An apparatus and method of assessing a narrowing in a blood vessel is disclosed. The vessel has a fluid flow pressure wave with a distal (backward-originating) pressure component and a proximal (forward-originating) pressure component. These two components are derived without taking a flow velocity measurement. The method includes the steps of taking pressure measurements in the tube； separating the pressure components into the backward-originating pressure component and the forward-originating pressure component； identifying a time window when the differential of flow velocity (dU) is minimal or absent； and deriving the backward and forward pressure components for pressure measurements taken in at least that time window.
