For Referring Physicians
Newsletter

July 1999

What is a "Doppler"?

The term "Doppler" is extremely general and often over used. Many times we have vascular tests ordered as a "Doppler of the leg", which is not enough information for us to know what test is needed. Christian Doppler is the name of the person whose theory is the basis of much of the technology we use today. His theory, proposed in 1842, is known as the Doppler effect and has wide applications in radar, sonar, and ultrasound. You have heard this Doppler effect as the change in pitch of a train whistle or a car horn as they approach and then move away. When the sound source is moving toward you, the sound waves are closer together and the pitch sounds higher. When the sound source is moving away from you, the sound waves are farther apart and the pitch sounds lower.

Vascular laboratories use the Doppler effect by bouncing sound waves off of moving blood cells. The reflected sound frequency from the moving blood is changed (Doppler effect) and the amount of change is related to the direction and speed of the blood. The instruments we use are generally referred to as "Dopplers", but are actually better defined as blood flow detectors. In the 1960's investigators started working with different plethysmographic techniques or pulse volume recording (PVR) to quantify arterial occlusive disease in the legs. The combination of Doppler and PVR represents an important advancement in instrumentation and led to the rapid expansion of the development of noninvasive vascular studies.

Continuous-wave Doppler detectors are the simplest systems. The probe has two separate crystals, one continuously transmitting and one receiving. The shifted frequency obtained from a blood vessel is within the audible frequency range (<20kHz), so the data can be presented as an audible signal, or most commonly referred to as the "Doppler sound".

Measurements of brachial artery blood pressures have been performed for many years using a sphygmomanometer and listening to Korotkoff sounds with a stethoscope. Although this is universally used for measurement of arm blood pressures, this technique is less practical in the lower extremity due to the size and location of the blood vessels. The most basic noninvasive lower extremity evaluation is to obtain ankle blood pressures and compare them to the arm blood pressure, which is defined as the ankle/brachial index (ABI). If this value is less than 1.0, it indicates clinically significant occlusive disease proximal to the point of measurement, but the diseased segment or segments can be anywhere from the level of the heart to the ankle.
Additional information or localization of occlusive disease of the extremities can be obtained by noninvasive diagnostic tests that we call the Lower Extremity Doppler (LED) or Upper Extremity Doppler (UED). These examinations measure pressure and pulsation (PVR) at different levels by applying cuffs at the thigh, upper calf, ankle, and forefoot for the lower extremity and brachium and forearm for the upper extremity. If necessary, pressure and flow recordings can also be performed on the digits for evaluation of distal small vessel disease, often seen in diabetics.

Pulse volume recording (PVR) can be obtained using the same recording cuffs as the segmental pressures. During systole the blood entering a limb normally causes an increase in the total volume of the extremity with a return to resting volume during diastole. These minor pulse pressure differences are recorded on a hard copy tracing and have been demonstrated to be quite similar to arterial pressure waves measured directly. The primary diagnosis of arterial occlusive disease is based on the qualitative evaluation of the PVR waveform.

The combination of segmental pressures and PVR can provide accurate assessment of infrainguinal and/or upper extremity arterial occlusive disease localization and severity. It is limited in that it detects only occlusive lesions that are sufficiently advanced to reduce the systolic pressure and flow and cannot localize proximal aorto-iliac disease or subclavian/axillary disease. Additional noninvasive testing modalities utilizing color flow duplex ultrasound are necessary to further evaluate and localize atherosclerotic arterial occlusive disease processes.

So, remember that when you are ordering a "Doppler of the leg", we assume that you want a Lower Extremity "arterial" Doppler study. Further information on duplex ultrasound will be forthcoming in the next Vascular Newsletter.

Thank you for your continued support!