cardiovascular disorders
Reentrant Supraventricular Tachycardias (SVT) Including Wolff Parkinson White Syndrome
Pathophysiology of Reentrant SVTs
The reentry pathway (see figure) in supraventricular tachycardia may be Within the atrioventricular (AV) node (about 50%) An accessory bypass tract (40%) OR Confined to the atria or sinoatrial (SA) node (10%) occurs most often in otherwise healthy patients. It is most commonly triggered by an Atrial premature beats . involves tracts of conducting tissue that partially or totally bypass normal AV connections (bypass tracts). They run most commonly from the atria directly to the ventricles and less commonly from the atrium to a portion of the conduction system or from a portion of the conduction system to the ventricle. They can be triggered by atrial premature beats or Ventricular Premature Beats (VPB) . WPW (preexcitation) syndrome is the most common accessory pathway SVT, occurring in about 1 to 3/1000 people. WPW syndrome is mainly idiopathic, although it is more common among patients with Hypertrophic Cardiomyopathy or other forms of Classic Concealed antegrade conduction occurs over both the accessory pathway and the normal conducting system during sinus rhythm. The accessory pathway, being faster, depolarizes some of the ventricle early, resulting in a short PR interval and a slurred upstroke to the QRS complex (delta wave—see figure Classic Wolff-Parkinson-White (WPW) syndrome ). Leads I, II, III, V3through V6show classic features of WPW syndrome, with a short PR interval and a delta wave during sinus rhythm. The delta wave prolongs QRS duration to > 0.12 second, although the overall configuration, apart from the delta wave, may appear normal. Depending on the orientation of the delta wave, a pseudoinfarction pattern Q-wave may be present. Because the early depolarized parts of the ventricle also repolarize early, the T-wave vector may be abnormal. Inconcealed WPW syndrome,the accessory pathway does not conduct in an antegrade direction; consequently, the above electrocardiographic (ECG) abnormalities do not appear. However, it conducts in a retrograde direction and thus can participate in reentrant tachycardia. In the most common form of reentrant tachycardia (called orthodromic reciprocating tachycardia), the circuit uses the normal atrioventricular (AV) conduction pathway to activate the ventricles, returning to the atrium via the accessory AV connection. The resultant QRS complex is thus narrow (unless Bundle Branch Block and Fascicular Block coexists) and without a delta wave. Orthodromic reciprocating tachycardia is typically a short RP tachycardia with the retrograde P wave in the ST segment. Rarely, the reentrant circuit revolves in the opposite direction, from the atrium to the ventricle via the accessory AV connection, and returning from the ventricle in the retrograde direction up the normal AV conduction system (called antidromic reciprocating tachycardia). The QRS complex is wide because the ventricles are activated abnormally. In patients with 2 accessory AV connections (not uncommon), a reciprocating tachycardia using one accessory connection in the antegrade direction and the other in the retrograde direction may occur. Tachycardias in WPW syndrome may begin as or degenerate into Atrial Fibrillation and Wolff-Parkinson-White Syndrome (WPW) (AF), which can be very dangerous if there is antegrade conduction over the accessory pathway (manifest WPW). In this case, the normal rate-limiting effects of the atrioventricular (AV) node are bypassed, and the resultant excessive ventricular rates (sometimes 200 to 300 beats/minutes) may lead to 