Dynamics of the ventilatory response to hypoxia in humans

Dynamic responses of the ventilatory system to rapid variations in isocapnic hypoxia were studied in five subjects. Sawtooth-shaped inputs were presented at constant amplitude with periods of 120, 90, 60, 45 and 30 sec, and square-wave inputs at different amplitudes with periods of 120, 60 and 30 sec. A breath-by-breath model fitting technique was used to assess whether any of a number of first order models of hypoxic ventilatory dynamics could fit the data adequately. The following was found: 1) An equation for the desaturation of haemoglobin provided a better expression for hypoxia in the model than did a hyperbolic function of PO2. 2) The gain and/or offset model parameters varied significantly between experiments, but the time constant and pure delay terms did not. 3) The time constants, and to a lesser extent the pure delays, were found to vary significantly between sawtooth experiments of different frequencies. The failure of a single set of dynamic parameters to describe all the responses suggests that the model is incomplete. 4) There was significant asymmetry in the hypoxic response with the on-transient dynamics faster than the off-transient dynamics. The results of the model fitting study suggest that a first order model cannot fully describe the hypoxic ventilatory dynamics.