During trauma resuscitation, two patients with blunt abdominal trauma are being evaluated. Patient A has a positive FAST (free fluid in Morrison's pouch and pelvis) but is haemodynamically stable after 2 litres of IV crystalloid. Patient B has a negative FAST but is haemodynamically unstable (SBP 85 mmHg, HR 128 bpm) despite 3 litres of crystalloid and ongoing transfusion. Which finding best discriminates between these two presentations and determines the need for immediate laparotomy?

Explanation

## Discriminating Feature: Haemodynamic Response and Ongoing Instability ### Rationale for Correct Answer **Key Point:** **Haemodynamic instability despite adequate resuscitation** is the single most important discriminator for immediate laparotomy, regardless of FAST findings. A negative FAST with shock mandates laparotomy; a positive FAST with stability permits observation. **High-Yield:** FAST is a **screening tool**, not a decision tool. The **clinical response to resuscitation** (vital signs, urine output, lactate clearance) determines operative need. ### Comparison: FAST-Positive Stable vs. FAST-Negative Unstable | Feature | FAST-Positive + Stable | FAST-Negative + Unstable | Discriminator | |---------|------------------------|--------------------------|---------------| | FAST result | Positive (free fluid) | Negative (no free fluid) | Not discriminatory | | SBP after resuscitation | > 90 mmHg | < 90 mmHg (refractory) | **Haemodynamic stability** | | HR | < 100 bpm | > 120 bpm (persistent) | **Ongoing shock** | | Management | SNOM, ICU monitoring | **Immediate laparotomy** | **Instability overrides FAST** | | Bleeding source | Intra-abdominal (visible) | Extra-abdominal OR occult intra-abdominal | Shock mandates exploration | ### Clinical Decision Algorithm ```mermaid flowchart TD A[Blunt abdominal trauma]:::outcome --> B{Haemodynamically unstable?}:::decision B -->|Yes| C[Resuscitate: 2 L crystalloid + blood]:::action C --> D{Response to resuscitation?}:::decision D -->|Stable| E[FAST + CT imaging]:::action D -->|Unstable/refractory| F[Immediate laparotomy]:::urgent B -->|No| G[FAST examination]:::action G --> H{FAST positive?}:::decision H -->|Yes| I[SNOM protocol]:::action H -->|No| J[Exclude other sources of shock]:::action F --> K[Explore abdomen, control bleeding]:::action ``` ### Why Haemodynamic Instability Overrides FAST **Clinical Pearl:** A FAST-negative patient in refractory shock has occult intra-abdominal bleeding (retroperitoneal, mesentery, or small bowel) that FAST cannot detect. Laparotomy is mandatory to control bleeding. **Warning:** Do NOT delay laparotomy waiting for CT or repeat FAST in an unstable patient. Shock refractory to 2 litres of crystalloid + blood products = **automatic indication for laparotomy**. 1. **FAST-negative + unstable** → Bleeding is present but not in dependent areas (Morrison's pouch, pelvis). Retroperitoneal or mesenteric bleeding is likely. 2. **FAST-positive + stable** → Bleeding is controlled by tamponade or is self-limited. Observation is safe. 3. **Haemodynamic response** is the true discriminator because it reflects the balance between bleeding rate and resuscitation efficacy. ### Resuscitation Thresholds - **Crystalloid:** 2 L as initial bolus - **Transfusion trigger:** If unstable after 2 L crystalloid, begin PRBC (1:1:1 ratio with FFP and platelets in massive transfusion protocol) - **Refractory shock:** Persistent SBP < 90 mmHg or HR > 120 bpm after 2 L crystalloid + 2 units PRBC = **laparotomy** [cite:ATLS 10e Ch 5, Harrison 21e Ch 297]