Altered vascular function in healthy normotensive pregnant women with bilateral uterine artery notches

OBJECTIVES

To assess endothelial function and vascular mechanical properties in normotensive pregnant women with high resistance in the uteroplacental circulation.

DESIGN

Cross-sectional prospective study.

SETTING

Doppler ultrasound laboratory at university department of obstetrics and gynaecology referral centre for high risk pregnancies.

PARTICIPANTS

Forty-two caucasian normotensive pregnant women: 23 with uncomplicated pregnancies and 19 with bilateral uterine artery notches.

METHODS

Flow-mediated dilatation of the brachial artery was measured by ultrasonography at 25 gestational weeks. Concentrations of nitrite and nitrate in the plasma were established at 25 and 32 gestational weeks. The elastic properties of the common carotid artery, abdominal aorta and popliteal artery were measured with an ultrasonic echo-tracking system.

RESULTS

Flow-mediated dilatation at two minutes after cuff deflation was significantly lower in the bilateral notch group compared with the control group, 8.3% and 13.7%, respectively (P = 0.0007). The ability to sustain vasodilatation was reduced in the bilateral notch group (P = 0.02). Lower values of nitrite and nitrate in the plasma were found at 32 gestational weeks in the bilateral notch group than in the control group (mean 24.76 microM/L (SD 5.6) and 30.93 microM/L (8.2), respectively; P = 0.008). Nitrite and nitrate levels tended to be lower in the bilateral notch group even at 25 gestational weeks (29.45 microM/L (8.3) and 35.73 microM/L (11.0) in the bilateral notch and control group, respectively; P = 0.09). There was no difference in aortic, carotid or popliteal elasticity between the two groups.

CONCLUSIONS

Healthy normotensive pregnant women with bilateral uterine artery notches show impaired endothelial function, but no differences in vascular mechanical properties.

Ventriculo-vascular interaction in the normal development of the fetal circulation

OBJECTIVES

To examine cardiovascular physiology in the healthy fetus during normal development.

DESIGN

Twenty normal fetuses were studied longitudinally from 20 weeks to term. Serial echocardiography was performed, and arterial and venous diameter pulse wave characteristics and aortic pulse wave propagation velocity (PWV) were examined in the thoracic descending aorta (AoD) and inferior caval vein (IVC) using an ultrasonic phase-locked echo-tracking system. Statistical analyses included ANOVA, paired t-test and logistic regression where appropriate.

RESULTS

Aortic PWV, maximum incremental and late decremental velocities increased with gestation while the relative pulse amplitude decreased, reflecting falling distal impedance. There was a linear increase in cardiac preload and relative pulse amplitude in the IVC with gestation that correlated significantly with the presence of end-diastolic flow in the pulmonary artery and improvement in right ventricular diastolic function.

CONCLUSIONS

Non-invasive concurrent assessment of preload, ventricular function and impedance are possible in the fetus and may prove useful in the longitudinal study of fetal adaptation to pathophysiological changes.

Reproducibility of ultrasonic fetal volume blood flow measurements

The intraobserver reproducibility of ultrasonic volume blood flow measurements in the human fetus was evaluated in this study. A new approach, simultaneous measurement of the vessel diameter and the flow velocity with a pulsed-wave Doppler ultrasound synchronized with a real-time ultrasound phase-locked echo-tracking system, was used to estimate volume blood flow (VBF) in the fetal descending aorta. Measurements were performed in a longitudinal study on 20 normally grown fetuses. Intraobserver reproducibility of repeated estimations of mean blood flow velocities throughout gestation was very good, with high values of intraclass correlation coefficient (IntraCC 0.80-0.91) and low values of coefficient of variation (CV 4-11%). The IntraCC of repeated vessel diameter measurements throughout gestation was low (0.30-0.68), whereas the values of CV were acceptable (< 12%), with the exception of the period between 140 and 167 gestational days (CV > 12%). The lower reproducibility of vessel diameter measurement contributed directly to the relatively low reproducibility of VBF estimations overall (IntraCC 0.25-0.70; CV 17-28%), as these are calculated from a formula using both flow velocity and vessel diameter. Nevertheless, the synchronized approach gives absolute values of vessel diameter, flow velocity and VBF comparable with values reported in the human fetus previously. The new method provides, by taking the vessel wall pulsations into consideration and by measuring diameter and velocity simultaneously, a more complete information on fetal haemodynamics and fetal physiology.

