Apparatus and method of assessing a narrowing in a fluid filled tube

A method of assessing a narrowing in a blood vessel, the method comprising:

• positioning at least one pressure-sensing probe within a blood vessel;
• obtaining, with an intravascular processing system having a processor in communication with the at least one pressure-sensing probe, pressure measurements from within the blood vessel using the at least one pressure-sensing probe, the pressure measurements being obtained not during hyperaemia;
• identifying, with the intravascular processing system, a wave free period corresponding to a time window when a differential of flow velocity is minimal or absent;
• and
• calculating, with the intravascular processing system, a pressure ratio using the pressure measurements obtained during the wave free period to provide an assessment of a severity of the narrowing in the blood vessel.