THE EFFECT OF EXOGENOUS ADENOSINE IN PATIENTS WITH NEURALLY-MEDIATED SYNCOPE AND SICK SINUS SYNDROME

The effects of a 20-mg IV, bolus of adenosine 5' triphosphate (ATP) on the heart rhythm was studied in 79 patients affected by neurally-medi ated syncope (26 cases) or sick sinus syndrome (22 cases) or both synd romes (31 cases) and in 31 healthy control subjects in order to examin e the sensitivity of cardiac purinoceptors in such circumstances. Duri ng ATP infusion, the sinus cycle lengthened to > 2 seconds in no contr ol, in 1 (4%) patient with neurally-mediated syncope, in 5 (23%) patie nts with sick sinus syndrome, and in 13 (42%) patients with both neura lly-mediated and sick sinus syndromes (P = 0.01). Atrioventricular blo ck occurred in 14 (45%) of controls, in 10 (38%) patients with neurall y-mediated syncope, in 4 (18%) patients with sick sinus syndrome, and in 13 (42%) patients with both neurally-mediated syncope and sick sinu s syndrome (n.s.). Thus, exogenous ATP exerts different effects on pat ients with neurally-mediated syncope and patients with sick sinus synd rome. In fact, intrisic disease of the sinus node is necessary to modu late an abnormal adenosine-mediated sinus arrest, whereas patients aff ected by neurally-mediated syncope alone show a normal sensitivity to the drug administration. The effect of ATP on atrioventricular conduct ion is greater than that on sinus node and is of similar magnitude in patients and controls; thus the clinical meaning of ATP induced atriov entricular block remains uncertain.