They may be iatrogenic, from medications such as Verapamil, Diltiazem, Amiodarone, and Adenosine, or from cardiac surgeries and catheter ablations for arrhythmias.
AV blocks are further classified according to the degree of blockage and include first degree AV block, second degree AV block, and third-degree AV block.
The measurement of conduction time between the atria and ventricles is represented by the PR interval on electrocardiograms ECG. This component includes the intra-atrial conduction, represented by the P wave, and the conduction from the AV node into the His-Purkinje system.
Prolongation of the PR interval of more than milliseconds is considered to be a first-degree AV block. These can be due to structural abnormalities within the AV node, an increase in vagal tone, and drugs that slow conduction such as digoxin, beta-blockers. It is important to note that in first degree AV block, no actual block occurs. Second-degree atrioventricular blocks are occasional non-conducted P waves with prolonged RR intervals.
There are two types under this classification. Mobitz type I Wenckebach occurs when there is an intermittent conduction block within the AV node that results in a failure to conduct an impulse from the atria into the ventricles.
The impaired nodal conduction is progressive to the point that there is a total block. Mobitz type I is a benign condition that rarely causes hemodynamic instability; asymptomatic patients need no further treatment. The site may continue to function, but may not display properly. This section considers all the important disorders of impulse conduction that may occur within the cardiac conduction system illustrated in the above diagram.
Heart block can occur anywhere in the specialized conduction system beginning with the sino-atrial connections, the AV junction, the bundle branches and their fascicles, and ending in the distal ventricular Purkinje fibers. Disorders of conduction may manifest as slowed conduction 1st degree , intermittent conduction failure 2nd degree , or complete conduction failure 3rd degree. In Type I block there is decremental conduction which means that conduction velocity progressively slows down until failure of conduction occurs.
Type II block is all or none. The term exit block is used to identify conduction delay or failure immediately distal to a pacemaker site. Sino-atrial SA block is an exit block. This section considers conduction disorders in the anatomical sequence that defines the cardiac conduction system; so lets begin.
There are two types, although because of sinus arrhythmia they may be hard to differentiate. Furthermore, the differentiation is electrocardiographically interesting but not clinically important. The following 3 rules represent the classic rules of Wenckebach, which were originally described for Type I AV block.
The rules are the result of decremental conduction where the increment in conduction delay for each subsequent impulse gets smaller until conduction failure finally occurs. Second-degree type I AV block is specifically characterized by an increasing delay of AV nodal conduction until a P wave fails to conduct through the AV node. This is seen as progressive PR interval prolongation with each beat until a P wave is not conducted. There is an irregular R-R interval. Sometimes when the block is consistent, the QRS complexes are said to demonstrate "group beating.
Enlarge A second-degree type I AV block occurs when conduction within the AV node itself is delayed in this progressive manner. It does not necessarily indicate intrinsic conduction disease, and rarely requires a pacemaker to be implanted.
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