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Control, Upper Airway, and Sleep Apnea

In a recent article, Younes (1) quantified the relative contributions of upper airway mechanics and control mechanisms to severity of obstructive sleep apnea (OSA). The severity of the mechanical abnormalities of the passive pharynx was assessed from the magnitude of reduction in flow when continuous positive airway pressure (CPAP) was acutely withdrawn, and from an estimate of the CPAP required to obtain stable breathing in the absence of compensatory mechanisms. Severity of OSA was assessed from the usual apnea–hypopnea index and from the fraction of time spent in stable breathing. Younes (1) found that only a third of the variance in severity could be explained by the variance in passive mechanical properties. I had understood from this that much of the variance in severity was related to differences in effectiveness of compensatory mechanisms.

In an accompanying editorial, Naughton (2) indicates that differences in the shape and size of the facial bony structure likely explain most of the severity not accounted for by the upper airways mechanics. I was quite confused by this since I had assumed that the mechanical load, as measured by Younes (1), would certainly include any contribution made by the factors that Naughton mentions.

Editorialists should clarify and not confuse. They are supposed to be experts in the subject of the editorial. Faced with a discrepancy in interpretation, the average reader (such as myself) is likely to question his/her own interpretation and accept the editorialist's view. In this case, I am reluctant to accept Naughton's assessment because it is so obvious that, to the extent they affect OSA severity, differences in anatomy should have been included in Younes' indices of passive mechanics and, by extension, do not contribute to the residual variance. Naughton should have realized that such a statement is counterintuitive and would be confusing unless supported by compelling explanation (such as how anatomic abnormalities can increase severity without manifesting as more upper airway collapsibility). I believe a clarification is in order.

Joseph Braidy

Centre Hospitalier Universitaire de Montréal Montréal, Québec, Canada

FOOTNOTES

Conflict of Interest Statement: J.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Dr. Younes was given an opportunity to respond to this letter but declined to do so.

REFERENCES

1. Younes M. Contributions of upper airway mechanics and control mechanisms to severity of obstructive apnea. Am J Respir Crit Care Med 2003;168:645–658.[Abstract/Free Full Text]
2. Naughton MT. Cycling sleep apnea: the balance of compensated and decompensated breathing. Am J Respir Crit Care Med 2003;168:624–625.[Free Full Text]

From the Author:

The study by Younes (1) reported that increasing age, male sex, increasing body mass index (BMI), supine sleeping position, and REM sleep explained 33% of the variance of the severity of obstructive sleep-disordered breathing. By extrapolation, he has assumed that the remaining 66% of the variance can be explained by effective compensatory properties. My concern (2) was that other common variables such as upper airway bony and soft tissue structures (such as retrognathia or enlarged tonsils), which would influence the baseline intraluminal diameter and thus predisposition to sleep-disordered breathing, were not taken into consideration. Indeed Younes also acknowledges the limitations of his work by stating "differences may exist among patients in the extent of upper airway viscoelastic behavior" and "the relationship between mechanical load and OSA severity (r²) may be somewhat higher than the 34% value found using CPAPmin." In addition, I took the liberty to speculate as to other compensatory mechanisms, such as neck posture, and new insights into intraluminal surface properties, such as surfactant, which might also provide compensatory mechanisms. What is the importance of my editorial for the "average reader"? Look beyond BMI and neck circumference when clinically assessing for obstructive sleep-disordered breathing.

Matthew T. Naughton

Alfred Hospital and Monash University Melbourne, Victoria, Australia

FOOTNOTES

Conflict of Interest Statement: M.T.N. has served on the Australian Medical Advisory board for ResMed and receives an honorarium for attendance. M.T.N. has received an unconditional grant for 2 years to employ staff and conduct a clinical trial. This grant was applied for after partial funding was awarded from the Australian National Health and Medical Research Council. He has no other conflicts of interest.

REFERENCES

1. Younes M. Contributions of upper airway mechanics and control mechanisms to severity of obstructive apnea. Am J Respir Crit Care Med 2003;168:645–658.
2. Naughton MT. Cycling sleep apnea: the balance of compensated and decompensated breathing. Am J Respir Crit Care Med 2003;168:624–625.

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