A five-parameter logistic equation for investigating asymmetry of curvature in baroreflex studies

Baroreceptor reflex curves are usually analyzed using a symmetric four-para meter function. We wished to ascertain the validity of assuming symmetry in the baroreflex curve and also of constraining the curves to pass through t he resting blood pressure and heart rate (HR) values. Therefore, we have in vestigated the suitability of a new five-parameter asymmetric logistic mode l for analysis of baroreflex curves from rabbits and dogs. The five-paramet er model is an extension of the usual four-parameter model and reduces to t hat model when the fitted data are symmetrical. Using 30 data sets of blood pressure versus renal sympathetic nerve activity (RSNA) and HR from six co nscious rabbits, we compared the five-parameter curves with the four-parame ter model. We also tested the effect of forcing these baroreflex curves thr ough the resting point. We found that the five-parameter model reduced the unexplained variation and gave small but important improvements to the esti mates of plateaus for RSNA and HR and the HR gain. Although forcing the HR curves through the resting values had little effect, this procedure, when a pplied to RSNA, produced a worse curve fit by increasing the unexplained va riation with alteration to most of the estimated curve parameters. The mean arterial pressure-MR baroreflex relationship from six conscious dogs was a lso analyzed and showed clear evidence of systematic asymmetry. We conclude that the asymmetric model is a valuable extension to the symmetric logisti c model when examining baroreceptor reflexes, giving improved estimates of the parameters and a new approach to examining the mechanisms contributing to baroreflex curve asymmetry. Furthermore, forcing the curves through the resting value is a statistically questionable practice when analyzing RSNA, because it affects the parameter estimates.