The disclosure of the present application provides for systems and methods for determining a phasic change in a vessel and vessel compliance. In at least one exemplary method for determining a phasic change in a vessel, the method comprises the steps of introducing a device into a site within a vessel, operating the device in connection with two or more fluid injections in the vessel to obtain two or more conductance values, calculating a parallel conductance value and a total conductance value, and calculating a phasic change in at least one vessel parameter based in part upon the calculated parallel conductance value and the calculated total conductance value. In an exemplary method for determining vessel compliance, the method comprises, in part, the steps of calculating a first vessel parameter and a second vessel parameter based on at least two conductance values, calculating a change in vessel parameter based upon the first and second vessel parameters, and calculating vessel compliance based upon the relationship between the change in vessel parameter and a change in pressure during a cardiac cycle.