Cerebral Doppler blood flow velocimetry and central hemodynamics in the ovine fetus during hypoxemia-acidemia

The purpose of this study was to assess the effects of hypocapnic hypoxia, acidemia and the combination of hypoxia/acidemia on blood flow velocity variables in the fetal cerebral circulation. Chronically instrumented fetal sheep were used and the ewes were induced to breathe a hypoxic gas mixture for about 90 min. This caused an initial period of hypoxemia followed by a period of mixed hypoxemia/acidemia. When the ewe was reoxygenated, the fetus experienced a period of normoxic acidemia. The fetal cerebral circulation was assessed by recording Doppler blood flow velocity waveform variables in a cerebral vessel and the umbilical artery, using standard ultrasound equipment. External carotid artery blood flow was maintained during hypoxic and hypoxic/acidotic periods despite a fall in cardiac output. In the cerebral vessel, mean maximum velocity (time-averaged maximum velocity), minimum diastolic velocity and maximum systolic velocity manifested increases during hypoxic and hypoxic-acidotic periods, but pulsatility index did not change due to the effect of reduced heart rate on pulsatility index. Umbilical artery pulsatility index increased in the hypoxic and hypoxic-acidotic periods, despite unchanged mean maximum velocity, minimum diastolic velocity and maximum systolic velocity. With acute hemodynamic changes, the measurement of pulsatility index can yield misleading results. For clinical and experimental research on the fetal cerebral circulation, more attention should be paid to the individual Doppler variables, especially to the mean maximum velocity, than to the pulsatility index alone. Changes in mean maximum velocity recorded from the cerebral artery seem to reflect changes in the cerebral arterial flow.

Aortic vessel wall characteristics and blood pressure in children with intrauterine growth retardation and abnormal foetal aortic blood flow

Blood pressure and pulsatile diameter changes of the abdominal aorta were measured in 68 children (mean age 9 years), with varying degrees of intrauterine growth retardation who were previously examined in their intrauterine life with Doppler velocimetry of the thoracic descending aorta. Diastolic blood pressure was lower (p < 0.05) and pulse pressure was increased (p < 0.01) in children with a birthweight small for gestational age as compared to those with a birthweight appropriate for gestational age. Systolic blood pressure was positively associated with relative increase in weight from birth up to the time of examination (p < 0.01), but not to early catch-up growth. Aortic vessel wall diameters were smaller in children born small for gestational age, both before and after correction for current body surface area (p < 0.01). Blood pressure and aortic vessel wall characteristics exhibited no relationship to the foetal aortic Doppler wave-form. Changes in foetal haemodynamics associated with intrauterine growth retardation do not appear to contribute to a later increase in blood pressure. Within a group of foetuses with suspected growth retardation, increasing foetal weight deviation and a birthweight small for gestational age is associated with lower diastolic blood pressure at 9 years of age.

Assessment of the fetal PO2 changes by cerebral and umbilical Doppler on lamb fetuses during acute hypoxia

The objective of the present study was to validate one or a combination of fetal Doppler parameters in order to assess acute fetal hypoxia in an ovine model. Acute hypoxia was induced by reducing umbilical, or maternal aortic flow (approx. 70%). A CW Doppler probe was fixed on the fetal cervical skin, facing the internal carotid artery and the fetal abdominal skin adjacent to the umbilical arteries. (The angle between Doppler beam and flow vector remained constant.) A "Doptek 3000" spectrum analyser was used to measure the maximal and mean Doppler frequencies. Heart rate (HR), umbilical blood flow (UBF), carotid blood flow (CBF), umbilical RI (URI), cerebral RI (CRI) and cerebroplacental ratio (CPR = CRI/URI) were calculated in real time. A catheter was inserted into the fetal femoral artery, for blood gas (PO2, PCO2 pH) and blood pressure (BP) measurements. After 1 min of aorta compression (70% aortic flow reduction), the URI increased by 10% (P < 0.05), and the UBF decreased by 10% (P < 0.05), but the CRI decreased by 20% (P < 0.02), and the CBF did not change significantly. Fetal PO2 and CPR fell down after 1 min (59% and 38%, respectively; P < 0.001), although strong fetal heart rate decelerations were observed. The blood pressure, PCO2 and pH did not change significantly during this test. Throughout the 12 min of cord compression (70% umbilical flow reduction) the URI increased (70% to 80% P < 0.001), and the UBF decreased (approx. 60%; P < 0.001), but the CRI decreased (approx. 25%; P < 0.01), and the CBF remained constant (+/- 5%; ns). Fetal PO2 and CPR all decreased during the compression (30% to 44% and 40% to 60%, respectively; P < 0.001). HR, pH and PCO2 did not change significantly. During cord compression the blood pressure did not change significantly. In both cases, the CPR decreased significantly (P < 0.001) with the PO2 in the same direction and with a comparable amplitude (-30% to -50%). Nevertheless, the drop in CPR was greater during cord compression than during aorta compression, probably because the compression of the cord induced a central hypovolemia in addition to the hypoxia. The CPR was found to be the hemodynamic parameter that followed most closely the PO2 acute changes. The amplitude of the variations of this parameter (-30% to -50%) were quite similar to those of the PO2 during the period of acute hypoxia.(ABSTRACT TRUNCATED AT 400 WORDS)

