Bradycardia
Bradycardias are caused by two general problems: disease in the sinus node, or disease in the specialized conduction system. (Click here for a review of the cardiac electrical system.)The sinus node is the pacemaker of the heart. The electrical impulse that generates the heart beat arises in the sinus node. Disease in the sinus node, therefore, can result in the lack of sufficient electrical impulses (and thus a lack of sufficient heart beats) to maintain the body’s needs. Sinus node disease that leads to symptoms due to a slow heart rate is called “sick sinus syndrome.” Most sinus node disease is related to simple deterioration in sinus node function caused by aging. Sick sinus syndrome is the most common indication for implanting a permanent pacemaker. (Click here for a discussion of pacemakers.)
The specialized conduction system is responsible for transmitting the heart’s electrical impulses from the atria to the ventricles. Disease in the AV node, bundle of His, or the bundle branches can lead to a condition called “heart block.” Heart block occurs when the electrical impulses in the atria are stopped from reaching the ventricles. The heart rate can reach dangerously low levels when heart block is present. A permanent pacemaker, however, takes care of the problem.
Tachycardia
Tachycardias also come in two general varieties: supraventricular tachycardias, and ventricular tachycardias.Supraventricular tachycardias (SVT)
The SVTs are generally benign (that is, non-life-threatening) tachycardias that either arise in the atria (that is, “supra” the ventricles), or involve the atria in the mechanism of the tachycardia. Many SVTs are due to extra, abnormal electrical connections between the atria and the ventricles. Individuals with SVT are often born with these extra pathways. The existence of such extra pathways (often called “bypass tracts”) allow the formation of “reentrant” arrhythmias, in which an electrical impulse is established that spins continuously between the atria and the ventricles, thus causing one form of SVT.Wolff-Parkinson-White syndrome is a common example, but there are several other varieties of bypass tracts that can cause episodes of SVT.Figure 1 - A bypass tract. A bypass tract is an extra connection between the atria and ventricles.
Figure 2 - SVT caused by a bypass tract.The purple line indicates the pathway followed by this reentrant electrical impulse.
Other varieties of SVT involve the formation of reentrant arrhythmias within the atria themselves (that is, without involving bypass tracts). Common examples of these “intra-atrial SVTs” include atrial fibrillation and atrial flutter. (Click here for a discussion of atrial fibrillation and its treatment.)
Figure 3 - Atrial flutter. In atrial flutter, a large reentrant circuit is formed within the right atrium (indicated by the purple line.)
Most forms of SVT are episodic in nature. They commonly cause severe palpitations, anxiety, and lightheadedness, and if they are frequent enough or last long enough can prove quite disruptive one’s lifestyle. But they only rarely pose a significant risk to life and limb.
Because most forms of SVT involve definite pathways for the abnormal electrical impulses that cause the arrhythmias, they are often amenable to radiofrequency ablation. This is a procedure conducted in the electrophysiology laboratory, in which the electrical pathways are carefully mapped using electrode catheters inserted within the heart, and critical areas within those pathways (such as bypass tracts) are ablated – that is, cauterized – by passing radiofrequency energy through the catheters.
