasting—dehydration, insulin resistance, patient discomfort, and metabolic stress. 1, 5