This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The term 'cardiac memory' has been used to describe changes that occur on the electrocardiogram after a change in the vector (or direction) of depolarization of the heart. For example, after a period of pacing the heart, the T wave will change its morphology or shape despite a return to the normal depolarization vector. This change will persist for many days. The T wave is representative of cardiac repolarization. Recently, changes in repolarization have been shown to be associated with increased mortality. Measures of repolarization have been developed which quantify these changes. The investigator hypothesizes that T wave changes induced by changes in the vector of depolarization induce repolarization abnormalities associated with increased risk. Patients with sinus node dysfunction traditionally receive dual chamber pacemakers and ventricular pacing is instituted as a matter of routine. Such patients do not need ventricular pacing and only require atrial pacing. Thus repolarization characteristics of a group of patients after atrial pacing as well as after ventricular pacing will be compared. The study design will be a double blinded, crossover trial. Patients with dual chamber pacemakers, implanted for sinus node dysfunction, will be evaluated for the ability to undergo atrial pacing safely. Patients with atrioventricular block are excluded. Assignment to an atrial pacing group or a dual chamber pacing group will be random. After greater than forty days of pacing in this mode, the patient will return and measurement of repolarization and depolarization will be made. All of these measurements will be done noninvasively. The patient's pacemaker will be reprogrammed to the alternative mode and the patient will return for repeat measurements after 40 additional days. Patients will be selected for the ability to be paced only from the atrium. Additionally, the ability to pace at normal rates from this chamber will be tested by pacing the heart at a relatively high rate and observing for AV block.
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