Viewed as a common menace to sleep and an intolerable deterrent to intimacy and bed sharing, snoring is likely the most common sleep complaint.1 The prevalence of snoring in the general population is estimated to exceed 50%.2,3 Interest in understanding the mechanism and implications of snoring preceded the advent of polysomnography and the characterization of sleep-disordered breathing (SDB).4 Studies of snoring have often suffered from use of subjective self-report, lack of standardized measurement techniques, and inadequate adjustment for underlying obstructive sleep apnea (OSA).5,6 When snoring occurs in the absence of SDB (apnea hypopnea index < 5 events per hour), it is termed “primary snoring” and not considered to require treatment.7 Nevertheless, the impact of snoring events on sleep quality, daytime sleepiness, and hypertension in the absence of significant OSA has been described.8,9 Importantly, while accepted as a possibly associated with some morbidity when it occurs without OSA, snoring is not considered to be an indicator of additional severity in patients with OSA.
Two main questions appear to pervade this field of research. One is whether a measurement technology can be developed for the quantification of snoring and simultaneous assessment of concomitant SDB. The second question is whether snoring has unique pathophysiological consequences that include sleepiness and cardiovascular disease risk that are independent of the severity of concomitant SDB or intermittent hypoxia. This latter question has been addressed by studies that yielded conflicting results. The association between snoring and hypertension, for example, has been long recognized.10 Most studies that used subjective measurement and reported association with hypertension often with inadequate adjustment for underlying SDB.11 Other studies suggested a local role for snoring in accelerating atherosclerosis in the carotid artery.12 Nevertheless, a relation between isolated snoring and cardiovascular risk independently of concomitant SDB is not well established.
The acoustic properties of snoring include intensity, regularity, and frequency profiles that can be measured and analyzed. The location of recording, tracheal, or oronasal, the measured output, analysis method, and the ability to exclude ambient noise are all factors that influence the quality of snoring analysis. Tracheal sound recording can detect periods of low tracheal sound intensity below an arbitrary threshold and quiet periods that can presumably distinguish between two snorers as defined by a high-intensity sound above certain threshold. Nevertheless, the diagnostic accuracy of these measurements has been reported to be relatively low and lacking consistency.13
Against this background, we examine the study by Furukawa et al. in this issue of the Journal of Clinical Sleep Medicine.14 The investigators used tracheal sound recordings and their proprietary analysis technique to derive a measurement of tracheal sound energy and an estimated respiratory disturbance index (RDI) in a group of men who have no previous diagnosis of SDB. After excluding the subjects who were deemed to have SDB based on the tracheal sound recording analysis, the investigators found a relationship between tracheal sound energy and morning blood pressure measurement in snorers. Both the RDI and the main outcome, tracheal sound energy, are derived from the same measurement. In other words, it remains probable that the tracheal sound energy is a surrogate measurement of SDB in this population. While the authors cite their previous work as adequate validation of the snoring-derived apnea-hypopnea index (AHI), significant concerns remain. For example their previous study from 2004 suggests a significant difference between the AHI derived from polysomnography measurements and tracheal sound recording.
It is true that an important contribution of this present study is the use of an objective measurement of snoring and describing a practical application that can, provided further testing and validation, be an inexpensive SDB screening tool. Issues that have not been addressed adequately in this study and in others using tracheal sound analysis include inadequate validation against the polysomnography. Several issues persist in this study and others using isolated snoring recordings for the detection of SDB. A noisier home environment or multiple snorers may interfere with this technology. Significant SDB such as in central or mixed sleep apnea may go undetected by sound analysis. Furthermore, what would the clinician do with the patients who are diagnosed with such technology? Are they considered to have OSA; and what cutoffs of severity in RDI should be used to trigger treatment with positive airway pressure as opposed to other OSA treatment modalities?
Inadequate validation prevents any conclusion that tracheal sound recording is actually measuring a spectrum of SDB or identifying an additional severity parameter of SDB. It would be difficult to accept the conclusion of the authors that the reported relation between tracheal snoring and morning blood pressure is an indicator of disease severity independent of underlying SDB. Even if their tracheal sound analysis method is detecting a unique form of mild SDB, it is difficult to conceptualize how it would be associated with hypertension independently from airway obstruction, arousals, hypoxia, and sympathetic activity; the established component of SDB pathophysiology. Nevertheless, their approach to correlating snoring with hypertension, while not novel, can contribute to identifying phenotypes of SDB that are associated with important clinical consequences such as hypertension.
Evaluation of the acoustic properties of snoring as a convenient and inexpensive tool for the diagnosis and stratification of SDB has been central to snoring research.6 However, the utility of using one parameter only for the screening or diagnosis of SDB is questionable in the presence of increasingly sensitive home sleep testing technologies that measure several parameters simultaneously in a convenient and well validated testing approach.15 Tracheal sound analysis is already incorporated in some home and in-lab sleep testing systems. However, it is not clear if it adds any significant diagnostic value to the other recorded channel in these systems.
In summary, this study contributes to the body of work linking spectra of SDB with cardiovascular consequences and provide a promising convenient method for SDB screening, albeit requiring significant validation.
Dr. Khayat has received research grants from Philips Respironics in 2014 and is a member of the scientific advisory board for Respicardia Inc. Dr. Jafari has indicated no financial conflicts of interest.
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