Supraventricular tachycardia is a general term that describes tachycardias arising from within the atria or the atrioventricular node. It is the preferred term as the exact cause of the tachycardia is not always obvious from reviewing the ECG.
Paroxysmal supraventricular tachycardia defines a type of rapid supraventricular arrhythmia characterised by a sudden onset and often abrupt termination.
Three of the most common supraventricular tachycardias are:
- atrial tachycardia,
- atrioventricular reentrant tachycardia, and
- atrioventricular nodal reentrant tachycardia.
The likeliest cause of atrial tachycardia is rapid and repetitive depolarisations from a focus of cells within the atria that lie outside of the sinus node.
The ECG trace below shows four sinus beats (arrowed), at a rate of 106 bpm (the RR interval is approximately three boxes), followed by a fast, regular heart rate at 147 bpm (the RR interval is approximately two boxes).
Atrioventricular Reentrant Tachycardia
Atrioventricular reentrant tachycardia is a supraventricular rhythm whose conduction pathway is a circuit that includes both the atria and the ventricles, using an additional atrioventricular pathway for at least one limb of that circuit.
As atrial conduction does not precede ventricular conduction, the P wave may appear inverted or even be hidden in the QRS complex.
Orthodromic atrioventricular reentrant tachycardia proceeds antegrade (from the atria to the ventricles) through the atrioventricular node, and retrograde (from the ventricles to the atria) over the additional pathway.
Antedromic atrioventricular reentrant tachycardia proceeds in the reverse direction, appearing on the ECG as a wide QRS complex tachycardia.
The rhythm in the following ECG trace is regular and rapid at 196 bpm. The P waves are not visible as they are hidden in the QRS complex. Note the absence of a P wave in the longer (shaded) RR interval.
Atrioventricular Nodal Reentrant Tachycardia
The most common cause of supraventricular tachycardia is atrioventricular nodal reentrant tachycardia. It is caused by two electrical pathways within the atrioventricular node that create a continuous circuit of conduction.
The tachycardia is normally initiated by an atrial ectopic beat arriving at the atrioventricular node. Depolarisation is conducted towards the ventricles down one pathway and then back to the atria through the other pathway. This sequence can perpetuate, assuming the pathways have repolarised before the next impulse arrives.
In the following example, paroxysmal tachycardia is present for four beats (shaded) at a rate of 180 bpm. The tachycardia is initiated by atrial bigeminy (beats two and four) and reverts to sinus rhythm. Normally conducted (sinus) beats are arrowed.