Analysis of the pressor dose response.

(1) When incremental infusions of drugs that increase blood pressure are given to human subjects to assess "pressor responsiveness," only the lower part of the sigmoid dose-response curve can be obtained. (2) Fitting a quadratic function does not involve discarding data points, which is usually the case with a linear fit, and it provides a more satisfactory fit to the lower part of a sigmoid dose-response curve. (3) In the presence of a competitive antagonist, a pressor dose-response curve will be shifted to the right. In this situation the dose-response curves obtained before and after treatment with antagonist should be fitted simultaneously to a quadratic model in which the parallel shift is one of the parameters. (4) The use of quadratic fitting is illustrated by reference to clinical experiments to obtain the following three curves for drugs that modify peripheral alpha adrenoceptors: norepinephrine pressor response curves after placebo and doxazosin, an alpha 1 antagonist; norepinephrine pressor response curves after placebo, labetalol, and medroxalol (drugs with combined alpha 1 and beta blocking properties); and phenylephrine pressor response curves before and after prazosin. (5) Fitting a quadratic function is the appropriate initial step in the analysis of pressor dose-response curves in man.