Obstructive sleep apnea (OSA) and congestive heart failure (CHF) are conditions characterized by derangements of repolarization and autonomic cardiovascular regulation. Typically, there is an augmentation in sympathetic activity and vagal activity is depressed. OSA often coexists with CHF but it is not clear whether it has additional effects on the neural control of heart rate and repolarization.It was found that CPAP increased high frequency heart rate variability an indicator of vagal modulation of heart rate and improved ventricular repolarization, as manifested by a decrease in the QT variability index.Sleep is accompanied by increases in vagal activity and reductions in sympathetic tone. However, periodic apneas can reverse this and can have lasting effects into wakefulness. We performed a randomized controlled trial involving CHF patients with OSA to test the hypothesis that treatment of OSA by continuous positive airway pressure (CPAP) would improve vagal heart rate modulation and ventricular repolarization.