Wagner_Marriot's Practical Electrocardiography, 12e

A

I

B F I G U R E 2 2 . 6 .

Lead II rhythm strips from elderly patients receiving digitalis therapy. The ven- tricular rates are in the normal (60 to 100 beats per minute) (A) and bradycardic (15 to 40 beats per minute) (B) ranges. Note the prolonged pause in the ventricular rhythm (3.5 seconds) in B .

When determining the clinical significance of second-degree AV block, the atrial rate should be considered. As discussed in Chapter 17, conduction of some, but not all, atrial impulses is essential for clinical stability in the presence of atrial flutter/fibrillation. Chapter 16 (see Fig. 16.6) indicates that second-degree AV block commonly occurs along with atrial tachycardia, particularly when there is digitalis toxicity. When “AV block” occurs in the presence of an atrial tachyarrhythmia, the block itself is considered a normal occurrence and not an additional arrhythmia (Fig. 22.6A) unless the ventricular rate is re- duced into the bradycardic range (see Fig. 22.6B). When both second-degree AV block and sinus pauses are present (see Chapter 21), the cause is most likely not within the heart itself but rather in its autonomic nervous control. Second-degree AV block usually occurs in the AV node 4,5 and is associated with reversible conditions such as the acute phase of an inferior myocardial infarction or treat- ment with digitalis, a -adrenergic blocker, or a calcium channel–blocking drug. Because second-degree AV block is generally a transient disturbance in rhythm, it seldom progresses to complete AV block. However, in one study of 16 children manifesting second-degree AV block, 7 developed complete block. 6 Chronic second-degree AV block may occasionally occur in many conditions, including aortic valve disease, atrial septal defect, amyloidosis, Reiter syndrome, and mesothelioma of the AV node.

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CHAPTER 22: Atrioventricular Block