What is the fractional flow reserve technique?
Fractional flow reserve (FFR) is a technique used for measuring the blood pressure in the coronary arteries to determine the extent of blockage or narrowing in those arteries. It is measured by inserting a fine wire into the artery to measure the blood pressure before and after the point of narrowing, and then calculating the ratio of the two pressures. FFR readings can help determine the need for angioplasty or stenting in a patient with coronary artery disease. It is a widely used technique for evaluating and managing these patients.
What are the limitations of FFR?
Whilst FFR is a valuable technique for evaluating coronary lesions, it does have some limitations. These limitations include:
FFR measurements require the insertion of a pressure sensor/wire into the coronary artery, which is an invasive procedure that carries a certain level of risk.
Adenosine must be administrated quickly to achieve a hyperemic state, which can be challenging in some patients. It is also costly and can cause uncomfortable side effects for the patient.
FFR is a snapshot measurement of coronary blood flow, and does not account for changes in flow that may occur over time.
FFR results can have low reproducibility. They may be affected by incorrect measurement technique, inadequate pressure curve interpretation, or unreliability of equipment.
Can we overcome those limitations?
One technique that has the potential to reduce the risks, costs and discomfort associated with the traditional, invasive FFR technique is PIE Medical’s vessel fractional flow reserve (vFFR) software.
vFFR uses two angiographic projection to reconstruct a 3D model of the coronary artery. This method reduces the effects of foreshortening, out-of-plane magnification, and non-symmetric coronary lesions. Instantaneous calculations occur to reveal the pressure drop using physical laws such as viscous resistance and separation loss effects present in coronary flow behavior. As obtained during the catheterisation procedure, patient-specific aortic pressure is included in the pressure drop value.
The resulting vFRR measurement can give physicians insight into whether or not a stenotic lesion is causing ischemia and guide treatment decisions, such as whether to proceed with stenting or medical therapy. The software also quantifies the percentage of stenosis and lesion length.
So, what are the differences between vFFR and FFR?
Whilst FFR measures the pressure difference across a stenosis from pharmacologically inducing hyperemia, vFFR does not require the use of adenosine or a pressure wire. The vFFR value shows information on blood flow patterns and can provide a more accurate assessment of coronary morphology, compared to FFR (see validation studies below).
Thus, while both FFR and PIE Medical’s vFFR provide important information about coronary artery disease, PIE Medical vFFR may be a more accurate and comprehensive assessment of hemodynamic significance.
Validation studies for virtual fractional flow reserve?
There are many validation studies that look at the ability of vFFR to identify significant stenosis assessed by FFR, all of which are summarised and linked below:
The FAST 1 / FAST 1 Extend study examined vFFR vs FFR retrospectively using 294 patients. The analysis showed very good correlation (r=0.89), high diagnostic accuracy (AUC=0.94) and very high reproducibility (r=0.95).
The FAST II study looked at vFFR and FFR, prospectively using 334 patients in 6 centres across Europe, Japan and USA. They found high diagnostic accuracy, AUC=0.93 assessed by corelab and high diagnostic accuracy AUC=0.91 assessed by site. There was a high reproducibility between site and core lab (r=0.97).
Looking at vFFR vs FFR post-PCI retrospectively, the FAST Post study showed good correlation (r=0.88), high diagnostic accuracy (AUC=0.98) and very high reproducibility (r=0.95) among 100 patients.
The Taipei Veterans General Hospital, Taiwan studied vFFR vs FFR across 258 lesions. They found good correlation (r=0.708) and good diagnostic accuracy (AUC=0.87).
vFFR vs FRR was analysed in 50 post-PCI patients undergoing successful CTO PCI retrospectively. A good correlation (r=0.82) and high diagnostic accuracy for post-PCI ≤0.90 (AUC=0.97).