Direct Measurement of Flow Velocity using the Doppler Shift

The Doppler shift has been used since the 1970s to determine coronary blood flow velocity as the basis for further calculations of coronary flow reserve (CFR) and coronary resistance.[10] A Doppler wire is inserted into the coronary artery to be measured. A sonic wave of a defined transmitting frequency is sent from the tip of the Doppler wire, which is then reflected by the erythrocytes, which alter the wave’s frequency depending on the flow velocity of the erythrocytes.[11] The main limitation of the technique is that it may be difficult to obtain an optimal Doppler signal.[12,13] However, with careful repositioning it is possible to finally receive useful Doppler signals in most patients.[14] Sometimes it may be necessary to stabilise the position of the Doppler wire using an intracoronary microcatheter. Despite these challenges and alternative techniques available, intracoronary Doppler measurements are still successfully used in daily clinical practice.[3,15]