Transcutaneous detection of relative changes in artery diameter.

The extent of the excursions of the arterial walls during the cardiac cycle depends on both the compliance of the vessel wall and the local pressure fluctuations. Simultaneous assessment of the relative change in artery diameter in combination with the velocity distribution along the vessel cross section can reveal the cause of loss of distensibility. As will be demonstrated, a multigate pulsed Doppler system with a high spatial resolution can perform simultaneously both functions. The relative change in diameter during the cardiac cycle is obtained by taking the ratio of the distension and the diameter of the artery as observed along the ultrasound beam. It can be shown that this ratio will be angle independent. Statistical and experimental evaluations demonstrate that the system allows the assessment of the relative change in diameter of major peripheral arteries as a continuous function of time with an accuracy of about 0.5%.

Methods to evaluate the sample volume of pulsed Doppler systems.

The spatial resolution of a pulsed Doppler system heavily depends on both the characteristics of the ultrasound transducer and the processing circuitry. The local characteristics of the ultrasound beam give the lateral sample function while the way the signals are processed affects the axial shape of the sample volume. A simple way to explore the size of the sample volume is the employment of a running string target. However, the detected Doppler signals will be the sum of all the Doppler signals returned by the string along its course through the sample volume. Therefore, it will be complicated to separate the axial and lateral contribution to the actual size of the sample volume. A scanning system capable of visualizing the cross-sectional distribution of the ultrasound intensity allows the assessment of the sample sensitivity in lateral direction. A single vibrating point target enables the exploration of both the axial and lateral sampling function. The latter approach is especially suited to evaluate routinely the spatial performance of pulsed Doppler systems in clinical practice.

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.

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.

Variation in early fetal growth.

OBJECTIVE: To obtain more information on interindividual variation in early fetal growth by using early fetal ultrasound parameters. DESIGN AND SUBJECTS: Individual fetal growth was assessed prospectively in 109 pregnancies by serial ultrasound measurements of CRL and BPD. From the resulting biometry data, individual CRL and BPD growth curves for each fetus were constructed and compared. RESULTS: A high correlation was found between growth rate and the start of growth of CRL and BPD, r = 0.79 and r = 0.84, respectively. CONCLUSION: The high correlation between growth rate and the start of growth of CRL and BPD might imply that a fetus with an impeded start of growth generally has a higher growth rate. This could be called an early pregnancy 'catch-up growth'. This interindividual variation of starting time and fetal growth rate may influence the assessment of gestational age in early pregnancy, if a CRL-curve obtained from cross-sectional measurements is used as a reference. BPD seems to be a more constant parameter with less variation in growth rate.

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.

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.

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.

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.

Umbilical artery flow velocity waveforms during acute hypoxemia and the relationship with hemodynamic changes in the fetal lamb.

The contributions of the variables of the fetal circulation to changes in the pulsatility index of the umbilical artery flow velocity waveform have not been assessed. Acute fetal hypoxemia was induced by 60 to 90 seconds of total occlusion of the maternal common internal iliac artery in six sheep. Mean fetal PO2 levels decreased from 26.0 to 18.3 mm Hg (p less than 0.01) after occlusion of uterine blood flow. Fetal heart rate decreased from 188 to 121 beats per minute at the end of occlusion (p less than 0.05). Placental vascular resistance did not change during the heart rate deceleration. The pulsatility index increased from 0.86 during the control period to 1.27 at the end of occlusion (p less than 0.05). After fetal parasympathetic blockade with atropine, fetal heart rate and placental vascular resistance did not change during occlusion. The pulsatility index did not change during occlusion after parasympathetic blockade. It is concluded that the changes in the umbilical artery pulsatility index during late decelerations in the fetal heart rate pattern appear to be primarily associated with changes in fetal heart rate and bear no relationship with placental vascular resistance.

Comparison of blood velocity waveform indices recorded by pulsed Doppler and continuous wave Doppler in the umbilical artery.

A comparison was made between the AB ratio, resistance index (RI), and pulsatility index (PI), calculated from tracings obtained by pulsed Doppler (PD) and continuous wave (CW) Doppler devices in 14 uncomplicated singleton pregnancies. The AB ratio, RI, and PI were a slightly higher for the CW Doppler compared to the PD system, although not significantly (p = 0.18, p = 0.21, and p = 0.44, respectively). The difference in signal to noise ratio (S/N ratio) between the PD and CW Doppler systems was felt to be the reason for the discrepancy in the calculated velocity waveform indices.

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 response of the umbilical and femoral artery pulsatility indices in fetal sheep to progressively reduced uteroplacental blood flow.

Fetal artery Doppler velocimetry may provide noninvasive information on the state of fetal oxygenation. It was hypothesized that during decreasing fetal oxygenation, the pulsatility index in the femoral artery will increase, whereas the pulsatility index in the umbilical artery will not change. Decreasing fetal oxygenation was induced in ten chronically-instrumented fetal sheep by progressive occlusion of the maternal common internal iliac artery. The pulsatility index in the umbilical artery was serially measured in six fetuses (group I, n = 6) and the pulsatility index in the femoral artery was serially measured in four fetuses (group II, n = 4). Fetal arterial oxygen content decreased by 72% in group I (P less than 0.0001) and by 79% in group II (P less than 0.0001). Fetal heart rate did not change. Fetal blood pressure increased by 11% in group I (P less than 0.02) and by 15% in group II (P less than 0.005). The umbilical artery pulsatility index (group I) did not significantly change during decreasing fetal oxygenation, whereas the femoral artery pulsatility index (group II) increased by 150% (P less than 0.005). It is concluded that progressively reduced uteroplacental blood flow results in fetal hypoxaemia, which is associated with increased pulsatility index in the femoral artery, while the pulsatility index in the umbilical artery does not change.

Pulsatility index and its relationship to placental vascular resistance during partial umbilical venous occlusion: a study in fetal lambs.

To study the effect of partial occlusion of the umbilical vein upon umbilical artery velocity waveforms, 4 chronically instrumented pregnant sheep have been subjected to measurement of the relevant haemodynamic parameters, i.e. the arterial inflow pressure, placental venous outflow pressure (Pv), venous blood flow (Quv) and arterial blood velocity waveform. The pulsatility index (PI) of the velocity waveform increases significantly for a Quv reduction of 40% or more (p less than 0.05). The Quv correlates well with the PI (r = 0.61) and the Pv (r = 0.71). The correlation between fetal heart rate (FHR) and Quv, FHR and PI is 0.75 and 0.64, respectively. The placental vascular resistance (R) can be calculated using the Poiseuille equation. There is not significant correlation between R and PI. It can be concluded that the increase in PI in the umbilical artery during partial venous occlusion is very likely caused by an increased Pv rather than a change in R.