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. There are two entirely separate clues, either of which alone would be enough to confirm their origin. 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 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. 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. Inc. All rights reserved.