Obstructive Sleep Apnea (osa) Is a Common Dis- Order of Breathing That Occurs during Sleep and Affects over 4% of Men and 2% of Women1

959 OBSTRUCTIVE SLEEP APNEA (OSA) IS A COMMON DISORDER OF BREATHING THAT OCCURS DURING SLEEP AND AFFECTS OVER 4% OF MEN AND 2% OF WOMEN1 and is an independent risk factor for cardiovascular disease.2 Sudden cardiac death (SCD) amongst OSA patients occurs predominantly at night, in contrast to the general population.3 A study of 400 OSA patients revealed that nearly half of them develop cardiac arrhythmias of some kind during sleep.4 OSA is characterized by repetitive partial or complete closure of the upper airway. Obstructive episodes elicit visceral changes, including blood gas disturbances, large negative intrathoracic pressure changes that increase cardiac preand afterload, surges in sympathetic neural activity, alterations in heart rate, and surges in arterial blood pressure. These acute and repetitive events are thought to contribute to fatal nocturnal cardiovascular events.2 Cardiac autonomic activity in OSA patients has been studied mainly using heart rate variability (HRV) analysis. HRV refers to beat-to-beat fluctuations in the RR interval and provides a noninvasive means to assess autonomic heart control mediated by vagal and sympathetic efferents. HRV analysis is usually performed using a set of measures in the time and frequency domains (see Methods for details). Abnormal HRV is an independent predictor of cardiac mortality in different patient populations.5,6 It has been demonstrated that autonomic heart rate modulation is altered in OSA patients,7 and different indices of HRV have been proposed as a screening tool for OSA.8 Cardiac vulnerability is heightened during ventricular repolarization that could be indirectly assessed by measuring QT interval duration on the surface ECG. The few studies that have examined QT interval properties in OSA patients were based on the QT dispersion index that has, however, a questionable validity.9 A novel approach has been proposed to assess repolarization lability using the QT variability index (QTVi) to quantify beat-to-beat changes in the QT interval duration.10 The QTVi is increased in CHF patients11 and is a powerful predictor of arrhythmias and cardiac mortality.10,12 In the present study, we evaluated QT variability in OSA patients and characterized the relationship between the severity of OSA and QT variability.