Use of nitroglycerin for uterine relaxation

Data from human and experimental animal research indicate that nitric oxide (NO), a novel messenger, formed during the nitric oxide synthase-catalyzed oxidation of L-arginine to L-citrulline, is involved in maintaining normal uterine tone during gestation. There are demonstrated and potential benefits of manipulating the L-arginine-NO system during pregnancy. Several recent case reports and case series have described the effective use of nitroglycerin (GTN), a NO donor compound, antenatally, intrapartum, and postpartum for acute uterine relaxation. Therapeutic indications for GTN range from facilitating external cephalic version, difficult vaginal or cesarean section delivery, and manual exploration of the uterus, to its use as a tocolytic. The intravenous regimen of GTN required to obtain the desired degree of uterine relaxation is extremely variable; intravenous bolus doses of 50 micrograms to 500 micrograms GTN with up to three repeated injections of 50 micrograms to 250 micrograms have been reported. Other methods of GTN administration include transdermal patches and sublingual spray. GTN, when used in low doses, may provide safe and effective uterine relaxation with no clinically apparent fetal or maternal adverse effects. However, clinical trials with use of objective methods of evaluating uterine tone and comparing GTN to other tocolytic agents are required before widespread use in advocated.

Autacoid interactions in the regulation of blood flow in the human placenta

1. Interactions between autacoids may play important roles in the regulation of blood flow in the foetal placenta. In order to investigate this aspect of placental haemodynamics, human normal-term placentae were perfused in vitro and the responses of the foetal vessels to various combinations of vasoactive agents were determined. 2. Vasoconstriction responses to 5-hydroxytryptamine (5-HT) were potentiated in the presence of endothelin-1 (ET-1), the thromboxane A2-mimetic U46619 and a nitric oxide synthase inhibitor, N-nitro-L-arginine (NOLA), but not in the presence of angiotensin II. 3. N-Nitro-L-arginine caused vasoconstriction of the perfused placenta and indomethacin attenuated this effect and blocked the potentiation of the 5-HT response by NOLA. 4. Indomethacin did not affect ET-1-induced pressure increases and infusion of U46619 had no effect on release of ET-like immunoreactivity into the foetal placental circulation. 5. The present study provides evidence of interactions between several autacoids in human perfused placentae in vitro. These interactions may play important roles in foetal placental haemodynamics in normal or pathological situations.

The effect of a nitric oxide donor on Doppler flow velocity waveforms in the uterine artery during the first trimester of pregnancy

Our objectives were to study the effect of a nitric oxide donor on uterine blood flow velocity waveforms and maternal heart rate and blood pressure during the first trimester of pregnancy. Eleven women were examined at 8-12 weeks' gestation, prior to elective termination of pregnancy. Each woman was randomly given a sublingual tablet containing 5 mg of isosorbide dinitrate or placebo, on two consecutive days. Measurements of maternal blood pressure, heart rate and Doppler flow velocity waveforms of the ascending branch of the main uterine artery were obtained before, and every 2 min (for 20 min) after taking the drug or placebo. Following administration of isosorbide dinitrate, the mean arterial blood pressure fell from a control value of 82.5 +/- 5.6 mmHg to a nadir of 73 +/- 4.1 mmHg at 16 min (p < 0.0001). The mean maternal heart rate increased from a control value of 80.9 +/- 2.8 beats/min to a peak of 90.4 +/- 4.6 beats/min at 10 min (p < 0.001). The mean resistance index in the uterine artery fell from a control value of 0.80 +/- 0.02 to a nadir of 0.73 +/- 0.03 at 10 min (p < 0.0001). Analysis of covariance was employed to compensate for the effect of heart rate on the resistance index. The drug had a significant effect on the resistance index in the uterine artery, independent of maternal heart rate. These results suggest that drugs such as isosorbide dinitrate may have a potential benefit under such conditions where the production of nitric oxide (i.e. endothelium-derived relaxing factor) is decreased.

Effect of long-term cocaine administration to pregnant ewes on fetal hemodynamics, oxygenation, and growth

OBJECTIVE

To assess uterine and fetal blood flows by Doppler velocimetry and fetal growth and oxygenation in pregnant ewes treated daily with cocaine and to determine whether cocaine impairs fetal cardiac and cerebral reactivity.

