Increased Propensity for Central Apnea in Patients with Obstructive Sleep Apnea Effect of Nasal Continuous Positive Airway Pressure

Rationale There is increasing evidence of increased ventilatory instability in patients with obstructive sleep apnea (OSA), but previous investigations have not studied whether the hypocapnic apneic threshold is altered in this group. Objectives: To compare the apneic threshold, CO2 reserve, and controller gain between subjects with and without OSA matched for age, sex, and body mass index. Methods: Hypocapnia was induced via nasal mechanical ventilation for 3 minutes. Cessation of mechanical ventilation resulted in hypocapnic central hypopnea or apnea depending upon the magnitude of the hypocapnia. The apnea threshold (PETCO2-AT) was defined as the measured PETCO2, at which the apnea closest to the last hypopnea occurred. The CO2 reserve was defined as the change in PETCO2 between eupneic PETCO2,and PETCO2-AT. Controller gain was defined as the ratio of change in VE between control and hypopnea or apnea to the Delta PETCO2. Measurements and Main Results: Eleven pairs of subjects were studied. There was no difference in the PETCO2-AT between the two groups. However, the CO2 reserve was smaller in the subjects with OSA (2.2 +/- 0.6 mm Hg) compared with the control subjects (4.5 +/- 1.4 mm Hg; P < 0.001). The controller gain was increased in the subjects with OSA (3.7 +/- 1.3 L/min/mm Hg) compared with the control subjects (1.6 +/- 0.5 L/min/mm Hg; P < 0.001). Controller gain decreased and CO2 reserve increased in seven subjects restudied after using continuous positive airway pressure for 1 month. Conclusions: Ventilatory instability is increased in subjects with OSA and is reversible with the use of continuous positive airway pressure.