Alternating Wenckebach periods and allied arrhythmias.

Alternating Wenckebach periods (AWPs) are episodes of 2:1 block during which the PR, AH, or AV intervals of the conducted beats gradually increase until a greater degree of block ensues. Most episodes occur at the AV node, but some have also been reported in other structures. AWPs are usually attributed to multilevel block due to transverse (horizontal) dissociation. This assumption was initially based on a method in which the solutions to difficult electrocardiographic rhythms were arrived at by analysis and deduction based on the knowledge existing at that particular time. Subsequently, it was reinforced by information extrapolated from intracardiac recordings performed in patients with documented multilevel block in separate anatomical structures (atria, AV node, and His bundle), as well as from microelectrode studies and computer simulations. Although AWPs are frequently observed in clinical tracings, those occurring at the AV node are best categorized during incremental atrial stimulation because then they occupy a specific point in the wide spectrum of tachycardia dependent AV nodal conduction disturbances. In fact, the A:H ratios occurring in the episodes where the degree of block increases can be represented by "universal" mathematical formulas. However, in the clinical setting, drugs affecting the electrophysiology of the node can alter the pacing induced symmetry by producing additional differential effects on the various levels. The latter still requires further elucidation.