Wagner_Marriot's Practical Electrocardiography, 12e

I

II

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V1

F I G U R E 2 2 . 1 0 . Leads I, II, and V1 rhythm strips from a patient receiving digitalis therapy for congestive heart failure. Arrows indicate P-wave locations (the irregularity is due to sinus arrhythmia) and an asterisk indicates a QRS complex produced by atrial capture.

Often, the AV dissociation produced by third-degree AV block is “isoarrhythmic,” with similar atrial and ventricular rates and with P waves and QRS complexes occurring al- most simultaneously. Insight into the presence or absence of AV block can be attained only when a P wave appears at a time sufficiently remote from a QRS complex that the ventricular refractory period would be expected to have been completed. In Figure 22.10, there is AV dissociation during the first three cycles, in which the independent sinus and ventricular rhythms are similar. Then, when variation in rate caused by respiration (sinus arrhythmia) accelerates the sinus rate but does not affect the ventricular escape focus dur- ing the fourth cycle, atrial capture occurs. This event proves AV conduction to be possible and eliminates complete AV block as a contributor to the AV dissociation. Block in both the RBB and LBB (level C in Fig. 22.1), rather than block at the AV node or in the His bundle, is usually the cause of chronic complete AV block. 7–10 Idiopathic fibrosis, called either Lev disease or Lenègre disease, is the most common cause of chronic complete AV block. 7,11 Acute complete AV block within the AV node results from inferior myocardial infarction, digitalis intoxication, and rheumatic fever. 12 Acute complete AV block within the bundle branches results from extensive septal myocardial infarction. 13,14 Complete AV block may also be congenital, as when it results from maternal anti-Ro antibodies affecting the AV node. 15 In the presence of chronic LBB or RBB block, the individual is at some risk of suddenly developing complete AV block. After this occurs, the ventricles either remain inactive (ventricular asystole; see Fig. 22.8) and the patient experiences syncope or even sudden death, or a more distal pacing site takes over (see Fig. 22.7B) and controls the ventricles (ventricular escape). In this event, the atria continue to beat at their own rate and the ventricles beat at a slower rhythm. This independence (AV dissociation due to AV block) is readily recognized in the ECG recording from the lack of relationship between the in- frequent QRS complexes and the more frequent P waves. Each maintains its own rhythm.

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CHAPTER 22: Atrioventricular Block