ECG - Supraventricular tachycardia SVT

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Supraventricular tachycardia: ECG

• Rate: exceeds upper limit of sinus tachycardia (>120 beats/min); seldom <150 beats/min; up to 250 beats/min
• Rhythm: regular
• P waves: seldom seen because rapid rate causes P wave loss in preceding T waves or because the origin is low in the atrium
• QRS complex: normal, narrow (=0.10 sec usually)

AV reentry

Supraventricular tachycardia commonly presents in two forms - AV reentry and AV nodal reentry. In AV reentry (below), the SVT presents as a regular tachycardia originating outside the ventricular myocardium. In this type of SVT, the AV node is used for impulse conduction to the ventricles, while an accessory pathway is used to return electrical conduction back to the atria. The heart rate is usually regular, at a rate of 170 to 250 bpm (below = 188 bpm). In this type of SVT, P waves are always present outside of the QRS complex, while their polarity depends on the atrial insertion of the accessory pathway. The QRS complex is narrow with a duration of less than 0.2 seconds and an atrioventricular conduction ratio of 1:1. In 25% - 30% of patients demonstrating AV reentry, QRS alternans is present (varying amplitudes of the QRS complex in all leads except V4). AV reentry is not usually associated with structural heart disease and commonly presents as a variety of symptoms including palpitations, nervousness, anxiety, syncope or heart failure.

AV nodal reentry

AV nodal reentry is a form of SVT that establishes its atrioventricular circuit entirely within the AV node. The heart rate is usually regular, between 150 to 250 bpm (below = 186 bpm). The QRS complex is narrow with a duration of fewer than 0.2 seconds and a conduction ratio of 1:1. P waves are buried within the QRS complex (as illustrated below), although they may be visible at the end of the complex as a distortion of the terminal forces. Due to the fact that atrial activation originates from the inferior aspect of the right atrium, P wave polarity is negative in leads II, III and aVF. This form of SVT is usually benign and is easily converted to sinus rhythm by vagal manoeuvres.

This web-based module was developed and edited by Adam Szulewski based on content written by Dr. Bob McGraw, Dr. Jason Lord, Matthew Westendorp, Lisa Evans and Jordan Chenkin for the Queen's University Technical Skills Program and Department of Emergency Medicine. The module was created using exe : eLearning XHTML editor with support from Lynel Jackson and the Queen's University School of Medicine MedTech Unit.