ASPIRATION RISK
Gastric ultrasound
When to use it
- Check whether the stomach is empty or full when fasting is uncertain: emergencies, diabetes with gastroparesis, GLP-1 agonist drugs (such as semaglutide), opioids, pregnancy, trauma, or when the patient can't give a reliable history.
- Decide between a routine induction, a rapid sequence induction, postponing surgery, or emptying the stomach.
- Probe
- Curved, low frequency (linear in thin adults and children)
- Position
- Supine, then right lateral decubitus
- Depth
- 8–12 cm
How to scan
- Probe in the sagittal plane in the epigastrium, just to the right of the midline. Find the liver at the top, and the aorta or inferior vena cava deep down.
- The antrum sits between the liver and the pancreas: a round or oval structure with a layered wall. Scan supine first, then turn the patient onto the right side, which moves fluid into the antrum.
- Look at what's inside: empty (flat or 'bull's-eye'), clear fluid (black), or solid (frosted glass at first, then mixed and bright).
- If there is clear fluid, measure the antrum's cross-sectional area (CSA) in the right lateral position, between contractions. Estimate the volume: Volume (mL) = 27 + 14.6 × CSA (cm²) − 1.28 × age (years).
Sagittal epigastric view: the antrum lies under the left lobe of the liver, in front of the pancreas, SMA and aorta.
- Liver
- left lobe of the liver, in front of the antrum
- Antrum wall
- gastric antrum: layered muscular wall
- Fluid
- clear fluid in the lumen: black (anechoic). Solid food looks like frosted glass, then mixed and bright
- Rectus
- rectus abdominis
- Panc
- pancreas
- SMA
- superior mesenteric artery
- Ao
- aorta (landmark: scan where the aorta or IVC lies behind the antrum)
Original schematic illustration © Nerve Blocks. Not to scale.
What you see and what it means
| On the screen | What it means |
|---|---|
| Grade 0: empty in both positions | Empty stomach, low risk |
| Grade 1: fluid only when on the right side | Small volume, usually normal (under about 1.5 mL/kg) |
| Grade 2: fluid seen in both positions | Higher volume: treat as a full stomach |
| Solid or thick contents | Full stomach, whatever the volume |
| Clear fluid over about 1.5 mL/kg | Above the normal fasting range: higher risk |
Pearls
- Scanning the right side is the key step: a small amount of fluid can be missed with the patient supine.
- Air in a recently filled stomach can hide everything ('frosted glass' or nothing visible). Treat an unclear view as not empty.
- The volume formula was validated in non-pregnant adults with BMI under 40. Use the grades, not the formula, outside that group.
Videos
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References
- Perlas A, Davis L, Khan M, et al. Gastric sonography in the fasted surgical patient: a prospective descriptive study. Anesth Analg 2011;113:93–7. PubMed
- Perlas A, Mitsakakis N, Liu L, et al. Validation of a mathematical model for ultrasound assessment of gastric volume by gastroscopic examination. Anesth Analg 2013;116:357–63. PubMed
- Van de Putte P, Perlas A. Ultrasound assessment of gastric content and volume. Br J Anaesth 2014;113:12–22. PubMed
