POCUS / Interactive tutorial
Gastric ultrasound.
Find the antrum. Read the scan. Plan the next step.
In this tutorial
- 01–02 · Why scan · I-AIM4 min
- Part I · Acquisition · aorta vs IVC9.5 min
- Part II · Interpretation · CSA → volume8.5 min
- Part III · Five peri-operative cases11 min
- Quick check · Questions2 min
One rule throughout: never call a stomach empty from the supine view alone.
Fasting time is a proxy. The antrum is the target.
A fasting history tells you when food went in. It cannot tell you whether it has left.
Pulmonary aspiration is uncommon, but it was the single most common cause of death among the anaesthetic airway events in NAP4.
Slow emptying — GLP-1 receptor agonists, diabetes with autonomic neuropathy, opioids, pain and trauma, Parkinson's disease, renal failure.
Unreliable history — dementia, language barriers, emergencies, "just a sip of water".
Something else in the stomach — blood from an upper GI bleed.
Four questions, always in this order.
A structure for doing — and teaching — gastric point-of-care ultrasound (Perlas et al., BJA 2016).
- IIndicationIs the aspiration risk uncertain, and would the answer change your plan?
- AAcquisitionCurvilinear probe, sagittal epigastrium, supine then right lateral decubitus.
- IInterpretationQualitative first — what is it? Then quantitative — if clear fluid, how much?
- MMedical decision-makingLow risk: proceed. High risk: postpone, protect the airway, or change the technique.
| Scan | Fasting status unknown, unclear or unreliable. Fasted, but a condition that slows emptying (GLP-1 RA, diabetic autonomic neuropathy, opioids, trauma, Parkinson's, renal failure). Clear fluid taken late and the volume is uncertain. |
| Don't scan | The answer will not change management: bowel obstruction, a crash induction, an RSI that is already indicated for other reasons. |
| Content | Looks like | Then |
|---|---|---|
| Empty | Flat or target-shaped antrum, thick dark muscle layer, both positions | Low risk |
| Clear fluid | Anechoic, distended, thin walls; bubbles look like a "starry night" | Measure |
| Thick fluid | Homogeneous, echogenic content | High risk |
| Solid | "Frosted glass" early; heterogeneous, mixed echogenicity later | High risk |
Find the antrum.
Liver in front, pancreas behind, a great vessel underneath. And if you cannot find the vessel, find the spine.
Curvilinear · Supine then RLD · Sagittal sweep · Antrum at the aorta
Start supine. Finish in right lateral decubitus.
Curvilinear 2–5 MHz, abdominal preset. Start deep (≈ 15–16 cm) to see the spine and the great vessels, then shallow up once you have the antrum.
Supine (or 45° head-up if they cannot lie flat), then right lateral decubitus. Gravity moves fluid into the antrum and air out of it.
Sagittal, marker to the head. Sweep from the left to the right subcostal margin. The antrum is superficial — about 3–4 cm deep.
Liver in front. Pancreas behind. Aorta underneath.
The antrum is the most reliably imaged part of the stomach, and its cross-sectional area tracks total gastric volume.
Mucosa–lumen interface bright · deep mucosa dark · submucosa bright · muscularis propria dark, thickest · serosa bright.
Measure from serosa to serosa — the full thickness of the wall.
Two tubes behind the antrum. Know which one you are on.
| Feature | Aorta | IVC |
|---|---|---|
| Side | Left of midline, on the vertebral body | Right of midline, behind and through the liver |
| Wall | Thick, bright, parallel walls | Thin, sometimes barely visible |
| Pulsation | Expansile, systolic | Flickering, venous |
| Sniff | No change | Calibre falls — collapses in spontaneous breathing |
| Probe pressure | Holds its shape | Flattens |
| Branches | Coeliac trunk and SMA arise anteriorly | Hepatic veins join it; it runs into the right atrium |
| Colour | Pulsatile, high-velocity flow | Phasic, low-velocity flow |
Use several features together. A ventilated patient cannot sniff; tricuspid regurgitation makes the IVC pulsatile.
Find it in cross-section, then rotate into its long axis.
- 01Go transverse and go deepMarker to the patient's right, just below the xiphoid, depth 18 cm. Find the vertebral body — a bright arc with a black shadow behind it.
- 02The aorta sits on the spine, left of midlineRound, thick-walled, pulsatile — on the screen's right. The IVC is to the patient's right (screen left): oval, thin-walled, breathing. The SMA, in its bright collar of fat, sits in front of the aorta.
- 03Centre the aortaSlide the probe until the aorta sits under the midline marker.
- 04Rotate 90°, marker to the headKeep it centred while you turn. The circle opens into a tube.
