Atrioventricular Block

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Atrioventricular Block




On its way from the sinus node to the ventricles, an electrical impulse passes through the atrioventricular (AV) conduction system comprising atrial myocardium, the AV node, the penetrating AV bundle (bundle of His), and the network of Purkinje fibers. Regular cardiac function requires some AV conduction delay to allow the ventricles to relax before the next atrial contraction. AV block is defined as further delay or block of impulse transmission that occurs mostly in the AV node. The AV node forms a heterogeneous and highly complex three-dimensional lattice of distinct cell types with different molecular make-up and therefore different functional behavior (Figure 106-1). Large atrial cells connect to midsized transitional cells, which, in turn, contact the small, closely packed cells of the compact node. It must be emphasized that the compact part represents one component of the AV node, which comprises all the different cell groups that determine its functional properties.



Many morphologic and functional factors like action potential characteristics, cell-to-cell coupling, and autonomic innervation determine impulse transmission and control AV conduction (see the previous chapter in this textbook1). Numerous pathologic processes, congenital or acquired, can affect this delicate system.


First, an overview of the different types of AV block will be provided, followed by congenital and acquired causes. Then, the ventricular consequences of chronic complete AV block obtained in a canine model will be described. Finally, future therapy considerations will be discussed in light of the findings described.



Electrocardiographic Characteristics and Definitions of AV Block


AV block can be defined anatomically—supra- (i.e., atrial or AV nodal), intra-, or infra-His (see Figure 106-1)2—or electrocardiographically—first-, second-, and third-degree (complete) AV block.


Thorough analysis of the electrocardiogram (ECG) can provide an estimation of the anatomical site of the block. If ventricular depolarization is undisturbed and QRS complexes appear normal, the site of delay or block is usually supra-His (usually AV nodal). Variations of the PR interval point to the AV node as the source of delay and block. When abnormal ventricular depolarization with prolonged QRS duration or left or right bundle branch block is present, the AV node can still be the substrate, but involvement of the His-Purkinje system distal to the AV node is likely. The site of conduction delay can be determined more precisely by endocardial recordings of AH and HV intervals. If impulse transmission is impaired in the atrium or in the AV node, the AH interval will be prolonged (>125 ms) and the HV interval will be normal (see Figure 106-1). A prolonged HV interval (>55 ms) indicates conduction disturbances in the His-Purkinje system (see Figure 106-1). Supra-His AV conduction delay or block usually has a favorable prognosis, whereas intra- or infra-His AV conduction delay or block implies a more serious prognosis.2,3 In the next section, a description of each specific AV block will be combined with data concerning incidence, prognosis, and treatment.



First-Degree AV Block


On the surface ECG, the PR interval is measured from the onset of atrial depolarization (P wave) to the beginning of ventricular depolarization (QRS complex) and ranges normally between 0.12 and 0.20 seconds. In first-degree AV block, every non-premature atrial impulse is conducted to the ventricles with a PR interval that exceeds 0.20 seconds. The incidence of first-degree AV block in asymptomatic subjects is low, ranging between 0.5% and 1.6%.4,5 The prognosis for PR prolongation in the presence of a normal QRS complex until recently was thought to be benign. However, an observation from the Framingham Heart Study showed increased risk of development of atrial fibrillation (hazard ratio [HR], 2.06) and of pacemaker implantation (HR, 2.89) and revealed a moderately increased risk of death with a hazard ratio of 1.4.5


Asymptomatic patients with first-degree AV block do not require treatment. Patients should be followed to detect progression to higher-degree AV block. In patients with a marked PR interval increase (>0.3 s) that approaches the preceding RR interval, symptoms can result from impaired ventricular filling and synchronous atrial and ventricular contraction (“cannon waves”) that may necessitate pacemaker implantation.



Second-Degree AV Block


Second-degree AV block is subclassified into type 1 (Wenckebach or Mobitz type I), type 2 (Mobitz type II), or advanced block.



