Vagal Vagaries

Report:

Sinus rhythm

Second degree AV block, Möbitz 1

Ventriculophasic effect in 2:1 conduction (top strip)

Period of advanced (high-grade) second degree AV block

Comment:

Either Wenckebach (Möbitz 1) or Möbitz 2 AV block can progress from "low-grade" (>50% conduction) through 2:1 to "high-grade" (<50% conduction) AV block.

Note the absence of significant slowing (or, for that matter, acceleration) of the sinus rate throughout the episode of second degree AV block. Sometimes there is a dissociation between various vagally mediated responses. Maybe the tendency to sinus bradycardia was balanced by the adrenergic surge provoked by drop in cardiac output.

During 2:1 conduction (or block) the P waves sandwiching a QRS are closer together than those with no QRS between them. This is dignified by the name of ventriculophasic sinus arrhythmia (or effect). It may be due to the effect of ventricular contraction on the right atrium, change in the perfusing pressure, or some mysterious electrical facilitation speeding up the SA node.