Conclusion
Differentiating causes of narrow complex tachycardia can be challenging. Evaluation usually involves two primary components: assessment of the patient for symptoms and signs of hemodynamic stability (or instability), and assessment of the patient’s ECG for clues to the type of tachycardia present. Observation of the warmup or cool down phenomenon on ECG can help clinicians correctly identify the cause of narrow complex tachycardia when the arrhythmia is difficult to define.