METHODS

The study groups received 70 mg (n = 7) or 140 mg (n = 7) of cocaine and the control group (n = 7) received placebo injected intramuscularly daily on days 60-134. Hemodynamic data were measured at rest and during two acute hypoxic tests at cesarean delivery performed on day 134.

RESULTS

The fetal heart rate (FHR) and umbilical and uterine resistance indices (RIs) were higher in the cocaine groups than in the control group (FHR: 187 +/- 8 and 166 +/- 8 beats per minute at 83 and 123 days, respectively, in controls and 9-11% higher in cocaine groups; umbilical RI: 0.79 +/- 0.06, 0.60 +/- 0.04, and 0.52 +/- 0.06, at 83, 105, and 123 days, respectively, in controls and 11-17% higher in the cocaine groups [P < .01]; and uterine RI: 0.40 +/- 0.05, 0.40 +/- 0.04, and 0.37 +/- 0.04, at 83, 105, and 123 days, respectively, in controls and 13-35% higher in cocaine groups [P < .05]). At delivery on day 134, the following characteristics were found to be different in the cocaine groups: fetal weight (4.03 +/- 0.2 kg in controls and 15-21% lower in the cocaine groups [P < .02]), partial pressure of oxygen (26.5 +/- 1.4 mmHg in controls and 15-16% lower in cocaine groups [P < .05]), umbilical RI (0.40 +/- 0.03 in controls and 11-17% higher in cocaine groups [P < .01]), cerebral RI (0.61 +/- 0.03 in controls and 9-15% lower in cocaine groups [P < .01]), and cerebral-umbilical ratio (1.52 +/- 0.04 in controls and 22-23% lower in cocaine groups [P < .001]). During the hypoxic tests, the cerebral RI (P < .05) and the cerebral-umbilical ratio (P < .05) decreased significantly less in the two cocaine groups. The FHR response was reduced significantly in the two cocaine groups (P < .05).

CONCLUSION

Long-term exposure to cocaine induces uterine and fetal blood flow disorders, fetal growth restriction, and hypoxia. It reduces the capability of the cerebral vessels to vasodilate and the heart rate to increase during acute hypoxia.

An overview of the influence of ACE inhibitors on fetal-placental circulation and perinatal development

The renin-angiotensin system is associated with a variety of pathophysiological processes in many organ systems, and is known to be involved in the normal regulation of blood pressure and in the pathogenesis of renovascular hypertension. Angiotensin II is a multifunctional hormone that manifests its properties by interacting with two major subtypes of cell surface receptors (AT1 and AT2). Angiotensin converting enzyme (ACE) inhibitors are able to modify the actions of the renin-angiotensin system, and are indicated for the treatment of hypertension and heart disease. The antihypertensive effects of ACE inhibiting drugs are related to their ability to block the conversion of the decapeptide, angiotensin I, to the potent pressor octapeptide, angiotensin II. ACE inhibitors have been implicated in fetopathies in humans and perinatal mortality in rats, rabbits, sheep and baboons. Human fetopathies were seen when ACE inhibitors were given around the 26th week of gestation. The major adverse effects in babies include: oligohydramnios, renal tubular dysgenesis, neonatal anuria, calvarial and pulmonary hypoplasia, mild to severe intrauterine growth retardation, persistent patent ductus arteriosus and fetal or neonatal death. These developmental anomalies are thought to be partly due to a direct action of ACE inhibitors on the fetal renin-angiotensin system and partly due to the ischemia resulting from maternal hypotension and decreases in fetal-placental blood flow and oxygen/nutrient delivery to the fetus. The purpose of this review is to briefly discuss the pathophysiological role of the renin-angiotensin system, the therapeutic uses of ACE inhibitors in pregnant patients and to focus primarily on the major fetotoxic effects of ACE inhibitors encountered in humans and animal models. I will also review our recent data which show that capozide (captopril + hydrochlorothiazide) not only produces oligohydramnios but also disturbs the balance of glucose and NaCl in the maternal plasma and amniotic fluid of the rat.

