First Degree AV Block

Report:

Sinus rhythm 60/min

First degree AV block

PR interval 0.62”

Indeterminate axis

Absolute small voltage

Minor T wave changes

Comment:

The PR interval is considerably longer than the R-P interval, confusing the computer into diagnosing junctional rhythm (at other times, especially during 2:1 conduction, it tends to favour long QT interval, including the P wave into the T wave). In this trace the two waves tend to merge and separate slightly with sinus arrhythmia.

The QRS complexes are small throughout and the frontal axis indeterminate (QRS sum approximately zero in all 6 leads). This could not be accounted for from the data available.

The patient had normal thyroid function and the digoxin level was therapeutic 1.5 (0.6 – 2.3) nmol/L. However, with low potassium, the AV block is not improved. Hypokalæmia alone can cause AV blocks, but it rarely does that without digitalis.

If the long (>0.30”) PR interval persists, one could make a case, in some circumstances (e.g., heart failure) for pacing this patient60. While lesser degrees (PR 0,22 – 0.23”) of latent AV block have a benign outlook61, marked PR interval prolongation is, probably, more likely to progress to 2o or higher block. Even then, some remain stable over decades62.

Fig 91. 89 year old man in Casualty with urinary retention. There is no cardiac history.