The effective refractory period of the Purkinje fibers is prolonged at slow heart rates (Fig. 16.16). However, the effective refractory period of AV node cells remains relatively stable within the normal range of heart rates and increases at very rapid heart rates. Therefore, during high repetition rates of atrial excitation, the AV node serves as a protective mechanism to prevent excessively high frequencies from reaching the ventricles.

Impulses from the AV node initially stimulate the interventricular septum (except the basal portion) and the papillary muscles. Activation then spreads through the septum from both the left and right endocardial surfaces. Early contraction of the septum provides rigidity and acts as an anchor for the remaining ventricular myocardium to contract. Additionally, the early contraction of the papillary muscles prevents the AV valves from turning inside out into the atria during ventricular systole.

While both ventricles are rapidly activated on their endocardial surfaces, the wave of excitation spreads slower from endocardium to epicardium (approximately 0.3 to 0.4 m/sec). The epicardial surface of the right ventricle is activated earlier than the left ventricle due to its thinner wall. Moreover, the apical and central epicardial regions of both ventricles are activated slightly earlier than their respective basal regions. The posterior basal epicardial regions and a small zone in the basal portion of the interventricular septum are the last regions to be excited.

Fig. 16.23 illustrates the conditions necessary for reentry. In each panel, a single bundle of cardiac fibers divides into a left and right branch, with a connecting bundle between them. Under normal circumstances, the impulse travels down the left and right branches of the fiber bundle. When it reaches the connecting bundle, it enters from both sides and is extinguished at the point of collision. The impulse cannot proceed from the left side because the tissue beyond is absolutely refractory, having just been depolarized from the opposite direction. Similarly, the impulse cannot pass through the connecting bundle from the right side due to the same reason.

If an antegrade block occurs in the left and right branches of the fiber bundle (Fig. 16.23B), the impulse cannot complete the circuit. Additionally, if a bidirectional block exists at any point in the loop, such as in branch R (Fig. 16.23C), the impulse cannot reenter.

Reentry is a phenomenon in which a cardiac impulse may re-stimulate a myocardial region through which it previously passed. This phenomenon, responsible for various clinical arrhythmias, can occur in an ordered or random fashion. In ordered reentry, the impulse follows a fixed anatomical path, whereas in random reentry, the path continues to change.

A necessary condition for reentry is the presence of unidirectional block at some point in the loop. Unidirectional block allows the impulse to pass in one direction but not in the other.
Fig. 16.16 ). At slow heart rates, the effective refractory period of the Purkinje fibers is especially prolonged.
In contrast to Purkinje fibers, the effective refractory period of AV node cells does not change appreciably over the normal range of heart rates and actually increases at very rapid heart rates. Therefore, when the atrium is excited at high repetition rates, it is the AV node that normally protects the ventricles from these excessively high frequencies.
The first portions of the ventricles to be excited by impulses arriving from the AV node are the interventricular septum (except the basal portion) and the papillary muscles. The activation wave spreads into the substance of the septum from both its left and right endocardial surfaces. Early contraction of the septum makes it more rigid and allows it to serve as an anchor point for contraction of the remaining ventricular myocardium. Furthermore, early contraction of the papillary muscles prevents eversion of the AV valves into the atria during ventricular systole.
The endocardial surfaces of both ventricles are activated rapidly, but the wave of excitation spreads from endocardium to epicardium at a slower velocity (≈0.3 to 0.4 m/sec). The epicardial surface of the right ventricle is activated earlier than that of the left ventricle because the right ventricular wall is appreciably thinner than the left. In addition, the apical and central epicardial regions of both ventricles are activated somewhat earlier than their respective basal regions. The last portions of the ventricles to be excited are the posterior basal epicardial regions and a small zone in the basal portion of the interventricular septum.
The conditions necessary for reentry (defined in the following box) are illustrated in
Fig. 16.23 . In each of the four panels, a single bundle of cardiac fibers splits into a left branch and a right branch. A connecting bundle runs cSimilar directional changes in the refractory period also occur in ventricular myocytes in response to changes in heart rate.
•Fig. 16.23 The Role of Unidirectional Block in Reentry. A, Anexcitationwavetravelingdownasinglebundle(S)offiberscontinuesdowntheleft(L)andright(R)branches.Thedepolarizationwaveenterstheconnectingbranch(C)frombothendsandisextinguishedatthezoneofcollision.B, ThewaveisblockedintheLandRbranches.C, AbidirectionalblockexistsintheRbranch.D, AunidirectionalblockexistsintheRbranch.Theantegradeimpulseisblocked,buttheretrogradeimpulseisconductedthroughandreenterstheSbundle.
between the two branches. Normally, the impulse moving down the single bundle is conducted along the left and right branches (see
Fig. 16.23A ). As the impulse reaches the link to the connecting bundle (
Fig. 16.23C), it enters from both sides and becomes extinguished at the point of collision. The impulse from the left side cannot proceed because the tissue beyond is absolutely refractory; it has just been depolarized from the other direction. The impulse also cannot pass through the connecting bundle from the right for the same reason.
Fig. 16.23B shows that the impulse cannot complete the circuit if an antegrade block exists in the left and right branches of the fiber bundle. Also, if a bidirectional block exists at any point in the loop (e.g., branch R in
Fig.
16.23C ), the impulse also cannot reenter.
Incertainconditions,acardiacimpulsemayreexcitesomemyocardialregionthroughwhichithadpassedpreviously.Thisphenomenon,knownasreentry, isresponsibleformanyclinicalarrhythmias(disturbancesincardiacrhythm).Thereentrymaybeorderedorrandom.Inorderedreentry,theimpulsetraversesafixedanatomicalpath,whereasinrandomreentry,thepathcontinuestochange.
A necessary condition for reentry is that at some point in the loop the impulse can pass in one direction but not in the other. This phenomenon is called unidirectional block. As shown in