Compliance and diameter in the human abdominal aorta--the influence of age and sex

OBJECTIVES

Females develop cardiovascular diseases and abdominal aortic aneurysms later in life than males. In this study we investigated the diameter and compliance of the distal abdominal aorta in healthy females of varying ages. The results were compared with those obtained previously from healthy males in order to assess potential sex-related differences in the aging process of the abdominal aorta.

METHODS

An ultrasound phase-locked echo-tracking system was used to determine differences in diameter and pulsatile diameter change of the distal abdominal aorta in 69 Caucasian females from 4 to 74 years. Pressure strain elastic modulus (Ep) and stiffness (beta) were calculated from diameter, pulsatile diameter change and blood pressure obtained by the auscultatory method. Compliance was defined as the inverse of Ep and beta. The pressure dependence of Ep and beta was evaluated in 10 females with intraarterial blood pressure measurement at rest and during isometric exercise.

RESULTS

The diameter of the distal abdominal aorta increased not only in the period between the ages of 5 and 25 years, but also by about 24% between 25 and 70 years (p < 0.0001). From about the age of 25 years the diameter was smaller in females than in males (p < 0.01). Ep and beta increased nearly linearly with advancing age in females from 0.18 x 10(5) to 1.17 x 10(5) N/m2 (Ep) and from 1.85 to 8.51 (beta). In males the increase in Ep and beta was greater and exponential in nature (p < 0.001). Ep but not beta increased significantly during blood pressure increase (p < 0.05). Stiffness (beta) may therefore be a more useful index of arterial compliance than Ep.

CONCLUSIONS

This investigation demonstrates age and sex-related differences in diameter and compliance in the normal human abdominal aorta and implies that degenerative changes appear later in females than in males.

Fetal aortic diameter pulse wave response to changes in maternal position after membrane rupture

An ultrasonic phase-locked echo-tracking system was used in order to study human fetal hemodynamics in relation to maternal postural changes after membrane rupture. Diameter pulse waves were recorded in the descending aorta of 17 fetuses presenting by the vertex in normal singleton pregnancies, with the mother in three different positions. The first measurements were made with the mother in the slightly left lateral recumbent (horizontal), then upon transition to the upright (70 degrees), and finally upon return to the horizontal. Change from the horizontal to the upright position produced a moderate but significant increase (p < 0.05) in maternal heart rate, while the systolic and diastolic blood pressures remained unchanged. No significant changes were seen in the fetal pulse wave variables (i.e. end-diastolic diameter, relative pulse amplitude, maximum incremental velocity, late decremental velocity, pulse duration, relative crest time and pulse wave velocity). The absence of fetal hemodynamic changes possibly reflects the presence of a protective effect against I gravitational stress to the fetal cardiovascular system provided by the remaining volume of amniotic fluid and the abundant fetal extracellular water.

Blood flow velocity and pulsatile diameter changes in the fetal descending aorta: a longitudinal study

Blood flow velocity and pulsatile diameter changes in the fetal descending aorta were measured subsequently with pulsed Doppler ultrasonography and phase-locked echo-tracking technique, respectively. Blood flow velocity and pulse-wave curves with equal beat-to-beat intervals were synchronized by external fetal electrocardiogram. The systolic increase in blood flow velocity and in vessel diameter started simultaneously. The velocity peak preceded the diameter peak by a significant margin (p in the range 0.0001 to 0.0014). The time interval was more pronounced in the thoracic than in the abdominal part of the aorta and was slightly smaller near term. The effective diameter, calculated from the synchronized integrals of the blood flow velocity and the diameter curves, was significantly greater (p in the range 0.0001 to 0.0016) than the mean diameter. The difference tended to decrease with advancing gestational age and was more pronounced in the thoracic than in the abdominal aorta. The maximum error incurred by the use of nonsimultaneous measurements of blood flow velocity and vessel diameter when calculating volume blood flow was found to be less than 8% and thus probably of little practical importance.