Slow-response fibers in the cardiac system have a unique characteristic known as postrepolarization refractoriness. This means that even after the cell has completely repolarized, it may still be difficult to evoke a propagated response for some time. The relative refractory period in these fibers often extends well beyond phase 3. When excitation occurs very late in phase 3 or early in phase 4, it induces a small, nonpropagated local response. As phase 4 progresses, a propagated response with small amplitude and a less steep upstroke can be elicited. This response is conducted very slowly. Eventually, full excitability is regained in phase 4, and the response displays normal characteristics. The timing of early beats can determine their clinical consequence. If they occur late in the relative refractory period or after full repolarization, they are inconsequential. However, if premature depolarizations originate early in the relative refractory period, conduction of the premature impulse is slow, and reentry may occur. Irregular reentry, such as ventricular fibrillation, can lead to ineffective pumping of the heart and potential death. Action potentials evoked early in the relative refractory period have smaller amplitudes and less steep upstroke slopes. As action potentials are elicited later in the relative refractory period, their amplitudes and upstroke slopes improve. Recovery of full excitability is much slower than recovery of the fast response. Impulses arriving early in the relative refractory period are conducted much more slowly than those arriving late in that period. The long refractory periods can also cause conduction blocks, where only a fraction of impulses are propagated. Changes in cycle length, which is the time between successive action potentials, can alter the duration of action potentials in cardiac cells and change their refractory periods. As a result, changes in cycle length play a significant role in the initiation or termination of certain arrhythmias.
•Fig. 16.15 Effects of Excitation at Various Times After the Initiation of an Action Potential in a Slow-Response Fiber. Inthis fiber,excitationverylateinphase3(orearlyinphase4)inducesa small,nonpropagated(local)response(wavea).Laterinphase4,a •Fig. 16.14 Changesinactionpotentialamplitudeandupstrokeslopeasactionpotentialsareinitiatedatdifferentstagesoftherelativerefractoryperiodoftheprecedingexcitation.(RedrawnfromRosenMR,etal.Am Heart J. 1974;88:380.)
In slow-response fibers, the relative refractory period frequently extends well beyond phase 3 (see
Fig. 16.1B ). Even after the cell has completely repolarized, it may be difficult to evoke a propagated response for some time. This characteristic of slow-response fibers is called postrepolarization refractoriness.
propagatedresponse(waveb)canbeelicited,butitsamplitudeissmallandtheupstrokeisnotverysteep;thisresponseisconductedveryslowly.Stilllaterinphase4,fullexcitabilityisregained,andtheresponse(wavec)displaysnormalcharacteristics.(ModifiedfromSingerDH,etal.Prog Cardiovasc Dis. 1981;24:97.)
Fig.16.32 ),thetimingoftheseearlybeatsmaydeterminetheirclinicalconsequence.Iftheyoccurlateintherelativerefractoryperiodoftheprecedingdepolarizationorafterfullrepolarization,theprematuredepolarizationisinconsequential.However,iftheprematuredepolarizationsoriginateearlyintherelativerefractoryperiodoftheventricles,conductionoftheprematureimpulsefromthesiteoforiginisslow,andhenceacardiacimpulseismorelikelytoreexcitesomemyocardialregionthroughwhichithadpassedpreviously(aphenomenonknownasreentry).Ifthatreentryisirregular(i.e.,ifventricularfibrillationensues),theheartcannotpumpeffectively,anddeathmayresult.
Action potentials evoked early in the relative refractory period are small, and the upstrokes are not very steep (Fig.
16.15 ). The amplitudes and upstroke slopes progressively improve as action potentials are elicited later in the relative refractory period. Recovery of full excitability is much slower than recovery of the fast response. Impulses that arrive early in the relative refractory period are conducted much more slowly than those that arrive late in that period. The long refractory periods also lead to conduction blocks. Even when slow responses recur at low frequency, the fiber may be able to conduct only a fraction of these impulses; for example, in certain conditions, only alternate impulses may be propagated.
•Fig. 16.16 Effectofchangesincyclelength(CL)ontheactionpotentialduration(APD)ofPurkinjefibers.(ModifiedfromSingerD,TenEickRE.Am J Cardiol. 1971;28:381.)
Effects of Cycle Length
Cycle length is the time between successive action potentials. Changes in cycle length alter the duration of the action potential in cardiac cells (
Fig. 16.16
Fig. 16.5 ) and thus change their refractory periods. As a consequence, changes in cycle length are often important factors in the initiation or termination of certain arrhythmias (irregular heart rhythms).
The changes in action potential duration produced by stepwise reductions in cycle length from 2000 to 200 msec in a Purkinje fiber are shown in