Ventricular Fibrillation
The most likely mechanism of syncope in this patient is ventricular fibrillation (VF), as evidenced by the fact that the AED delivered a shock—a device that only recommends shocks for shockable rhythms like VF or pulseless ventricular tachycardia. 1
Clinical Reasoning
The key diagnostic clue is that the AED delivered a shock, which is definitive evidence of a shockable rhythm at the scene. 1 AEDs are highly accurate rhythm analysis systems with >90% sensitivity for VF and 99-100% specificity for nonshockable rhythms, meaning they will not recommend a shock unless VF or rapid ventricular tachycardia is present. 1
Why This is Ventricular Fibrillation:
AEDs only shock VF or pulseless VT: The device analyzed the rhythm and determined it was shockable, which by definition means VF or rapid VT was present. 1
Blunt chest trauma mechanism: This patient experienced commotio cordis—a phenomenon where blunt chest impact during the vulnerable period of the cardiac cycle (just before the T wave peak) can trigger VF. 1 This typically occurs in young athletes struck in the chest by projectiles like baseballs.
Immediate collapse after chest impact: The sudden loss of consciousness immediately following chest trauma, requiring defibrillation, is pathognomonic for VF induced by mechanical chest wall impact. 1
Successful defibrillation with return to normal rhythm: The patient responded to a single shock and now has normal sinus rhythm, which is consistent with VF that was successfully terminated by defibrillation. 1
Why NOT the Other Options:
Asystole (Option A): AEDs do not deliver shocks for asystole—this is a nonshockable rhythm. 1 The device would have instructed rescuers to perform CPR only, without shock delivery.
Vagal reaction (Option B): Vagal syncope causes bradycardia or brief asystole, not a shockable rhythm. 2 An AED would not recommend a shock for a vagal episode.
Ventricular contusion (Option C): While chest trauma can cause myocardial contusion, this does not directly cause syncope requiring defibrillation. 3 Contusion might cause arrhythmias over time, but the immediate collapse with shockable rhythm indicates VF, not contusion alone.
Clinical Context: Commotio Cordis
This presentation is classic for commotio cordis, where:
- A projectile strikes the precordium during a narrow vulnerable window (10-30 milliseconds before the T wave peak). 1
- This mechanical impact triggers VF in a structurally normal heart. 1
- Immediate defibrillation is lifesaving, with survival rates approaching 90% when defibrillation occurs within the first minute. 1
- The patient is typically a young, healthy individual with no underlying cardiac disease. 1
Post-Resuscitation Management
Following successful defibrillation from VF:
- Monitor for recurrent arrhythmias: VF can recur after initial successful defibrillation. 1
- Evaluate for structural heart disease: Although commotio cordis occurs in structurally normal hearts, echocardiography should be performed to rule out underlying cardiac abnormalities. 2, 3
- Check cardiac biomarkers: Troponin and CK-MB should be measured, though they are typically normal or minimally elevated in commotio cordis. 1
- Consider ICD evaluation: While not typically indicated for isolated commotio cordis in structurally normal hearts, any recurrent VF or underlying structural disease would warrant ICD consideration. 1, 2
Key Clinical Pearl
The delivery of a shock by an AED is diagnostic evidence that VF or pulseless VT was present. 1 AEDs have been extensively validated and will not recommend shocks for nonshockable rhythms like asystole, bradycardia, or organized rhythms with pulses. 1 In this clinical scenario of chest trauma with immediate collapse requiring defibrillation, VF is the only plausible mechanism.
Answer: D. Ventricular fibrillation