Measurement of intervillous and myometrial blood flow by an intravenous 133Xe method

Hemodynamic Complications in Pregnancy: Preeclampsia and Beyond

Normal pregnancy is a complex and dynamic process that requires significant adaptation from the maternal system. Failure of this adaptive process in pregnancy contributes to many pregnancy related disorders, including the hypertensive disorders of pregnancy. This article discusses placental development and how abnormalities in the process of vascular remodeling contribute to the multisystem maternal and fetal disease that is preeclampsia and fetal growth restriction. We review some of the consequences of this condition on the mother and fetus, aspects of the clinical management of preeclampsia and how it can influence both mother and infant in the postnatal period and beyond.

Gestation-Specific Changes in the Anatomy and Physiology of Healthy Pregnant Women: An Extended Repository of Model Parameters for Physiologically Based Pharmacokinetic Modeling in Pregnancy

BACKGROUND

In the past years, several repositories for anatomical and physiological parameters required for physiologically based pharmacokinetic modeling in pregnant women have been published. While providing a good basis, some important aspects can be further detailed. For example, they did not account for the variability associated with parameters or were lacking key parameters necessary for developing more detailed mechanistic pregnancy physiologically based pharmacokinetic models, such as the composition of pregnancy-specific tissues.

OBJECTIVES

The aim of this meta-analysis was to provide an updated and extended database of anatomical and physiological parameters in healthy pregnant women that also accounts for changes in the variability of a parameter throughout gestation and for the composition of pregnancy-specific tissues.

METHODS

A systematic literature search was carried out to collect study data on pregnancy-related changes of anatomical and physiological parameters. For each parameter, a set of mathematical functions was fitted to the data and to the standard deviation observed among the data. The best performing functions were selected based on numerical and visual diagnostics as well as based on physiological plausibility.

RESULTS

The literature search yielded 473 studies, 302 of which met the criteria to be further analyzed and compiled in a database. In total, the database encompassed 7729 data. Although the availability of quantitative data for some parameters remained limited, mathematical functions could be generated for many important parameters. Gaps were filled based on qualitative knowledge and based on physiologically plausible assumptions.

CONCLUSION

The presented results facilitate the integration of pregnancy-dependent changes in anatomy and physiology into mechanistic population physiologically based pharmacokinetic models. Such models can ultimately provide a valuable tool to investigate the pharmacokinetics during pregnancy in silico and support informed decision making regarding optimal dosing regimens in this vulnerable special population.

When the Heart Stops

Cardiac arrest is a rare occurrence in pregnancy and may be related to obstetric or medical causes. Pregnancy is associated with profound physiologic changes that prepare the gravida for the challenges of labor and delivery, and resuscitation of the pregnant patient needs to take these changes into consideration. Cardiac output and plasma volume increase in pregnancy and distribute differently with the uterine circulation receiving approximately 17% of the total cardiac output. On the other hand, cardiac output is sensitive to positional changes in the second half of pregnancy but may improve with a lateral tilt of the gravida. Both oxygen reserve and upper airway size decrease in pregnancy, leading to difficulties surrounding airway management. Changes in the volume of distribution, renal and hepatic clearance may impact drug effects and need to be recognized. This review will discuss an overview of pregnancy physiology that is relevant to cardiac resuscitation, detail the challenges in the various resuscitative steps including a synopsis on perimortem delivery, and describe obstetric and nonobstetric causes of mortality and cardiac arrest in pregnancy.

Maternal uterine vascular remodeling during pregnancy

Sufficient uteroplacental blood flow is essential for normal pregnancy outcome and is accomplished by the coordinated growth and remodeling of the entire uterine circulation, as well as the creation of a new fetal vascular organ: the placenta. The process of remodeling involves a number of cellular processes, including hyperplasia and hypertrophy, rearrangement of existing elements, and changes in extracellular matrix. In this review, we provide information on uterine blood flow increases during pregnancy, the influence of placentation type on the distribution of uterine vascular resistance, consideration of the patterns, nature, and extent of maternal uterine vascular remodeling during pregnancy, and what is known about the underlying cellular mechanisms.

