EFFECTS OF SLEEP-INDUCED INCREASES IN UPPER AIRWAY-RESISTANCE ON VENTILATION

To determine the effects of the sleep-induced increases in upper airway resistance on ventilatory output, we studied five subjects who were habitual snorers but otherwise normal while awake (AW) and during non-rapid-eye-movement (NREM) sleep under the following conditions: 1) stage 2, low-resistance sleep (LRS); 2) stage 3-4, high-resistance sleep (HRS) (snoring); 3) with continuous positive airway pressure (CPAP); 4) CPAP + end-tidal CO2 partial pressure (PET(CO2)) mode isocapnic to LRS; and 5) CPAP + PET(CO2) isocapnic to HRS. We measured ventilatory output via pneumotachograph in the nasal mask, PET(CO2), esophageal pressure, inspiratory and expiratory resistance (RL,I and RL,E). Changes in PET(CO2) were confirmed with PCO2 measurements in arterialized venous blood in all conditions in one subject. During wakefulness, pulmonary resistance (RL) remained constant throughout inspiration, whereas in stage 2 and especially in stage 3-4 NREM sleep, RL rose markedly throughout inspiration. Expored minute ventilation (V̇E) decreased by 12% in HRS, and PET(CO2) increased in LRS (3.3 Torr) and HRS (4.9 Torr). CPAP decreased RL,I to AW levels and increased end-expiratory lung volume 0.25-0.93 liter. Tidal volume (VT) and mean inspiratory flow rate (VT/TI) increased significantly with CPAP. Inspiratory time (TI) shortened, and PET(CO2) decreased 3.6 Torr but remained 1.3 Torr above AW. During CPAP (RL,I equal to AW), with PET(CO2) returned to the level of LRS, VT/TI and V̇E were 83 and 52% higher than during LRS alone. Also on CPAP, with PET(CO2) made equal to HRS, VT, VT/TI, and V̇E were 67, 112, and 67% higher than during HRS alone. These changes, between low- and high-resistance sleep vs. CPAP with PET(CO2) held at the same level as LRS and HRS, represent the ventilatory output equivalent of the sleep-induced increase in airway resistance. We conclude that an increased airway resistance contributes significantly to the CO2 retention which accompanies light and deeper stages of NREM sleep.