Figure 4.

Ischemic canine proximal His-Purkinje system after ligation of the anteroseptal artery shows Mobitz type II block and TD-PAVB. Top, in vivo experiment; intracardiac His bundle electrograms from both left and right sides of the ventricular septum (Hb-L and Hb-R) showing a split His potential (H₁ and H₂). After the first two sinus beats, incremental atrial pacing started (P₁) resulting in 2:1 AVB followed by TD-PAVB in the His bundle. Bottom, in vitro recordings from an ischemic His-Purkinje preparation. Inset, arrangement of electrodes: stimuli applied to proximal His bundle (S); intracellular recording electrode in penetrating portion of His bundle (Hb). Note the slow approach to threshold preceding each action potential (foot potentials) as a result of slow conduction through the bundle; extracellular bipolar recordings from proximal right (Rb) and left (Lb) bundles. A: 1:1 conduction from His bundle to both bundle branches (arrows point at the extracellular recording spikes) at a slow driving rate (40/min). B: Increasing the rate to 75/min resulted in 3:2 response with slight (6 ms) increment of Hb-Lb conduction time before the blocked beat. The Hb recording showed only a local potential during block. C: Further increase of the rate to 111/min resulted in complete conduction failure; the Hb recording showed repetitive local potentials with failure of inscription of a regenerative action potential. Note that conduction failure occurred at cycle lengths (CLs) that far exceeded the action potential duration (i.e., postrepolarization refractoriness). The Rb and Lb electrograms have been retouched for clarity. (Reprinted with permission of the American College of Cardiology from El-Sherif et al, Am J Cardiol 1974;35:421– 434).