Atrio-ventricular re-entry tachycardia (AVRT)

AVRT differs from atrioventricular nodal re-entry tachycardia (AVNRT) in that the latter consists of cycles of depolarisation occurring in the AV node itself, whereas in AVRT the cycle of depolarisation involves depolarisation from one of the ventricles re-entering the AV node. AVRT is seen more often in men, whereas AVNRT is more commonly in women. AVRT is almost always seen in patients with Wolff-Parkinson-White syndrome (WPWS), in which an accessory pathway exists in the heart, allowing electrical communication between the atria and ventricles. Normally, the atria and ventricles are electrically isolated from one another, and the only pathway through which action potentials can reach the ventricles is via the atrio-ventricular (AV) node. Action potentials are delayed slightly at the AV node to allow the atria to completely contract. Most of the time, patients with WPWS have a normal sinus rhythm and the tendency for pre-excitation is apparent in the form of delta waves – a slow depolarisation preceding the R wave. This represents the spread of the action potential “leaking” down the accessory pathway, where it is conducted more slowly than the action potential carried by the bundle of His.

Figure 1: The accessory pathway in WPWS allows the action potential in the atrium to leak into a ventricle, as well as being conducted by the bundle of His. Depolarisation of the accessory pathway shows up on the ECG as delta waves – slow upward deflections preceding the RS waves. The P-R interval is short or non-existent in WPWS, because although the action potential is delayed at the AV node (as usual), the leak down the accessory pathway occurs immediately.

Orthodromic AVRT

As in AVNRT, tachycardia occurs in patients with WPWS syndrome when a freak premature atrial contraction (PAC) occurs at just the right time and in the right place. After conducting an action potential, the accessory pathway is briefly refractory to further stimulation. A PAC that results in depolarisation of the AV node at that moment will fire an action potential down the bundle of His in the normal manner. By the time the ventricles have nearly finished depolarising, the accessory pathway is no longer refractory and conducts the action potential back towards the atria, starting the cycle over once again. Because the spread of action potentials is in the normal direction, this type of AVRT (representing about 95% of cases) is known as orthodromic AVRT. Delta waves disappear, because the accessory pathway is no longer acting as an alternative pathway to depolarise the ventricles. Instead, the accessory pathway is closing the loop on a cycle of depolarisation. The P wave is usually obscured by – or appears just after - the QRS complex, since atrial depolarisation occurs at the end of each cycle, rather than preceding it.

Figure 2:Orthodromic AVRT. Unlike the sinus rhythm in WPWS, during episodes of tachycardia, the accessory pathway conducts the action potential towards the atria, rather than away. To establish this cycle of depolarisation, a PAC needs to arrive at the AV node while the accessory pathway is refractory to stimulation (i.e. just after a recent action potential). Once an action potential is able to activate the bundle of His while the accessory pathway is briefly refractory, a simple cycle is allowed to establish.

Antidromic AVRT

Antidromic AVRT is much less common (about 5% of cases) and involves a cycle of depolarisation occurring in the opposite direction. For this to occur, a PAC must depolarise the accessory pathway while it is not refractory, but at a time when the AV node is refractory. Hence, the accessory pathway conducts an action potential to the ventricles, where it passes backwards up the bundle of His to depolarise the SA node. This produces a big delta wave, and because conduction is slower through muscle than the bundle of His, the QRS complex becomes widened as a result. Antidromic AVRT can produce an ECG signal that is difficult or impossible to distinguish from ventricular tachycardia.

Figure 2:Antidromic AVRT. In this rarer variant, the accessory pathway conducts the action potential to the ventricle and back up the septum to the AV node. The sluggish conduction through muscle widens the QRS complex

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