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A 64 year old women was admitted because of palpitations and a rapid pulse rate. She had a history of coronary heart disease, diabetes, and arterial hypertension. Her medication comprised ramipril, aspirin, amitriptyline, and insulin. This ECG was taken upon admission and shows a tachycardia (136 beats/min) with a left bundle branch block and left axis morphology. A premature, more narrow complex was recorded (arrow) during the tachycardia. It was classified as a supraventricular capture beat, consistent with a diagnosis of ventricular tachycardia. However, the QRS morphology during tachycardia and the right axis vector of the presumed capture beat cast doubt on the diagnosis. Carotid sinus massage had no effect. A 12 mg dose of intravenous adenosine was administered that led to complete atrioventricular block, cessation of the tachycardia, disclosure of atrial flutter, and persistence of the left bundle branch block morphology of the QRS complex, as seen during tachycardia.
The most likely explanation for the “pseudocapture beat” is supernormal conduction of an atrial flutter beat along the left anterior bundle, with persistence of block along the left posterior bundle. Supernormal excitability and conduction are observed during a short period at the end of the recovery phase of the action potential, when an otherwise subthreshold stimulus can depolarise the membrane and be propagated faster than during normal resting potential.