The Doppler phenomenon occurs when the sound source (the transducer) and the object reflecting the source (blood cells or myocardium) are moving relative to one another as opposed to the case where the transducer and the reflector are both immobile. In the case of moving reflectors, the frequency of the returning sound wave will be different than the transmitted wave. The Doppler equation uses the measured frequency difference between the transmitted and reflected sound waves to derive the velocity of the reflector. In this way, the velocity and direction of blood flow may be measured within the cardiovascular system. The angle of incidence between the direction of blood flow and the ultrasound beam is of utmost importance in making Doppler measurements. If the angle of incidence is < 20°, the velocity measurement is of acceptable accuracy. At values above this, the velocity will be increasingly underestimated, such that at an incident angle of 90°, the velocity measurement will be zero (Fig 1). The operator, therefore, must align the axis of Doppler interrogation as close as possible with direction of blood flow. Doppler measurement of blood flow velocity will never overestimate velocity. It can easily underestimate it, however, if the operator is not successful in optimal beam alignment. Under some circumstances, the absolute velocity of blood flow is not as important as changes in the velocity, as might occur during the respiratory cycle when making serial semiquantitative measurements of SV. When making serial measurements of velocity variation, the operator must focus on obtaining the same angle with each measurement. Otherwise, changes in velocity might be caused by changes of the incident angle rather than by changes in physiologic function.