In Response

We thank Drs. Bouvet and Chassard1 for their interest in our article.2 However, we respectfully disagree with their assumptions that our grading system is not clinically relevant and that any gastric volume over 0.8 mL/kg (50 mL for an average adult) poses a significant aspiration risk.1 In fact, the minimum gastric volume posing a risk of aspiration is currently controversial. Early extrapolations from animal models suggested thresholds of 0.4 mL/kg.3 More recently, Dr. Engelhardt et al.4 demonstrated that installation of 0.4 to 0.6 mL/kg of hydrochloric acid directly into monkeys’ tracheas causes clinical changes but not death and that the LD50 of hydrochloric acid instilled into the trachea of monkeys is 1.0 mL/kg. Based on these animal findings, they speculate that a volume of 0.8 mL/kg or approximately 50 mL may be a critical volume for severe aspiration.5 However, many experts have long questioned the validity of these extrapolations and they have suggested that arbitrary “thresholds” based on hydrochloric acid directly instilled into animals’ tracheas are not valid surrogates of gastric volumes and should be abandoned.5 Moreover, a plethora of evidence from clinical studies comprising more than 1000 patients shows that 0.8 mL/kg lies within the normal range of fasting gastric secretions and does not pose a significant aspiration risk.6–11 Human data consistently show that the mean gastric volume in fasted adults is 0.4 to 0.6 mL/kg and the upper limit of normal is approximately 1.5 mL/kg (or about 100 mL in the average adult).6–11 Therefore, a threshold of 0.8 mL/kg (corresponding to a supine antral area of 3.4 cm2) as suggested by Drs. Bouvet and Chassard would grossly overestimate aspiration risk in a large proportion of low-risk patients. Our own data from several studies is consistent with previous reports. In a prospective study (n = 200), 44% of fasted surgical patients presented a supine antral CSA >3.4 cm2.12 Similarly, 56% of fasted patients presenting for elective gastroscopy had a supine antral CSA >3.4 cm2, which again suggests this antral size is a normal finding.2 Therefore, although the threshold of gastric volume that increases aspiration risk is still debatable, clinical data strongly suggest it is much higher than that extrapolated from animal models, and likely greater than 1.5 mL/kg (approximately 100 mL for the average adult). Thus, a grading system than can differentiate volumes above and below this threshold is clinically relevant. This grading system, however, only gives a rough estimate of volume. To obtain a more precise estimate, we apply a simple mathematical model that is valid for a wide demographic range (nonpregnant adults with BMI up to 40).2 Having an accurate volume estimate, the clinician can then consider the clinical context of the individual patient, the presence of other risk factors, comorbidities, and the benefits and risks of alternate management strategies. Despite our disagreement as to what constitutes a “risky” threshold of gastric volume, we share Drs. Bouvet and Chassard’s enthusiasm for gastric sonography. As the first validated, noninvasive tool to assess the nature and volume of gastric content at the bedside, it is a great new resource for both researchers and clinicians. We are confident that this tool has now come of age, and it will soon demonstrate its full potential to help guide anesthetic management and prevent this most devastating perioperative complication.

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