Use of intrauterine microdialysis to investigate methanol-induced alterations in uteroplacental blood flow

Methanol is teratogenic in rodents; it has been postulated that this teratogenicity may be mediated in part by conceptal hypoxia. To construct a model to predict conceptal risk following maternal methanol exposure, conceptal disposition of methanol must be determined and any effects of such exposure on blood flow must be quantitated. In the present study, these toxicokinetic and toxico-dynamic parameters were evaluated by in vivo intrauterine microdialysis. Microdialysis probes were inserted into the uteri of Gestational Day (gd) 20 rats; methanol was administered as either an iv bolus (100 or 500 mg/kg) or infusion (100 or 1000 mg/kg/hr). In separate experiments, methanol (100 or 500 mg/kg) and 3H2O (20 microCi/kg) were administered iv to gd 20 and 14 rats and gd 18 mice. In both experiments, maternal blood and uterine microdialysate were collected and analyzed for methanol or 3H2O content. The methanol concentration-time data were consistent with saturable maternal elimination and apparent first-order transfer between maternal and conceptal compartments; at distribution equilibrium, conceptal methanol concentrations exceeded those in the dam by approximately 25%. The initial rate of conceptal permeation of methanol was proportional to the reciprocal of maternal blood methanol concentration (r2 = 0.910). Methanol also reduced significantly the rate of 3H2O uptake into the conceptus in a concentration-dependent fashion in gd 14 and 20 rats and gd 18 mice. These data indicate that methanol may decrease uteroplacental blood flow, decreasing methanol presentation to the conceptus and possibly producing conceptal hypoxia.

Single bolus compared with a fractionated dose injection technique of bupivacaine for extradural Caesarean section: effect on uteroplacental and fetal haemodynamic state

We studied 26 healthy parturients undergoing elective Caesarean section, allocated randomly to receive extradural block with 0.5% plain bupivacaine in a double-blind manner in either a single bolus or fractionated doses. After a 3-ml test dose, an additional 20 ml of bupivacaine were given over a 5-min period in the single bolus group (n = 13) and over a 25-min period in the fractionated dose group (n = 13). We studied the effects of bupivacaine on blood flow velocities in the maternal placental and non-placental uterine and fetal umbilical arteries before and four times during establishment of extradural block using a pulsed colour Doppler technique. Median sensory block reached T3 in the single-dose group compared with T4 in the fractionated-dose group. Two subjects in each group required i.v. ephedrine to correct transient hypotension (systolic arterial pressure < 90 mm Hg). Blood flow velocity waveform indices of the uterine and umbilical arteries did not differ significantly within or between groups during the study. There was no significant difference in neonatal outcome, as assessed by Apgar scores and umbilical artery pH values. In conclusion, we observed no deterioration in uteroplacental circulation after administration of a single bolus dose of bupivacaine.

Effects of L-arginine on utero-placental circulation in growth-retarded fetuses

BACKGROUND

To evaluate the effects of L-arginine (ARG) infusion, the nitric oxide as substrate, on the utero-placental circulation at third trimester.

METHODS

Three groups of nine pregnant women each were infused i.v. with 30 g ARG, for 30 minutes. One group served as control, and the two remnants were composed by patients with intrauterine growth retardation with (IUGR-B) or without (IUGR-A) increased resistances in the utero-placental circulation. Changes of blood flow velocity waveforms of both uterine arteries and umbilical artery were recorded for 60 minutes. Blood pressure, serum nitrites/nitrates and growth hormone levels were also measured.

RESULTS

No hemodynamic changes in utero-umbilical circulation were observed during infusion in any of the three groups. Considering the uterine arteries separately as placental and non-placental sided we found a significant decrease of non-placental side resistances in IUGR-B women. Indeed, the pulsatility index was lowered by 14%, in respect of baseline value. Serum nitrites/nitrates as well as serum growth hormone levels were significantly increased by ARG, in every woman, irrespective of the presence of fetal growth retardation. Blood pressure remained unaffected during infusion in every woman.

CONCLUSIONS

These findings suggest that L-arginine infusion affects utero-placental circulation in patients with IUGR associated with increased uterine resistances. Such an action is specific and appears possibly to be mediated by a release of nitric oxide.

