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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 PR interval is increased to greater than 0.20 seconds. In the tracing below, the PR interval is 0.24 sec. Rate is not altered by the prolonged PR interval because it is still being controlled by the SA node. This type of block can be caused by enhanced vagal tone, digitalis, beta-blockers, calcium channel blockers, or ischemic damage to the nodal tissue.
First degree AV block
Second-degree AV nodal block - the conduction velocity is slowed to where some impulses from the atria cannot pass through the AV node. This results in P waves that are not followed by QRS complexes. There are two subtypes of second-degree AV blocks: Mobitz Type I and Mobitz Type II. In Mobitz I (also called "Wenckebach"), the PR interval gradually increases over several beats until it is sufficiently prolonged (that is, AV conduction is sufficiently impaired) that the impulse cannot pass into the ventricles (i.e., a QRS will not follow the P wave). In the first tracing below, the PR interval for the first beat is 0.16 sec and increases to 0.24 sec by the third beat. In the fourth beat, the P wave is not followed by a QRS; therefore, the ventricular beat is dropped. The fifth beat starts this cycle over again. 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 in the second tracing below. This is an example of a 2:1 rhythm because there are two P waves for each QRS. Other ratios (e.g., 3:2) may occur. In either type of second-degree block, the ventricular rate will be less than the normal sinus rhythm (indicated by green arrows in figures).
Also note that the QRS complexes appear normal in shape and duration because they are still being triggered by impulses from the atria passing through the AV node.
Second degree type 1 AV block Second degree type 2 AV block


Third-degree AV nodal block - conduction through the AV node is completely blocked so that no impulses can be transmitted from the atria to the ventricles. QRS complexes still occur (escape rhythm), but they originate from within the AV node, bundle of His, or other ventricular regions. Therefore, QRS complexes will not be preceded by P waves. There will be complete asynchrony between the P wave and QRS complexes. Atrial rate and rhythm may be completely normal (green arrows indicate P waves, although in the last beat it is obscured by the QRS), but ventricular rate will be reduced the extent of which depends on the location of the site generating the ventricular rhythm. Ventricular rates typically range from 30 to 40 beats/min if the ventricular foci generating ventricular rhythm is below the bundle of His. The following figure shows QRS complexes having an abnormal shape and prolonged duration because ventricular depolarization does not follow normal conduction pathways.
Third degree AV block


Ventricular premature beat

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

Revised 01/31/2023




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