A major complication related to anesthesia in the perioperative period is aspiration of gastric contents. Multiple population level studies have found that aspiration among the leading causes of death from airway complications associated with anesthesia. In order to minimize the risk of aspiration, patients are often fasted prior to surgery if possible with the goal of emptying the stomach. Guidelines from the American Society of Anesthesiologists suggest that the existing evidence supports a 2 hour fast from clear fluids, a minimum of 6 hours from light foods such as toast, and even greater fasting times such as 8 hours or more from heavier foods such as meat. However, there is often a lack of clarity as to whether patients are adequately fasted prior to surgery especially elective surgery patients coming from home. This presents a dilemma of whether additional precautions need to be taken during surgery or even if the case needs to be cancelled.

One tool that has been used by anesthesiologists to clarify the risk of a full stomach preoperatively is an estimation of gastric volume using bedside ultrasound. The most commonly utilized method of estimating gastric volume is done by laying the patient in the right lateral decubitus position, measuring the diameter of stomach at the level of the antrum, and estimating a volume using the formula stomach volume (mL) = 27 + 14.6\* antral cross sectional area (cm2) - 1.28\*age (years). Volumes up to 1.5 mL/kg are generally thought to be consistent with baseline gastric secretions, while higher volumes or the presence of solids on ultrasound are indicative of a relatively full stomach and elevated aspiration risk. Gastric ultrasound and this method of estimating volume have been found to be very sensitive and specific for identifying a full stomach including in pregnant and severely obese patients.

Despite the proven accuracy of bedside ultrasound in estimating gastric volume in a variety of populations, all studies in the literature excluded patients who had previous gastric surgeries. There is a single published case study of an attempt to obtain an ultrasound estimate of gastric volume in a patient who had a previous roux-en-Y gastric bypass. This report found that the patient's stomach appeared to be empty on ultrasound regardless of ingested volume because the antrum of the stomach is no longer continuous with the remainder of the gastrointestinal tract after the operation. This is not the case in gastric sleeve procedures in which the greater curvature of the stomach is resected but the flow of material in the tract is left intact. Notably, an estimated 280,000 bariatric procedures were performed in the United States in 2022 with the vast majority of these being gastric sleeves. Given the large and ever increasing population of patients who have had gastric sleeve procedures and the proven utility of gastric ultrasound in assessing appropriate preoperative fasting it is important to determine whether traditional models of estimating gastric volume retain their accuracy in this population.

Participants will be asked to fast from solids for 8 hours and fluids for 2 hours prior to presenting for the study to ensure an empty stomach at baseline. They will be asked to lay on a flat surface on their right side. A curvilinear ultrasound probe will be placed on their abdomen and the antrum of the stomach will be identified with the liver and aorta in view at which point 3 measurements of both the anteroposterior and craniocaudal diameter of the stomach will be taken. These measurements will be used to calculate a gastric volume. Participants will then be asked to drink 100 mL of water. A second set of measurements of the gastric antrum will be obtained at 5 minutes after ingestion of the water as previously described. Serial measurements will be taken at thirty-minute intervals until 2 hours have past since ingestion. Participants will then be asked if they would be willing to repeat the above process at a later date. If they agree to go forward with an additional date, they will be asked to undergo the same process detailed above but with ingestion of 200 mL of water. If they are amenable to presenting a third time they will undergo a final trial with 300 mL of water as described above.