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Cardiovascular Physiology Concepts

Richard E. Klabunde, PhD


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Abnormal Rhythms - Definitions

General Terms:

Specific Arrhythmias:

First-degree AV nodal block - the conduction velocity is slowed so that the P-R interval is increased to greater than 0.2 seconds.  Can be caused by enhanced vagal tone, digitalis, beta-blockers, calcium channel blockers, or ischemic damage.

first degree AV block

Second-degree AV nodal block - the conduction velocity is slowed to the point where some impulses from the atria cannot pass through the AV node.  This can result in P waves that are not followed by QRS complexes.  For example, 1 (as shown below) or 2  P waves may occur alone before one is followed by a QRS.  When the QRS follows the P wave, the P-R interval is increased.  In this type of block, the ventricular rhythm will be less than the sinus rhythm.

There are two subtypes of second-degree AV blocks: Mobitz I and Mobitz II. In Mobitz I (Wenkebach block), the P-R interval gradually increases over several beats until it is sufficiently prolonged (that is, AV conduction is sufficiently impaired) that the impulse fails to pass into the ventricles (i.e., a P wave will not be followed by a QRS). Mobitz II occurs is when the P-R interval is fixed in duration, but some P waves are not followed by a QRS (as illustrated below).
second degree AV block

Third-degree AV nodal block - conduction through the AV node is completely blocked so that no impulses are able to be transmitted from the atria to the ventricles.  QRS complexes will still occur (escape rhythm), but they will originate from within the AV node, bundle of His, or other ventricular regions.  Therefore, QRS complexes will not be preceded by P waves.  Furthermore, there will be complete asynchrony between the P wave and QRS complexes.  Atrial rhythm may be completely normal, but ventricular rhythm will be greatly reduced depending upon the location of the site generating the ventricular impulse.  Ventricular rate typically range from 30 to 40 beats/min.

third degree AV block

For information on the pharmacologic treatment of arrhythmias, go to:

Revised 03/16/10

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.