Doppler technology embraces a common phenomenon and a familiar part of our world. We all come to appreciate its principles when we are stopped at a train crossing: We hear the train's horn at one tonal pitch as the train approaches, and at a lower tonal pitch as it passes us and moves away. The change in pitch is a manifestation of the change in sound-wave frequency that our ears can detect, and—when it is transformed into mathematical equations and scientific formulas—the change in frequency is what forms the basis of Doppler technology.
Blood flow is like the moving train that passes a particular point, which in a clinical context is an ultrasound transducer. As blood rushes past it, the transducer detects changes in frequency.
The speed of blood flow, or the resistance the blood encounters downstream, can thus be assessed using these universal Doppler principles.
Doppler technology has been used quite effectively in medicine for a variety of clinical circumstances, from the assessment of thrombi to the assessment of blood flow to the fetus or resistance in the placenta.
Over the past few decades, this technique has been applied to both normal and problem pregnancies with the expectation that as blood-flow changes are detected by Doppler techniques, such detection might be effective in the assessment of fetal well-being and, more importantly, the loss of fetal well-being. The question, however, is how we can best apply Doppler technology in our general practice.
This month's guest professor is Dr. Christopher Harman, who is a well-known international expert in the area of ultrasound and Doppler technology.
Dr. Harman is professor and vice chairman of the department of obstetrics, gynecology, and reproductive sciences at the University of Maryland, Baltimore, as well as director of the school's maternal-fetal medicine division. He will discuss the application of Doppler in general obstetric practice.
Roles and Limitations of Doppler Ultrasound
What is Doppler good for, many of us wonder. And how might it be useful in routine obstetric care and the general, uncomplicated pregnancy?
To answer these questions, it's helpful to understand the basis for Doppler abnormalities, as well as the ways in which this technology is useful in detecting and managing specific clinical problems.
The evolution of Doppler has significantly expanded our understanding of many fetal disease processes, and it is now clear that Doppler is a useful tool for both evaluation and diagnosis. As part of an integrative, contextual approach to evaluation, it has earned a broad role that is not limited to application in intrauterine growth restriction.
The Doppler parameters that are used to evaluate pregnancies at risk for placental problems focus on the relationship between resistance in the blood vessels and its effect on blood flow. Especially important is Doppler's depiction of resistance and flow through the maternal uterine arteries and the fetal umbilical arteries.
When the placenta is developing properly, blood-flow resistance on both sides of this exchange surface falls significantly with advancing gestational age. In the uterine arteries, this phenomenon involves both direct mechanical change in the arteries themselves—the vessels lose their muscular coats and their reactive capability—and a change in the volume of blood that's able to run through them into a low-resistance placental bed.
These changes, signifying normal development of the uterine artery circulation, are easily depicted by Doppler ultrasound. The technology shows progressively lower speeds required to perfuse the uterine arteries, as well as significantly more blood flow during diastole. (See
Blood-flow resistance on the other side of the placenta—that is, in the fetal umbilical arteries—begins its progressive drop a little later than does resistance in the maternal uterine arteries. High resistance on the fetal side is a normal feature of first-trimester fetal circulation. (See
Ultimately, however, the development of successive generations of villous vessel branching offers more area for blood to run through, and resistance in the fetal umbilical arteries drops progressively. Much work has been done correlating the anatomical and structural elements of placental development with blood-flow resistance, and Doppler has been shown again and again to be an excellent method of evaluating the function of the placenta.
Specific Clinical Roles
Elevated resistance and persistence of notching in the uterine arteries throughout pregnancy signify a placenta that is not developing well—and often, an intrauterine growth-restricted fetus that has to cope with and invoke compensations for this placental insufficiency.
These compensations can at least in part be depicted by Doppler ultrasound through a more detailed evaluation of the fetal circulation. Here we must examine two additional vascular beds: the middle cerebral artery (MCA), which is used to evaluate brain blood flow, and the ductus venosus (DV), a unique fetal vessel that funnels a proportion of nutrient-rich umbilical venous return directly into the right atrium. Because this direct connection allows reflection of the atrial impulse, we can use the DV to evaluate cardiac status. Because the DV is very sensitive to fetal oxygen status, we can also use it to evaluate fetoplacental respiratory status.