Cardiac autonomic control in obstructive sleep apnea - Effects of long-term CPAP therapy

To determine how long-term treatment with continuous positive airway pressu re (CPAP) affects cardiac autonomic function, we measured R-R interval (RRI ), respiration, and blood pressure in 13 awake patients with moderate-to-se vere obstructive sleep apnea (OSA) in both supine and standing postures, be fore and after 3 to 9 mo of home therapy. Using visual feedback, the subjec ts controlled their respiration to track a randomized breathing pattern. Fr om the RRI spectrum, we computed high-frequency power and the ratio of low- frequency to high-frequency power (LHR). To correct for differences in brea thing, the average transfer gain relating respiration to RRI changes (G(RSA )) and the modified low-frequency to high-frequency ratio (MLHR) were also derived. CPAP therapy did not change the conventional spectral indices of h eart rate variability (HRV). However, G(RSA) increased with average nightly CPAP use in supine (p < 0.01) and standing (p < 0.03) postures whereas MLH R decreased with CPAP compliance during standing ( < 0.03). Supine mean hea rt rate decreased with compliance (p < 0.03). None of the estimated paramet ers was correlated with duration of therapy when actual CPAP use was not ta ken into account. These results suggest that CPAP treatment improves vagal heart rate control in patients with OSA and that the degree of improvement varies directly with compliance level.