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Adenosine has been proposed as a useful diagnostic agent in patients with narrow complex regular tachycardia of uncertain origin. Its effects are usually transient owing to its extremely short plasma half life and, as a consequence, it is thought to be safer than other drugs used in the acute treatment of such arrhythmias. However, adenosine had a proarrhythmic effect when administered to a patient in order to confirm the diagnosis of atrial flutter. As expected, a transient increase in atrioventricular block was seen but this was followed by a doubling of the ventricular rate and haemodynamic compromise requiring immediate DC cardioversion. It is postulated that the secondary catecholamine-mediated effects of adenosine were responsible for this phenomenon.