Doppler echocardiography

Doppler echocardiography measures the motion of blood by listening for a pitch change in the reflected ultrasound — the same effect that makes an ambulance siren sound higher as it approaches and lower as it recedes. Blood cells moving toward the probe shift the echo to a higher frequency; cells moving away shift it lower. From the size of that shift, the machine calculates how fast and in which direction blood is flowing inside the heart and great vessels.

Several display modes serve different purposes. Color Doppler paints flow onto the moving image — conventionally red toward the probe and blue away — so leaks and abnormal jets become visible at a glance. Spectral Doppler (pulsed and continuous wave) traces velocity against time at a chosen spot, allowing precise numbers. By converting a measured velocity into a pressure difference, Doppler lets clinicians estimate the gradient across a narrowed valve, the severity of a regurgitant leak, and pressures inside the heart and lungs — all without a catheter.

Doppler is what turns echo from a picture into a quantitative tool, but its numbers depend on alignment: the beam must run nearly parallel to the flow, or velocities are underestimated. Results are also angle- and operator-dependent and assume idealized flow. Skilled integration of several Doppler measurements, rather than any single number, gives the reliable answer.