Contrast Sonography, Video Densitometry and Intervillous Blood Flow: A Pilot Project

PURPOSE

To examine the feasibility of constructing time-intensity (TI) curves from the intervillous space with an intravascular ultrasound contrast agent and computer assisted video densitometry.

STUDY DESIGN

We sedated nine pregnant baboons, optimized the grey scale and color Doppler images of their placentas, and then fixed the transducers in place. For each injection of contrast, we recorded images on videotape without changing the ultrasound image processing functions. Video images were captured using a Macintosh personal computer equipped with a video-capture board using image analysis software (Image 1.4, W Rasband, NIH). For each injection, we sampled digitized images of a fixed region of interest at regular intervals. After computing the mean video density of each image, we used the sampling frequency to construct TI curves depicting any change over time as the contrast agents washed into and out of the intervillous space.

RESULTS

Three of four agents tested produced changes in the video density of the placenta. TI curves were established using both grey scale and color Doppler signal augmentation. As expected, intra-arterial agents produced rapid accumulation and decay. Intravenous agents produced more protracted effects secondary to bolus dilution and transit through the right heart and pulmonary vascular bed.

CONCLUSION

TI curves may be generated from the intervillous space with the use of a transpulmonary ultrasound contrast agent and video densitometry. If validated by further study, this may allow investigators to apply ultrasound and indicator-dilution theory to intervillous blood flow.

Evaluation of the uncertainty in an oral reference dose for methylmercury due to interindividual variability in pharmacokinetics

An analysis of the uncertainty in guidelines for the ingestion of methylmercury (MeHg) due to human pharmacokinetic variability was conducted using a physiologically based pharmacokinetic (PBPK) model that describes MeHg kinetics in the pregnant human and fetus. Two alternative derivations of an ingestion guideline for MeHg were considered: the U.S. Environmental Protection Agency reference dose (RfD) of 0.1 microgram/kg/day derived from studies of an Iraqi grain poisoning episode, and the Agency for Toxic Substances and Disease Registry chronic oral minimal risk level (MRL) of 0.5 microgram/kg/day based on studies of a fish-eating population in the Seychelles Islands. Calculation of an ingestion guideline for MeHg from either of these epidemiological studies requires calculation of a dose conversion factor (DCF) relating a hair mercury concentration to a chronic MeHg ingestion rate. To evaluate the uncertainty in this DCF across the population of U.S. women of child-bearing age, Monte Carlo analyses were performed in which distributions for each of the parameters in the PBPK model were randomly sampled 1000 times. The 1st and 5th percentiles of the resulting distribution of DCFs were a factor of 1.8 and 1.5 below the median, respectively. This estimate of variability is consistent with, but somewhat less than, previous analyses performed with empirical, one-compartment pharmacokinetic models. The use of a consistent factor in both guidelines of 1.5 for pharmacokinetic variability in the DCF, and keeping all other aspects of the derivations unchanged, would result in an RfD of 0.2 microgram/kg/day and an MRL of 0.3 microgram/kg/day.

The investigation of placental relaxation and estimation of placental perfusion using echo-planar magnetic resonance imaging

