Discussion
The differential diagnosis for this case includes atrial flutter with 2:1 AV block, atrio-ventricular nodal reentry tachycardia (AVNRT), atrio-ventricular reentry tachycardia (AVRT) and atrial tachycardia. Making the correct diagnosis is important because it implicates patient management. We identify the disease process and showcase the rationale used to make the diagnosis.
The diagnosis of typical atrial flutter with 2:1 AV block was initially considered. Typical atrial flutter is caused by a macro-reentry circuit bound by the Cavo-tricuspid isthmus inferiorly and the right atrial roof or the supero-posterior right atrium.[4] This produces an atrial rate of 240-350 bpm and is usually accompanied by a 2:1 AV block with a ventricular response of 120-150 bpm.[4] In the case presented, the patient had a regular supra-ventricular tachycardia with a ventricular rate of 150 bpm during admission. This ventricular rate, so characteristic of typical atrial flutter with a 2:1 AV block, made us feel that this diagnosis was most likely and that the atrial flutter waves were simply masked by the rapid ventricular repolarization/depolarization.
With that said, it was difficult to rule out atypical AVNRT and AVRT from the differential. Atypical AVNRT is caused by a re-entry circuit localized to the AV node and produces a narrow complex tachycardia with long RP P-waves.[5] Orthodromic AVRT is a macro-reentry circuit passing antegrade through the AV node and His-Purkinje system and retrograde through an accessory pathway, and this also produces a narrow complex tachycardia with retrograde long RP P-waves.[3]. This made exclusion of AVNRT and AVRT challenging.
Atrial tachycardia was also considered. Atrial tachycardia is caused by one of several etiologies, including enhanced automaticity, triggered potential or micro-reentry circuit.[1] The presence of discernable P-waves, a long R-P interval and an isoelectric baseline between atrial deflections differentiates atrial tachycardia from other causes of supraventricular tachycardia in most cases.[3]
Ultimately, observation of the cool down phenomenon elegantly confirms the diagnosis of focal atrial tachycardia due to enhanced automaticity. Cool down and warm up describe a more gradual deceleration/acceleration with changes in heart rate, typically lasting a few seconds. Cool down and warm up are only seen in cases where enhanced automaticity is the underlying cause of the arrhythmia. [1,6] The EKG during admission shows the cool down phenomenon as pointed out in Figure 2. Observing this essentially ruled out arrythmias caused by reentry circuits, including atrial flutter with 2:1 AV block, atypical AVNRT and orthodromic AVRT, and allowed us to make the diagnosis of focal atrial tachycardia.