What is the most appropriate investigation for a 75-year-old female with a history of atrial fibrillation (A.fib) and recent unintentional weight loss, presenting to the emergency room with severe abdominal pain that is out of proportion, a soft but tender abdomen, and decreased bowel sounds?

Abdominal CT Angiography is the Most Appropriate Investigation

In a 75-year-old female with atrial fibrillation presenting with severe abdominal pain out of proportion to physical examination findings, abdominal CT angiography (CTA) with triple-phase imaging is the diagnostic test of choice to evaluate for acute mesenteric ischemia. 1, 2

Clinical Reasoning

This patient's presentation is pathognomonic for acute mesenteric ischemia (AMI):

• Pain out of proportion to examination findings is the hallmark clinical feature of AMI 1, 2, 3
• Atrial fibrillation is present in nearly 50% of patients with embolic AMI and represents the most critical risk factor for mesenteric arterial embolism 1
• Unintentional weight loss may suggest chronic mesenteric ischemia progressing to acute thrombotic occlusion 1, 3
• Soft abdomen with decreased bowel sounds indicates absence of peritoneal signs, suggesting potentially viable bowel with a window for revascularization 1, 3

Why CT Angiography is Superior

Triple-phase CTA (non-contrast, arterial, and portal venous phases) provides comprehensive evaluation that simultaneously:

• Identifies arterial occlusion (embolus vs. thrombosis) with 94% sensitivity and 95% specificity 1, 3
• Evaluates bowel wall changes and viability 1, 2
• Detects complications such as pneumatosis, portal venous gas, or perforation 1
• Excludes alternative diagnoses 1
• Guides treatment planning for endovascular or surgical intervention 1, 3

Why Other Options Are Inadequate

Lactate levels (Option A) are not appropriate as the initial investigation because:

• Elevated lactate indicates advanced disease with bowel gangrene already present, not early ischemia 3
• Normal lactate does not exclude AMI 1
• Laboratory parameters lack sufficient accuracy to identify or exclude ischemic bowel 1

Plain X-ray (Option B) is strongly not recommended because:

• Plain radiography has limited diagnostic value and only becomes positive when bowel infarction has already developed 1
• Normal findings do not exclude mesenteric ischemia 1, 3
• The American College of Radiology gives plain X-ray a rating of "usually not appropriate" for evaluating intestinal ischemia 1

Abdominal ultrasound (Option C) is inappropriate because:

• Ultrasound cannot adequately evaluate mesenteric arterial occlusion or bowel ischemia 1
• Results vary considerably with operator expertise and bowel gas 1
• It is not mentioned in any guideline as appropriate for suspected AMI 1

Critical Time-Sensitive Considerations

Time to diagnosis is the most important predictor of outcome, with mortality approaching 50% overall but improving dramatically with intervention within 5-10 hours of symptom onset 3. In patients aged 75 years or older, AMI is more prevalent than appendicitis as a cause of acute abdomen 1.

Do not delay CTA for elevated creatinine—the risk of missing mesenteric ischemia far outweighs contrast nephropathy concerns 3. Patients with atrial fibrillation presenting with acute abdominal pain have a 37% likelihood of CT findings related to their arrhythmia, including end-organ ischemia or infarction in 27% 4.

Immediate Next Steps After Imaging

Once CTA confirms the diagnosis:

• Immediate surgical consultation is required regardless of imaging findings 2
• Systemic anticoagulation should be initiated as adjunct therapy (rated 8/9 appropriateness) 1, 3
• Endovascular intervention (angiography with aspiration embolectomy or transcatheter thrombolysis) is preferred as first-line treatment for AMI without peritoneal signs 1, 2, 3