Supraventricular tachycardias

Supraventricular tachycardias

Some supraventricular tachycardias can be missed clinically as the AV conduction may not be 1:1 and the heart rate may be in the normal range. The clue to atrial rate may be obtained by examining the jugular venous pulse. All tachycardias originating above the bifurcation of the bundle of His or using atrial tissue in a reentrant circuit are included in supraventricular tachycardias.

Tachycardias can be classified clinically into paroxysmal, persistent or chronic. Depending on mechanism, it can be grouped into those due to reentry, triggered activity or enhanced automaticity. The former is a disorder of impulse conduction while the latter two are disorders of impulse production.

Reentrant tachycardias can be reliably reproduced by programmed stimulation. Presence of a loop circuit, unidirectional block due variation of refractory period and a zone of slow conduction are the requirements for reentry.

Alpha and beta pathways were later called slow and fast pathways. Now we know that there are no separate pathways, but slow and fast inputs to the AV node (posterior and anterior) from the atrial tissue.

Slow pathway modification is a common method for ablation of atrioventricular nodal reentrant tachycardia [1]. In atrial echo beats the atria gets activated through the fast pathway and the P wave is usually buried within the terminal part of the QRS. The retrograde activation of the atria is delayed in atrioventricular reentrant tachycardia as part of the path is slow conducting ventricular muscle.

P waves are narrower if the activation is from the centre of the heart as in AVNRT or septal accessory pathways because of simultaneous activation of both atria. Sinus P waves and P waves due to retrograde activation through lateral pathways have broader P waves as there is only sequential activation of atria.

Reference

1. Hugh Calkins, V K Ajit Kumar, Johnson Francis. Radiofrequency catheter ablation of supraventricular tachycardia. Indian Pacing Electrophysiol J. 2002 Apr 1;2(2):45-9.