The effect of hydrostatic pressure on invasive measures of coronary physiology

Introduction: Coronary arteries are at differing vertical heights in a supine patient relative to the aortic root. Pressure within an artery varies based on distance from the aorta due to hydrostatic effect. This could impact pressure-based indices of stenosis severity, as the vertical distance between distal and proximal pressure sensors creates a baseline pressure difference. This is neglected in clinical practice, as distal and proximal sensors are considered at the same vertical level. Methods: Pd/Pa, instantaneous wave free ratio (iFR), fractional flow reserve FFR and doppler flow velocity were recorded in 23 coronary stenoses in the standard supine patient position, and in the prone position. Measurements between positions were compared using a Student’s t test for matched pairs. Results: There were significant differences in mean Pd/Pa (0.05), iFR (0.06) and FFR (0.06) when comparing prone and supine positioning (p<0.05). When inferior to the aorta, mean Pd/Pa, iFR and FFR were 0.96±0.05, 0.93±0.11 and 0.84±0.10 respectively. When superior, mean Pd/Pa, iFR and FFR were 0.91±0.07, 0.87±0.11 and 0.78±0 respectively. Resting and hyperaemic doppler flow measurements did not change significantly when comparing prone and supine patient position. 26% of all FFR and 36% of all iFR values were re-classified across a treatment threshold when hydrostatic effect was corrected. Conclusion. Patient position alters physiological stenosis severity as quantified by invasive coronary pressure measurements. Coronary stenoses positioned inferiorly to the aorta, produce significantly higher Pd/Pa, iFR and FFR values when compared to a superior position. Conversely, patient position did not influence coronary doppler flow velocity. This is the first study to quantify the effect of hydrostatic pressure on invasive measures of coronary stenosis. The data supports hydrostatic effect as a potential confounding factor leading to inaccurate lesion assessment.