Echo-planar imaging (EPI) is a form of magnetic resonance imaging (MRI) which acquires images in milliseconds rather than minutes as with conventional MRI. The images produced using EPI are affected by the physiological environment in which the hydrogen atoms producing the signals are found, a process referred to as relaxation. Also by producing images a matter of milliseconds apart, quantification of perfusion within the tissue being imaged is feasible. The objective of this study was to investigate T1 and T2 relaxation times along with perfusion in placentae from normal pregnancies at different gestations and also to compare these to pregnancies complicated by abnormal placental function. A cross-sectional study of normal and compromised pregnancies from 20 weeks to term and a longitudinal study of normal pregnancy were performed. Placental T1, T2 relaxation times, and perfusion were measured using echo-planar magnetic resonance imaging. Placental T1 and T2 relaxation times decreased in normal pregnancy (P<0.001). Relaxation times in pregnancies associated with placental pathology appeared to be reduced for that gestation although the numbers were too small to allow any statistical validation. No differences in placental perfusion with gestation or between normal and compromised pregnancy were demonstrated using this technique. This is the first demonstration of placental magnetic resonance relaxation and perfusion measurements in normal pregnancy using echo-planar magnetic resonance imaging. In the future it may be possible to identify compromised pregnancies by differences in placental T1 and T2 relaxation times, using this novel non-invasive technique.

The effects of regional anaesthesia for caesarean section on maternal and fetal blood flow velocities measured by Doppler ultrasound

We studied the effects of spinal anaesthesia (Group S), epidural anaesthesia (Group E), and combined spinal and epidural anaesthesia (Group SE), on maternal and fetal blood flow in 24 healthy parturients (n = 8/group) with uncomplicated singleton pregnancies using Doppler technique. Prior to the induction of anaesthesia, the patients were prehydrated with balanced electrolyte solution 15 ml.kg-1 over a period of 15 min. After the induction of regional anaesthesia, the systolic blood pressure was maintained within 15% limits of the preoperative values using prophylactic etilefrine infusion in Groups S and SE. The flow velocity waveforms of the maternal femoral artery, the main branch of the uterine artery (placental side), the foetal umbilical and middle cerebral arteries were recorded by Doppler technique before and after prehydration as well as after onset of T7 analgesia and the pulsatility indices (PI) were derived. Rapid intravenous prehydration had no effects on uteroplacental or fetal circulation as indicated by unaltered uterine, umbilical, and fetal middle cerebral artery PIs. After the onset of T7 analgesia, the uterine artery PI was increased in Group S indicating increased uterine vascular resistance while no changes occurred in Groups E and SE. No adverse effects were observed on the neonates as indicated by the Apgar score and the umbilical artery and vein acid-base status in any of the groups.

Anaesthesia and the sick foetus

The primary cause of foetal illness is placental pathology. The use of diagnostic ultrasound allows for a better understanding of how the foetus may be compromised by inadequate placental function. Anaesthesia affects placental function by changing uteroplacental and umbilical placental perfusion. Regional anaesthesia may have a beneficial effect on these circulations although general anaesthesia may be adapted to produce minimal disturbance of placental circulatory dynamics. The advantages and disadvantages of particular anaesthetic techniques in the delivery of the sick foetus are discussed.

The effect on uteroplacental blood flow of epidural anaesthesia containing adrenaline for caesarean section

The effect on uteroplacental blood flow of an epidural anaesthesia containing adrenaline for caesarean section was investigated in ten healthy women using dynamic placental scintigraphy with indium-113m and a computer-linked gamma camera. The epidural anaesthesia was performed with 18-22 ml bupivacaine 5 mg/ml with adrenaline 2.5 micrograms/ml followed by an i.v. balanced electrolyte infusion of 10 ml/kg b.w. A significant median decrease in the total maternal placental blood flow of 34% was found (P less than 0.01). There was also a significant decrease in maternal mean blood pressure of 3 mmHg (0.4 kPa) (P less than 0.05) and a significant negative correlation between the change in maternal blood pressure and the change in uteroplacental blood flow (r = -0.69, P less than 0.05).

Uteroplacental blood flow measured by placental scintigraphy during epidural anaesthesia for caesarean section

The uteroplacental blood flow was measured before and during epidural anaesthesia for caesarean section in 11 women. The blood flow was measured with dynamic placental scintigraphy. After an i.v. injection of indium-113m chloride, the gamma radiation over the placenta was recorded with a computer-linked scintillation camera. The uteroplacental blood flow could be calculated from the isotope accumulation curve. The anaesthesia was performed with bupivacaine plain 0.5%, 18-22 ml and a preload of a balanced electrolyte solution 10 ml/kg b.w. was given. The placental blood flow decreased in eight patients and increased in three with a median change of -21%, not being statistically significant. No correlation between maternal blood pressure and placental blood flow was found.

