Hydrostatic pressure and FFR measurements 

All traditional sensor-tipped pressure wires are affected by the physical phenomenon causing the hydrostatic pressure error (1, 2, 3). The measuring error varies, as it depends on the anatomy of the vessel path, and it can sometimes affect your result significantly.

 

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).

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.

Detailed information: Hydrostatic pressure errors in traditional sensor-tipped wires

Hydrostatic error occurs due to the height difference between the ostium and the distal measuring point (1, 2, 3).

 

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:

  • The Pd-value will be presented on average 2 mmHg higher than the true value in LCX

  • Correspondingly, Pd will be 4mmHg too low in LAD (2, 3)

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Data on effect of hydrostatic pressure error

  • The average error in all arteries is 3.5 mmHg (= 0.035 Pd/Pa) taking into account sex and vessel prevalence of stable CAD (1-4)

  • In the male LAD (most common case), the average height difference is 5.8 cm (4.5 mmHg) (2)

  • Hydrostatic pressure errors causes up to 24% vessel misclassification (1-5)

- 16% for hyperemic measurements

- 22% for dPR measurements

- 24% for resting Pd/Pa​

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