Sleep apnea, as noted earlier, is classified as central (if the cause stems from a disorder affecting the central respiratory drive) or obstructive (if a physical defect leads to blocked airways). The more sleep researchers learn about these conditions, however, the more the distinctions between them become blurred. "Pure" central sleep apnea (CSA)—the total absence of breathing effort and airflow through the nose and mouth—and "pure" obstructive sleep apnea (OSA)—decreased or absent airflow coupled with increased breathing effort—are rarely seen. More often the problem is a mixture of the two: a central malfunction that in turn causes some kind of airway blockage. CSA was discussed in the previous chapter because it often results in nighttime awakenings. Here we will focus on OSA because of its association with excessive daytime sleepiness.
To be considered clinically significant, the lack of airflow must last for at least ten seconds; some reports describe patients who stop breathing for up to a minute and a half. (For some reason, you are able to hold your breath longer when you are asleep than when you are awake.) Normally everyone experiences four or five apneic episodes per hour of sleep, but patients with sleep-disordered breathing may have five hundred, eight hundred, even a thousand episodes in an eight-hour period. People who experience a thousand microarousals over the course of a night—an average of one every thirty seconds—understandably feel, on rising, that they didn't sleep at all. However, unless these victims have been aroused for ten to fifteen seconds, they are not likely to realize that their sleep has been disturbed. Consequently they may wonder why they feel sleepy during the day.
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