Labetalol does not alter the placental and fetal blood flow or maternal prostanoids in pre-eclampsia

The effect of intravenously administered labetalol (1 mg/kg) on placental and fetal blood flow was studied in 13 pre-eclamptic women. Although the maternal blood pressure decreased, no changes occurred in the blood flows in the intervillous space, the umbilical vein or the fetal descending aorta, nor did the indices of peripheral vascular resistance in the fetal aorta change, but the placental vascular resistance did decrease. Labetalol had no effect on prostacyclin or thromboxane A2 as measured by urinary 6-keto-prostaglandin F1 alpha and serum thromboxane B2 respectively. These findings are clinically relevant since they suggest that labetalol reduces maternal blood pressure without interfering with the placental or fetal blood flow.

Ritodrine infusion during late pregnancy: effects on fetal and placental blood flow, prostacyclin, and thromboxane

To study the effects of ritodrine on fetal and placental blood flow and maternal prostacyclin and thromboxane A2, 14 women with premature uterine contractions between the thirty-first and thirty-sixth weeks of pregnancy were treated with intravenous infusions of ritodrine, with incremental doses up to 200 micrograms per minute. The intervillous and the umbilical vein blood flows were measured before and after 1 hour of infusion of ritodrine, with the xenon 133 method and with a combination of real-time and Doppler ultrasonic equipment, respectively. Ritodrine decreased maternal diastolic and mean arterial pressures, as well as placental vascular resistance, but caused no significant changes in intervillous and umbilical vein blood flows. Ritodrine stimulated the synthesis of vasodilatory prostacyclin, as seen from a rise in maternal plasma 6-keto-prostaglandin F1 alpha, but inhibited the platelets' capacity to generate the vasoconstrictor thromboxane A2. Thus, apart from maternal hemodynamic changes, the intervillous and umbilical circulations are maintained during short-term administration of ritodrine in normotensive pregnancies.

Failure of exogenous prostacyclin to change placental and fetal blood flow in preeclampsia

Seven patients with acute preeclampsia and six with superimposed preeclampsia were infused intravenously with incremental doses of prostacyclin (up to 8 ng/min/kg during 80 minutes). Prostacyclin infusion was accompanied by significant decreases in maternal blood pressure and consistent rises in maternal plasma or urinary 6-keto-prostaglandin F1 alpha, but it caused no changes in maternal or fetal pulse rate or uterine contractility. Moreover, prostacyclin did not change the placental and umbilical blood flow, which were measured before and at the end of infusion. All women experienced facial flushing and two complained of headache during infusion. There was no difference in prostacyclin effects between women with acute or superimposed preeclampsia. These results may be taken as evidence that intravenous prostacyclin is not a specific therapy to increase placental or umbilical blood flow in preeclampsia.

Evaluation of the Xe-133 clearance method for indirect placental blood flow determination

The Xe-133 clearance method, chosen by several groups for indirect placental blood flow (PBF) determination, is evaluated using a placenta-fetus compartment model first introduced by Moll (1973). The placenta is modelled as a strong concurrent, counter-current or cross-current exchange system. The fetal circulatory system is simplified by dividing it into two major branches: the umbilical circulation and the fetal tissue or systemic circulation. Following an arbitrary labelling of maternal arterial blood, Xe-133 concentrations in myometrium, placenta and fetus are derived from the model and are used to calculate placental clearance rates. The dependence of clearance rates on maternal and fetal PBF, on the distribution of fetal cardiac output, and on the effect of recirculation times, is considered in the three different arrangements of maternal and fetal placental vessels.

