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.

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

  • In the LAD (most common case), the average height difference is 5.8 cm (4.5 mmHg) in men and 4.3 cm (3.3 mmHg) in females (2)

  • Hydrostatic pressure errors causes up to 18% vessel misclassification (2, 4, 5)

- 14% for hyperemic measurements

- 18% for dPR measurements

- 18% for resting Pd/Pa​ measurements

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The effect of removing hydrostatic error on existing study results

​Studies (6, 7) show that post-PCI FFR is generally much lower in LAD than in non-LAD arteries. Hydrostatic-error free measurements would have made the rate of successful post-PCI FFR higher in the LAD and lower in non-LAD, and therefore the success rate in LAD and non-LAD would have been more similar.  

  • Hwang et al. (6) studying 835 patients concluded the optimal cut-off values of post-PCI FFR for predicting target vessel failure were 0.82 and 0.88 in the LAD and non-LAD, respectively. These cut-offs correspond to the difference in hydrostatic error between LAD and non-LAD (2, 3).

  • Collison et al. (7) studying 260 patients found that the proportion of patients achieving a final post-PCI FFR value ≥0.90 were 7.2% of the LAD, 74% of the LCX and 64% of the RCA arteries. If the hydrostatic error would have been avoided, the results would have been more equal for LAD and non-LAD.

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