(Chest. 2003;124:1628-1630.)
© 2003
American College of Chest Physicians
Getting the Most Out of Nocturnal Pulse Oximetry
Konrad E. Bloch, MD, FCCP
Zürich, Switzerland
Dr. Bloch is Director of the Sleep Laboratory, Pulmonary Division, University Hospital of Zürich.
Correspondence to: Konrad E. Bloch, MD, FCCP, Pulmonary Division, University Hospital Zurich, Rämistrasse 100, CH-8091 Zürich, Switzerland; e-mail: pneubloc{at}usz.unizh.ch
Due to its noninvasive nature and the convenient patient interface consisting of a small sensor clipped or taped onto the skin, pulse oximetry is widely used in pulmonary medicine, critical care, and anesthesia. In sleep medicine, pulse oximetry is an essential tool for tracking the rapid fluctuations in arterial oxygen saturation that are characteristic for the unstable ventilation in patients with sleep apnea. Pulse oximetry has provided early insights into sleep-related breathing disturbances,1
and has opened the way for subsequent systematic investigations of sleep apnea. Today, the technique is an integral component of polysomnography and, by itself, is commonly used as a simple tool in the evaluation of sleep apnea. The clinical relevance of indexes derived from pulse oximetry alone or combined with other techniques that detect nocturnal breathing disturbances is emphasized by their correlation with impaired cognitive performance, cardiovascular diseases, and other consequences of sleep apnea.2
3
Over the last few years, the development of pulse oximetry has progressed in three major domains: first, the size and weight of the hardware have been reduced so that most devices are now portable, and some are even available in miniaturized form; second, paper chart recordings have been replaced by digital signal acquisition; third, novel mathematical and statistical techniques are being applied to computer-assisted analysis of pulse oximetry recordings. This is particularly useful and efficient for processing overnight recordings extending over several hours. While conventional analysis of nocturnal pulse oximetry has consisted of visual inspection of tracings for identification of desaturation events, baseline and minimal values, and patterns of fluctuations in oxygen saturation,4
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this kind of subjective interpretation is being replaced by computerized scoring to automatically and objectively derive the variables previously obtained by tedious manual scoring. Additionally, computer analysis provides the opportunity to process pulse oximetry data by powerful statistical and mathematical methods. Several indexes derived by these techniques are listed in the Table 1
, along with references to studies that have evaluated the diagnostic performance of nocturnal pulse oximetry with computer-assisted analysis for identification of patients with sleep apnea. The sensitivity of pulse oximetry ranged from 88 to 98%, and the specificity from 40 to 88% if an apnea/hypopnea index of 10/h or 15/h measured during polysomnography was defined as the upper limit of normal.5
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The introduction of various novel approaches to quantify pulse oximetric information requires an evaluation of the relative utility of derived indexes in the diagnosis of sleep-disordered breathing. In the current issue of CHEST (see page 1694), Magalang and coworkers report a systematic comparison of several indexes derived from computer-assisted nocturnal pulse oximetry. Three large cohorts of patients with suspected sleep apnea were examined at two different institutions. The investigation confirms that the Delta index (Table 1)
, and desaturation indexes that reflect the variability of oxygen saturation, have a similarly high diagnostic accuracy for detection of patients with the obstructive sleep apnea syndrome. Moreover, the authors demonstrate that combining several pulse oximetry-derived variables to an aggregated model enhances the diagnostic yield over that of individual indexes. The study represents an important contribution to the validation of computerized analysis and interpretation of nocturnal pulse oximetry. Nevertheless, the efforts to get the most out of this fascinating noninvasive technique should continue. With further technical developments, and the advancement in our understanding of sleep apnea, new questions arise. For example, it is not clear to what extent the dynamic response characteristics,16
artifact detection and rejection algorithms,17
and other technical specifications that differ among various brands of pulse oximeters affect their performance in sleep apnea diagnosis. Finally, the principal discussion over the reference standard against which novel techniques in sleep apnea diagnostic should be evaluated is also relevant for pulse oximetry. It seems that future evaluations of pulse oximetry would be more meaningful if performed in regard to major clinical outcomes of sleep apnea rather than in comparisons with polysomnography, a technique that has its own limitations, and depends itself on pulse oximetry for the detection of breathing disturbances.
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