Pearls of Knowledge: Second-degree AV Blocks

Dysrhythmias: Second-degree AV Blocks 

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A defect in the AV node causes sinus rhythm with second-degree AV block, type I. Transmission takes longer with each successive impulse until one fails to be conducted to the ventricles. The PR interval gets progressively longer until some sinus impulses are blocked by the AV node. The rhythm usually occurs in a cyclic pattern such as 3:2, 5:4, etc. It is usually a temporary disorder and does not necessitate a pacemaker because it is not progressive. 

A defect below the AV node in the Bundle of His or bundle branch system due to ischemia or injury causes second- degree AV block, type II. The sinus impulses are usually blocked at regular intervals, allowing only every second, third, or fourth impulse to be conducted to the ventricles. The PR intervals which are present will be constant. 2:1 conduction is common in second-degree, type II, resulting in a slow heart rate and symptoms. It is a progressive rhythm and pacemaker therapy should be implemented. 

Second-degree, type I 

• P waves are regular at a rate of 72 BPM.
• R to R intervals are irregular due to the 3:2 conduction.
• PR intervals become progressively longer, until a sinus impulse is blocked.

Second-degree, type II 

• P waves are regular at a rate of 83 BPM. Ventricular rate 41 BPM.
• R to R intervals are regular due to the 2:1 conduction.
• PR intervals that are present are constant.