Hydrostatic pressure and FFR measurements
What is hydrostatic pressure?
Hydrostatic pressure is naturally exerted by the weight of the fluid above a certain measurement point. Hydrostatic pressure is what you feel in your ears when diving deep into deep water.
The more traditional explanation says:
"Hydrostatic pressure is the pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. Hydrostatic pressure increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above." — dictionary.com
Blood pressure changes with height
The incremental change in blood pressure due to gravity accounts for 0.77 mmHg per cm height difference (1).
Since both aortic and venous pressure is equally affected by gravity, the hydrostatic pressure will not affect flow.
How does hydrostatic pressure affect measuring accuracy?
Traditional pressure wires have a pressure sensor located 3cm from the tip. Before performing measurements, wire pressure (Pd) and aortic pressure (Pa) are equalized, with the wire pressure sensor at the same proximal position in the vessel as the guide catheter tip.
As the wire sensor is inserted in a distal RCA, it travels lower in the body, and Pd pressure becomes higher, due to hydrostatic pressure
As the wire sensor is inserted in a distal LAD, it travels higher in the body, and Pd pressure becomes lower, due to hydrostatic pressure
Why does the hydrostatic pressure error not affect the fluid filled Wirecath® wire?
Fluid-filled wires/catheters are immune to hydrostatic error, since their saline-filled interior compensates for hydrostatic pressure in the body. The fact that the pressure transducer is external - located outside of the body, and always positioned at the same level, is also critical for measuring accuracy.
Hydrostatic pressure errors in traditional sensor-tipped wires
Usually, equalization minimizes the pressure difference between Pa and Pd before the procedure starts. However, since the pressure sensor in a sensor-tipped wire is advanced into the distal coronary vessel after equalization, the height difference between the sensor (Pd) and the catheter (Pa) can lead to a significant pressure error.
This is of particular importance since this hydrostatic error is not obvious to the physician.
Pd in distal vessels
When using sensor-tipped wires:
Kawaguchi Y. et al. Impact of Hydrostatic Pressure Variations Caused by Height Differences in Supine and Prone Positions on Fractional Flow Reserve Values in the Coronary Circulation. J Interv Cardiol. 2019; 2019: 4532862.