Key points are not available for this paper at this time.
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) is a scientific organization that encourages sound clinical practice, teaching and research related to diagnostic imaging in women's healthcare. The ISUOG Clinical Standards Committee (CSC) has a remit to develop Practice Guidelines and Consensus Statements as educational recommendations that provide healthcare practitioners with a consensus-based approach for diagnostic imaging. They are intended to reflect what is considered by ISUOG to be the best practice at the time at which they are issued. Although ISUOG has made every effort to ensure that Guidelines are accurate when issued, neither the Society nor any of its employees or members accepts any liability for the consequences of any inaccurate or misleading data, opinions or statements issued by the CSC. They are not intended to establish a legal standard of care because interpretation of the evidence that underpins the Guidelines may be influenced by individual circumstances and available resources. Approved Guidelines can be distributed freely with the permission of ISUOG (info@isuog.org). This document is a Practice Guideline on how to perform Doppler ultrasonography of the fetoplacental circulation. It is of the utmost importance not to expose the embryo or fetus to unduly harmful ultrasound energy, particularly in the earliest stages of pregnancy. At these stages, Doppler recording, when clinically indicated, should be performed at the lowest possible energy levels. ISUOG has published guidance on the use of Doppler ultrasound at the 11 to 13 + 6-week fetal ultrasound examination1. When performing Doppler imaging, the displayed thermal index should be ≤ 1.0 and the exposure time should be kept as short as possible, usually no longer than 5–10 min. It is not the intention of this Guideline to define clinical indications, specify appropriate timing of Doppler examination in pregnancy or discuss how to interpret findings or the use of Doppler in fetal echocardiography. The aim is to describe pulsed Doppler ultrasound and its different modalities: spectral Doppler, color flow mapping and power Doppler, which are commonly used to study the maternal–fetal circulation. We do not describe the continuous-wave Doppler technique, because this is not usually applied in obstetric imaging; however, in cases in which the fetus has a condition leading to very high-velocity blood flow (e.g. aortic stenosis or tricuspid regurgitation), it might be helpful in order to define clearly the maximum velocities by avoiding aliasing. The techniques and practices described in this Guideline have been selected to minimize measurement error and improve reproducibility. They may not be applicable in certain clinical conditions or for research protocols. Details of the grades of recommendation used in this Guideline are provided in Appendix 1. Reporting of levels of evidence is not applicable to this Guideline. All Doppler modalities are based on three fundamental principles. (1) Moving structures change the frequency and amplitude of reflected ultrasound signals. Moving structures include not only blood, but also fetal vessels or tissues. This can generate a shift in the backscattered signals. (2) Analysis of the components of the reflected signals are utilized for different Doppler modalities: the shift in frequency for directional color and spectral Doppler, and the shift in amplitude for power Doppler ultrasound (PDU). (3) All color and power Doppler modalities are pulsed techniques, while spectral Doppler can be pulsed or continuous. PRF, or scale, is the frequency at which the ultrasound signals (pulses) are emitted; a low PRF allows signals from slow-moving targets to reach the transducer before the next pulse is emitted, whereas a high PRF will allow only high velocities to reach the ultrasound transducer before the next pulse. The wall filter is a barrier defined by a specific threshold frequency below which signals are not displayed in the Doppler image. Gain is the amplification of signals. The quality and reproducibility of the recordings can be improved by knowledge of these Doppler settings and how to adjust them. Using real-time color Doppler ultrasound, the main branch of the uterine artery is located easily at the cervicocorporeal junction. Doppler velocimetry measurements are usually performed near to this location, either transabdominally2 or transvaginally3-5. While absolute velocities are of little or no clinical importance, semiquantitative assessment of the velocity waveforms is commonly employed. Measurements should be reported independently for the right and left uterine arteries, and the presence of notching should be noted. (GOOD PRACTICE POINT) Notching is defined qualitatively as reduced early diastolic velocities before the maximum diastolic velocity in the Doppler waveform. The severity of notching is defined by the difference between the lower early and the maximum diastolic velocities6. Note that, in women with