However, the presence of a narrow QRS complex escape rhythm and a second‐degree Wenckebach‐type AV block suggests that the block is located in the AV junction, whereas the presence of a wide QRS complex favors evidence of a block below the bundle of His.įigure 17.10 Differential diagnosis (Lewis diagrams) between 3 : 2 sinoatrial block and atrial parasinus bigeminy with very similar or identical P′ waves, and a virtually identical ECG pattern. The exact location of the block (suprahisian, hisian, or infrahisian block) is accurately determined only by intracardiac studies (see later Figure 17.13). In rare cases, it may be congenital (see Chapter 24, Advanced AV block). The etiology is due to a degenerative, infection, inflammatory or ischemic process of the AV junction. As in other types of heart block, we shall refer to first‐, second‐, and third‐degree blocks (see Chapter 14, Heart block). We have discussed all the aspects related to blocks at atrial level in Chapter 9.Ĭonduction block is observed in the AV junction. Therefore, the ECG recording shows an AV junctional or ventricular escape rhythm (see Figure 14.19D). No atrial depolarization due to sinus stimuli is observed. Figure 17.10 explains the key steps for this differential diagnosis (Bayés de Luna et al. This type of block does not modify the PR interval because the block is at the sinoatrial level.įrom an electrocardiographic point of view, the 3 : 2 sinoatrial Wenckebach block shows a sequence and ECG characteristics similar to those of atrial bigeminy due to premature parasinus atrial impulses (P′ wave is identical or almost identical to the sinus P waves). The block increment is gradually less until the complete block initiates a pause. Figure 17.9 explains why in the second‐degree sinoatrial block (Wenckebach‐type), the RR interval progressively shortens. Usually, this presents as a 3 : 2 ( Figure 18.6E) or 4 : 3 block ( Figures 14.19B and 17.9). Second‐degree sinoatrial block (Wenckebach‐type) may also be observed. If the 2 : 1 block disappears with exercise, an abrupt increase of the heart rate, usually more than double due to increase in sinus rate, may be observed. If the 2:1 second‐degree block is fixed, the ECG recording will be similar to that of a sinus bradycardia due to automaticity depression.
In the presence of an intermittent‐type 2 : 1 second‐degree sinoatrial block, the ventricular rate is half the sinus rate (BC = 2 AB in Figure 14.19C). 880 ms), as this is the difference between the sinoatrial conduction increases of the first and second cycles. The second RR interval is 50 ms shorter than the first (930 vs. If the AV junction shows depressed automaticity, an idioventricular rhythm at a slower rate ( 2–3). When the heart rate is slow as a result of depressed sinus automaticity, sinoatrial block, or atrioventricular (AV) block, an AV junction pacemaker at a normal discharge rate (40–50 bpm) may pace the electrical activity of the heart by delivering one or more pacing stimuli (escape beat or complex and escape rhythm) ( Figures 17.1– 17.5). This chapter will describe the most important ECG characteristics of the different passive arrhythmias according to the classification in Table 14.1.
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