Gastric Ultrasound
Gastric point-of-care ultrasound (Gastric POCUS or Gastric USG) is a bedside ultrasound technique used to determine:
- Whether the stomach is empty or contains contents
- Nature of gastric contents (empty, clear fluid, thick fluid, solid)
- Estimated gastric volume
- Aspiration risk
- Suitability for anesthesia, procedural sedation, extubation, or enteral feeding
The technique was pioneered by Perlas et al., and most current practice is based on the Canadian Gastric Ultrasound Group protocols.
Why perform Gastric POCUS?
- Many critically ill patients have delayed gastric emptying.
- Traditional fasting guidelines often fail in ICU patients.
A patient fasting for 10 hours may still have a full stomach.
Conversely,a patient fed 3 hours ago through NG tube may actually have an empty stomach.
- Therefore gastric ultrasound directly evaluates gastric contents rather than assuming gastric emptying.
Clinical Applications
1. Aspiration Risk Assessment(Most common indication)
2. Before Emergency Intubation
3. Before Procedural Sedation
4. ICU Determine Feed intolerance
5. Enteral Nutrition
6. Extubation
Anatomy
The target structure is the gastric antrum.
Why?
Because:
- most dependent portion
- changes predictably with filling
- easy to visualize
- correlates with gastric volume
Technique
Probe Selection Adults-Curvilinear probe,2–5 MHz
Machine Settings
- Depth 10–15 cm
- Gain Moderate
- Focus At antrum level
Patient Position
Two positions are mandatory.
1. Supine-First examination
2. Right Lateral Decubitus (RLD)
Gold standard
This is the most important position.
Why? Fluid moves into the antrum under gravity. Thereforesmall fluid volumes become visible.Sensitivity increases dramatically.
Probe Position
Epigastrium,Just below xiphoid,Slightly right of midline and Marker toward patient’s head (Longitudinal orientation)
Important Landmarks
The antrum lies between Anteriorly Left lobe of liver and Posteriorly Pancreas below it Aorta
Identifying the Antrum
Look for Thick muscular wall,Oval shape Between liver and pancreas,Peristalsis,Changes with respiration.
Sonographic Appearance
|
Perlas Gastric Ultrasound Grade |
Clinical Significance |
|
Grade 0 (Empty Stomach) |
Appearance: Empty antrum with a bull’s-eye/target sign, flat and collapsed, minimal or no visible lumen, no fluid. Interpretation: Empty stomach. Aspiration Risk: Lowest risk. |
|
Grade 1 (Small Gastric Fluid Volume) |
Appearance: Small amount of clear fluid visible only in the Right Lateral Decubitus (RLD) position and not visible in the supine position. Interpretation: Usually represents normal physiological gastric secretions (low gastric volume). Aspiration Risk: Low risk. |
|
Grade 2 (High Gastric Fluid Volume) |
Appearance: Fluid-filled gastric antrum visible in both the supine and Right Lateral Decubitus (RLD) positions. Interpretation: Indicates a high gastric fluid volume (often clinically significant, especially when estimated gastric volume exceeds ~1.5 mL/kg). Aspiration Risk: High risk. |
Qualitative Assessment
There are four major patterns.
|
Gastric Ultrasound Finding |
Appearance |
|
Empty Stomach |
Appearance: Target sign (bull’s-eye sign), flat/collapsed antrum, thick anterior and posterior walls apposed (“kissing walls”), no visible fluid. Interpretation: Empty stomach with lowest aspiration risk. |
|
Clear Fluid |
Appearance: Black (anechoic), round/distended antrum, compressible, fluid moves with respiration or gentle probe pressure, often shows posterior acoustic enhancement. Interpretation: Clear gastric fluid; aspiration risk depends on volume (small physiological volume vs. large volume). |
|
Thick Fluid |
Examples: Milk, enteral tube feed, blood, pus, protein-rich solutions. Appearance: Gray, heterogeneous, hypoechoic but not completely anechoic (not completely black), with internal echoes. Interpretation:More viscous gastric contents with a higher aspiration risk than clear fluids. |
|
Solid Contents |
Examples: Food particles (rice, meat, vegetables). Appearance: Classic “frosted glass” appearance with heterogeneous echogenic material, multiple bright internal echoes, no posterior acoustic enhancement. Interpretation: Solid stomach contents indicate non-fasted stomach and carry a very high aspiration risk. |
Gastric Volume Estimation
The best validated model in adults is the Perlas equation, applicable in the right lateral decubitus position:
Predicted gastric volume (mL) = 27 + 14.6 × CSA (cm²) − 1.28 × Age (years)
This equation is validated primarily for clear fluids in non-pregnant adults.
Cross-Sectional Area (CSA)
- CSA is the basis for gastric volume estimation.
- Measure at End expiration Between peristaltic contractions
- Measure AP diameter and CC diameter
CSA = AP × CC × 0.785
Where:
- AP = anteroposterior diameter
- CC = craniocaudal diameter
Gastric Residual Volume (GRV)
Traditional practice:
- Aspirating gastric residual volume through a nasogastric tube is inaccurate and often discouraged as a routine strategy in modern ICU nutrition guidelines.
Gastric ultrasound:
- Non-invasive
- Repeatable
- Can estimate gastric volume and trends
- May complement (but not universally replace) clinical assessment and feeding protocols
Gastric POCUS vs Gastric Residual Aspiration
|
Gastric Ultrasound |
NG Aspiration |
|
Non-invasive |
Invasive |
|
Estimates total gastric volume |
Underestimates if tube position is suboptimal |
|
Repeatable |
Variable |
|
Dynamic assessment |
Single measurement |
|
No interruption of feeding |
Requires aspiration |
Common Pitfalls
- Mistaking the transverse colon for the antrum
- Measuring during active peristalsis
- Measuring in the supine position only
- Not using the right lateral decubitus position
- Including the gastric wall in diameter measurements
- Applying volume equations to solid gastric contents (they are validated mainly for clear fluids)
Guideline Recommendations
Difficult Airway Society (DAS)
- Gastric ultrasound can aid aspiration risk assessment when fasting status is uncertain.
European Society of Anaesthesiology and Intensive Care (ESAIC)
- Supports gastric ultrasound as an adjunct in selected high-risk patients rather than routine use in every patient.
American Society of Anesthesiologists (ASA)
- Fasting guidelines remain the standard of care.
- Gastric ultrasound may help individualize management when fasting history is uncertain or gastric emptying is suspected to be impaired.