Endothelin-1 constricts fetoplacental microcirculation and decreases fetal O2 consumption in sheep

Endothelin-1 produced by umbilicoplacental tissues may regulate fetal placental perfusion. To investigate its site of action, we measured segmental resistance in this bed in unanesthetized fetal sheep near term during fetal endothelin-1 infusion. A 15-min intravenous infusion of endothelin-1 at 1 micrograms/min significantly increased fetal blood pressure in the aorta (+33%), cotyledon artery and vein, and inferior vena cava, and endothelin-1 decreased fetal heart rate (-40%). Vascular resistance in the placental microcirculation increased significantly (+332%), but smaller increases in resistance of the umbilical artery and vein were not significant. Nevertheless, the stiffness of the umbilical arterial wall appeared to increase because vascular input impedance increased significantly both at the heart rate frequency (+85%) and when averaged > 2 Hz (characteristic impedance; +138%). Mean blood flow in the umbilical artery decreased by 64%, and the flow pulsatility index increased 137% (P < 0.05 for both). Despite the large decrease in placental perfusion, there was no significant change in descending aortic oxygen tension or oxygen content, because fetal oxygen consumption was reduced by 40%. We conclude that endothelin-1 is a potent constrictor of the placental microcirculation in sheep. Endothelin-1 also decreases fetal oxygen consumption by an unknown mechanism.

Prenatal toxicity studies in rats and rabbits with the calcium channel blocker diproteverine

In prenatal toxicity studies, diproteverine, a calcium channel blocker with demonstrated antianginal properties, produced an unusual pattern of digital, heart, tail, and vertebral defects in rat fetuses from mothers treated during the major period of organogenesis, but only a very low incidence of heart abnormalities was seen in the rabbit. Heart changes were rarely seen in association with digital defects. The findings were consistent with those seen with other calcium channel blockers and add weight to the suggestion of Danielsson and colleagues (5) that digital malformations are a class effect for this type of compound, the effects being related to reduced uteroplacental blood flow. In addition, it is proposed that cardiovascular malformations are also a class response with calcium channel blockers. The distribution of fetal death and hemorrhages and the varying association between cardiovascular, digital, and tail abnormalities seen in the rat with increasing doses of diproteverine fits the pattern of changes reported following hypoxia in the chick embryo. Reduced uteroplacental blood flow with resultant embryonic hypoxia secondary to pharmacologic action is considered a probable mechanism of action for the induction of abnormalities produced by calcium channel blockers.

Effect of maternal hyperoxygenation on experimentally produced uteroplacental insufficiency in the rat

The paper examines the effects of maternal hyperoxygenation on uteroplacental insufficiency produced by ligation of the uterine artery. Maternal hyperoxygenation did not significantly affect experimentally produced growth retardation or survival of the fetuses from the ligated horn. Analysis of the vascular anatomy revealed that additional oxygen improves the survival of fetuses compromised by uteroplacental insufficiency only in the presence of anastomosis between the uterine and ovarian arteries. The study demonstrated the importance of that anastomosis in evaluating the results obtained by this experimental model.

Effect of crystalloid and colloid preloading on uteroplacental and maternal haemodynamic state during spinal anaesthesia for caesarean section

We have studied the effects of crystalloid 1 litre (lactated Ringer's) or colloid 0.5 litre (hydroxyethyl starch) preloading in 26 healthy parturients undergoing elective Caesarean section under spinal anaesthesia. Maternal placental uterine artery circulation was measured using a pulsed colour Doppler technique with simultaneous measurement of maternal haemodynamics. A high incidence of maternal hypotension was observed during spinal anaesthesia in the crystalloid group (62%) but the incidence was lower in the colloid group (38%). Central venous pressure was increased significantly in both groups after preload but decreased shortly after induction of spinal anaesthesia to baseline values. The mean pulsatility index (PI) in the uterine arteries did not change during preload or spinal block. A surprising finding was the widespread variation and some high values for the uterine artery PI after spinal anaesthesia. These individual increases in PI were transient and always returned to baseline values within 2 min. These results suggest that preloading with either solution is ineffective in preventing maternal hypotension and that changes in maternal heart rate, systolic arterial pressure and central venous pressure during spinal anaesthesia were not associated with rapid individual increases in uteroplacental vascular resistance. These changes seemed not to have any major effect, however, on the clinical condition of the newborn, as assessed by Apgar scores and umbilical artery pH values.

