A 62-year-old woman is brought to the emergency department after witnessed collapse at home. CPR is initiated immediately. The cardiac monitor shows pulseless electrical activity (PEA). The resuscitation team begins searching for reversible causes. Which is the most common reversible cause of PEA in the out-of-hospital setting?

## Most Common Reversible Cause of PEA **Key Point:** Hypovolemia (from acute hemorrhage or severe dehydration) is the most common reversible cause of PEA in the out-of-hospital setting, accounting for approximately 20–30% of PEA arrests. ### The "H's and T's" Mnemonic for PEA Causes **Mnemonic: H's and T's** - **H's:** Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypothermia, Hypoglycemia - **T's:** Tension pneumothorax, Tamponade, Thrombosis (PE/MI), Toxins, Thrombosis (coronary) ### Frequency Distribution of Reversible PEA Causes | Cause | Frequency | Reversibility | Prognosis | |-------|-----------|---------------|----------| | **Hypovolemia** | 20–30% | **High** | Better if treated early | | Hypoxia | 15–20% | High | Depends on cause | | Tension pneumothorax | 5–10% | High | Good if decompressed | | Pulmonary embolism | 3–5% | Low (thrombolysis/embolectomy) | Poor | | Cardiac tamponade | 2–5% | High | Good if drained | | Hypothermia | 1–2% | Very high | Excellent if rewarmed | ### Why Hypovolemia is Most Common 1. **Out-of-hospital setting:** Trauma with uncontrolled hemorrhage is a leading cause of cardiac arrest in the prehospital environment. 2. **Mechanism:** Severe blood loss reduces preload → decreased cardiac output → loss of perfusion pressure → PEA. 3. **Reversibility:** Aggressive fluid resuscitation (or hemorrhage control + transfusion) can restore perfusion and return of spontaneous circulation (ROSC). 4. **Recognition:** Clinical clues include pale/mottled skin, absent pulses, cold extremities, and history of trauma or acute illness with fluid loss. **High-Yield:** In trauma-related PEA, hypovolemia is the presumed cause until proven otherwise. The "Resuscitative Endovascular Balloon Occlusion of the Aorta" (REBOA) and resuscitative hysterotomy are now considered in non-compressible torso hemorrhage (NCTH) protocols. ### Clinical Pearl **Clinical Pearl:** PEA with a rate >60 bpm has a better prognosis than PEA with a rate <60 bpm (bradycardic PEA). Hypovolemic PEA often presents with bradycardia and poor perfusion, necessitating rapid fluid administration and hemorrhage control. ### Differential Approach ```mermaid flowchart TD A[PEA on Monitor]:::outcome --> B{Clinical Clues?}:::decision B -->|Pale, cold, trauma history| C[Hypovolemia]:::action B -->|Unilateral breath sounds, JVD| D[Tension pneumothorax]:::urgent B -->|JVD, muffled heart sounds| E[Cardiac tamponade]:::action B -->|Hypoxia on ABG, poor oxygenation| F[Hypoxia]:::action B -->|Sudden onset, risk factors| G[Pulmonary embolism]:::action C --> H[Rapid IV fluids + hemorrhage control]:::action D --> I[Needle decompression + chest tube]:::urgent E --> J[Pericardiocentesis]:::action ``` [cite:American Heart Association ACLS Guidelines 2020, Resuscitation Council UK Guidelines]