Performance of a compact end-tidal forcing system.

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 (PET O2) and end-tidal CO(2) partial pressure (PET O2) control at two different levels (PET O2 at 55 and 75 mmHg; and PET CO2 at 4 and 7 mmHg above resting). The ETF system maintained the mean PET CO2 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 PET CO2 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 PET CO2 and PET O2 were not significantly different from their target values. These data demonstrate the performance of a portable, compact, economical system that controls PET CO2 within 1 mmHg and PET O2 within 2 mmHg of their respective target values.