At what gastric volume and pH are patients considered at risk for aspiration?

Gastric Volume and pH Thresholds for Aspiration Risk

Patients are considered at risk for aspiration if their gastric volume is greater than 1.5 ml/kg and gastric pH is less than 2.5. 1

Historical Context and Evolution of Aspiration Risk Parameters

• The original parameters for aspiration risk were set at 25 ml (>0.4 ml/kg) and pH <2.5 by Roberts and Shirley, but these were based on findings from a single rhesus monkey 1
• Further systematic animal experiments suggested a higher critical volume of 50 ml (0.8 ml/kg) 1
• The most recent evidence supports a gastric volume threshold of 1.5 ml/kg as the critical value for aspiration risk, based on pragmatic evaluation of gastric volumes in fasting patients and the low incidence of aspiration 1

Clinical Significance and Assessment Methods

• Gastric volume can be assessed using point-of-care ultrasound, which has good correlation with isotope scintigraphy and provides a practical approach to individualized risk assessment 1, 2
• Antral cross-sectional area (CSA) can be measured in the lateral decubitus position, and gastric fluid volume calculated using the formula: Volume (ml) = 27.0 + 14.6 CSA - 1.28 age 1
• Recent studies show that pre-operative gastric ultrasound helps identify high-risk (solid, mixed or gastric fluid volume >1.5 ml/kg) and low-risk (empty, gastric fluid volume ≤1.5 ml/kg) situations in patients at risk of aspiration 2

Risk Modification Strategies

• Pharmacological interventions can effectively modify gastric pH and volume:

  • H2 receptor antagonists (e.g., famotidine) effectively increase gastric pH during the perioperative period 1, 3
  • Nonparticulate antacids can increase gastric pH but have inconsistent effects on gastric volume 1
  • Prokinetic agents (e.g., metoclopramide) can reduce gastric volume 4, 5

• Combination therapy with H2 blockers and prokinetic agents provides the most effective risk reduction:

  • Patients given both metoclopramide and cimetidine showed mean pH of 6.9 and mean volume of 2.0 ml, significantly reducing aspiration risk parameters 4, 5

Important Caveats and Considerations

• Despite many patients having gastric volumes >0.4 ml/kg, the actual incidence of aspiration is very low (approximately 1 in 3,000 general anesthetic cases) 6, 7
• Morbidly obese patients have a higher risk of regurgitation and pulmonary aspiration compared to lean patients when no prophylaxis is used (30% vs. 5% meeting critical volume and pH criteria) 5
• The American Society of Anesthesiologists recommends that H2 receptor antagonists should not be routinely administered to all patients but reserved for those at increased risk of pulmonary aspiration 1, 3
• Risk factors for aspiration include emergency surgery, obesity, pregnancy, difficult airway, gastroesophageal reflux disease, and delayed gastric emptying 3

Clinical Application

• For high-risk patients, consider:

  • Administering H2 receptor antagonists (e.g., oral famotidine 40 mg at least 3 hours before surgery) 3
  • Adding a prokinetic agent for patients with delayed gastric emptying 1, 5
  • Using nonparticulate antacids when indicated 1
  • Performing pre-operative gastric ultrasound to guide anesthetic management 2

• Recent evidence suggests that pre-operative gastric ultrasound may allow more liberal management in approximately 15% of patients and more conservative management in 4% of patients, potentially improving individualized care and patient safety 2