An apparatus and method of assessing a narrowing in a fluid filled tube having a fluid flow pressure wave having a backward-originating pressure component and a forward-originating pressure component without taking a flow velocity measurement, comprising: 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 the time window.
