Paroxysmal AV block

Paroxysmal AV block or phase 4 block is a condition when 1:1 AV conduction suddenly changes to complete AV block.

Mechanism of phase 4 block

 * 1) Cells electrically capable of pacing slowly reduce their transmembrane potential during phase 4 of the action potential
 * 2) When the time of diastole is prolonged, cells may reduce their membrane potential below that capable of conducting (below -65 mV)
 * 3) With reduction in cycle length, the membrane threshold potential decreases toward zero resulting in inability to excite that tissue.  Hence this phenomena is also referred to as "bradycardia dependent block"

Most often occurs after critical lengthening of PP interval often following:

 * 1) APD eithers conducted or non conducted - Figure 1
 * 2) Retrogradely conducted VPBs - Figure 2
 * 3) His bundle extrasystole retrogradely conducts to atrium but with block to the ventricle during CMS
 * 4) Vagal maneuvers, such as valsalva and carotid sinus massage - Figure 3

Characteristics of Paroxysmal AV block

 * 1) Occurs after critical lengthening of PP interval
 * 2) Localized to sub AV node
 * 3) Mechanism is phase 4 block
 * 4) Dangerous because of unreliable escape mechanism
 * 5) Paroxysmal and often difficult to diagnose

Phase 4 Block, Figure 1

 * Phase 4 block in the His-Purkinje system occurring after an atrial premature beat that conducts to the ventricles. Notice the delayed sinus beat that results in a critical P-P lengthening with subsequent Phase 4 block in the His.

Figure 2

 * Complete AV block following a ventricular premature beat that conducts to the atrium. The following sinus beat results in a long P-P interval.  Given the baseline RBBB, left anterior hemiblock, phase 4 block likely occurs in the left posterior fascicle.

Figure 3

 * Carotid massage resulting in complete AV block. Although this maneuver most often results in block in AV node, this likely results from phase 4 block infranodally.  Note the lengthening of the P-P interval with no increase in PR interval.