Coronary artery height differences and their effect on fractional flow reserve. 
Fractional flow reserve (FFR) uses pressure-based measurements to assess the severity of a coronary stenosis.  Distal pressure (Pd) is often at a different vertical height to that of the proximal pressure (Pa).  The difference in pressure between Pd and Pa due to hydrostatic pressure, may impact FFR calculation.  One hundred computed tomography coronary angiographies were used to measure height differences between the coronary ostia and points in the coronary tree.  Mean heights were used to calculate the hydrostatic pressure effect in each artery, using a correction factor of 0.8 mmHg/cm.  This was tested in a simulation of intermediate coronary stenosis to give the "corrected FFR" (cFFR) and percentage of values, which crossed a threshold of 0.8.  The mean height from coronary ostium to distal left anterior descending (LAD) was +5.26 cm, distal circumflex (Cx) -3.35 cm, distal right coronary artery-posterior left ventricular artery (RCA-PLV) -5.74 cm and distal RCA-posterior descending artery (PDA) +1.83 cm.  For LAD, correction resulted in a mean change in FFR of +0.042, -0.027 in the Cx, -0.046 in the PLV and +0.015 in the PDA.  Using 200 random FFR values between 0.75 and 0.85, the resulting cFFR crossed the clinical treatment threshold of 0.8 in 43% of LAD, 27% of Cx, 47% of PLV and 15% of PDA cases.  There are significant vertical height differences between the distal artery (Pd) and its point of normalization (Pa).  This is likely to have a modest effect on FFR calculation and the results in values crossing the treatment threshold.  Operators should be mindful of this phenomenon when interpreting FFR values.