CARDIAC MOTION CAN ALTER PROXIMAL ISOVELOCITY SURFACE-AREA CALCULATIONS OF REGURGITANT FLOW

Objectives. This study addressed the hypothesis that motion of the sur face containing a regurgitant orifice relative to the Doppler ultrasou nd transducer can cause differences between actual flow rate and calcu lations based on the proximal flow convergence technique. Background. In vitro studies quantitating regurgitant flow rate by proximal how co nvergence have been limited to stationary orifices. Clinically, howeve r, valve leaflets generally move relative to the ultrasound transducer during the cardiac cycle and can move at velocities important relativ e to the measured color aliasing velocities. The transducer therefore senses the vector sum of actual flow velocity toward the orifice and o rifice velocity relative to the transducer. This can cause potential o verestimation or underestimation of true flow rate, depending on the d irection of surface motion. Methods. The hypothesis was explored compu tationally and tested by pumping fluid at a constant flow rate through an orifice in a plate moving at 0 to 8 cm/s (velocities comparable to those described clinically for mitral and tricuspid annulus motion to ward an apical transducer). Results. Surface motion in the same direct ion as flow caused overestimation of the aliasing radius and calculate d flow rate. Surface motion opposite to the direction of flow (typical for mitral and tricuspid regurgitation viewed from the apex or esopha gus) caused underestimation of actual flow rate. The underestimation w as greater for lower aliasing velocities (36 +/- 11% for 10 cm/s vs. 2 3 +/- 6% for 20 cm/s). Correcting for surface motion provided excellen t agreement with actual values (y = 0.97x + 0.10, r = 0.99, SEE = 0.17 liters/min). Conclusions. Physiologic motion of the surface containin g a regurgitant orifice can cause substantial differences between actu al flow rate and that calculated by the proximal flow convergence tech nique. Low aliasing velocities used to optimize that technique can mag nify this effect. Such errors can be minimized by using higher aliasin g velocities (compatible with the need to measure the aliasing radius) or eliminated by correcting for surface velocity determined by an M m ode ultrasound scan.