Ventricular Tachycardia
The most likely diagnosis is C) Ventricular tachycardia. In a 78-year-old man presenting with syncope 24 hours after acute myocardial infarction with a regular wide-QRS tachycardia at 160 bpm, ventricular tachycardia is the presumptive diagnosis until proven otherwise.
Clinical Reasoning
In the post-MI setting, any wide-QRS tachycardia should be assumed to be ventricular tachycardia, especially when accompanied by hemodynamic compromise such as loss of consciousness. 1 The guidelines explicitly state that the vast majority of post-MI VT and VF occur within the first 48 hours of myocardial infarction, placing this patient squarely in the highest-risk window 1.
Why Not the Other Options?
Sinus tachycardia (A): Would show narrow QRS complexes unless pre-existing bundle branch block exists. A rate of 160 bpm with wide QRS and syncope in the post-MI period makes this extremely unlikely.
Atrial fibrillation with bundle branch block (B): The ECG shows a regular rhythm. Atrial fibrillation is characteristically irregular, making this diagnosis incompatible with the clinical presentation.
Atrial tachycardia (D): Would typically produce narrow QRS complexes and is uncommon in the acute post-MI period. The regular wide-QRS pattern with syncope points away from this diagnosis.
AVNRT (E): Also produces narrow QRS complexes in the absence of aberrant conduction and is not associated with acute MI. The clinical context makes this diagnosis highly improbable.
Key Diagnostic Features Supporting VT
The ACC/AHA guidelines emphasize that monomorphic VT with rates greater than 150 bpm causing hemodynamic compromise (evidenced by syncope) requires immediate intervention 1. The regular rhythm at 160 bpm fits the classic presentation of sustained monomorphic VT in the post-infarction period 1.
Critical pitfall: While the guidelines note that monomorphic VT at rates less than 170 bpm are unusual as a post-MI arrhythmia and may suggest a more chronic substrate 1, a rate of 160 bpm with syncope occurring within 48 hours of MI still represents acute VT requiring urgent treatment.
Immediate Management Approach
Given the loss of consciousness, this represents hemodynamically compromising VT requiring urgent electrical cardioversion 1:
- Immediate synchronized cardioversion with 100 J for monomorphic VT with rates greater than 150 bpm 1
- If the patient were hemodynamically stable, brief trials of lidocaine or procainamide could be considered first
- However, syncope indicates hemodynamic instability, mandating immediate electrical conversion
Post-Cardioversion Management
After successful cardioversion 1:
- Correct electrolyte abnormalities (potassium, magnesium)
- Administer beta-blockers to reduce sympathetic tone and prevent ischemia
- Consider antiarrhythmic infusion (lidocaine 2-4 mg/min) for 6-24 hours only, then reassess
- Evaluate for ongoing ischemia or other reversible causes
The answer is C) Ventricular tachycardia - this diagnosis demands immediate recognition and treatment to prevent deterioration to ventricular fibrillation and cardiac arrest.