Adenosine. An evaluation of its use in cardiac diagnostic procedures, and in the treatment of paroxysmal supraventricular tachycardia.

Adenosine (adenine riboside), administered either as the free base or as the 5'-triphosphate (ATP) by rapid intravenous bolus, depresses atrioventricular (AV) nodal conduction, resulting in transient AV block. Adenosine is the active agent and ATP is rapidly converted to adenosine after exogenous administration. By blocking the anterograde AV nodal limb of a re-entrant circuit, adenosine 6 to 12 mg (or ATP 10 to 20 mg) converts almost all episodes of paroxysmal supraventricular tachycardia (PSVT) involving the AV node within 30 seconds of administration. This is at least equivalent in efficacy to verapamil in adults, and superior to lanatoside C in children, with a considerably more rapid onset of action. Furthermore, if a dose of adenosine is ineffective, the exceptionally short plasma half-life of the adenyl nucleosides (less than 10 sec) allows rapid upward dosage titration until PSVT is terminated. Because the induced conduction block primarily affects the AV node, adenosine is a useful diagnostic tool in patients with broad or narrow QRS complex tachycardia; it terminates arrhythmias dependent on the AV node, unmasks other supraventricular mechanisms during transient AV block, but almost always has no effect on ventricular tachycardia. Noncardiac adverse effects, i.e. flushing, dyspnoea and chest pain, may occur during acute arrhythmia termination or diagnosis with adenosine, and arrhythmias may develop; however, these effects are usually transient (lasting less than 1 minute). Adenosine has also been used to induce coronary vasodilation in patients undergoing thallium-201 single photon emission computed tomography (201Tl SPECT), 2-dimensional echocardiography or positron emission tomography to evaluate suspected coronary artery disease. Intravenous infusion of adenosine 140 micrograms/kg/min for 6 minutes was generally associated with only mild adverse effects. These usually resolved within 1 to 2 minutes of discontinuing adenosine, although occasionally patients required aminophylline and/or nitroglycerin (glyceryl trinitrate). Diagnoses based on the results of scintigraphy were of a sensitivity, specificity and predictive accuracy comparable to those achieved with exercise- or dipyridamole-201Tl SPECT. Adenosine is therefore particularly suitable for the diagnosis of tachycardias and the acute management of PSVT involving the AV node in all age groups, without the risks of cardiac arrest and hypotension associated with verapamil. Furthermore, intravenous adenosine infusion may be used to induce coronary vasodilation in patients unable to perform exercise stress tests for 201Tl scintigraphy, and is well tolerated.