DYNAMICS OF THE VENTILATORY RESPONSE TO HYPOXIA IN HUMANS

Dynamic responses of the ventilatory system to rapid variations in iso capnic hypoxia were studied in five subjects. Sawtooth-shaped inputs w ere presented at constant amplitude with periods of 120, 90, 60, 45 an d 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 hypoxi c ventilatory dynamics could fit the data adequately. The following wa s found: 1) An equation for the desaturation of haemoglobin provided a better expression for hypoxia in the model than did a hyperbolic func tion of P(O2). 2) The gain and/or offset model parameters varied signi ficantly between experiments, but the time constant and pure delay ter ms did not. 3) The time constants, and to a lesser extent the pure del ays, were found to vary significantly between sawtooth experiments of different frequencies. The failure of a single set of dynamic paramete rs 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 resu lts of the model fitting study suggest that a first order model cannot fully describe the hypoxic ventilatory dynamics.