Spectral indices of cardiac autonomic function in obstructive sleep apnea

Spectral analysis of heart rate variability (HRV) is useful as a noninvasiv e means of assessing autonomic function in patients with obstructive sleep apnea (OSA). However, standard spectral measures, such as the ratio of low- frequency to high-frequency power (LHR) and normalized high-frequency power (NHFP), can be confounded by the abnormal breathing patterns that occur du ring sleep. To circumvent this limitation, we employed an autoregressive mo deling approach to partition the RR time-series into a component that is co rrelated with respiration and a respiration-independent component. From the se components, we derived two new spectral indices: the modified LHR (MLHR) and the average gain relating respiration to RR changes (G(RSA)) Six norma ls and seven OSA patients were studied in relaxed wakefulness and stage 2 s leep; during sleep, the OSA patients were studied without and with continuo us positive airway pressure (CPAP) therapy. All four spectral indices showe d significant differences between OSA patients and normals in both wakefuln ess and sleep, although the changes in MLHR and G(RSA) were substantially l arger and less variable: MLHR (p < 0.0003)and G(RSA) (p < 0.0001) vs. LHR ( p < 0.005) and NHFP (p < 0.004). However, in the OSA subjects, LHR and NHFP were unchanged by CPAP. By contrast, CPAP produced a highly significant in crease in G(RSA) (p < 0.0004), as well as a decrease in MLHR (p < 0.03). Th us, by compensating for the effects of breathing pattern differences, MLHR and G(RSA) unmasked the effects of CPAP therapy, which has been shown in pr evious studies to reduce sympathetic activity and increase vagal cardiac mo dulation.