Oximetry spectral analysis in the diagnosis of obstructive sleep apnoea

Using spectral analysis of oximetry data, we prospectively evaluated the validity of this methodology in patients clinically suspected of suffering from obstructive sleep apnoea (OSA). A total of 233 outpatients were studied. Nocturnal oximetry was performed simultaneously with conventional polysomnography for all participants. The power density of oxygen saturation was analysed using Fast-Fourier transformation of the oximetric signal. Nocturnal oximetry test results were considered as abnormal (suspicion of OSA) if a peak in the spectrum between the period boundaries 30 and 70 s was observed. A normal test result was defined as the absence of the 30-70 s peak from the spectrum. Single-blind evaluation was performed by three independent observers, and agreement of two or more of these was considered definitive. The peak amplitude and the ratio of the area enclosed in the 30-70 s peak to the total area of the spectrum (r(S)) were measured. The presence of a peak has a sensitivity of 78%, a specificity of 89%, a positive predictive value of 89% and a negative predictive value of 78%. Apnoea-hypopnoea indexes were correlated significantly with peak amplitude (r=0.74; P<0.001) and with r(S) (r=0.69; P<0.001). For a peak amplitude threshold of 0.7%(2), the sensitivity was 94% and the specificity was 65% for OSA diagnosis. Using a threshold for r(S) of 0.15, the sensitivity was 91% and the specificity was 67%. Thus the spectral analysis of nocturnal oximetry and identification of a peak at 30-70 s could be useful as a diagnostic technique for OSA subjects.