We will continue now with the bradyarrhythmias. We will begin with sinus node dysfunction, and then the AV blocks, including a discussion of escape rhythms and pacemakers ECG’s.

The two mechanisms for bradyarrhythmias are also related to automaticity and altered conduction. A decreased rate of impulse formation (decreased automaticity), is the cause of the very common sinus bradycardia. The other mechanism is decreased impulse conduction (or block). Note that if conduction from the atria into the ventricles is only slowed, there may not be a bradycardia (e.g., first degree AV block at a rate of 75 beats per minute). However, when conduction from the atria to the ventricles is completely blocked (e.g., third degree AV block), bradycardia may occur.