A supraventricular impulse results in a narrow QRS complex by virtue of rapid synchronous conduction through the His-Purkinje system, finally reaching the ventricular myocardium. As we have learned previously, a fixed conduction delay or interruption in the bundle branches or their divisions produces abnormalities of QRS complex morphology that we know as bundle branch block or fascicular block. The ECG findings include alterations in QRS complex duration, amplitude, configuration, and axis. The term aberration (or aberrant ventricular conduction) is used to describe transient bundle branch block and fascicular block that are unrelated to preexisting bundle branch block, preexcitation, or the effects of drug or electrolyte abnormalities. This temporary alteration of conduction, also called phasic aberrant ventricular conduction, is due to differential refractoriness of the bundle branches combined with variances in cycle length, concepts that we will now review.
In Chapter 3 we learned that the action potential of a nonpacemaker myocyte is divided into five phases (0,1,2,3,4) (Figure 16-1). Phase 0 (rapid depolarization) is the rapid upstroke from the negative resting membrane potential (RP) that begins depolarization. A stimulus from an adjacent cell opens sodium channels to reach the threshold potential (TP), further inducing the rapid influx of positively charged sodium ions. Phase 1 (early rapid repolarization) is the initial reversal of the transient positive overshoot of depolarization from phase 0. Phase 2 (plateau) is the delay in repolarization resulting from a balance of inward-flowing calcium ions and outward-flowing potassium ions. Phase 3 (terminal rapid repolarization) continues repolarization toward the baseline membrane potential. Phase 4 (resting) is the time between action potentials, during which the stable negative resting potential is established, preparing the cell for the next stimulus.
Action potential of a nonpacemaker cell showing the phases and refractory periods (see text for details). ARP = absolute refractory period, ERP = effective refractory period, RP = resting potential, RRP = relative refractory period, TP = threshold potential, (*) = stimulus.
During each action potential, there is a period of time when the cell is either partially or completely unable to respond to a new stimulus. These refractory periods are dependent on the number of sodium channels available for opening, which are depleted with depolarization and restored with repolarization. The absolute refractory period (ARP) comprises phases 1 and 2 of the action potential, during which time the cell cannot generate a new action potential, no matter how great the stimulus. The effective refractory period (ERP) includes the ARP and the early portion of phase 3. Any arriving stimulus produces a weak, ineffective response that will not produce a new depolarization (Figure 16-2). Recall that I like to think of this portion of the action potential as the “ineffective” refractory period because any impulse arriving during this time will ...