Cardiac arrhythmias (heart rhythm problems) can be divided into two general types: Those that make the heart rhythm too slow -- known as bradycardias -- and those that make the heart rhythm too rapid -- known as tachycardias. This article briefly reviews the tachycardias.
Tachycardias also can be sub-divided into two general types: supraventricular tachycardias, which arise in the atria of the heart, and ventricular tachycardias, which arise in the ventricles.
Supraventricular Tachycardias (SVT)
SVTs, which are generated in atria of the heart, are most often seen in young, otherwise healthy individuals. Most forms of SVT occur as random episodes that begin and end suddenly. SVT commonly causes palpitations, anxiety, and lightheadedness, and if they are frequent enough or last long enough, they can prove quite disruptive to one's lifestyle. But they only rarely pose a significant risk to life and limb. There are several varieties of SVT. The most common of these include:
- Inappropriate sinus tachycardia (IST) is caused by a sinus node that generates electrical signals too rapidly. IST is discussed in detail here.
- AV-nodal reentrant tachycardia, and SVT caused by bypass tracts (also known as Wolff-Parkinson-White syndrome). These types of SVT are caused by abnormal electrical pathways that are present in the hearts of some people from birth. These abnormal pathways can be mapped precisely during a catheter procedure called the electrophysiology study, and "ablated" by cauterization through a catheter (small tube). In other words, these forms of SVT can usually be cured if they are causing significant symptoms.
- Atrial fibrillation (and its close relative, atrial flutter) are the most common types of SVT. Atrial fibrillation is usually seen in older patients, who have other types of cardiovascular disease. In addition to palpitations, atrial fibrillation produces a risk of blood clots that can lead to stroke. Atrial fibrillation is discussed in detail here.
Ventricular Tachycardia (VT)
VT is a rapid heart rhythm originating within the ventricles. VT tends to disrupt the orderly contraction of the ventricular muscle, so that the ventricle's ability to eject blood is often significantly reduced. That, combined with the excessive heart rate, can reduce the amount of blood actually being pumped by the heart during VT to dangerous levels. Consequently, patients with VT often experience -- in addition to palpitations -- extreme lightheadedness, loss of consciousness, or even sudden death.Ventricular fibrillation (VF) is closely related to VT. However, VF is a much more disorganized rhythm that immediately brings to a halt all meaningful ventricular contractions. Blood immediately stops flowing, and loss of consciousness occurs within seconds. Unless cardiopulmonary resuscitation is started within a few minutes of the onset of VF, sudden death will occur.
Patients with a high risk of life-threatening VT or VF today are best treated by insertion of an implantable defibrillator.
Heart Disease Patients and Tachycardia
Both VT and VF are usually seen in patients who have significant underlying heart disease, most often after heart attack or heart failure. The risk of sudden death after surviving a heart attack is an especially important problem, and unfortunately (since it is preventable) it is one that doctors all too frequently overlook.
An exception to the rule that serious ventricular arrhythmias are usually seen in patients with underlying heart disease is long QT syndrome (LQTS).
Ssudden death in apparently healthy young people, especially young athletes, is sometimes seen in a condition called hypertrophic cardiomyopathy, or HCM.
