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Editor,—Coronary pressure derived fractional flow reserve (FFR), as reviewed by Pijls and Bruyne,1 provides an excellent and reproducible technique to estimate the severity of a coronary lesion, and is a significant advance over coronary flow reserve. Three points need to be raised.
First, the arterial pressure measurement should be taken during diastole as most coronary flow is during diastole (not strictly true for the right ventricle). Using mean arterial pressure will induce significant errors.
Second, calculations of FFR without full assessment of the central venous pressure (CVP) may incur significant errors, as the vast majority of patients do not have a CVP of 0. The percentage error incurred when the CVP is not included can be calculated from equation 1. Percentage error in FFR = (1)
Where Pa = arterial pressure; Pd = distal pressure; and Pcvp = central venous pressure.
This is graphically illustrated in fig 1A, which shows that the percentage error incurred is significant.
Finally, even though catheter technology has advanced significantly over the past decade resulting in decreased physical size and increased flexibility, sources of error resulting from the physical size of the catheters should be appreciated if correct interpretations of the readings and derived FFR they produce are to occur. Figure 1B shows the percentage error incurred in vessels of varying diameters with a 30%, 40%, and 50% stenosis. The percentage error can be approximated by equation 2. Percentage error = (1-(Rv2.Ps2(Rv2.Ps.2-Cd2/2)-2)2)*100 (2) Where Cd = catheter diameter; Ps = percentage stenosis/100; and Rv = vessel radius.
This letter was shown to the authors, who reply as follows:
We acknowledge the comments by Dr Poullis and his careful thoughts about coronary pressure measurement. During rest, in the left coronary artery blood flow occurs predominantly during diastole. However, in the presence of maximum coronary vasodilation systolic blood flow also occurs, and in a normal artery this systolic flow constitutes about 25% of total flow. Therefore, as coronary pressure measurements should be performed during maximum coronary vasodilation, the systolic component cannot be neglected. We have demonstrated in our earlier animal studies that it is the mean coronary pressure in the distal coronary artery that determines the FFR by direct comparison to Doppler flow velocimetry in animals and by positron emission tomography in human subjects.1-1 1-2
We also showed in that study that measuring only diastolic pressure would not make much difference, but the problem in clinical practice is that there is not one diastolic pressure, and therefore mean pressure is more suitable and practical. Again, all validation studies used mean coronary pressure and it is this pressure that has the best correlation with blood flow. For the right coronary artery this issue is even more applicable because here blood flow is often equally distributed between systole and diastole.
The second issue, the implication of central venous pressure, is well regarded but has little practical implications for decision making in the catheterisation laboratory. As nicely illustrated by Dr Poullis's equation, at an FFR of 0.75 (cut off point at diagnostic catheterisation) neglecting central venous pressure results in an error in fractional flow reserve of 2.5% if central venous pressure is as high as 10 mm Hg. Only at very low values of FFR is central venous pressure more important, but decision making in such low values of FFR is trivial. At clinical decision making after a coronary intervention (cut off point 0.9), there is an error of less than 1.5% even in the presence of central venous pressure as high as 10 mm Hg. Therefore, for practical decision making, with respect to the question whether a particular stenosis should be dilated, and whether the result of an intervention is optimal, central venous pressure can be neglected if it is not extremely raised.1-3 1-4
When fractional collateral blood flow is assessed, it is advisable to include central venous pressure because in that case larger mistakes can be made.1-5
With respect to the possible overestimation of stenosis severity by the presence of the wire, we also considered these theoretical considerations and have examined this in an in vitro study.1-6 1-7 We showed that the influence of the wire on stenosis haemodynamics and gradient is negligible in the range of stenoses where functional assessment is desirable. Only in cases of very severe stenoses will considerable overestimation of the gradient occur, but in that type of stenosis there will be a large gradient and FFR will be well below 0.75. Therefore, in the range of stenoses in which measurement of FFR is desirable, no mistakes are made by the presence of the wire.
In summary, to calculate FFR, mean distal coronary pressure at hyperaemia should be measured; central venous pressure can be neglected as long as it is not very high, unless fractional collateral blood flow is calculated; and in the range of stenoses where physiological measurements are needed, no additional significant gradient is introduced by the presence of the pressure wire.