What is the most appropriate initial diagnostic step for a patient with massive lower gastrointestinal (GI) bleeding and no evidence of upper GI bleeding, as indicated by a negative nasogastric tube (NGT) aspirate?

Management of Massive Lower GI Bleeding in a Patient with Negative NGT

For a patient with massive lower GI bleeding and no blood in the nasogastric tube (NGT), CT angiography (CTA) is the most appropriate initial diagnostic step.

Rationale for CT Angiography

CTA offers several advantages in this clinical scenario:

• High sensitivity (80-90%) for detecting active bleeding at rates as low as 0.3-1.0 mL/min 1
• Provides excellent anatomical localization of the bleeding source
• Can be performed rapidly in an unstable patient
• Can guide subsequent interventions (endoscopic, angiographic, or surgical)

While the negative NGT suggests a lower GI source, it's important to note that 10-15% of severe hematochezia cases can still have upper GI sources 1. However, in this case of massive lower GI bleeding with a negative NGT, CTA is the preferred first-line diagnostic approach.

Diagnostic Algorithm for Massive Lower GI Bleeding

1. Initial assessment: Confirm hemodynamic status and severity of bleeding

   • Negative NGT suggests lower GI source but doesn't completely rule out upper GI source

2. First diagnostic step: CT angiography

   • Provides rapid assessment with high sensitivity for active bleeding
   • Can detect bleeding rates as low as 0.3 mL/min 2, 1
   • Offers excellent anatomical localization to guide subsequent interventions

3. If CTA is positive:

   • Proceed to targeted intervention based on location:
     • Angiographic embolization for accessible arterial bleeding
     • Urgent colonoscopy for colonic sources
     • Surgery if bleeding is massive and uncontrollable by other means

4. If CTA is negative or inconclusive:

   • Perform upper endoscopy to definitively rule out upper GI source
   • Consider colonoscopy after adequate bowel preparation
   • Consider nuclear medicine studies (tagged RBC scan) for intermittent or slow bleeding

Why Not Upper Endoscopy First?

While upper endoscopy is often the first diagnostic procedure in GI bleeding, in this specific case of massive lower GI bleeding with a negative NGT, CTA offers several advantages:

• More rapid assessment in an unstable patient with massive bleeding
• Better visualization of the entire GI tract, including small bowel
• Can detect active bleeding that may be missed by endoscopy
• Avoids the risks of sedation in an unstable patient

The British Society of Gastroenterology guidelines specifically recommend CTA as the first-line investigation for patients with suspected acute lower GI bleeding 2. If CTA doesn't identify the bleeding source, then upper endoscopy should be performed to definitively exclude an upper GI source.

Common Pitfalls to Avoid

• Delaying diagnosis: Massive lower GI bleeding requires rapid diagnosis and intervention
• Assuming a lower GI source: Despite negative NGT, up to 15% of severe hematochezia can be from upper GI sources
• Proceeding directly to colonoscopy: In massive bleeding, visualization is often poor without prior bowel preparation
• Blind surgical exploration: Without localization, surgery has higher morbidity and may not identify the source

In summary, for a patient with massive lower GI bleeding and a negative NGT aspirate, CT angiography is the most appropriate initial diagnostic step to rapidly localize the bleeding source and guide subsequent management.