While forcing of end-tidal gases by regulating inspired gas concentrations is a common technique for studying cardio-respiratory physiology, independently controlling end-tidal gases is technically challenging. Feedforward control methods are challenging because end-tidal values vary as a dynamic function of both inspired gases and other non-regulated physiological parameters. Conventional feedback control is limited by delays within the lungs and body tissues, and within the end-tidal forcing system itself. Consequently, modern end-tidal forcing studies have generally restricted their analysis to simple time courses of end-tidal gases, and to resting steady-state conditions. To overcome these limitations, we have designed and validated a more generalized end-tidal forcing system that removes the need for manual tuning and rule-of-thumb based control heuristics, while allowing for accurate control of gases along spontaneous and complicated time courses, and under non-steady physiological conditions. On average during resting, steady walking, and walking with time varying speed, our system achieved step changes in PETCO2 within 3.0±0.9 (mean±std) breaths and PETO2 within 4.4±0.9 breaths, while also maintaining small steady-state errors of 0.1±0.2 mmHg for PETCO2 and 0.3±0.8 mmHg for PETO2. The system also accurately tracked more complicated changes in end-tidal values through a bandwidth of 1/10 the respiratory (sampling) frequency, a practical limit of feedback control systems. The primary mechanism enabling this controller performance is a generic mathematical model of the cardio-pulmonary system that captures the breath-by-breath relationship between inspired and end-tidal gas concentrations, with secondary contributions from reduced delays in controlled air delivery.
- end-tidal forcing
- cardio-pulmonary model
- Copyright © 2016, Journal of Applied Physiology