After a pause, both bundle branches recover from refractoriness producing a normal QRS 5. LBBB aberration is also observed during QRS 4, likely secondary to concealed transseptal conduction causing perpetuation of aberration. Once 1:1 AV conduction occurs (QRS 3), left bundle branch block (LBBB) aberration develops (caused by long-short cycle sequence, phase 3 block). Left, AT with 2:1 AV conduction and normal QRS morphology is observed. 2, 3, 6, 8 – 10 ,12įIGURE 10-2 Atrial tachycardia (AT) with variable atrioventricular (AV) conduction and intermittent aberrancy.
This explains the tendency of aberration to be in the form of RBBB when premature excitation occurs during normal heart rates and in the form of LBBB when it occurs during fast heart rates. However, RB ERP shortens to a greater degree than LB ERP, so that the duration of the refractory periods of the two bundles crosses over, and LB ERP becomes longer than that of the RB. At faster heart rates, the ERP of both bundle branches shortens. See text for discussion.Īt normal heart rates, the effective refractory period (ERP) of the right bundle branch (RB) exceeds the ERP of the AV node (AVN), HB, and left bundle branch (LB). Note that the aberrantly conducted complex (blue arrows) during AF occur at variable coupling intervals to the preceding beats. During AF, long-short cycle sequences occur repeatedly and are associated with RBBB aberrancy (Ashman phenomenon, phase 3 block). Note that the PAC is conducted with right bundle branch block (RBBB) aberrancy (phase 3 block). A premature atrial complex (PAC, red arrow) during sinus rhythm induces atrial fibrillation (AF). Phase 3 block constitutes the physiological explanation of several phenomena, including aberration caused by premature excitation, Ashman phenomenon, and acceleration-dependent aberration.įIGURE 10-1 Ashman phenomenon. The block usually occurs in the very proximal portion of the bundle branch. 3, 11 Transient left BBB (LBBB) is less common than RBBB (only 25% of phase 3 aberration is of the LBBB type). Phase 3 aberration can also occur pathologically if electrical systole and the refractory period are abnormally prolonged (with refractoriness extending beyond the action potential duration or the QT interval) and the involved fascicle is stimulated at a relatively rapid rate.
This is commonly seen with very early premature atrial complexes (PACs) that conduct aberrantly. 3, 11įunctional or physiological phase 3 aberration can occur in normal fibers if the impulse is sufficiently premature to encroach on the physiological refractory period of the preceding beat, when the membrane potential is still reduced. Manifestations of phase 3 block include BBB and fascicular block, as well as complete atrioventricular (AV) block.
Phase 3 block, also called tachycardia-dependent block, occurs when an impulse arrives at tissues that are still refractory caused by incomplete repolarization. Consequently, the Na + current and phase 0 of the next action potential are reduced, and conduction is then slower.
#SUPRAVENTRICULAR TACHYCARDIA WITH IVCD FULL#
Therefore, when stimulation occurs during phase 3 of the action potential, before full recovery and at less negative potentials of the cell membrane, a portion of Na + channels remains refractory and unavailable for activation. The more negative the membrane potential is, the more sodium (Na +) channels are available for activation, the greater the influx of Na + into the cell during phase 0, and the greater the conduction velocity. These factors, in turn, depend on the membrane potential at the time of stimulation. Conduction velocity depends, in part, on the rate of rise of phase 0 of the action potential (dV/dt) and the height to which it rises (Vmax).