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AV blocks can be classified into three degrees, as depicted in Figure 16.31. First-degree AV block is characterized by a prolonged PR interval. In most cases of first-degree block, the delay occurs above the His bundle (in the AV node), with a prolonged atrium-to-His interval and a normal His-to-ventricle interval. Second-degree AV block is characterized by P waves preceding all QRS complexes, but not all P waves are followed by QRS complexes. The ratio of P waves to QRS complexes is usually the ratio of two small integers (such as 2:1, 3:1, or 3:2). The block can occur above or below the His bundle, with a block below the bundle being more serious and likely to progress to third-degree block. In such cases, an artificial pacemaker is often implanted. Third-degree AV block, also known as complete heart block, occurs when the impulse is completely unable to pass through the AV conduction pathway from the atria to the ventricles. The most common sites of complete block are distal to the bundle of His. In incomplete heart block, the atrial and ventricular rhythms are independent, leading to inadequate blood pumping, especially during exercise. Third-degree block is often associated with syncope (lightheadedness) due to insufficient cerebral blood flow. It is one of the most common conditions requiring the implantation of artificial pacemakers (Figure 16.31).

Figure 16.32 illustrates premature atrial depolarization and premature ventricular depolarization. Premature atrial depolarization is characterized by an inverted P wave and normal QRS complexes and T waves. The interval after the premature atrial depolarization is not significantly longer than the usual interval between beats. Premature ventricular depolarization, on the other hand, is characterized by bizarre, inverted QRS complexes and elevated T waves, followed by a compensatory pause (Figure 16.32B).

Premature depolarizations generally result from a reentry phenomenon or enhanced automaticity in an ectopic focus. If a premature depolarization occurs at regular intervals or integral multiples of those intervals, it is referred to as parasystole.

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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