Supraventricular techycardia (SVT) is a type of arrhythmia (an abnormal heart rhythm) which originates in the top heart chambers (i.e. supra = above, ventricles = bottom heart chambers). Episodes of SVT start and stop suddenly and the fast and regular heart beat can last from seconds up to hours. It may happen several times a day or only a few times a year or even less. Normal heart rate in adults is about 60-90 beats per minute (bpm) at rest. In supraventricular tachycardia, the heart rate is usually between 140-180 bpm and sometimes can exceed 200 bpm. Unlike in atrial fibrillation, the pulse is regular in SVT.
Under normal circumstances, the heart muscle contracts and ejects the blood into circulation, then relaxes, is filled with blood and the whole process is repeated. In SVT the contractions of the heart muscle are so fast that the heart cannot properly relax and fill with blood. The heart is therefore running half-empty during SVT which reduces the amount of blood pumped into circulation, causing palpitations (awareness of unpleasant rapid pulse), breathlessness, tiredness, dizziness and sometimes blackouts.
The abnormal electrical impulses, that make the heart beat fast in SVT, start in the top heart chambers, override normal heart rhythm and spread around the heart. There is no obvious cause of SVT and the episodes of palpitations may occur randomly without obvious trigger. Sometimes, SVTs are associated with exertion, emotional upset, change in posture, coffee and alcohol. SVTs may occur at any age but are more common in young adults.
In most cases the episodes of SVT are unpleasant but prognostically benign. The episodes usually last only a short period of time and terminate spontaneously without treatment. Most SVTs can be stopped by Valsalva manoeuvre; i.e. forceful attempted exhalation against a closed airway, usually done by closing mouth and pinching nose shut while pressing out as if blowing up a balloon. If Valsalva manoeure is unsuccessful, SVT can be stopped with medication (e.g. adenosine, beta-blockers and calcium antagonists) to slow or block the abnormal electrical signals. Most patients with frequent SVTs undergo catheter ablation which breaks the abnormal circuit and permanently cures the arrhythmia.