GI Bleed Imaging When Endoscopy Cannot Be Done
For hemodynamically unstable patients (shock index >1), CT angiography (CTA) should be performed immediately as the first diagnostic test, followed by catheter angiography with embolization within 60 minutes if positive. 1, 2
Hemodynamic Status Determines Imaging Strategy
Unstable Patients (Shock Index >1)
CT angiography is the mandatory first-line imaging modality because it provides the fastest, least invasive means to localize bleeding without requiring bowel preparation, with sensitivity of 79-95% for detecting bleeding rates as low as 0.3 mL/min. 1, 2, 3
Perform arterial phase imaging only—do not use delayed or portal-venous phase, and never administer positive oral contrast before CTA as it masks extravasation. 1
Following positive CTA, proceed to catheter angiography with embolization within 60 minutes in centers with 24/7 interventional radiology to maximize technical success (40-100% immediate hemostasis rate). 1, 2, 3
If CTA shows no lower GI source, perform upper endoscopy immediately because 10-15% of severe hematochezia originates from the upper GI tract, especially with hemodynamic instability. 1, 2, 3
Colonoscopy is explicitly contraindicated when shock index >1 because it requires 4-6 liters of polyethylene glycol over 3-4 hours, sedation that worsens shock, and does not address massive bleeding. 1, 2, 3
Stable Patients (Shock Index ≤1)
For suspected small bowel bleeding after negative upper and lower endoscopy, CT enterography (CTE) should be performed as the first-line imaging test. 1
Use multiphase CTE technique (at least arterial and enteric/portal venous phases) in patients >40 years where vascular lesions are common causes. 1
Administer neutral enteric contrast in divided doses beginning 1 hour before CTE to optimize small bowel visualization. 1
CTE should be performed instead of CTA in hemodynamically stable patients with ongoing suspected small bowel bleeding after negative colonoscopy and esophagogastroduodenoscopy. 1
CTE is the first-line study when small bowel neoplasm is suspected or when patients are at increased risk for video capsule retention. 1
Alternative Imaging Modalities
Nuclear Medicine RBC Scan
If CTA is negative but bleeding persists, consider Tc-99m-labeled RBC scintigraphy which can detect bleeding rates as low as 0.05-0.1 mL/min (versus 0.3 mL/min for CTA). 1, 2, 3
SPECT/CT imaging improves localization accuracy to 75-100% compared to planar imaging alone. 2
Nuclear scintigraphy offers 60-93% sensitivity for intermittent or slow bleeding when CTA and colonoscopy are negative. 2
RBC scan is most useful for intermittent bleeding when other methods have failed to detect the source. 2, 4
Meckel Scan
A Meckel scan can be considered for unexplained intermittent GI bleeding in children and adolescents after negative endoscopic evaluation (including capsule endoscopy if available) and cross-sectional evaluation of the small bowel. 1
Critical Pitfalls to Avoid
Never delay CTA while attempting endoscopy in unstable patients—this delays definitive localization and potential life-saving embolization. 2, 3
Do not assume bright red rectal bleeding is always lower GI—up to 15% may be upper GI sources, particularly with hemodynamic instability, peptic ulcer disease, or portal hypertension. 2, 3
Avoid blind surgical resection without prior imaging localization—this carries rebleeding rates up to 33% and mortality of 33-57%, versus ~10% when bleeding is first localized radiologically. 2, 3
Never perform CTA if extensive positive oral contrast is already present in the bowel—it will render the examination nondiagnostic by masking extravasation. 1
Mortality Context
Overall in-hospital mortality for lower GI bleeding is 3.4%, rising to 20% in patients requiring ≥4 units of red blood cells, with mortality more related to comorbidity than exsanguination. 2, 3
Emergency total colectomy without localization carries 27-33% mortality versus ~10% operative mortality when bleeding is first localized. 2, 3