A 52-year-old woman with chronic obstructive pulmonary disease presents to the emergency department with sudden onset severe epigastric pain radiating to the back. She is tachycardic (heart rate 118 bpm), hypotensive (blood pressure 95/55 mmHg), and appears in severe distress. On abdominal examination, there is rigid epigastric tenderness. Plain abdominal radiograph (upright view) shows a thin crescent of lucent gas beneath the right hemidiaphragm. What is the most likely diagnosis?

Explanation

## Diagnosis: Perforated Peptic Ulcer **Key Point:** Free intraperitoneal air (pneumoperitoneum) beneath the diaphragm on upright or left lateral decubitus radiograph is pathognomonic for a perforated hollow viscus, most commonly a peptic ulcer. ## Radiographic Signs of Pneumoperitoneum | Finding | Appearance | Sensitivity | |---------|-----------|-------------| | **Subdiaphragmatic air (upright)** | Lucent crescent under diaphragm on erect CXR or abdominal film | 70–80% | | **Rigler's sign** | Both sides of bowel wall visible (air inside and outside lumen) | Highly specific | | **Pneumatosis intestinalis** | Air in bowel wall (linear or cystic lucencies) | Suggests ischemia or perforation | | **Portal venous gas** | Branching lucencies in liver | Indicates severe ischemia | | **Left lateral decubitus view** | Air rises to left hemidiaphragm if supine film is negative | Increases sensitivity | ## Clinical Presentation of Perforated Peptic Ulcer **High-Yield:** Classic triad: 1. **Sudden severe epigastric pain** ("like a knife") — due to chemical peritonitis from gastric/duodenal contents 2. **Rigid abdomen** — acute peritoneal irritation 3. **Hypotension and tachycardia** — third-space fluid loss and sepsis ## Pathophysiology ```mermaid flowchart TD A[Peptic ulcer erosion]:::outcome --> B[Perforation of anterior wall]:::action B --> C[Gastric/duodenal contents leak]:::action C --> D[Free intraperitoneal air]:::outcome D --> E[Chemical peritonitis]:::urgent E --> F[Bacterial peritonitis within 6-12 hrs]:::urgent F --> G[Septic shock if untreated]:::urgent ``` **Clinical Pearl:** The patient's COPD is a risk factor for peptic ulcer disease (increased gastric acid, use of NSAIDs or steroids). The sudden onset of severe pain with hemodynamic instability is classic for perforation, not simple peptic ulcer disease. **Mnemonic: FREE AIR** — **F**ree intraperitoneal air, **R**igler's sign, **E**pigastric pain, **A**cute peritonitis, **I**nstability (hemodynamic), **R**igid abdomen.