The rhythm in this ECG is suggestive of
a) Sinus tachycardia
b) Atrial fibrillation
c) Supraventricular tachycardia
d) None of the above
Correct answer: c) Supraventricular tachycardia
Supraventricular tachycardia is seen here at a rate of around 150 / min (RR interval 10 mm or 400 msec). The QRS is narrow in supraventricular tachycardia without any aberrant conduction as in this case. If there is aberrant conduction, it can be wide and usually showss a right bundle branch block pattern as aberrancy is more common in right bundle. The P waves are not very evident in this case and may be buried within the QRS complex. This can occur in AV nodal re-entrant tachycardia (AVNRT) and junctional tachycardia due to simultaneous activation of the atria and ventricles. When there is simultaneous contraction of the atria and ventricles, clinical examination will reveal cannon waves.
When a supraventricular tachycardia at a rate of 150 per minute is seen, atrial flutter with 2:1 conduction should also be borne in mind as the flutter waves may not be evident in all leads. Carotid sinus massage may alter the AV conduction ratio to make the flutter waves evident, if they are within the QRS or T waves.
The two common forms of supraventricular tachycardia are AVNRT and atrioventricular re-entrant tachycardia (AVRT) of WPW syndrome. The latter often manifest the typical pattern of WPW syndrome (short PR interval and delta wave) after termination of the tachycardia.