- 05Fine-tune until the walls run parallelShort, tapering segment = still oblique. Rotate or fan a few degrees until the tube crosses the whole screen.
- 06Shallow up and find the antrumDepth ≈ 12 cm. The antrum lies in front, between liver and pancreas. Need the IVC plane? Slide ≈ 3 cm to the patient's right.
Read the antrum.
What is in it — and, only if it is clear fluid, how much.
Qualitative · Grade 0 · 1 · 2 · CSA → volume · Decide
Read the content before you measure anything.
- 00Empty Low riskFlat or round "target": bright centre, thick dark muscle ring. Both positions.
- 01Clear fluid MeasureAnechoic, distended, thin walls, bright through-transmission behind.
- 02Clear fluid with bubbles MeasureRecent drink — a "starry night" of bright dots rising in black fluid.
- 03Thick fluid High riskHomogeneous, echogenic content: milk, feeds, blood.
- 04Solid — early High risk"Frosted glass": bright front edge, shadow behind, posterior wall hidden.
- 05Solid — later High riskHeterogeneous, mixed echogenicity; the antrum is distended and lumpy.
Same stomach, two positions.
For clear fluid: where you see it predicts how much there is (Perlas et al., Anesth Analg 2011).
| Grade | Supine | RLD | Usually means |
|---|---|---|---|
| 0 | Empty | Empty | Empty stomach · low risk |
| 1 | Empty | Fluid | Small volume, consistent with baseline secretions |
| 2 | Fluid | Fluid | Larger volume, often > 1.5 mL/kg — measure it |
Clear fluid? Measure the antrum, then calculate.
RLD · antrum at the level of the aorta · freeze between peristaltic contractions · free-trace the serosa (full wall thickness).
Perlas et al., Anesth Analg 2013. Adult, non-pregnant model, validated for 0–500 mL. Clear fluid only — it does not apply to solid or thick content.
Quick table · volume (mL) by age and CSA
Low risk: proceed. High risk: change the plan.
Empty · or clear fluid ≤ 1.5 mL/kg
Consistent with baseline secretions. Proceed with the planned technique.
Clear fluid > 1.5 mL/kg · thick fluid · solid
Elective — postpone; for clear fluid, wait and rescan.
Urgent — RSI with a cuffed tube; or regional/neuraxial with minimal sedation; NG decompression for fluid (it will not clear solids); a prokinetic and a rescan if time allows.
Cannot find or cannot read the antrum
Not a negative scan. Decide on clinical grounds; treat as full if risk factors are present.
Document: position(s), plane, content, grade, CSA and volume if measured, and the decision.
Five patients.
Images and clips from my scan library. Commit to an answer before each reveal.
Indication · Acquisition · Interpretation · Medical decision
Clinical details are teaching vignettes built around real library images; images within a case are not necessarily from one patient.
Every image and clip in the deck.
Eight questions before we finish.
Tap an answer.
Scan the easy stomachs, so you can read the hard ones.
Thank you.
Any questions?
“If you love what you do, you will teach yourself. If you do not love what you do, others will teach you.” — Yukitaka Yamaguchi
Key references
- Perlas A, Van de Putte P, Van Houwe P, Chan VW. I-AIM framework for point-of-care gastric ultrasound. Br J Anaesth 2016;116:7–11.
- Perlas A, Chan VW, Lupu CM, et al. Ultrasound assessment of gastric content and volume. Anesthesiology 2009;111:82–9.
- Perlas A, Davis L, Khan M, et al. Gastric sonography in the fasted surgical patient. Anesth Analg 2011;113:93–7.
- Perlas A, Mitsakakis N, Liu L, et al. Validation of a mathematical model for ultrasound assessment of gastric volume. Anesth Analg 2013;116:357–63.
- Van de Putte P, Perlas A. Ultrasound assessment of gastric content and volume. Br J Anaesth 2014;113:12–22.
- Perlas A, Arzola C, Van de Putte P. Point-of-care gastric ultrasound and aspiration risk assessment. Can J Anaesth 2018;65:437–48.
- Xiao MZX, Englesakis M, Perlas A. Gastric content and perioperative pulmonary aspiration in patients with diabetes mellitus. Br J Anaesth 2021;127:224–35.
- Sen S, Potnuru PP, Hernandez N, et al. GLP-1 receptor agonist use and residual gastric content before anesthesia. JAMA Surg 2024;159:660–7.
- Kindel TL, Wang AY, Wadhwa A, et al. Multisociety clinical practice guidance for the safe use of GLP-1 receptor agonists in the perioperative period. 2024.
- gastricultrasound.org — full reference list
Simulations are purpose-built teaching models, not diagnostic images. Anatomy, echogenicity and grading thresholds are simplified.