Cohort 1 will include subjects who have a peripheral intravenous line placed in the pre-op area per routine care. This cohort is on average age 7 years and older. Procedures for cohort 1 will include: (a) gastric US measurement of the antral CSA and gastric volume using the Perlas US qualitative grading assessment by an expert and a novice operator to determine the inter-rater class reliability and repeat measurements by each operator to assess the intra-rater reliability (Aim 1), and (b) serial abdominal ultrasounds for gastric assessment and the acetaminophen absorption test to determine the correlation between gastric US and a gold-standard for gastric emptying (Aim 2). The acetaminophen absorption test requires administering a single weight-based enteral dose of acetaminophen with 6 oz of water and drawing of blood samples through a peripheral intravenous line.

Cohort 2 will include subjects for whom a peripheral intravenous line placement is not part of routine pre-operative care. Average age for cohort 2 is age less than 7 years old. Procedures for Cohort 2 will include a gastric US measurement of the antral CSA and gastric volume using Perlas US qualitative grading assessment by an expert and a novice operator to determine the inter-rater class reliability and repeat measurements by each operator for the intra-rater reliability (Aim 1). This cohort is included to examine the reliability of measurements across age groups.

The patient's abdomen will be scanned using the ultrasound device by an expert scanner and novice scanner for baseline assessment of the antral CSA and gastric volume using Perlas US qualitative grading assessment.

Measured using the following quantitative and qualitative ultrasound assessments between a novice and an expert gastric ultrasound operator:

• Qualitative using the previously published Perlas' US qualitative grading assessment
• Grade 0- no contents visualized in supine and lateral position
• Grade 1- contents visualized in the lateral decubitus position (RLD) only
• Grade 2- contents visualized in the supine and RLD.
• Quantitative assessment using the cross-sectional area as published by Spencer AO et al.
• Measurement of the CSA: longitudinal diameter (D1), and the antero-posterior diameter (D2) in both supine and RLD. Then these will be used to calculate the cross-sectional area (CSA) = (π*D1*D2)/4.
• Quantitative assessment using the cross-sectional area as published by Spencer AO et al.
• Measurement of the CSA: longitudinal diameter (D1), and the antero-posterior diameter (D2) in both supine and RLD. Then these will be used to calculate the cross-sectional area (CSA)

Measured using the following quantitative and qualitative ultrasound assessments between a novice and an expert gastric ultrasound operator:

• Qualitative using the previously published Perlas' US qualitative grading assessment
• Grade 0- no contents visualized in supine and lateral position
• Grade 1- contents visualized in the lateral decubitus position (RLD) only
• Grade 2- contents visualized in the supine and RLD.
• Quantitative assessment using the cross-sectional area as published by Spencer AO et al.
• Measurement of the CSA: longitudinal diameter (D1), and the antero-posterior diameter (D2) in both supine and RLD. Then these will be used to calculate the cross-sectional area (CSA) = (π*D1*D2)/4.

Outcome will be gastric emptying assessed by the change in antral CSA (measured in cm2) over time after acetaminophen administration, and this will be correlated to gastric emptying over the same period of time by the acetaminophen absorption test PK parameter, AUC60 mcg*min/mL. As such, each patient will act as their own control. Correlation analysis will be applied to examine the performance of CSA measurements overtime using gastric ultrasound to determine gastric emptying compared to the acetaminophen absorption test as the gold standard.

Fasting times have been correlated to post-anesthetic outcomes such as post-operative nausea/ vomiting, and discomfort, and it is known that some patients fast longer than required by their clinical team, which can result in adverse effects. Reported fasting times for all patients and confirmatory assessment by gastric US will be examined along with data pulled from the medical record about post-operative nausea/ vomiting, post-operative discomfort, satisfaction, and whether the patient met their expected post-operative trajectory or not (e.g. unplanned admission).