The purpose of the present study was to develop and validate a new compact, portable end-tidal forcing (ETF) system capable of reliably controlling end-tidal gases. The system consists of compressed gas sources (air, N 2 and CO 2) that are connected via three solenoid valves to a humidification chamber and an inspiratory reservoir bag from which the participant breathes. This computer-controlled system compares actual end-tidal gas partial pressures with target pressures and mixes the gases on a breath-by-breath basis. This leaves no unused exhaust gas and keeps gas requirements to a minimum. Eight participants underwent two different 30-min protocols that included each possible combination of end-tidal O 2 partial pressure ( PE T O 2 ) and end-tidal CO 2 partial pressure ( PE T C O 2 ) control at two different levels ( PE T O 2 at 55 and 75 mmHg; and PE T C O 2 at 4 and 7 mmHg above resting). The ETF system maintained the mean PE T C O 2 at 0.13 mmHg from the target values, with a pooled S.D. across conditions of ±0.91 mmHg and a 95% confidence interval (CI) of ±0.63 mmHg. The mean PE T O 2 was held at 0.49 mmHg from its target values, with a pooled S.D. across conditions of ±1.31 mmHg and a 95% CI of ±0.91 mmHg. To demonstrate suitability of this system for measuring chemosensitivity to hypoxia, hypoxic ventilatory response (HVR) tests were conducted in a subset of five participants. During a 20-min HVR test both PE T C O 2 and PE T O 2 were not significantly different from their target values. These data demonstrate the performance of a portable, compact, economical system that controls PE T C O 2 within 1 mmHg and PE T O 2 within 2 mmHg of their respective target values.
