Atrial Pacemaker: First Degree AV Block

Report:

Atrial pacemaker rhythm 90/min 4

AV interval 0.40 sec 2

Borderline intraventricular conduction delay (QRS 0.12 sec) 1

Inferior infarction, probably old 2

Nonspecific ST/T changes 1

Comment:

The pacemaker is probably in the AAI mode; there is no programmed or set AV interval. If one was set in, e.g., the DDD mode, it would be set somewhere between the physiological 0.12 and 0.20 sec but the native QRS complexes would never appear, being replaced by the ventricular-paced ones. Unlike sinus rhythm, atrial pacing tends to prolong the AV (equivalent of PR) interval at lower rates, and more. This is due to the lack of a concomitant rise in adrenergic tone. It is fairly certain that this patient would go into second degree block at pacing rates not much higher than the present one.

The patient had of course sustained an inferior infarction prior to CABGs (Fig 12a), most likely due to disease in the RCA territory which includes the AV node in 90% subjects. This may well be another factor in this unusually prolonged AV conduction.