Wagner_Marriot's Practical Electrocardiography, 12e

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F I G U R E 2 2 . 1 3 . A continuous recording of lead II from a patient with acute inferior myocardial infarction. Arrows indicate both the obvious and the assumed locations of sinus-originated P waves.

When second-degree AV block appears during an acute inferior myocardial infarction, the elevation of the ST segment in the ECG may obscure many of the P waves, as seen in Figure 22.13. The visible P waves with prolonged PR intervals during the pauses allow diagnosis of first-degree block, but only the typical RR-interval pattern allows a diagnosis of second-degree AV nodal block. The features described are typical of a classic Wenckebach period, but AV nodal block rarely fits this pattern, because both the sinus rate and the AV conduction are under the constant influence of the autonomic nervous system. 17,18 Among common variations from the classic pattern are (a) the first incremental increase in PR interval may not be the greatest, (b) the PR intervals may not lengthen progressively, (c) the last PR increment may be the longest of all, and (d) a nonconducted atrial beat may not occur. 17 The only criterion needed to identify the form of AV block that typically occurs in the AV node is a variation in the PR intervals. The term Mobitz type I or simply type I AV block is used when variation of the PR intervals is virtually diagnostic of block in the AV node.

The earlier an impulse arrives during the prolonged partial refractory period of the AV node, the longer the time required for conduction of the impulse through to the ventri- cles. Therefore, when the AV node remains in its refractory period, the shorter the interval between a conducted QRS complex and the next conducted P wave ( RP interval ), the lon- ger is the following conduction time (PR interval). This inverse or reciprocal relationship between RP and PR intervals is illustrated schematically in Figure 22.14. A ladder diagram illustrating the effect of progressively earlier entry of atrial impulses 1 to 5 into the AV node ( AV ) . The light stippled area indicates the AV node’s relative refrac- tory period, during which impulses 2 , 3 , and 4 encounter progressively slower conduction. The dark stippled area indicates the node’s absolute refractory period, during which impulse 5 cannot be con- ducted to the ventricles. F I G U R E 2 2 . 1 4 .

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CHAPTER 22: Atrioventricular Block