calculated blood vessel contractile force measurement concentration of vasoconstrictor at which the force of blood vessel contraction is half the maximum value (EC50) 2014-06-09T18:33:14Z A calculated value for the concentration of a vasoconstrictor at which the energy produced by the active reduction in the diameter of a blood vessel in response to the application of the stimulus is halfway between the baseline value and the highest achievable value of such stimulus-induced contractile energy. A vasoconstrictor is any agent or condition that narrows blood vessels by constricting the smooth muscle in the vessel walls. CMO:0002464 JSmith clinical_measurement_ontology vasoconstrictor half maximal effective concentration (EC50)