DIAGNOSIS: Sinus rhythm with occasional premature atrial beats [PACs], precipitating paroxysmal supraventricular tachycardia [PSVT]. Left axis deviation of minus 30 degrees. Non-specific ST-T changes.

EXPLANATION: Although relatively straightforward, this was such a classic and beautiful tracing, that I couldn't help but include it. The traditional teaching that PACs are benign is not always correct. Especially in the elderly, PACs can precipitate atrial fibrillation or PSVT.

Note the clear P waves in lead II. The 7th beat in the rhythm strip is early and is preceded by a P wave indenting the ST-T segment. The QRS is conducted with a prolonged PR, an example of RP-PR reciprocity. (You may recall this terminology from the 3rd case. Briefly, the AV node has a relatively short absolute and a long relative refractory period. The deeper into the relative refractory period an impulse occurs, the longer it takes to get through the node. It may be surmised that the closer an atrial impulse is to the prior ventricular beat, the more refractory will be the AV node from that prior beat, and the longer the PR interval to the next ventricular beat. Hence, the PR interval is inversely or reciprocally related to the preceding RP interval..)

The 13th beat is again early, is again conducted with a prolonged PR, but is this time followed by a run of a narrow complex regular tachycardia. This is probably PSVT, however, there is a bit of irregularity; consequently atrial fibrillation is certainly a possibility.

In this elderly man who presented with a stroke, the tachyarrythmia was initiated as noted above, lasted for a few minutes, and spontaneously stopped on multiple occasions during his stay in the ED.

In Emergency Medicine, we rarely have the luxury of observing the initiation of such tachycardias. More usually we are presented with a patient in whom the rhythm is already established. Differentiating those that are paroxysmal from those which are sustained becomes problematic. Although the accelerated pacemaker type of sustained tachycardias are a small minority [perhaps 10%] of all SVTs, it is clinically important to differentiate them from the more usual re-entry or paroxysmal type, simply because they will not respond to the usual treatments we apply. Stay tuned for an example of this.

Editor: Sol Nevins MD FACEP. Faculty, Emergency Medicine Residency, Morristown Memorial Hospital, Morristown, N.J.