Depressed Conduction Through The Atrioventricular Node Causes Heart Blocks

Conduction through the atrioventricular (AV) node is vulnerable to disease. When this is depressed we refer to it as a heart block. In heart block's mildest form, conduction can be simply slowed. The PR interval represents the time from the start of the P wave to the first appearance of the QRS complex. The PR interval is the sum of conduction time from the SA node to the AV node plus the time required to traverse the AV node. Conduction through the AV node comprises most of this time. We are forced to use the PR interval as an index of AV nodal conduction because there is no way from the ECG tracing to determine precisely when the depolarization wave enters the AV node. Normal values for the PR interval range from 0.12-0.21 sec. If the PR interval is longer than 0.21 sec and if every P wave is followed by a QRS complex, we would suspect abnormally slow conduction through the AV node, which is termed first-degree heart block. Figure 9 shows a normal ECG in panel A and a first-degree heart block in panel B. If the process affecting the AV node intensifies, a second-degree heart block can occur. In second-degree block, conduction through the AV node intermittently fails so that some but not all of the P waves are followed by a QRS complex. Panel C shows a second-degree heart block. Finally, injury can be so extensive to the AV node that it does not conduct at all. In that case, we call it a third-degree or complete heart block. Panel D shows a complete heart block. Notice that the atria (P waves) and ventricles (QRS complexes) are beating independently of each other. The ventricles will beat more slowly because they are responding to a latent pacemaker in the ventricle. If complete heart block is acquired as an adult, the heart rate will be too low to maintain an adequate cardiac output. Interestingly patients born with complete heart block increase their stroke volume so that they have normal cardiac outputs despite the low heart rate.

Although the preceding discussion might imply that the AV node's vulnerability makes it a liability, that is hardly the case. Not only does the AV node provide a delay between atrial and ventricular contraction to aid ventricular filling, but it also protects the ventricle from

A Normal rhythm

B First degree block C Second degree block

D Third degree block

Indicates P wave

FIGURE 9 ECG tracings from (A) a normal individual, (B) a patient with first-degree heart block, (C) a patient with second-degree heart block, and (D) a patient with third-degree heart block. All four recordings are from lead I.

atrial tachyarrhythmias. The long refractory periods in the AV node prevent rapid reactivation of the ventricle. At a normal heart rate the AV node is barely out of its refractory period before the next beat. When the discharge rate of the SA node is increased by sympathetic stimulation, the refractory period of the AV node is also reduced, which allows the ventricle to follow the increased rate.

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