ECG. The ECG was recorded from a 71 year old woman admitted for cataract surgery. While awaiting the operation she developed the tachycardia shown in the rhythm strip. On direct questioning she admitted to a lifelong history of discrete attacks of palpitation but had never sought medical advice.
Morphology. (Lead VI)The rhythm strip shows the initiation of a narrow complex tachycardia (QRS duration less than .10 seconds), at a rate of 180 per minute. The tachycardia arises above the bifurcation of the bundle of His and is therefore a supraventricular tachycardia. The first two beats in the trace are sinus beats.The third beat shows a premature P wave of different configuration to the sinus P waves. This beat arises from an ectopic focus in the atria (ie it is an atrial premature beat) and this initiates the tachycardia.
Comment. This patient was subsequently shown to have ventricularpre-excitation on her EGG and the typical features of the WolffParkinson White syndrome. The tachycardia is described as an atrio-ventricular re-entrant (AVRT) tachycardia. In sinus rhythm atrial depolarization reaches the ventricle after passing through both the AV node as well as the accessory pathway; the ventricles are activated through both routes. An atrial premature beat may find the accessory pathway refractory but be able to reach the ventricles through the AV node. As depolarization subsequently spreads through the conducting system and ventricular myocardium, it reaches the distal end of the accessory pathway. If this is no longer refractory depolarization will spread retrogradely to the atria ands ubsequently be conducted back into the ventricles through the AV node. If this circuit is maintained, a sustained tachycardia results. A similar circus movement can also be initiated by a ventricular premature beat that activates the atria retrogradely through the accessory pathway.
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