Estimation of peripheral chemoreflex gain from spontaneous sigh responses

A method is proposed for quantifying the responsiveness of the peripheral chemoreflex loop to CO2 by utilizing the natural fluctuations in ventilation and end-tidal PCO2 which occur subsequent to the appearance of spontaneous sighs. The advantage of this method lies in its simplicity and noninvasiveness: the need for administering inhaled mixtures with high CO2 content is eliminated. Using autoregressive moving-average (ARMA) analysis, we demonstrate that post-sigh responses can be adequately described by a simple chemoreflex model that contains first-order dynamics and a pure time delay. The effective gain of this model is shown to reflect peripheral chemosensitivity closely when the estimation procedure is applied to 'data' obtained from computer simulations of the respiratory control system. Although central chemosensitivity affects the absolute values of effective gain, the slope of the linear correlation between effective and peripheral gains remains unchanged. Application of the procedure to spontaneously breathing anesthetized dogs shows that, in every case, effective gain increased with the induction of hypoxia, which is known to enhance peripheral chemosensitivity.