[The changes of respiratory mechanics in patients with obstructive sleep apnea-hypopnea syndrome during sleep]

To study the changes of respiratory mechanics in patients with obstructive sleep apnea hypopnea syndrome (OSAHS) during sleep.

Eighty-one consecutive subjects, 70 males and 11 females, mean age (43.9 +/- 12.6), were recruited in this study. The respiratory mechanics during sleep were studied by using the side stream spirmeter (SSS) technique. All subjects underwent respiratory mechanics monitoring with Novametrix Medical Systems (connected with full face mask) during nocturnal polysomnographic (PSG) study. Then by the apnea-hypopnea index (AHI), the subjects were divided into the OSAHS group (AHI >or= 15/h) and the control group (AHI < 15/h).

Fifty-four subjects were confirmed to have OSAHS [AHI = (57.6 +/- 24.9) h] by PSG, and 27 as the control group [AHI = (4.4 +/- 4.1) h]. In the OSAHS group, inspiratory tidal volume/expiratory tidal volume (V TI/V TE) was 1.37 +/- 0.18 during the first respiratory cycle after apnea, which was significantly greater than that (0.99 +/- 0.04) during wakefulness. The mean V TI/V TE (0.86 +/- 0.09) during the 5 respiratory cycles and that (0.72 +/- 0.19) of the last respiratory cycle before the next apnea decreased significantly. These indicated that the functional residual capacity (FRC) and upper airway size were reduced gradually before apnea. Compared with the measurements before sleep, V TI [(463 +/- 122) ml vs (554 +/- 134) ml], V TE [(466 +/- 127) ml vs (565 +/- 147) ml], and expired minute volume [(6.4 +/- 1.6) L/min vs (8.3 +/- 1.9) L/min] were decreased significantly during light sleep without apnea. Tidal breath flow-volume loop showed that in the OSAHS group, the inspiratory resistance increased in 54 patients (100%), the expiratory resistance increased in 52 (96.3%), and both of them increased in 52 (96.3%).

OSAHS patients exhaled more air than inhaled gradually before apnea onset, and then this change was compensated by inhaling more air than exhaled in the first respiratory cycle following apnea. These resulted in decreases in FRC and upper airway size, which may play an important role in the pathophsiological mechanisms of sleep apnea. The tidal volume decrease suggested that the breathing drive was reduced in light sleep of OSAHS. In addition, a rise of both inspiratory and expiratory resistance was evident in a majority of patients with OSAHS.