© SUPPLEMENT OF JAPI  •  aPRIL 2007  •  VOL. 55

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Supplement

Treatment of Supraventricular Tachycardia R Samie, M Green

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INTRODUCTION

upraventricular tachycardias (SVT) may be defined as any tachycardias that requires the atrium or AV junction for its maintenance. Thus, any tachycardia using myocardial tissue above the level of the His-Purkinje system may be said to be a SVT. The classification of these tachycardias is arbitrary, however from a therapeutic point of view the use of the AV node may be a good distinguishing feature. A useful classification is:

A. AV Nodal dependant These tachycardias are comprised of a reentry circuit. that includes the AV node. The AV node is integral to the tachycardia and even transient block of the AV node will terminate the tachycardia.

B. AV Nodal independent These tachycardias originate in the atria or pulmonary veins and may be either due to increased automaticity, triggered activity or a reentrant circuit in the atria. The AV node is not an integral part of the tachycardia and block of the AV node will not terminate the arrhythmia. Transient block of the AV node may aid in the diagnosis of these tachycardias as atrial tachycardia or atrial flutter may be easier to visualize.

AV Nodal dependant

AV Nodal independent

AV nodal reentrant tachycardia (AVNRT)

Atrial Tachycardia

AV re-entrant tachycardia (AVRT)

Atrial flutter



Atrial fibrillation (AF)

A. AV Nodal Dependant 1. AV nodal reentrant tachycardia (AVNRT) This is due to a re-entry circuit that consists of a fast pathway and a slow pathway that have different properties of conduction and refractoriness. Duringtypical AVNRT, the slow pathway acts as the anterograde pathway and the fast pathway acts as the retrograde part of the circuit. In some atypical forms of AVNRT (occurring in < 5% of cases), the fast pathway may act as the anterograde pathway and the slow pathway as the retrograde pathway.1 The fast and slow pathways are found in the triangle of Koch, the borders being the coronary sinus os, the septal leaflet of the tricuspid valve and the tendon of Todaro.

2. AV re-entrant tachycardia (AVRT) These are due to re-entry circuits that involve the AV node and an accessory pathway. Accessory pathways (AP) are abnormal bands of muscle that connect the ventricle to the atrium and occur in various positions along the AV groove. If the accessory pathway is manifest on the electrocardiogram as pre-excitation and the patient has tachyarrhythmias then it is termed Wolff- Parkinson-White syndrome. Division of Cardiology, University of Ottawa Heart Institute, 40 Ruskin, Ottawa, Ontario, K1Y 4W7, Canada.

AVRT is termed orthodromic if the AV node acts as the anterograde pathway and an AP as the retrograde pathway. AVRT is termed antidromic if anterograde conduction occurs over the AP and retrograde conduction over the AV node or a second accessory pathway. As such, the antidromic tachycardias will have a wide QRS complex.

B. AV Nodal independent 1. Atrial Tachycardia They may occur in any part of the atrium, but commonly arise from the crista terminalis, atrial septum, pulmonary veins and the mitral valve annulus. They may be classified as automatic, triggered, or re-entrant. They usually have atrial rates between 100 and 250 bpm and have an isoelectric baseline. The AV node is not involved in the initiation or continuation of the tachycardia.

2. Atrial flutter Atrial flutter is an organized atrial rhythm with no clear isoelectric baseline, at a rate of 250 to 350 bpm. The most common atrial flutter, typical atrial flutter, is due to a counterclockwise macroreentrant circuit that has well defined boundaries in the right atrium and a critical area of slow conduction, the cavotricuspid isthmus. Nonisthmus dependant atrial flutter may occur in the right atrium and left atrium and may be associated with scar or previous surgery. In all atrial flutter circuits, the arrhythmia is independent of the AV node; i.e. the AV node is not required for the continuation of the flutter circuit.