Intervillous blood flow during caesarean section with prophylactic ephedrine and epidural anaesthesia

We administered a 15 mg i.v. bolus of ephedrine at the commencement of epidural blockade to nine healthy parturients scheduled for elective caesarean section. Nine other patients did not receive prophylactic ephedrine before epidural anaesthesia (control group). Lactated Ringer solution, 30 ml/kg, was infused before and during blockade, and left uterine displacement was used to minimize aortocaval compression. A133Xe i.v. technique was used to measure intervillous blood flow (IBF) before and 20-25 min after epidural block. The mean arterial pressure (MAP) decreased after epidural blockade in the ephedrine group by 0.67 +/- 0.8 (mean +/- s.d.) kPa and by 1.20 +/- 1.1 (mean +/- s.d.) kPa in the control group. In spite of the decrease in MAP, IBF increased by 6% in patients receiving ephedrine (N.S.), whereas it decreased by 11% in the control group (N.S.). In the ephedrine group there was in this preliminary study a trend to increasing IBF during falling perfusion pressure (MAP). The results of this preliminary study suggest that ephedrine will not affect IBF, but to prevent maternal hypotension ephedrine should be used as an i.v. infusion instead of a bolus injection.

Maternal and fetal effects of low-dosage bupivacaine paracervical block

Vasoconstriction of the uterine arteries, hypertonus of the uterus, and the direct toxic effects of a local anesthetic on the fetus or a combination of the above have been presented as etiological factors of fetal bradycardia following paracervical block. The reduce fetal side-effects a superficial and lowdosage technique of PCB have been advocated. We have studied the effects of 25 mg of bupivacaine PCB using the above technique on fetal heart rate pattern (FHR), fetal acid-base balance, uterine activity, placental blood flow and maternal and fetal plasma levels of bupivacaine in 38 patients. The analgesic effect of a single 25 mg of bupivacaine PCB was good in 76%, moderate in 12% and poor in 12% of the cases. No changes in maternal heart rate or in blood pressure were noted. Fetal bradycardia defined as a decrease of mean fetal heart rate of at least 20 bpm or an absolute rate less than 100 bpm and a duration greater than two minutes occurred in 12% of the cases. The mean amplitude of the baseline fetal heart rate variability decreased significantly after PCB and a silent pattern (an amplitude less than 5 bpm) was observed in 20% of the cases. The most frequent (27%) pathological finding in our study was the disappearance of FHR accelerations after PCB. Similarly early and late decelerations of FHR occurred more often after PCB than during the control period before the block. The fetal pH from scalp blood samples did not, on average, decrease after PCB, but did so in cases with fetal bradycardia. Intervillous blood flow as measured by the 133Xe washout method did not change when measured before and after PCB. In addition in three cases with fetal bradycardia the changes in the intervillous blood flow were minimal. No significant changes in the mean uterine tone, amplitude and frequency of contractions were observed after PCB. However, an obvious uterine hypertonus was observed after PCB was observed in three cases of fetal bradycardia but not in two other cases of bradycardia or in the 8 cases of silent FHR pattern. Mean maternal bupivacaine concentration 20 minutes after PCB was 0.14 +/- 0.06 microgram/ml and 0.07 +/- 0.04 microgram/ml at birth. Simultaneous fetal and umbilical venous and arterial concentrations were correspondingly 0.04 +/- 0.02 microgram/ml, 0.03 +/- 0.01 microgram/ml and 0.03 +/- 0.01 microgram/ml, and they were significantly lower than respective maternal concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of caffeine on placental and fetal blood flow in human pregnancy

The effects of maternal ingestion of two cups of coffee were investigated in 20 pregnancies during the last trimester. Maternal serum caffeine and epinephrine concentrations after 30 minutes were significantly elevated as compared with the fasting values (p less than 0.01). The intervillous placental blood flow decreased almost significantly (p less than 0.05). The fetal umbilical vein blood flow was unchanged. In patients with hypertensive pregnancy in the series there was reduced intervillous blood flow initially, and these values did not change after the maternal caffeine intake. The decrease of placental blood supply and increased maternal serum epinephrine levels associated with maternal coffee ingestion may be potential perinatologic risks, and more investigation about caffeine effects in human pregnancy is needed.

