ThreedegreesofAVblockcanbedistinguished,asshownin
Fig.16.31 .First-degreeAVblockischaracterizedbyaprolongedPRinterval.Inmostcasesoffirst-degreeblock,theatrium-to-HisintervalisprolongedandtheHis-toventricleintervalisnormal.Hence,thedelayinafirst-degreeAVblockislocatedabovetheHisbundle(i.e.,intheAVnode).Insecond-degreeAVblock,allQRScomplexesareprecededbyPwaves,butnotallPwavesarefollowedbyQRScomplexes.TheratioofPwavestoQRScomplexesisusuallytheratiooftwosmallintegers(suchas2:1,3:1,or 3:2).ThesiteofblockmaybelocatedaboveorbelowtheHisbundle.Ablockbelowthebundleisusuallymoreseriousthanoneabovethebundlebecausetheformerismorelikelytoevolveintoathird-degreeblock.Anartificialpacemakerisfrequentlyimplantedwhentheblockisbelowthebundle.Third-degreeAVblockisoftenreferredtoascomplete heart block becausetheimpulseiscompletelyunabletotraversetheAVconductionpathwayfromatriatoventricles.ThemostcommonsitesofcompleteblockaredistaltothebundleofHis.Incompleteheartblock,theatrialandventricularrhythmsareentirelyindependent.Becauseoftheslowventricularrhythmthatresults,thevolumeofbloodpumpedbytheheartisofteninadequate,especiallyduringmuscularexercise.Third-degreeblockisfrequentlyassociatedwithsyncope(pronouncedlightheadedness),whichiscausedprincipallybyinsufficientcerebralbloodflow.Third-degreeblockisoneofthemostcommonconditionsthatnecessitatetheimplantationofartificialpacemakers.
•Fig. 16.31 Atrioventricular (AV) Blocks. A, First-degreeblock;thePRintervalis0.28second(normal, <0.20sec).B, Second-degreeblock(ratioofPwavestoQRScomplexes,2:1).C, Third-degreeblock;notethedissociationbetweenthePwavesandtheQRScomplexes.
•Fig. 16.32 Premature Atrial Depolarization and Premature Ventricular Depolarization. Theprematureatrialdepolarization(A; secondbeat)ischaracterizedbyaninvertedPwave(justbelowthesecond“P”)andnormalQRScomplexesandTwaves.Theintervalaftertheprematureatrialdepolarizationisnotmuchlongerthantheusualintervalbetweenbeats.Thebriefrectangulardeflectionjustbeforethelastatrialdepolarizationisastandardizationsignal.Theprematureventriculardepolarization(B) ischaracterizedbybizarre,invertedQRScomplexesandelevatedTwavesandisfollowedbyacompensatorypause.
generally reflect a reentry phenomenon. A second type of premature depolarization occurs as the result of enhanced automaticity in some ectopic focus. This ectopic center may fire regularly, and a zone of tissue that conducts unidirectionally may protect this center from being depolarized by the normal cardiac impulse. If this premature depolarization occurs at a regular interval or at an integral multiple of that interval, the disturbance is called parasystole.
A tracing of a premature atrial depolarization is shown in
Fig. 16.32A . With a premature atrial depolarization, the normal interval between beats is shortened. In addition, the configuration of the premature P wave differs from that of the other normal P waves because the course of atrial excitation, which originates at some ectopic focus in the atrium, differs from the normal spread of excitation, which originates at the SA node. The QRS complex of the premature depolarization is generally normal because the ventricular excitation spreads over the usual pathways.
A tracing of a premature ventricular depolarization is shown in