ACC/AHA guidelines for implantation of cardiac pacemakers: how accurate are the definitions of atrioventricular and intraventricular conduction blocks?

A Case With Inferior Wall Myocardial Infarction and Conduction Abnormalities: Addressing the Diagnostic Challenges

Conduction disturbances are an important complication of ST-elevation myocardial infarction (STEMI). Conduction disturbances such as fascicular blocks and bundle branch blocks are associated with alteration of QRS morphology and secondary ST-T wave changes that can influence the diagnosis of acute myocardial ischemia. We report an interesting case where a patient presented with inferior wall myocardial infarction (MI), right bundle branch block (RBBB), and left anterior hemiblock (LAHB). We discuss the challenges in diagnosing MI in such patients, including the impact of QRS changes in RBBB and LAHB, their influence on diagnosis of STEMI, and differentiation of combined first-degree AV block and bifascicular block from trifascicular block.

2:1 Atrioventricular block: order from chaos

2:1 AV block can occur in either the AV node or the His-Purkinje system and cannot be classified into type I or type II second-degree AV block because there is only one PR interval to examine before the blocked P wave. It is inappropriate to use terms such as 2:1 or 3:1 type I or type II AV block because this characterization violates the accepted traditional definitions of type I and type II block based on electrocardiographic patterns and not on the anatomical site of block. Type I and type II second-degree AV block can progress to 2:1 AV block, and 2:1 AV block can regress to type I or type II block. Consequently, the site of the lesion in 2:1 block can often be determined by seeking the company 2:1 AV block keeps. An association with type I block and a narrow QRS complex almost always reflects AV nodal block but type I block with a wide QRS complex occurs more commonly in the His-Purkinje system than the AV node. Type II block, if correctly defined, is always infranodal. Outside of acute myocardial infarction, sustained 2:1 and 3:1 AV block with a wide QRS complex occurs in the His-Purkinje system in 80% of cases and 20% in the AV node. Administration of atropine in patients with His-Purkinje disease may increase the degree of AV block.

Prospective evaluation of infrahisal second-degree AV block induced by atrial pacing in the presence of chronic bundle branch block and syncope

The value of nonfunctional infrahisal second-degree atrioventricular (AV) block induced by incremental atrial pacing was prospectively examined in 192 patients with chronic bundle branch block (BBB) and syncope. We compared 174 (91%) patients with normal response to atrial pacing (Group I) to 18 (9%) patients with atrial pacing induced nonfunctional infrashisal second-degree AV block (Group II). Patients in group I had higher incidence of organic heart disease, ventricular tachycardia induction, and retrograde ventriculoatrial conduction (P < 0.001, P < 0.05, P < 0.01, respectively), while patients in group II had higher incidence of primary conduction disease and prolonged H-V intervals (P < 0.001, P < 0.01, and P < 0.001). During mean follow-up period of 65 +/- 34 months for group I, and 68 +/- 35 months for group II, a development of spontaneous second- or third-degree AV block was higher in group II (14/18 [78%]), than in group I (15/174 [9%]) (P < 0.001). The site of AV block was infrahisal in all patients in group II, and in 10 of 15 patients in group I. Because of the prophylactic pacing in all patients in group II, the incidence of sudden death was similar among the two groups, but patients in group I had higher incidence of cardiac death (P < 0.05).

CONCLUSION

In patients with chronic BBB and syncope, a nonfunctional infrashisal AV block induced by incremental atrial pacing identified patients with particularly high risk of development of spontaneous infrahisal AV block. Therefore, permanent cardiac pacing is absolutely indicated in these patients.