Sex hormone modulation of flow velocity in the parametrial artery of the pregnant rat

The objective of the study was to correlate the disparate distal/proximal state of maturation of uterine segments in the rat with the hemodynamics of the parametrial uterine artery. Flow velocity was determined by the methylene blue photometric analysis. In nonpregnant animals, the velocity in the distal (VD-->P) and proximal (VP-->D) portions of the parametrial artery were similar, but in late pregnancy, VD-->P was higher than VP-->D. The difference was associated with an increased width of the distal part of the parametrial artery. In addition, the distance from the proximal end of the parametrial artery where the D-->P and P-->D streams collided decreased in animals in late pregnancy, compared with nonpregnant animals. Treatment with estradiol and RU-486 reversed the pregnancy-related hemodynamic changes. The pregnancy-related shift in flow velocity in the parametrial artery is crucial to rat fetal survival in utero and may be a mechanism in vivo for promoting maturation of distal, compared with proximal, segments of the uterine horn.

The effect of alpha 1-blocking vasodilator on fetal growth and uteroplacental blood flow in streptozotocin-induced diabetic rats

For largely unknown reasons, severe diabetes mellitus in pregnant rats results in fetal growth retardation. In order to clarify the mechanism of this phenomenon, the relationship between the fetal growth and the uteroplacental blood flow, and the fetal serum insulin and IGF-1 concentrations were studied in streptozotocine-induced diabetic and control pregnant rats. As compared to control rats, diabetic rats had significantly reduced uteroplacental blood flow, and the mean body weight of the fetuses was significantly lower. Treatment of diabetic rats with an alpha1-blocking vasodilator, urapidil, restored uteroplacental blood flow and fetal body weight without affecting hyperglycemia. There was no significant difference between diabetic and normal rats with respect to insulin and IGF-1 concentrations in fetal plasma, although the mean value was slightly lower in the diabetic. We conclude that reduced uteroplacental blood flow is a major factor in the establishment of fetal growth retardation in diabetic pregnancies.

Effects of intravenous infusions of noradrenaline into the pregnant ewe on uterine blood flow, fetal renal function, and lung liquid flow

To determine the effects on the fetus of high maternal levels of noradrenaline, experiments were carried out in 17 pregnant ewes (123-137 days gestation). Intravenous infusion of 40 mg/min of norepinephrine to the ewe for 1.5 h increased maternal arterial pressure and significantly decreased maternal placental blood flow (p < 0.05). Fetal arterial pressure did not change, but fetal arterial PO2 fell (p < 0.01) and PCO2 rose (p < 0.01). Fetal urine flow fell and osmolality rose (p < 0.01), fetal lung liquid flow and osmolar excretion fell (p < 0.01, p < 0.05, respectively), and the lung sodium:potassium ratio changed. These effects of high levels of maternal noradrenaline were transient, i.e., 2.5 h after the infusion of noradrenaline had finished, fetal urine flow and lung liquid flow had both returned to control values and fetal PCO2 was significantly depressed relative to control values (p < 0.01). It is concluded that high levels of maternal catecholamines reduce placental blood flow and cause small changes in fetal oxygenation. These changes are sufficient to transiently affect fetal water excretion and to reduce lung liquid flow.

Nicotine- or epinephrine-induced uteroplacental vasoconstriction and fetal growth in the rat

We examined the relationship between nicotine-induced vasoconstriction in pregnant rat dams and fetal growth during the third trimester of pregnancy. Pregnant rats were continuously treated between days 13 and 19 of gestation with either nicotine (9.6, 4.8 or 2.4 mg/kg/day), epinephrine (0.72 microgram/kg/day), or saline via continuous infusion from a subcutaneously implanted osmotic minipump. Placental weights in rats treated with high dose nicotine and dams' body weights were severely reduced. However, fetal weights were not affected. Blood flows in uterus and placenta were quantified by measurement of tissue content of 85Sr-labelled microspheres injected via a carotid artery catheter. Both nicotine and epinephrine caused a significant reduction (> 40%) in uterine and placental blood flow. We conclude that vasoconstriction alone as a result of nicotine or epinephrine administration during the last trimester of gestation does not necessarily reduce nutrient supply to the fetus and does not affect fetal growth in rats.

