A 28-year-old woman is brought to the trauma bay following a stab wound to the right lower abdomen. She is haemodynamically unstable (BP 88/56 mmHg, HR 128/min) despite two units of O-negative blood. On examination, she has severe right-sided abdominal tenderness and guarding. Which is the most appropriate next investigation before proceeding to the operating theatre?

Explanation

## Investigation in Unstable Penetrating Abdominal Trauma **Key Point:** In a haemodynamically unstable patient with penetrating abdominal trauma and peritoneal signs, FAST is the rapid screening investigation of choice. A positive FAST (free fluid in peritoneal cavity) combined with instability and peritoneal signs mandates immediate exploratory laparotomy without further delay. ### Decision Algorithm for Penetrating Abdominal Trauma ```mermaid flowchart TD A[Penetrating abdominal trauma]:::outcome --> B{Haemodynamically stable?}:::decision B -->|Yes| C[FAST + CT abdomen/pelvis]:::action B -->|No| D[FAST only]:::action D --> E{Free fluid on FAST?}:::decision E -->|Yes| F[Exploratory laparotomy]:::urgent E -->|No| G[Selective non-operative management]:::action C --> H{Peritoneal signs or FAST+?}:::decision H -->|Yes| I[Exploratory laparotomy]:::urgent H -->|No| J[Observe, serial exams]:::action ``` ### Why FAST is Correct in This Scenario 1. **Haemodynamic instability** — BP 88/56 mmHg and HR 128/min indicate shock; time is critical. 2. **Peritoneal signs present** — Guarding indicates peritoneal irritation or bleeding. 3. **Penetrating wound** — High risk of hollow viscus or vascular injury. 4. **FAST speed** — Takes <2 minutes; does not delay operative intervention. 5. **Positive FAST + instability + peritoneal signs = mandatory laparotomy** — No further imaging needed. **Clinical Pearl:** In unstable penetrating trauma, FAST is not done to "rule out" injury—it is done to confirm free fluid and expedite surgical decision-making. A negative FAST does NOT exclude injury in penetrating trauma; clinical judgment and peritoneal signs drive the decision. [cite:ATLS 10th Edition] **High-Yield:** **Unstable + penetrating + peritoneal signs = FAST → Laparotomy**. Do not delay for CT or DPL. **Mnemonic:** **FAST-STAB** — Free fluid + Shock + Tenderness + Abdominal wound = Bypass imaging, go to theatre. ### Why Other Investigations Are Inappropriate Here | Investigation | Why Not in This Case | | --- | --- | | **CT with contrast** | Requires transport to CT suite and 10–15 min scan time; patient is unstable and would deteriorate. Contraindicated in shock. | | **DPL** | Invasive; takes time; FAST is faster and non-invasive. | | **Diagnostic laparoscopy** | Not a screening tool in trauma; requires general anaesthesia in unstable patient; does not assess retroperitoneum. |