ACCESSORY PATHWAY RECIPROCATING TACHYCARDIA

Patients who have an accessory pathway (AP) of atrioventricular (AV) c onduction may develop circus movement tachycardia otherwise known as a trioventricular re-entrant tachycardia (AVRT). Orthodromic AVRT is the most common form. It occurs as a result of antegrade conduction throu gh the normal AV conduction system and retrograde conduction to the at ria via the AP. Less commonly, conduction occurs in the opposite direc tion resulting in antidromic AVRT. Tachycardia may also involve multip le APs which may provide both antegrade and retrograde conduction and may alternate antegradely or retrogradely. Tachycardia may occur in wh ich the AP simply acts as a bystander, and does not participate in the tachycardia mechanism. When atrial fibrillation is conducted to the v entricles via an AP, the resultant ventricular rare may be extremely r apid, placing the patient at risk of developing ventricular fibrillati on and cardiac arrest. This paper reviews the anatomical and physiolog ical substrates involved in the pathogenesis of AVRT. The acute and lo ng-term management of patients who suffer from these arrhythmias will then be discussed. The normal AV annulus is composed exclusively of el ectrically inert fibrous tissue. The AV node and His bundle normally a ct as the sole route of electrical conduction. Accessory pathways occu r at all points along the AV ring, and usually occur as isolated abnor malities, although a proportion of patients have associated congenital abnormalities. This is particularly true of right-sided APs. Most APs exhibit non-decremental conduction properties, and conduct faster tha n normal AV conduction tissue. In many patients with APs the surface E CG reveals clear evidence of pre-excitation, and a good idea of pathwa y localization is possible using one or more of several algorithms whi ch have been developed. Patients with latent pre-excitation, intermitt ent pre-excitation, and patients with concealed APs have no evidence o f pre-excitation on a proportion or all of their surface ECGs. Patient s present with a history of paroxysmal palpitations, often with associ ated symptoms such as chest discomfort. Syncope is a rare presenting s ymptom. Unless bundle branch block is present, patients with orthodrom ic AVRT exhibit a narrow complex tachycardia on the surface EGG. Patie nts with pre-excited tachycardia including antidromic AVRT, and other forms of SVT in which the AP conducts to the ventricles as a bystander but does not participate in the tachycardia mechanism, present as bro ad complex tachycardias on the surface ECC which may be difficult to d istinguish from ventricular tachycardia. Adenosine is increasingly use d for this purpose since it is highly efficacious and has an extremely short half-life. Adenosine is also very useful in the diagnosis of br oad-complex tachycardia, and in unmasking latent preexcitation during sinus rhythm. Electrophysiology study in these patients is frequently performed at the same lime as an attempt at catheter ablation; it aims to diagnose, localize and determine the functional characteristics of an AP, and to characterize the role of the pathway in tachycardia. AV RT can be reliably terminated by effective AV nodal blockade. Drug the rapy for the prevention of AVRT is useful for temporary control whilst awaiting more definitive measures and in certain cases as long-term m anagement. No class of drug stands out as 'therapy of choice', and phy sician preference, pro-arrhythmic effects and associated conditions ne ed to be taken into account such that an individual choice can be made in each patient. The management of patients with AVRT has been revolu tionized in recent years with the advent of catheter-based techniques for their cure. Whilst this method of treatment is highly effective an d has low complication rates, pathways in particular locations such as the septal region remain challenging.