Gastric Volume and pH Thresholds for Aspiration Risk

Historical Context and Evolution of Aspiration Risk Parameters

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, according to the American Society of Anesthesiologists 1, 2, 3
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 3, 4
Further systematic animal experiments suggested a higher critical volume of 50 ml (0.8 ml/kg) 1, 2
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, 3

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
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

H2 receptor antagonists (e.g., famotidine) effectively increase gastric pH during the perioperative period, as recommended by the American Society of Anesthesiologists 5
Nonparticulate antacids can increase gastric pH but have inconsistent effects on gastric volume, according to the American Society of Anesthesiologists 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 5

Risk factors for aspiration include emergency surgery, obesity, pregnancy, difficult airway, gastroesophageal reflux disease, and delayed gastric emptying, as identified by the American Society of Anesthesiologists 6
The American Society of Anesthesiologists recommends considering the administration of H2 receptor antagonists for high-risk patients, such as those with risk factors for aspiration 5, 6