CHANGES IN OBSTRUCTIVE SLEEP-APNEA CHARACTERISTICS THROUGH THE NIGHT

It was our impression that the respiratory parameters in obstructive s leep apnea (OSA) worsened as the night progressed. To confirm this, we reviewed polysomnographic studies from 66 patients with apnea-hypopne a indices (AHI) of 40 to 125 events per hour, dividing bed time into e qual quartiles. As the night progressed, the mean apnea duration (MAD) increased from 27.2 s to 34.6 s (p<0.0001), mainly from increases dur ing NREM sleep. The proportion of time spent in apnea increased from 5 4 to 71%, (p<0.0001) due to increases in both MAD and the proportion o f REM sleep (from 2.8 to 14.7%, of the total sleep time). The ARI did not change significantly between quartiles. Even though preapneic oxyg en saturation did not change and apnea duration increased as the night progressed, the end-apneic saturation did not decrease, hence the rat e of oxygen desaturation decreased. Also, it was found that patients w ith an AHI greater than 65 events per hour increased their proportion of time spent in apnea significantly more than those with an AHI small er than 65, as the night progressed. In the patients with an AHI great er than 85, this was due to both an increase in MAD and AHI. In conclu sion, in patients with an AHI greater than 40 events per hour, the sev erity of apnea increased as the night progressed due to lengthening of MAD, increased proportion of REM sleep, and in the most severe patien ts, also an increase in AHI. Even though the exact pathophysiologic me chanisms for the observed changes are unknown, a decrease in respirato ry muscle effort with consequent decrease in oxygen consumption may ex plain both the lengthening of the apneas and the decrease in the rate of oxygen desaturation.