Analysis of Phenylephrine in Plasma: Initial Data About the Concentration-effect Relationship

A simple and sensitive assay for phenylephrine in plasma using HPLC with electrochemical detection is presented. Infusion of phenylephrine hydrochloride 0.5, 1.0, 2.0, and 4.0 micrograms X kg-1 X min-1 for 6 min at each dose level to 9 healthy subjects resulted in mean venous plasma concentrations of phenylephrine of 20, 56, 118 and 308 nM, respectively. The systolic and diastolic blood pressures were increased and the heart rate and venous plasma noradrenaline levels fell with increasing phenylephrine concentrations. There was a more than two-fold interindividual variation in the plasma concentration during infusion of the standardized doses of phenylephrine, with overlap of the concentrations achieved at the different dose levels. Estimates of the sensitivity to phenylephrine showed discrepancies within individuals when expressed as the concentration (PC20) or dose (PD20) required to increase systolic blood pressure by 20 mm Hg. Thus, evaluation of concentration-response curves should be more reliable than of dose-response curves when assessing phenylephrine sensitivity during i.v. infusions.