Circadian Variations of Electrical Properties of the Heart' zy JUAN CINCA, ANGEL MOYA, ALFRED0 BARDAJI, JORGE RIUS, AND JORGE SOLER-SOLER zyx Electrophysiology Laboratory Cardiology Service Valle de Hebrbn General Hospital 08035 Barcelona, Spain INTRODUCTION Recent technical advances in intracardiac electrophysiology and dynamic electro- cardiography led to the exploration of the influence of the circadian rhythms on some electrical properties of the human heart.'-s Using an appropriate electrocatheter technique we have been able to perform sequential bedside electrophysiologic testing over a period of 22 hours to investigate the existence of a possible daily variability in electrophysiologic In this presentation we are going to compile our data on sequential 22-hour measurements of the refractory periods of the normal cardiac structures and accessory AV conduction pathways with special emphasis on the effects of a possible daily variability on electrical induction of reciprocating supraventricular tachycardia (SVT). Additionally, we will deal with some clinical aspects that remain to be investigated. METHODS Patients Thirty-eight patients, 29 men and 9 women, with a mean age of 38 years (range 16 to 70 years) were studied to determine the mechanism of recurrent episodes of paroxysmal SVT in 25 or the origin of syncopal attacks in 13. All patients observed a normal nocturnal sleeping schedule and gave informed consent to participate in the study. Sequential Electrophysiologic Testing The first electrophysiologic study was performed at the laboratory between 11 AM and 2 PM in a nonsedated postabsortive state. Thereafter, the patients were transferred to a day-lit room to complete sequential bedside electrophysiologic measurements. The electrocatheters were introduced percutaneously under local anesthesia with 2% mepivacaine hydrochloride. zyxwv A quadripolar USCI no. 7F electrocatheter was inserted into the coronary sinus via the left subclavian vein. Two other electrocatheters, an octapolar and a bipolar USCI 7F probe, were introduced into the right femoral vein 'This work was supported zyxwvu by a grant from Fondo de Investigaciones Sanitarias de la Seguridad Social zyxwvutsr (86/1188). 222