Wagner_Marriot's Practical Electrocardiography, 12e

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F I G U R E 2 2 . 9 . Rhythm strips from two patients receiving digitalis therapy for congestive heart failure. A. Lead V1. B. Lead II. Note in A that the initial independent ventricular beat is a “fusion beat” produced partly by conduction from the atria and partly from the ventricular pacing site. Arrows indicate the varying relationships between adjacent P waves and QRS complexes, and asterisks indicate the regular ventricular rates. Note that the P waves are unusually small in A and unusually large in B .

Third-degree AV block always produces AV dissociation with its independent atrial and ventricular rhythms; however, AV dissociation may result from other sources than third-degree AV block. Decreased sinus automaticity (see Chapter 21), increased junctional and ventricular automaticity (see Chapter 16), and reentrant ventricular tachycardia (see Chapter 19) can all produce AV dissociation by creating the condition in which anterograde impulses fail to traverse the AV node. They encounter the refractoriness that follows AV nodal activation by retrograde impulses. Therefore, AV dissociation caused solely by im- paired function of the AV conduction system should actually be termed “AV dissociation due to AV block” (see Fig. 22.7A, B) and AV dissociation caused solely by an accelerated distal pacing site should be termed “AV dissociation due to refractoriness” (Fig. 22.9A). Both causes can coexist, producing “AV dissociation due to a combination of AV block and refractoriness,” when there are P waves that are obviously not conducted to the ven- tricles but the ventricular rate is slightly above the upper limit of the bradycardic range of 60 beats per minute (see Fig. 22.9B). The term “interference” is often used to describe the condition of refractoriness that either causes or contributes to the two types of AV dissocia- tion described here.

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SECTION III: Abnormal Rhythms