Atrial Pacemaker Rhythm: Wenckebach Periods

Report:

Dowling Dennis Raymond 55 15/4/00 106504

Atrial pacemaker rhythm 89/min 2

Second degree AV block, Möbitz 1 5

Left ventricular hypertrophy with ST/T changes 2

Probable old inferior infarction 3

Comment:

The spike-to-R interval (equivalent of PR or AV interval) stretches from about 0.26” at the beginning of a sequence to about 0.44” for the last conducted beat. Throughout, the spike-to-P interval remains relatively constant.

The Wenckebach cycles (two complete ones are present) are typically atypical in that there is a failure to accelerate the ventricular rate before the pause. More than 50% of Wenckebach cycles actually observed in clinical practice are atypical19. In this case, the increment in spike-R interval increases by more, rather than less, thus prolonging the R-R intervals.

Atrial pacing induces second degree AV block at relatively slow rates 130 – 140/m in normal subjects; this is due to lack of sympathetic drive that enhances nodal conduction with other causes of tachycardia. This patient’s rate was only 89/m when Wenckebach conduction occurred; this implies some AV nodal disease or drug effect.

The LVH is typical enough, with characteristically convex-up ST segment depression in 1 or V6. One criterion, not commonly thought of, is present: the R wave in V6 is larger than in V5. It’s even less sensitive than the commonly used voltage criteria, but quite specific (except when anterior infarction causes it).

Another point of interest is the presence of Q waves in the inferior leads, never a sure sign of infarction in the presence of LVH. The fact that they are there in all three inferior leads supports the diagnosis of previous inferior infarction (the patient was known not to have had one, anyway).

Finally, the T wave amplitude in V1 exceeding that of V6 (almost completely inverted here) may signify either ischæmia or LVH. This man had the latter.

Below (Fig 25a) shows paced and sinus rhythm in the same patient. The latter shows 1o AVB.