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In a proportion of patients with left free wall accessory connections, preexdtation is apparent only during atrial arrhythmias or atrial pacing (latent preexcitation). These patients may be at risk of a rapid ventricular response to atrial fibrillation despite the absence of preexcitation in sinus rhythm. The ability of intravenous adenosine to unmask latent preexcitation was evaluated in 22 patients with a history of documented supraventricular tachycardia and a normal electrocardiogram during sinus rhythm. Preexcitation was unmasked in response to adenosine in 4 patients: all 4 were shown to have latent preexcitation at electrophysiologic study. in 12 patients atrioventricular (AV) nodal conduction delay or block was induced without preexcitation after adenosine first-degree AV block in 8, second-degree block in 4): at subsequent electrophysiologic study none of these patients was found to have latent preexcitation. Five patients had little or no PR prolongation in response to adenosine: of these, 2 were shown to have latent preexcitation at electrophysiologic study. Atrial fibrillation was induced in 1 patient and a narrow complex regular tachycardia in another after intravenous adenosine. Intravenous adenosine during sinus rhythm is capable of producing AV nodal conduction delay or block in 73% of patients with a history of supraventricular tachycardia: in these patients adenosine provides a diagnostic test that is both 100% sensitive and 100% specific for latent preexcitation. in those patients in whom adenosine does not produce AV conduction delay or block, further investigation is required to establish or refute the diagnosis of latent preexcitation. (Am J Cardiol 1990;6S:868-873), Normal rhythmic contraction of the heart is the result of proper functioning of the heart's electrical conduction system, which involves special fibers in the heart muscle. Abnormalities of this conduction system can predispose the individual to sudden death if atrial fibrillation (AF) develops. AF is characterized by an irregular, random contraction of the atria, the chambers that receive blood from the body and lungs and transmit it to the ventricles. Abnormal accessory atrioventricular (AV) connections can exist, which are additional paths the electrical impulse can take as it moves from atria to ventricles. This can lead to preexcitation, or premature activation, of the ventricles, since the impulse that takes the accessory path may arrive at the ventricle before the impulse taking the normal path. Detection of preexcitation is important if treatment aimed at preventing ventricular fibrillation, a life-threatening development, is to be instituted. Some patients, however, in whom preexcitation exists, do not demonstrate it during sinus rhythm (the normal rhythm of the heart), and special techniques are necessary to diagnose the condition. Their condition is known as latent preexcitation. To investigate this matter, 22 patients with supraventricular tachycardia (rapid heart rate of more than 100 beats/minute) underwent electrophysiologic testing, during which electrical measurements were made at various locations in the heart using catheters carrying electrodes. Patients were given intravenous adenosine, a drug that can block the AV node, one of the major points in the normal conducting system. This made latent preexcitation, if present, more obvious. Of the patients tested, electrocardiographic records typical of preexcitement appeared in four patients, and the existence of preexcitement was confirmed by electrophysiology in all four. AV block was induced without preexcitation in 12 patients, and five other had little change in the large wave of the electrocardiogram. Of this latter group, two patients were found to have latent preexcitation upon physiological testing. It thus appears that intravenous adenosine can produce AV nodal conduction delay or block in 73 percent of patients with supraventricular tachycardia, and that it can aid in correct diagnosis of latent preexcitation. Since the incidence of people with supraventricular tachycardia is probably low, a less drastic diagnostic approach than invasive studies, such as the adenosine test reported here, may be more desirable. (Consumer Summary produced by Reliance Medical Information, Inc.)