Measurement of Blood Pressure Direct Measurement: extravascular transducer: connected to vasculature via saline-filled catheter intravascular transducer: placed directly inside vasculature, use semiconductor strain gauge Indirect Measurement: Pressure Cuff Methods Tonometry 1

Extravascular Pressure Measurement port can be used to flush system, remove air bubbles flexible diaphragm heparin-filled catheter blood vessel 2

Catheter Model sensor diaphragm distributed model 3

Catheter Model (cont.) fluid flowing in a catheter experiences hydraulic inertance, resistance, and compliance. hydraulic resistance: due to friction between sheering fluid molecules. or : pressure difference across segment, DL (Pa = N/m2) F: flow rate (m3/s) average fluid velocity (m/s) A: cross-sectional area (m2) 4

hydraulic resistance can be expressed in terms of catheter length: L: total catheter length (m) r: radius (m) hydraulic viscosity (Pa-s) valid for laminar flow 5

Hydraulic inertance is due to the mass of the liquid: a = acceleration (m/s2) m = mass (kg) r = density of liquid (kg/m3) 6

Catheter, Sensor Compliance compliance has to do with the elasticity of the catheter: 7