How to measure gastric volume or gastrointestinal motility?

Measuring Gastric Volume and Gastrointestinal Motility

Gastric scintigraphy using a standardized low-fat egg white meal labeled with 99mTc sulfur colloid remains the gold standard for measuring gastric emptying, while ultrasound provides the most practical non-invasive method for assessing gastric volume in clinical practice. 1

Measuring Gastric Volume

Point-of-Care Ultrasound (Preferred for Clinical Practice)

• Measure the antral cross-sectional area (CSA) in the lateral decubitus position and calculate gastric fluid volume using the validated formula: Volume (ml) = 27.0 + 14.6 × CSA - 1.28 × age 1
• Ultrasound correlates well with isotope scintigraphy and allows visualization of solid matter, which indicates a high-risk "full stomach" regardless of volume 1
• Two-dimensional ultrasound is validated for measuring emptying of liquids and semi-solids, while three-dimensional ultrasound provides more comprehensive imaging of the total stomach 1
• This method is radiation-free, user-friendly, can be frequently repeated for longitudinal studies, and does not influence normal GI physiology 2

Alternative Volume Measurement Methods

• Magnetic resonance imaging (MRI) offers excellent reproducibility for measuring gastric emptying and motility but is limited to research purposes due to cost and accessibility 1
• Gastric residual volume (GRV) measurement via nasogastric aspiration can assess gastrointestinal dysfunction during enteral feeding; consider delaying feeding when GRV exceeds 500 mL/6 hours 1

Measuring Gastric Emptying

Scintigraphy (Gold Standard)

• Use a standardized low-fat egg white meal labeled with 99mTc sulfur colloid, consumed with jam and toast as a sandwich with water 1
• This consensus protocol improves standardization between centers, which was previously a major limitation 1
• Scintigraphy can also evaluate intragastric distribution of a meal, which is frequently abnormal in diabetic patients 1
• Limitations include modest radiation exposure, relative expense, and need for specialist centers 1

Breath Testing (Practical Alternative)

• 13C-acetate or 13C-octanoic acid breath tests are safe, inexpensive, and correlate well with scintigraphy results 1
• These non-radioactive tests provide a practical alternative when scintigraphy is unavailable 1

Important Pre-Test Conditions

• Withdraw medications that influence gastric emptying for 48-72 hours prior to testing 1
• Avoid smoking on the test day 1
• Monitor blood glucose and maintain levels between 4-10 mmol/L (72-180 mg/dL) during testing, as hyperglycemia (16-20 mmol/L) substantially slows gastric emptying while hypoglycemia accelerates it 1
• Exclude other causes of gastroparesis before attributing symptoms to diabetic gastropathy 1

Measuring Gastrointestinal Motility

Small Bowel Manometry

• 24-hour ambulatory jejunal manometry using catheters with built-in miniature strain gauge transducers is the most established technique for diagnosing pseudo-obstruction and evaluating small bowel motor patterns 1
• Normal stereotypic migrating motor complex (MMC) activity is evident during nocturnal sleep; patients with pseudo-obstruction show distortion of the fasting MMC pattern 1
• Wireless motility capsule (WMC) allows ambulatory assessment of intraluminal pH, temperature, and pressure throughout the GI tract, measuring transit times and pressure profiles in the antro-duodenum 1
• The WMC is minimally invasive and allows standardized comparisons across centers, though capsule retention is a concern in patients with severe dysmotility 1

Ultrasound for Motility Assessment

• Intestinal ultrasound (IUS) with color Doppler can assess bowel wall thickness, vascularization, and motility using cine loops captured after 4-6 hours of fasting 1, 2
• Semi-quantitative grading of intra- and extramural blood flow correlates with disease activity (moderate-to-excellent reliability, κ = 0.60-0.93) 1
• Reduced motility detected with ultrasound correlates well with disease activity on MRE 1

Specialized Research Techniques

• Surface electrogastrography and pressure measurements assessing motor function should be considered research tools rather than clinical diagnostic methods 1
• Anorectal manometry, balloon expulsion test, and defecography are well-developed for clinical use in evaluating pelvic floor dysfunction and difficult bowel evacuation 1

Clinical Pitfalls and Caveats

Barostat Limitations

• The intragastric barostat, previously considered the gold standard for measuring gastric accommodation, actually induces non-physiological accommodation responses 3
• The barostat bag significantly increases both fasting (350 vs. 37 mL) and postprandial (852 vs. 361 mL) intragastric volumes compared to measurements without the bag 3
• While the barostat does not interfere with gastric emptying or contraction frequency, the accommodation response observed with the barostat is not present without it 3

Patient-Specific Considerations

• In patients with suspected POTS (postural orthostatic tachycardia syndrome), consider earlier testing of gastric motor functions because underlying autonomic dysfunction may predispose to gastroparesis 1
• Failure to demonstrate delayed gastric emptying does not exclude diabetic gastropathy as the cause of symptoms 1
• Evaluation of solid emptying is more sensitive than low-nutrient liquid or semi-solid meals, though some patients only exhibit delay with liquid meals 1

Prokinetic Testing

• When gastric feeding intolerance occurs, use intravenous erythromycin (100-250 mg three times daily) as first-line prokinetic therapy for 24-48 hours 1
• Alternatively, intravenous metoclopramide (10 mg two to three times daily) or combination therapy can be used 1, 4
• Effectiveness of erythromycin decreases to one-third after 72 hours and should be discontinued after three days 1