DIAGNOSIS: Left bundle branch block, sinus rhythm with frequent conducted and non-conducted premature atrial contractions. Left axis deviation.

EXPLANATION: The LBBB is apparent from the morphology in V1. Note the slick downstroke to an early nadir of the intrinsicoid deflection of less than 60 msec with a QRS duration of greater than 120 msec as well as the secondary T-wave changes with the T-wave axis in the opposite direction to the main QRS forces. Although one might initially mistake this rhythm for Wenckebach [particularly the first two grouped beats in the rhythm strip], the P-P interval in Wenckebach is constant; not the case in this example. Sinus rhythm is most likely present only in the second and 4th beats noted in the rhythm strip. The 3rd, 5th, and all subsequent beats are PACs, distinguished by the varying morphology of the p-waves as well as their early occurrence. Although subtle, the ST-T segment is deformed after the 3rd, 5th, 7th, 8th, and 9th beats indicating the PACs. Those which are non-conducted follow the 3rd, and the 5th beats. Those which are conducted reveal varying PR intervals; another example of RP-PR reciprocity. You may recall this terminology from a prior tracing at which time I mentioned that there would be more on this concept later. Now is at least a little later. 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. In fact, when the P-wave occurs very close to the prior QRS, the absolute refractory period of the AV node is reached; these are the non-conducted PACs. There will be more discussion and examples of this in the future. Thanks to Dr Leo Capobianco for the EKG.

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