Also Known As: Supraventricular tachycardia, SVT
Supraventricular tachycardia (SVT) is any rapid heart rhythm originating above the ventricular tissue. Supraventricular tachycardias can be contrasted to the potentially more dangerous ventricular tachycardias - rapid rhythms that originate within the ventricular tissue.
Although technically an SVT can be due to any supraventricular cause, the term is often used by clinicians to refer to one specific cause of SVT, namely Paroxysmal supraventricular tachycardia (PSVT). SVT is most commonly due to a loop of electrical current in the heart, which as it loops, causes the heart to beat quickly. Two common types of SVT are atrioventricular reciprocating tachycardia and AV nodal reentrant tachycardia. In the older adult population atrial fibrillation becomes one of the most common types of supraventricular arrhythmias - though this is typically considered separately. SVT is generally not life threatening, though it may cause worsening heart function if it is sustained for hours.
In general SVT are caused by two separate mechanisms. The first mechanism to cause SVT is re-entry, the second mechanism is automaticity. SVT due to re-entry (such as AV nodal reentrant tachycardia and atrioventricular reciprocating tachycardia), often present with a sudden, almost immediate onset  with a sudden increase in heart rate. A person experiencing PSVT may feel their heart rate go from 60 to 200 beats per minute or more. It typically also has a sudden termination from the fast heart rate back to normal rhythm. Wolff-Parkinson-White Syndrome is a combination of SVT with characteristic findings on ECG.
Automatic types of SVT (Atrial Ectopic Tachycardia, junctional ectopic tachycardia) more typically have a gradual increase and decrease of the heart rate. These are due to an area in the heart that generates its own signal.
Symptoms can come on suddenly and may go away without treatment. Stress, exercise, and emotion can all result in a normal or physiological increase in heart rate, but can also, though more rarely, precipitate SVT. Episodes can last a few minutes or as long as 1 or 2 days, sometimes persisting until treated. The rapid beating of the heart during SVT can make the heart a less-effective pump, decreasing cardiac output and blood pressure. The following symptoms are typical with a rapid pulse of 150–270 or more beats per minute: