Smoothed periodogram of oxyhemoglobin saturation by pulse oximetry in sleep apnea syndrome: an automated analysis.
Variability of oxyhemoglobin saturation (Spo(2)) during sleep has been utilized as a diagnostic index for sleep apnea. Spectral analysis with its graphical presentation, the periodogram, is an approach for measuring such variability. This work examined the parameters on a smoothed periodogram created from series data for Spo(2) obtained by pulse oximetry during a sleep study. Spo(2) was recorded during polysomnography study of 273 subjects. Clinical data of subjects were collected retrospectively. A novel automated algorithm was created to measure the low-frequency (< 0.1 Hz) peak and the slope of spectral density vs frequency in the frequency region of 0.1 to 0.5 Hz (slope(0.1-0.5)). Two successive modified Daniell smoothers with span lengths of 3 to 121 in odd numbers were applied to determine the effect of smoothing on these parameters. slope(0.1-0.5) was least affected by smoothing and had a sensitivity of 78% and a specificity of 80% in diagnosing sleep apnea defined by a value of apnea-hypopnea index >/= 5. Combining slope(0.1-0.5) with parameters of the low-frequency peak enlarged the area under the receiver operating characteristic curve. A composite indicator comprised of slope(0.1-0.5) and ratio of the area under the curve of the low-frequency peak to that of whole periodogram (AUCratio) had a positive likelihood ratio of 15.25 in identifying patients with moderate-to-severe obstructive sleep apnea. The algorithm was validated in another 206 patients undergoing polysomnographic studies. These analytical results demonstrate that the smoothed periodogram of Spo(2) is a useful tool for screening subjects with sleep apnea.