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.