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Laval Hospital Quebec, PQ, Canada
Correspondence to: Frédérick Sériès, MD, 2725 Chemin Sainte-Foy, Quebec, PQ, Canada G1V4G5
To the Editor:
It was with great interest that we read the review dedicated to pulse oximetry that was published in August issue of CHEST.1 This review nicely highlights the benefits and limitations of such a recording tool in the screening of patients suspected of having sleep apnea syndrome. We agree with the authors that the utility of such a simple recording in our investigation strategy definitively needs to addressed in this time when the referrals for sleep and breathing investigation are rapidly increasing. We were quite surprised to find out that the literature detailed in this review did not mention and discuss the results that we published in 1993 on the utility of home oximetry in a large sample of outpatients.2 This study was the first to evaluate a new interpretation procedure for oximetry tracings that looked at the desaturation/resaturation pattern without considering any threshold for arterial oxygen saturation (SaO2) fall or minimal SaO2 amplitude to be reached. This was justified by the inability of the test to confirm the diagnosis in a subset of sleep apnea patients when the interpretation is based on rigid criteria defining SaO2 decline.3 In these circumstances, the sensitivity of home oximetry in pioneer studies such as the one of Williams et al3 may be lower than the one reported in the review by Netzer et al1 (65% instead of 78%). Following the publication of our results2 and the recognition of their importance in clinical practice,4 the interpretation algorithm of subsequent studies evaluating the diagnosis value of oximetry was based on SaO2 variability analysis.5 In complement to the interpretation of the results of the literature detailed by Netzer et al,1 it should be mentioned that the poor specificity that we and others6 observed with this screening method dramatically depends on the definitions used to define breathing abnormalities and especially hypopneas. In fact, most of the articles cited in this review considered a minimal SaO2 fall as an obligatory event associated with flow reduction to consider the presence of an hypopnea. However, according to the recommendations of up-to-date guidelines,7 such SaO2 changes are no longer required to score such breathing abnormality. At the end of the spectrum, nonapneic, nonhypopneic events (respiratory effort-related arousals) can be observed in the absence of significant nocturnal desaturation. It is clear that if our data were reanalyzed with these new criteria, the specificity would be dramatically enhanced with a minor alteration of the sensitivity of our method. In this context, we believe that the analysis of SaO2 variability alone is more than ever of first importance in the interpretation of oximetry tracings. Prospective studies that will take into account for these new definitions of sleep-related breathing disorders need to be conducted to evaluate the accuracy of overnight oximetry in the diagnosis of the disease.
References
Walter Reed Army Medical Center Washington, DC
Correspondence to: Col. Arn Eliasson, MD, USA, Pulmonary and Critical Care Medicine Service, Department of Medicine, Walter Reed Army Medical Center, Washington DC 20307-5501; e-mail: aheliasson@aol.com 
To the Editor:
We appreciate the critical comments of Frédéric Sériès and acknowledge the importance of his publication. We were aware of this study but, by oversight, did not include it in our bibliography.
As Dr. Sériès mentions, and as we have outlined in our review, the specificity of overnight pulse oximetry in the diagnosis of sleep-disordered breathing depends on the type of breathing abnormalities. This is demonstrated with our data of oximetry samples from hypopneic and obstructive apneas. Calculations of breathing events based on the interpretation of pulse oximetry may underestimate or overestimate the number of hypopneas and central apneas. New calculation models for automatic readings should therefore include different forms of oxygen desaturation per time interval and not only count desaturations with a specified decline of 3% or 4% arterial oxygen saturation. Indeed, newer models for automated pulse oximetry readings are based on arterial oxygen saturation and pulse-rate variabilities. We agree with Dr. Sériès that these methods will enhance the sensitivity and specificity of pulse oximetry in the diagnosis of sleep-disordered breathing.
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