congenital uterine anomaly, assessment of uterine artery Doppler indices and their interpretation is unreliable, since all published studies have been on women with (presumed) normal anatomy. (GOOD PRACTICE POINT) There is a significant difference in Doppler indices measured at the fetal end (intra-abdominal)11, in a free loop and at the placental end of the umbilical cord12. The impedance is highest at the fetal end, and absent/reversed EDV is likely to be seen first at this site. Reference ranges for umbilical artery Doppler indices at each of these sites have been published11, 13. For the sake of simplicity and consistency, by convention, measurements should be made in a free cord loop. (GOOD PRACTICE POINT) The decision to use a free loop of the cord was made early in the history of Doppler ultrasound and has been applied with great clinical success. However, in multiple pregnancies, and/or when comparing repeated measurements longitudinally, recordings from fixed sites, i.e. fetal end, placental end or intra-abdominal portion, may be more reliable. Appropriate reference ranges should be used according to the site of interrogation. Figure 3 shows examples of acceptable and unacceptable velocity waveform recordings and Figure 4 illustrates the influence of the vessel wall filter. Note that, in multiple pregnancy, assessment of umbilical artery blood flow can be challenging, since there may be difficulty in assigning a cord loop to a particular fetus. It is therefore better to sample the umbilical artery just distal to the abdominal insertion of the umbilical cord. However, the impedance there is higher than that in a free loop and that at the placental cord insertion, so appropriate reference charts are needed. (GOOD PRACTICE POINT) Note also that, in a two-vessel cord, at any gestational age, the diameter of the single umbilical artery is larger than the arterial diameter would be if there were two arteries14. Due to the different hemodynamics, the recorded velocity waveform in such cases should be interpreted with caution when using conventional reference ranges. (GOOD PRACTICE POINT) S/D ratio, RI and PI are the three best known indices to describe arterial flow velocity waveforms. All three are highly correlated. RI and S/D ratio estimate the relationship between PSV and EDV in the Doppler waveform (RI = (S − D)/S, S/D ratio = S/D, where S is peak systolic velocity and D is end-diastolic velocity). PI takes into account the PSV, the EDV and the time-averaged mean of the maximum frequency shift over the cardiac cycle (PI = (S − D)/TAMX, where S is peak systolic velocity, D is end-diastolic velocity and TAMX is the maximum velocity recorded in the MVE averaged over the cardiac cycle; TAMX should not be confused with time-averaged intensity-weighted mean velocity (TAV or Vm)). In Doppler waveforms showing dynamic changes in the systolic or diastolic components (i.e. in case of uterine artery waveform with presence of notching, or reversed EDV in umbilical artery waveform), PI gives a better estimate of the characteristics of the waveform than do RI or S/D ratio. PI shows a linear correlation with vascular resistance, as opposed to both S/D ratio and RI, which show a parabolic relationship with increasing vascular resistance31. Additionally, PI does not approach infinity when there are absent or reversed diastolic values. PI is the index recommended for use in clinical practice and research. (GOOD PRACTICE POINT) There is currently no high-level evidence to indicate how either CPR or UCR should be utilized in clinical management. Two indices are described for pulsed-wave Doppler analysis of the veins. The most commonly used is the pulsatility index for veins (PIV)32. This is calculated as PIV = (Vs − Va)/TAMX, where Vs is the peak forward velocity during ventricular systole and Va is the lowest forward velocity or peak reversed velocity during atrial contraction (the ‘a-wave’). The peak velocity index for veins (PVIV) is reported less frequently and is not featured on most auto-measure packages. PVIV is calculated as (Vs − Va)/Vd, where Vd is the peak forward velocity during atrial contraction (diastole). The use of PIV is recommended in clinical practice. (GOOD PRACTICE POINT) This Guideline presents the most commonly used techniques in clinical obstetrics, backed by solid scientific documentation. We are aware of important uses and sections of the circulation not mentioned herein, although these vessels and measurements may be of crucial importance in certain individuals. These vessels include, for example, the umbilical vein, hepatic artery, left portal vein and superior vena cava. However, the principles presented in this Guideline are valid for all fetal Doppler examinations. A. Bhide, Fetal Medicine Unit, St George's University Hospital and St George's University of London, London, UK G. Acharya, Division of Obstetrics and Gynecology, Department of Clinical Science, Intervention and Technology, Karolinska Institutet 58: 331–339.’
Bhide et al. (Mon,) studied this question.