The influence of maternal oxygen inhalation on human placental and umbilical venous blood flow

The effect of maternal short-term inhalation of oxygen (51/min) on intervillous (IVBF) and umbilical vein blood flow (UVBF) was studied in 22 cases during the third trimester of pregnancy. The maternal paO2 levels increased significantly (P less than 0.001) after O2 inhalation. The mean IVBF level was 190 +/- 66 (SD) ml/min per 100 ml of the intervillous space before inhalation and 125 +/- 58 ml afterwards, the decrease being significant (P less than 0.01). UVBF maintained its original level after O2 inhalation. The human fetus seems not to compensate for alterations in oxygen delivery and reduced IVBF after maternal oxygen inhalation, by means of changes in UVBF.

Antepartum fetal heart rate variability and intervillous placental blood flow in association with smoking

Intervillous placental blood flow and indices of fetal heart rate variability were measured from seven healthy pregnant women in the last trimester of pregnancy, once before and twice after smoking one cigarette. A blood flow reduction was observed in seven, a rise in five, and no change in two measurements. When intervillous placental blood flow decreased both indices of variability decreased (p less than 0.001), and when it increased the short-term component of fetal heart rate variability increased more significantly (p less than 0.001) than did the long-term component (p less than 0.01).

Maternal prostacyclin, thromboxane, and placental blood flow

The vasoactive prostanoids--prostacyclin (PGI2) and thromboxane A2 (TxA2)--and their metabolites--6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TxB2), respectively--have been implicated as regulators of uteroplacental blood flow in animals. To study their roles in placental blood flow in human beings, plasma or serum samples were collected for the measurement of 6-keto-PGF1 alpha and TxB2 from 42 women during late pregnancy on the same occasion, when placental intervillous blood flow (IVBF) was determined with 133Xe isotope method. The concentrations of 6-keto-PGF1 alpha in plasma or those of TxB2 in plasma or serum were not related to the IVBF. The intravenous infusion of ritodrine for 1 hour up to the dose of 200 micrograms/min increased (p less than 0.05) the plasma 6-keto-PGF1 alpha concentrations, but caused no changes in the TxB2 levels or IVBF in seven women with premature uterine contractions. We conclude that if PGI2 and TxA2 participate in the control of placental blood flow, their changes are located in the fetoplacental unit and, thus, are not reflected by the levels of their metabolites in maternal circulation, and ritodrine may stimulate PGI2 synthesis in human beings.

The acute effect of smoking on uteroplacental blood flow in normotensive and hypertensive pregnancy

The acute effect of smoking on uteroplacental blood flow was studied by the 133Xe method in 12 normotensive and 11 hypertensive pregnant women at term. A standard cigarette caused an acute decrease in intervillous blood flow in the normotensives but normalized within 15 minutes. In the other group, smoking produced a progressive increase in this flow, whose basal presmoking value lower in the hypertensive women. The myometrial blood flow was equal in both groups before smoking but significantly higher in the hypertensive group immediately afterwards. Smoking produced a constant increase in systolic, diastolic and mean arterial blood pressure in the normotensive women. In the other, diastolic and mean arterial blood pressure increased only temporarily, while systolic blood pressure remained unchanged. Heart rate increased in both groups. Repeated decreases in the intervillous blood flow could explain growth retardation of the fetus and some other complications of pregnancy in normotensive smokers. The authors assume that postischemic reactive hyperemia in the uterine vessels caused the increased flow in hypertensive pregnant women after smoking but cannot recommend it as beneficial because of the increased placental transfer of the harmful elements found in tobacco smoke.

