Cause

The definition of tachycardia is a heart rate greater than 100 beats per minute (bpm). Supraventricular tachycardia (SVT) is an abnormally fast heart rhythm arising from improper electrical activity in the upper part of the heart. 

SVT starts from either the atria or atrioventricular node. It is generally due to one of two mechanisms: re-entry or increased automaticity. The other type of fast heart rhythm is ventricular arrhythmias—rapid rhythms that start within the ventricle.

Sinus tachycardia originates from the sinus node, also called Sinoatrial node or SA node. It is caused by the SA node firing at a rate of greater than 100 bpm. All other electrical conduction after the SA node is normal.

Symptoms

Symptoms may include palpitations, feeling faint, sweating, shortness of breath, or chest pain.

Diagnosis

 • Sinus tachycardia: Regularly occurring P waves (>50–60/min) because of stress, exercise, pain, drugs, or underlying disease. 

• Atrial flutter: Regularly occurring sawtooth flutter waves at about 180 to 250/min. 

• Atrial fibrillation: Fibrillation waves replace P waves. 

• Ventricular premature contractions (VPCs): Depending on the prematurity of a VPC, the intensity of the first heart sound may be normal or reduced (indistinguishable from APC) or increased (in contrast to an APC). ECG is needed for the final diagnosis. 

• Ventricular tachycardia: (Ir)regular RR intervals, QRS complexes with abnormal duration and morphology, and not associated with a preceding P wave 

• Diagnosis of SVT is typically by electrocardiogram (ECG), holter monitor, or event monitor. Blood tests may be done to rule out specific underlying causes such as hyperthyroidism or electrolyte abnormalities.