Failure to Generate a Signal - Automaticity Problems

          Sometimes, you find multiple problems associated with one underlying cause. In patients with diffusely diseased atrium, you can find combinations of atrial tachyarrhythmias and bradyarrhythmias. One such case is demonstrated here.  

          This patient presented to the lab with atrial flutter.  After the flutter broke with RF ablation, atrial asystole occurred. Pacing was initiated to support the patient. The strip above shows a recording when the pacing was turned off to determine if any underlying rhythm was present. Based upon these results, a permanent pacemaker was implanted.

Failure to Conduct - Heart Blocks

          The other reason the heart would beat too slow is that the signals being generated by the heart are being delayed or blocked before they can cause a complete contraction of all the chambers of the heart.  This type of bradycardia is called heart block and comes in three primary categories;
- First degree heart block: Signals generated by the sinus node are delayed as they progress from the atria to the ventricles.
- Second degree heart block: Signals generated by the sinus node are delayed and periodically blocked as they progress from the atria to the ventricles. Only some of the signals are blocked. If the delay is progressive until one of the signals is finally blocked, it is referred to as Type I 2nd degree heart block. This type of 2nd degree heart block is also know as Wenkebach or Mobitz I heart block.  If the progression of the delay is fast enough that at least every other beat is blocked, it is referred to as Type II 2nd degree heart block or Mobitz II heart block.
 - Third degree heart block: All the signals generated by the sinus node are blocked before the reach the ventricles. This is also known as complete heart block. When this occurs, the sinus node no longer has any control over heart rate. In this situation, one of the backup control centers of the heart kicks in at a slower rate. These secondary areas are located in the "juncitonal" region in the center of the heart or lower down in the ventricles. Which center takes control may be dependant upon where the signals from the sinus node are blocked. If the block occurs higher up near the AV node, the junctional center will usually kick in at a rate of 40-60 beats per minute. Patients who have this response may feel more fatigued than normal, though symptoms are not usually severe. If the block is below the AV node, the lower ventricular rate kicks in, usually at a rate of 20-40 beats per minute. This type of block may cause more severe symptoms due to the slower heart rate associated with it.

Complete Heart Block - Note ventricular rate completely independant from atrial rate - thus, no constant PR interval.

          The treatments for many types of bradycardia may be a cardiac pacemaker. These devices have helped thousands of people return to normal activities after having difficulties with slower than normal heart rates. As always, your physician is best qualified to recommend the most appropriate course of treatment.