Drift vs true gradient in FFR
Fractional Flow Reserve (FFR) is measured using a Doppler pressure wire with a transducer distal to the coronary stenosis. Proximal pressure in the guide catheter is measured and designated as Paand distal pressure is designated at Pd. During calibration, with the pressure wire transducer kept proximal to the lesion should show near identical tracings for both proximal and distal pressures. The ratio of Pa to Pd measured at maximal hyperemia induced by adenosine will give the FFR.
When there is ‘drift’ in FFR, the proximal and distal pressure tracings have same morphology so that the shift of the distal pressure curve is a parallel shift. The dicrotic notch is visible well in both tracings.
If it is a true gradient, the tracings have different morphology. The dicrotic notch is absent and the diastolic pressure is lower in the distal pressure tracing. Systolic pressure is lower in the distal tracing in both drift and true gradient. Further checking of proximal and distal tracing with the pressure transducer proximal to the lesion after the FFR measurement, will confirm whether it was drift or true gradient as the near-equalisation will not be seen.