C.J. Ruissen

Interexaminer Variability of Fetal Doppler Velocity Waveforms

A study was made of the interexaminer variability of Doppler velocity waveform determinations of the fetal circulation. Three investigators recorded Doppler ultrasound tracings of the blood velocity in the fetal thoracic and abdominal aorta and umbilical artery in 23 uncomplicated pregnancies. Of these recordings, a hard copy was made, and the pulsatility indices were computed. An analysis of variance was performed and the reliability calculated. The interexaminer variability was nonsignificant in measurements of the three blood vessels under examination. Both thoracic and abdominal aorta measurements showed poor reliability (intraclass correlation coefficient of reliability 0.30 and 0.19, respectively) due to high random errors. Umbilical artery measurements showed a better reliability (intraclass correlation coefficient of reliability 0.61), and, therefore, these hold the greatest clinical applicability.

A Prospective Study of the Umbilical Artery Waveform in Appropriate-for-Date and Growth-Retarded Fetuses

Blood velocity waveforms were obtained by a combination of real-time B-mode and pulsed Doppler (2 MHz) ultrasound from the umbilical artery. In a prospective study, velocity waveforms of the umbilical artery were analyzed from 36 patients with single pregnancies. Sonograms were performed every 2nd week from the 18th week menstrual age onwards till labor. A total of 331 velocity profiles were obtained. Reference values were obtained from 27 patients with uncomplicated pregnancies. Abnormal waveforms were found in 9 cases. A comparison was made between the normal and abnormal group with regard to placental impedance indices [resistance index (RI), A/B ratio and pulsatility index (PI)]. The sensitivity, specificity, predictive values of each mentioned index were determined. The sensitivity of the PI and the RI is the same: 77.8%, the sensitivity of the A/B ratio is 66.7%. The specificity is also slightly higher for the PI and RI than for the A/B ratio (81.5, 81.5 and 77.5%, respectively). The same can be said for the predictive values (predictive value with positive result, 58.3, 58.3 and 50.0%, predictive value with negative result, 91.2, 91.2 and 87.5%). On the basis of these results and theoretical considerations, the PI is preferable to the other two placental impedance indices. It was shown once more that, with certain restrictions, this noninvasive transcutaneous technique reflects fetal well-being.

The influence of maternal exercise on the pulsatility index of the umbilical artery blood velocity waveform

Doppler bloodflow velocity measurements in the umbilical arteries are quite generally used as a parameter for obstetrical management, notwithstanding the fact that restricted knowledge is available about the physiological and pathophysiological mechanisms of human umbilical circulation. The possible effects of maternal haemodynamic changes on the fetal circulation are only scarcely investigated. In a prospective study the effect of maternal exercise on the Pulsatility Index (PI) values measured in the umbilical artery with a pulsed Doppler device was assessed to investigate the need of standardization with respect to maternal exertion. The studied group consisted of 23 women with uncomplicated pregnancies of various gestational age, who underwent a moderate strain. Statistical analysis showed no significant difference in FHR and PI values before and after exercise, so it is concluded that it is not necessary to introduce a period of rest to precede a Doppler investigation.

Variability of PI calculations

In obstetrics the pulsatility index (PI) may be a useful parameter for the determination of fetal well-being. Although the PI is unambiguously defined, calculation from the measured data can be difficult. In order to study the comparability of PI values found by several investigators and with different methods tape recordings of Doppler signals of fetal vessels were used. The tapes were processed by several centres in Europe and the results were compared. Comparability is poor, although for the umbilical artery the results are in reasonable agreement. The method used has its drawbacks but shows that one must be careful in comparing results from different centres.

Technical Aspects of Fetal Doppler Measurements

Doppler ultrasound may be a valuable tool in the estimation of fetal well-being. Nevertheless, a good understanding of the basic principles of ultrasound and the applied circuitry is necessary to obtain useful information. Some physical principles and limitations of ultrasound are discussed and the basics of the electronic devices are mentioned. Furthermore, the parameters used in obstetrical investigations are reviewed.

Characteristics of the Umbilical Artery Velocity Waveform as Function of Measurement Site

In 30 uncomplicated singleton pregnancies, varying in duration between 24 and 40 weeks, the variability of the flow velocity waveform (FVW) along the course of the umbilical artery was investigated. Blood flow velocities were recorded at 4 locations in the vessel: within the fetal abdomen, 0-5 cm from the origin of the umbilical cord, in the free-floating part, and 0-5 cm from its insertion in the placenta. From the Doppler signals recorded, the pulsatility index (PI) and a parameter for the frequency distribution index (FDI) were calculated. PI values differed among the locations, but no unequivocal tendency could be demonstrated. Statistical analysis, including multiple regression analysis for maternal and menstrual age and fetal heart rate, showed no significant difference in PI and FDI values for any of the 4 locations. It can be concluded that in uncomplicated pregnancies, possible changes in FVW (quantified by PI) along the course of the umbilical artery have no clinical relevance. Therefore, standardization for the sampling site when measuring PI in this vessel seems to be unnecessary.

The blood flow velocity waveform index in the fetal thoracic aorta and its ability to detect fetal compromise in the small for gestational age fetus

In a prospective longitudinal study of 96 patients, mean values of blood velocity waveform indices of the fetal thoracic aorta are established with a combined linear array real-time and pulsed Doppler transducer. The course of the various blood velocity waveforms is elucidated. The most striking finding is the restricted variability of the pulsatility index (PI), resistance index (RI), A/B ratio, acceleration time (AcT) and acceleration time percentage (AcTP) during the last trimester of pregnancy. In 17 proven and evaluable SGA infants the various blood velocity waveform indices were compared with the established mean values. Fourteen of these 17 (82%) infants were proven to be intrauterine growth-retarded (IUGR). Only 3 of these 14 (21%) showed a significant increase in PI values.

Prospective Study of Velocity Waveforms in the Fetal Descending Thoracic and Abdominal Aorta in Fetuses Appropriate for Gestational Age and in Growth-Retarded Fetuses

Blood velocity waveforms were obtained by a combination of real-time B mode and pulsed Doppler (2 MHz) ultrasound from the fetal descending thoracic and abdominal aorta. In a prospective study 36 patients were studied. Throughout pregnancy 261 tracings of the thoracic aorta and 241 tracings of the abdominal aorta were evaluable for this study. Measurements were performed every 2nd week from a menstrual age of 18 weeks onwards till the 40th week. Reference values for the resistance index (RI), AB ratio and the pulsatility index (PI) were established in 27 patients with uncomplicated pregnancies. Abnormal waveforms were found in the small for gestational age (SGA) group, which consisted of 8 patients. A comparison was made between the 2 sample means of the PI in the abdominal and descending thoracic aorta in the group with appropriate for gestational (AGA) fetuses, as well as between the AGA and SGA groups. With regard to the 95% confidence interval significance is shown between the mean PI in the descending thoracic and abdominal aorta, as well as between the AGA and SGA groups. However, blood velocity measurements of the fetal aorta do not provide additional clinical information compared with measurements of the fetal aorta do not provide additional clinical information compared with measurements in the umbilical artery.