DIAGNOSIS: Atrial Fibrillation, rapid ventricular response, with occasional aberrantly conducted beats in a RBBB pattern; Ashman's phenomenon. Non-specific ST-T changes.

EXPLANATION: Narrow QRS complexes of 80 msec confirm the supraventricular origin of the tachy-arrythmia. The irregularly irregular ventricular rate of about 150 bpm, as well as the absence of organized atrial activity confirm atrial fibrillation. Only MAT and frequent PACs may be mistaken for this rhythm. The non-specific ST-T changes [flattening and inversion] are most likely rate related. Of more interest are the FLBs [funny looking beats]; #1, #14, #19, #23 in the rhythm strip. They are wide, 120 msec, and bizarre looking with secondary T-wave changes. How do we know that they are aberrant and not PVCs? There are two entirely separate clues, either of which alone would be enough to confirm their origin. The first is the morphology in V1; rsR` is classic for RBBB in this lead, being 90% specific for aberrancy. The second clue is more interesting and less well known. The distal conduction system, in contradistinction to the AV node, has a relatively short relative refractory period and a long absolute refractory period. Conduction therefore is usually all or none. In addition, as with all conduction tissue, the refractory period of the bundles is rate dependent; that is the slower the rate, the slower [and longer] the refractory period, the faster the rate, the faster [and shorter] the refractory period. In looking at this strip it is apparent that the FLBs always follow a short RR interval preceded by a long RR interval. The preceding long RR interval sets up a slow recovery period for the conduction tissue. The distal conduction tissue, when challenged by an early beat, is not entirely recovered; in this case the right bundle is still refractory. This is known as Ashman's phenomenon, and is relatively common in rapid atrial fibrillation and other irregular tachy-arrythmias. Occasionally these patients are given lidocaine in the mistaken belief that these beats are VPBs. Unfortunately, lidocaine will sometimes speed conduction through the AV node, increasing the rate from a relatively stable 150 bpm, to 180-200 bpm with cardiovascular collapse!

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