Negative airway pressure (Paw) (following airway occlusion), inspired volume, end-tidal PCO2, arterial oxygen saturation (SaO2), mean blood pressure (BP), and heart rate (HR) are plotted every 30 min during continuous inspiratory muscle pacing with HF-SCS. Results are plotted as means ± SE. Airway pressure and inspired volume generation remained constant throughout the initial 5.5 h period of stimulation. End-tidal PCO2 and oxygen saturation were also maintained between 40.1 ± 0.3 and 39.9 ± 0.7 mmHg and 94.3 ± 0.6 and 94.2 ± 0.9%, respectively. Mean blood pressure and heart rate also remained stable throughout this period. After 5.5 h of pacing, stimulus parameters were increased, resulting in an increase in inspired volume and fall in PCO2 to 29.3 ± 0.4 mmHg, which was sustained over a 30 min period.