Vaginal and intracervical prostaglandin E2 for cervical ripening: a Doppler study of hemodynamic effects

The hemodynamic effects of vaginally and intracervically administered prostaglandin E2 gel were evaluated by pulsed wave color Doppler ultrasound. Twenty term pregnant mothers were recruited. Mothers were randomized to receive either vaginal or intracervical prostaglandin E2 gel. The hemodynamics was assessed by repeated color Doppler velocimetry of the uterine artery and fetal umbilical and middle cerebral arteries; analysis of variance was used to test statistical significance. In the uterine artery a statistically significant rise (P < or = 0.001) of the pulsatility index (PI) was observed. A slightly higher rise of the PI was observed after intracervical administration, but the difference between the groups was not statistically significant (P = 0.4). The fetal PI remained unchanged both in the umbilical and cerebral vessels throughout the study. The rise in the uterine artery PI is probably caused by an increase in the uterine tone as prostaglandins of the E series usually cause vasodilation. No detectable fetal effects were evident.

Pressor effect of ethanol in the isolated perfused human placental lobule

Pharmacologically relevant concentrations of ethanol (0.01-0.3 g/%) were perfused via the fetal circulation in isolated human placental lobules. This resulted in a dose-related rise in fetal arterial pressure, which at 0.3 g/% ethanol, was 10.1 +/- 1.1 mmHg above the pre-drug baseline. The pressor responses to ethanol were (i) rapid in onset, reaching a stable plateau within 5-10 min following administration, (ii) readily reversible by perfusion with drug-free media, (iii) non-tachyphylactic and (iv) largely inhibited by the cyclo-oxygenase inhibitor, inhibitor, indomethacin (5 microM). This pressor action of ethanol in the fetal circulation may contribute to the pathogenesis of the fetal alcohol syndrome as well as represent an underlying mechanism of ethanol-induced hypertension.

The utero-placental circulation, eugenics and the prevention and treatment of high risk pregnancies

Through systematic experimental and clinical studies, the physiological regulation of utero-placental circulation and the relation of the disturbance in this acirculation to pathogenic mechanisms of high risk pregnancies-Intrauterine Growth Retardation (IUGR) and Pregnancy-induced hypertension (PIH) were explored. The pharmacological effects and mechanism of a Chinese herbal medicine-Qingxintong in improving, the utero-placental circulation and the therapeutic efficacy in treatment of IUGR and PIH, both accompanied by disturbance of utero-placental circulation, were investigated as well.

Does a paracervical block with bupivacaine change vascular resistance in uterine and umbilical arteries?

Paracervical block during labor following normal, uncomplicated pregnancy is occasionally accompanied by fetal bradycardia. To evaluate whether a paracervical block with bupivacaine causes changes in the vascular resistance of uteroplacental and umbilicoplacental blood flow, a total of 12 singleton, uncomplicated pregnancies in active labor at the end of pregnancy were included to this study. By using pulsed color Doppler ultrasound techniques the pulsatility indices were measured from both uterine and umbilical arteries before, one minute and 20 minutes after a paracervical block with bupivacaine. Pulsatility indices of both uterine and umbilical arteries remained unchanged throughout the study period, as did fetal and maternal heart rates. In two cases fetal bradycardia developed, causing a marked increase in the vascular resistance of the umbilical artery one minute after the paracervical block. In the uterine arteries there was only a minor increase. When the fetal bradycardia ceased the pulsatility indices returned to the levels at the beginning of the study. Paracervical block with bupivacaine in normal pregnancies without signs of chronic or acute fetal distress does not change the vascular resistance in the uterine or umbilical arteries. If fetal bradycardia develops, it seems to be due to the direct effect of bupivacaine on the fetus, mainly on the umbilical vessels.