3. Atrial fibrillation (AF) This is the most common sustained supraventricular arrhythmia with a prevalence of 0.1% among adults younger than 55 years to 9.0% in persons aged 80 years or older.2 Atrial fibrillation may be due to triggers arising from the PV, however in approximately 10% case the triggers may occur in the superior vena cava, crista terminalis or the coronary sinus. AF may also be due to multiple re-entry wavelets that occur in the atrium. The AV node is not involved in the initiation or perpetuation of atrial fibrillation.

TREATMENT AV Nodal Dependant SVT Immediate management The immediate management of any tachyarrhythmia depends on the clinical and hemodynamic status of the patient. A 12 lead EKG should be obtained prior to initiating treatment, provided the patient is stable., This would aid diagnosis of the arrhythmias as well as help determine the long-term management strategy. If the patient is conscious and hemodynamically stable then one can attempt physical maneuvers that increase vagal tone i.e. the valsalva maneuver or carotid sinus massage. Care should be taken when carotid sinus massage is attempted in the elderly as carotid artery disease or previous CVA/TIA is a contraindication. If this is unsuccessful, then pharmacological treatment is indicated. Initially adenosine 6mg IV may be rapidly injected, followed by 12 mg and then 18 mg. If this is unsuccessful, then a calcium channel blocker could be used, either verapamil 10 mg IV or diltiazem 0.25 mg/kg IV over 2 minutes. Alternatively a beta-blocker could be used eg metoprolol

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5 –10 mg IV or atenolol 5-10 mg IV. If the patient is hemodynamically unstable then immediate synchronized cardioversion is indicated. Once the AV node is blocked during an AV nodal dependent tachycardia, the re-entry circuit is interrupted and the arrhythmia is terminated. Occasionally AV nodal dependant tachycardias can restart after initially terminating. This may occur with adenosine or the Valsalva maneuver and is due to the short half-life of adenosine and short action of the Valsalva maneuver. A non-dihyropyridine calcium channel blocker or a long acting beta-blocker should then be used. Once the acute arrhythmia is terminated, the long-term management can be considered. These patients can generally be sent home from the emergency room and long-term management considered electively.

Long-term management No therapy Patients who have infrequent mild symptoms may opt for no pharmacological treatment and may use the valsalva maneuver intermittently, with attendance at an emergency department if the valsalva maneuver fails. This is an acceptable form of treatment, provided that the patient is not in a high-risk profession eg. airline pilot, commercial truck driver etc. If symptoms worsen, then pharmacological treatment or catheter ablation could be considered.

Fig. 1A : Initiation of Typical AV nodal reentry tachycardia with an atrial extra-stimulus (premature paced atrial beat) that blocks in the fast pathway, and conducts antegradely via the slow pathway and then reenters retrogradely via the fast pathway.

Pharmacotherapy Patients with infrequent prolonged episodes of AVNRT or AVRT, which are well tolerated and not associated with preexcitation, can take a “pill in the pocket” approach. Either a calcium channel blocker eg. diltiazem 120mg po or a beta blocker eg. metoprolol 50 mg po can be taken.1 Patients with frequent recurrent symptoms, who are not keen on catheter ablation, can be treated with either a calcium channel blocker or a beta-blocker chronically. If there are recurrent arrhythmias despite calcium channel blockers or beta blockers and the patient does not want catheter ablation, then antiarrhythmic agents such as flecainide, or propafenone or amiodarone can be used. As AVNRT is a benign arrhythmia, the latter agents should be used with caution, given the associated proarrhythmic risks and side effects.

Catheter Ablation Radiofrequency catheter ablation is the preferred treatment in young patients and patients with poorly tolerated tachycardia and can obviate the need for long-term pharmacotherapy. It can be performed for AVNRT and AVRT with success rates in the order of 95% and low complication rates with serious complications in the order of ≤ 1%.2

AVNRT During catheter ablation of AVNRT, initial attempts are made at ablation of the slow pathway, which usually lies anterior to the coronary sinus os. In earlier studies, the fast pathway was initially targeted but there was an incidence of heart block in the order of 5-10%. Subsequent studies demonstrated that by ablation of the slow pathway the incidence of heart block was approximately