Type 1 Second-Degree AV Block


Electrocardiographically, typical type 1 block is characterized by several features: (1) progressive prolongation of the PR intervals, (2) the greatest increase in PR interval noted in the second beat of a cycle, with the increment being progressively less in subsequent beats, (3) gradual shortening of RR intervals, (4) a P wave not followed by a QRS complex, (5) a pause with an RR interval less than the sum of two PP intervals, and (6) the first conducted atrial impulse after the pause showing a shorter or normal PR interval (Figure 106-2). Clinically, only a minor part of type 1 second-degree AV block complies with all criteria.3



Progressive (beat-to-beat) slowing of conduction velocity during a fast atrial rate, referred to as decremental conduction, can be attributed to the AV node and its atrial nodal extensions.1 Therefore, in type 1 second-degree AV block with normal QRS complexes, conduction delay and block occur most often within the AV node proximal to the His bundle.


Type 1 block can occur in 1% to 2% of healthy individuals as a result of enhanced efferent discharge of parasympathetic nerve fibers. This vagally mediated block, in combination with slowing of the sinus rate, can occur in normal persons, for instance, as a result of pain or during sleep, or in persons with high vagal tone, like endurance athletes at rest.6 It is functionally determined and usually benign and disappears with increased sympathetic tone. Syncope is rare, although it sometimes can lead to serious symptoms in athletes and can necessitate de-training.


In asymptomatic individuals without evident infra-nodal conduction disturbances, permanent pacemaker implantation is not recommended (class III).2 Progression to so-called advanced second-degree AV block or third-degree AV block is uncommon. If patients develop symptomatic bradycardia, permanent pacemaker implantation is indicated.2



Type 2 Second-Degree AV Block


In type 2 block, abrupt failure of a non-premature atrial impulse to reach the ventricles is not preceded by lengthening of the PR interval, meaning that the RR interval spanning the pause equals two PP intervals. The site of type 2 block is generally infra-nodal and, if the QRS complex appears normal, can be located within the His bundle. This indicates diffuse disease of the infra-nodal conduction system, especially if associated with a wide QRS. If failure of AV conduction is associated with conduction delay in the Purkinje system and/or bundle branch block, the site of block is distal to the His bundle.


If two or more non-premature atrial impulses fail to activate the ventricles, but some QRS complexes are related to preceding P waves, advanced or high-grade AV block is present. In this case, AV conduction is compromised, but some conduction is preserved. Sometimes, 2 : 1 AV block is referred to as high-grade AV block.


The incidence of type 2 block is rare in healthy individuals and increases with age and in the presence of structural heart disease. About 2% of people older than 70 years with structural heart disease present with type 2 block. It carries a relatively high risk of progression to high-grade or third-degree AV block and thus is associated with increased risk of mortality.


Implantation of a permanent pacemaker is recommended for patients with type 2 second-degree AV block and should be considered also in asymptomatic patients. An electrophysiological study can sometimes be useful for determining intra- or infra-His levels of block. Type 2 second-degree AV block during exercise in the absence of ischemia indicates a diseased His-Purkinje system and should lead to permanent pacemaker implantation.2



Third-Degree AV Block


In third-degree or complete AV block, no conduction from the atrium to the ventricle is possible, and all atrial impulses halt somewhere in the atrioventricular conduction axis. The term AV dissociation is often used to describe third-degree AV block. However, this term can be misleading because several situations can lead to dissociated atrial and ventricular activation in the presence of normal AV conduction. In principle, any junctional or ventricular rhythm with a rate that exceeds the sinus rate and without VA conduction may appear dissociated, especially if only a standard 12-lead ECG is recorded. Sinus bradycardia with a junctional escape rhythm or junctional or ventricular tachycardias with retrograde block can mimic AV dissociation too. Longer ECG recordings and thorough analysis are sometimes required for correct diagnosis of third-degree AV block.

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Jun 4, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Atrioventricular Block

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