Ventricular Fibrillation
The most likely initial rhythm on ECG in this patient is ventricular fibrillation (VF). This clinical scenario describes commotio cordis—sudden cardiac arrest from blunt chest trauma during a vulnerable period of the cardiac cycle—which characteristically presents with VF as the initial rhythm 1.
Clinical Context and Mechanism
This 15-year-old experienced immediate collapse after precordial impact, presenting pulseless and apneic. This presentation is pathognomonic for commotio cordis, where mechanical impact during the vulnerable repolarization phase (approximately 15-30 milliseconds before the T wave peak) triggers VF 1.
Key distinguishing features of this case:
- Immediate collapse following chest impact indicates a primary electrical event rather than structural cardiac injury
- Young, healthy athlete without underlying cardiac disease—the typical demographic for commotio cordis
- Witnessed arrest with immediate pulselessness strongly suggests a shockable rhythm (VF/VT) rather than asystole or PEA
Why VF is Most Likely
VF is the predominant initial rhythm in witnessed sudden cardiac arrest from commotio cordis 1. The guidelines consistently emphasize that VF/pulseless VT are the primary rhythms requiring immediate defibrillation, and modern resuscitation protocols prioritize rapid defibrillation for witnessed arrests 2.
The evidence supporting VF as the initial rhythm includes:
- Witnessed cardiac arrest in previously healthy individuals typically presents with VF, particularly when collapse is immediate 2
- Survival rates are highest when VF is treated within 3-5 minutes of collapse, which is why rapid defibrillation is emphasized 2
- Commotio cordis specifically has been documented to cause VF as the mechanism of sudden death 1
Why Other Rhythms Are Less Likely
Third-degree AV block and atrial flutter with 2:1 block would not cause immediate pulseless collapse. These bradyarrhythmias typically present with gradual hemodynamic deterioration, not instantaneous cardiac arrest 2, 3.
Ventricular tachycardia (VT) is possible but less likely than VF in this scenario. While pulseless VT is grouped with VF in resuscitation algorithms as a shockable rhythm 2, the immediate collapse without any preceding symptoms suggests the more chaotic VF rather than organized VT.
Pulseless electrical activity (PEA) would be atypical for this mechanism. PEA typically occurs in cardiac arrest from:
- Severe preexisting left ventricular dysfunction with acute ischemia (occurring within 2 minutes of coronary occlusion) 4
- Reversible causes like tension pneumothorax, cardiac tamponade, or massive pulmonary embolism 3
- Progressive deterioration from untreated VF 2
None of these mechanisms apply to a healthy adolescent with blunt chest trauma.
Immediate Management Implications
The recognition that VF is the likely rhythm mandates immediate defibrillation once a monitor/defibrillator is available 2. Current guidelines recommend:
- Single shock followed immediately by CPR rather than stacked shocks, given modern biphasic defibrillators have >90% first-shock efficacy 2
- Minimize interruptions in chest compressions—rhythm checks should occur only after approximately 2 minutes (5 cycles) of CPR 2
- Energy dose: 150-200 J for biphasic truncated exponential waveform or 120 J for rectilinear biphasic waveform 2
Prognostic Considerations
Aggressive resuscitation should continue in this patient despite the traumatic mechanism 1. Unlike most blunt trauma cardiac arrests (which have dismal outcomes), commotio cordis represents a primary electrical event in an otherwise healthy heart. The case report literature documents successful resuscitation with full neurological recovery when CPR and defibrillation are provided promptly 1.
The key prognostic factors favoring survival include: