Acceleration of the sinoatrial rate leading to complete heart block, an unusual mechanism for the Adams-Stokes syndrome

Bundle branch block after ventricular tachycardia: a manifestation of "fatigue" or "overdrive suppression"

Three cases of bundle branch block due to ventricular tachycardia are presented. Although the exact mechanism remains obscure, this phenomenon may be related to previously described acceleration-induced atrioventricular and bundle branch block.

The pathophysiology of tachycardia-dependent paroxysmal atrioventricular block after acute myocardial ischemia. Experimental and clinical observations

The pathophysiology of paroxysmal A-V block (PAVB) was studied in 20 anesthetized dogs after ligation of the anterior septal artery. Simultaneous recording of leads II and aV R , as well as intracardiac recordings from the His bundle (Hb) and both bundle branches, were monitored. In 18 of 20 experiments, PAVB was localized in the Hb. In all experiments, PAVB occurred subsequent to Mobitz type II A-V block. In eight experiments, PAVB occurred spontaneously during sinus rhythm and was preceded by a period of Wenckebach periodicity superimposed upon a 2:1 A-V block. Vagal-induced slowing of the sinus rate resulted in immediate resumption of 1:1 A-V conduction. In 18 experiments, PAVB was induced by atrial pacing at a critical heart rate in each case (180-300 beats/min). Evidence is presented that A-V conduction was consistently blocked below a critical H-H interval. Slowing the pacing rate, termination of pacinig or increasing the pacing rate until physiological A-V nodal block occurred, all could result in a longer H-H interval and immediate resumption of A-V conduction. When the critical heart rate for PAVB was maintained, a slow idioventricular escape rhythm occurred.

Five patients who developed PAVB after acute myocardial ischemia are also reported providing the clinical counterpart for the experimental observations. In all five cases, PAVB occurred on acceleration of the sinoatrial rate (105-140 beats/min) which was spontaneous in two and induced by drugs given for varied therapeutic indications in three (isoprenaline in two and atropine sulfate in one). In all five, PAVB was associated with Mobitz type II and/or 2:1 A-V block. These experimental and clinical observations suggest that PAVB after acute myocardial ischemia appears to be due to a tachycardia-dependent repetitive concealed conduction in the ischemic His-Purkinje system, probably mainly in the Hb. The clinical observations point out potential consequences of a rapid atrial rate in patients with acute myrocarcial ischemia and type II A-V block.

Depression of escape pacemakers associated with rapid supraventricular rate in patients with atrioventricular block

Intermittent failure of escape pacemakers to maintain an effective rate when the supraventricular rate was rapid was observed in three patients with complete, or high degree atrioventricular block. In two of these patients asystole or slowing of escape pacemaker rate to less than 20 per minute was reproduced by atrial pacing at rates from 125 to 150 per minute. We localized the site of the block and the site of escape pacemaker proximal to the bifurcation of the His bundle in all patients. We assume that a critical increase in atrial rate facilitates penetration of the nonconducted supraventricular impulses into the area of block causing discharge of the escape pacemaker. We suggest that such concealed discharge of escape pacemakers should be considered as a cause of slow and variable rate of escape pacemakers in patients with block proximal to the bifurcation of the His bundle.