Wagner_Marriot's Practical Electrocardiography, 12e

Second-Degree Atrioventricular Block

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II

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III

F I G U R E 2 2 . 3 . Leads I, II, and III rhythm strips from an elderly patient receiving digitalis ther- apy for chronic heart failure. Arrows indicate first-degree AV block and asterisks indicate second- degree AV block.

By definition, second-degree AV block is present when one or more, but not all, atrial im- pulses fail to reach the ventricles. Examples of atrial premature beats that are not conducted because they occur early were presented in Chapter 15 (see Figs. 15.7 and 15.8). This situ- ation is not considered AV block because it is normal. Figure 22.3 presents an example of both first- and second-degree AV block in which on-time P waves either have delayed AV conduction (first and second cycles of the series) or no AV conduction (third cycle).

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SECTION III: Abnormal Rhythms