More than half - up to 80% - of patients with heart failure suffer from concomitant sleep apnea (SA), which further worsens symptoms and prognosis. Cardiac resynchronization therapy may ameliorate sleep apnea, but only the central form of sleep apnea (CSA). However, only very small uncontrolled studies with mainly less than 20 patients have been reported so far concerning the interactions between CRT and sleep apnea, and no data are available in patients with conventional right ventricular pacing undergoing upgrading to CRT.