and Marcelo Zugaib4
(1)
São Paulo University, Bauru, Brazil
(2)
Parisian University, Bauru, France
(3)
Member of International Fetal Medicine and Surgery Society, Bauru, Brazil
(4)
Obstetrics, University of São Paulo, Bauru, Brazil
The possibility of assessing blood flow in different maternal and fetal territories is a recent development in the field of obstetrics, being a highly relevant innovation for the understanding of hemodynamic events involved in various physiological and pathological situations during the gestational period.
The pioneering application of the method in this discipline occurred in 1977, when Fitzgerald and Drumm managed to obtain ultrasound records of the umbilical vessels using a continuous Doppler device. Once this possibility had been established, in the first studies, authors subsequently tried to standardize the method, and the various types of abnormalities found in these tests were described. The preparation of normality curves for various vessels constituted a logical necessity.
An initial source of much enthusiasm and radicalism in their use, after a relatively short period of time, Doppler velocimetry in obstetrics made it possible to demonstrate various hemodynamic events in humans, particularly those related to placental insufficiency and fetal response to hypoxemia, well known in the past, but studied only in animals and in laboratory models.
20.1 Physical Basis of Doppler Velocimetry
Ultrasound beams with known frequencies are produced through vibrations of piezoelectric crystals. These beams, when directed at a blood vessel, are reflected by columns of red blood cells (solid components) flowing within it. The echoes received by the same crystal or others generate electrical signals sent to the processing unit of the Doppler device, where they are transformed into audio signals (sound) and presented on a video screen in graphical form called a sonogram.
Illustration 20.1
Representative illustration of a blood vessel subjected to Doppler velocimetry. (fD = Doppler frequency; f0 = emission frequency, 3–5 mHz; fR = repetitive frequency; O = wave sound angle in relation to studied blood vessel axis). V = blood flow velocity; C = acoustic impedance of fetal blood (1570 m/s)
Fig. 20.1
Normal flow velocity waves at the umbilical vein and artery in the first trimester of pregnancy—9 weeks gestation. Note the absent end diastolic flow in the artery—zero velocity. This is a normal feature for this early gestational age. ARTERIA = artery; VEIA = vein; 9 SEMANAS = 9 weeks; CORDAO = umbilical chord
Fig. 20.2
Normal flow velocity waves of umbilical vein and artery at 13-week pregnancy. Note the presence of end-diastolic flow in the artery and vein with pulses
Fig. 20.3
Ideal location for uterine artery insonation (ART). When positioning the transducer in the iliac fossa and using the color Doppler device, the uterine artery can be visualized at its junction with the artery and external iliac veins
Fig. 20.4
After locating the uterine artery, the Doppler device is triggered and the characteristic flow velocity wave is obtained. This image shows a normal sonogram of the uterine artery during pregnancy, observing a high diastolic flow (arrow)
Fig. 20.5
Sonogram of the uterine artery in a hypertensive pregnant patient presenting a notch (dotted line) and low flow during diastole. This situation, when found after the 26th week of gestation, reflects a high resistance in the placental bed
Fig. 20.6
Normal umbilical artery sonogram in the third trimester. Note that the diastolic flow is high, demonstrating low placental resistance (arrow)
Fig. 20.7
Abnormal umbilical artery sonogram in the third trimester. It is noted that diastolic flow, although present, is reduced. The high ratio systole/diastole (A/B = 5.15) indicates the increase in placental resistance