Measurement of bone blood flow with a 133 Xe washout method. A preliminary report

A method based on the intravenous 133Xe injection technique has been used for measurement of bone blood flow in man. The measurements were made from the greater trochanteric region of the femur of eight healthy subjects and three patients with bone marrow or a bone disease in which bone blood flow is known to be increased. The half-times of the fast and the slow compartments of the externally recorded two-exponential bone washout curves were 4.05 +/- 0.88 min and 45.4 +/- 7.4 min (mean +/- 1 sd) in the healthy subjects, 1.46 min and 20.1 min in the patient with chronic myeloid leukemia, 2.50 min and 22.9 min in metastic bone disease and 1.93 min and 18.1 min in the patient with osteosarcoma, respectively. The corresponding flow values were 11.5 +/- 1.4 ml/100 g/min (mean +/- 1 sd) in healthy subjects and 59.8, 28.3 and 34.0 ml/100 g/min in patients with bone disorders. The precision of the method estimated from the duplicate measurements in eight healthy persons is; for the fast compartment, 6.8%; and for the slow one, 3.2%. Because of the rapid washout of xenon and the very low radiation dose the measurements are easily repeatable.

Changes in placental intervillous blood flow measured by the 133xenon method during lumbar epidural block for elective caesarean section

The effect of lumbar epidural block on placental intervillous blood flow (IBF) was estimated with the i.v. 133Xe method in 24 cases and in 14 controls. All parturients had uneventful pregnancies and were scheduled for elective caesarean section. Lumbar epidural block caused a significant decrease in maternal mean arterial pressure (MAP), when no preloading infusion with a plasma expander was given. In contrast, the MAP remained unchanged during lumbar epidural block when the parturients received a preloading infusion (Haemaccel, 100 ml/10 kg body weight within 10 min immediately before lumbar epidural block). The IBF decreased to some extent in the non-preloaded group, but increased temporarily in the preloaded group 15 min after the administration of lumbar epidural block. The mean changes of IBF in the non-preloaded and the preloaded groups differed statistically significantly from each other during lumbar epidural block. The IBF of the preloaded group did not differ significantly from the IBF of the control group receiving no lumbar epidural block.

Epidural analgesia and placental blood flow during labour in pregnancies complicated by hypertension

Placental blood flow was measured during the first stage of labour in pregnancies complicated by essential hypertension or severe pre-eclampsia using a 133Xe clearance technique before and after segmental epidural analgesia. Analgesia was produced with 0.5 per cent plain bupivacaine in six patients with essential hypertension and in 11 patients with severe pre-eclampsia. After segmental epidural analgesia the placental blood flow was slightly but insiginificantly improved in most of the patients with pre-eclampsia and in half of the patients with essential hypertension, thus suggesting this analgesic method to be safe in hypertensive pregnancies in the absence of any substantial change in systemic blood pressure.

Placental intervillous blood flow measured by inhaled 133Xe clearance in relation to induction of epidural analgesia

A method of determining placental intervillous blood flow (IBF) using an inhaled 133Xe technique is described. The method was used to investigate the effect on IBF of epidural analgesia with an initial dose of 37.5 mg bupivacaine. There was no statistically significant reduction in IBF following the initial dose of bupivacaine in spite of a small but statistically significant reduction in mean blood pressure. There was no significant correlation between the change in mean blood pressure and the percentage change in IBF following epidural block although the single largest percentage reduction in IBF was associated with the largest fall in mean blood pressure.

The acute effect of smoking on intervillous blood flow of the placenta

Smoking a standard cigarette caused an acute decrease in intervillous placental blood flow. This change normalized within 15 minutes. At the time intervillous blood flow was depressed, heart rate and blood pressure were elevated and remained so throughout the study period. Repeated decreases in the intervillous blood flow could explain growth retardation of the fetus and some other complications of pregnancy in women who smoke.