Effects of prolonged infusion of beta-adrenergic agonists on uterine and umbilical blood flow in pregnant sheep

Hemodynamic effects of intravenous isoproterenol versus epinephrine in the chronic maternal-fetal sheep preparation

Isoproterenol 5 micrograms may be an effective marker of accidental intravascular injection in women in labor; however, before isoproterenol can be incorporated in routinely used epidural test doses, the safety and usefulness should be determined in an animal model. This study was designed to examine the hemodynamic effects of isoproterenol in comparison with epinephrine in the pregnant ewe. Five doses of isoproterenol were tested and compared with two doses of epinephrine in a randomized cross-over fashion. After administration of isoproterenol there was a small decrease of uterine blood flow (UBF) and maternal mean artery pressure (MMAP), which both almost immediately returned to baseline. When epinephrine was used a more pronounced and more prolonged decrease of UBF occurred. Increasing doses of isoproterenol resulted in dose-dependent increases in maternal heart rate (MHR), while with epinephrine this was not the case. A significant increase in the cardiac output was seen after isoproterenol. Neither isoproterenol nor epinephrine affected fetal heart rate (FHR), fetal mean arterial pressure (FMAP), amniotic fluid pressure (Amn-pr), blood gases, or acid base status in the mother and the fetus. Provided that neurotoxic effects are absent, isoproterenol might be a better alternative than epinephrine as a test dose for possible intravenous placement of an epidural catheter in pregnant women.

Effect of intravenous beta-sympathomimetic tocolysis on human fetal serum erythropoietin levels

OBJECTIVE

The major stimulus for erythropoietin production is tissue hypoxia. We sought to investigate the relationship of beta-sympathomimetic administration for tocolysis and fetal serum erythropoietin.

STUDY DESIGN

Umbilical cord blood was obtained from infants whose mothers received intravenous beta-sympathomimetic tocolysis and who were delivered at < or = 34 weeks' gestation. Serum erythropoietin was measured by radioimmunoassay. On the basis of the presumed 2- to 4-hour half-life of fetal erythropoietin, the infants were divided into two groups. In group 1 (n = 16) beta-sympathomimetic therapy was discontinued < 24 hours before delivery; in group 2 (n = 11) it was discontinued > or = 24 hours before delivery.

RESULTS

Group 1 fetuses had significantly higher erythropoietin levels than did group 2 fetuses (37.3 vs 13.9 mU/ml, p = 0.02). The duration of beta-sympathomimetic tocolysis and the maximum infusion rate were not different. The two groups did not differ in gestational age, birth weight, route of delivery, presence of labor, or duration of first or second stage of labor.

CONCLUSIONS

We speculate that intravenous beta-sympathomimetic tocolytic therapy stimulates fetal erythropoietin production by decreasing fetal oxygenation as a result of the reversible fetal metabolic effects of the tocolysis. These data suggest that beta-sympathomimetic tocolysis should be undertaken cautiously if fetal compromise is suspected, fetal well-being should be assessed carefully if tocolysis is undertaken, and treatment should be discontinued promptly if a clear benefit is not realized.

Maternal endocrine and fetal metabolic responses to heat stress

Heat-stressed pregnant ewes deliver intrauterine growth-retarded lambs. Selected maternal and fetal changes were investigated during acute heat stress in order to elucidate the mechanism for this growth retardation. Uterine blood flow decreased 20 to 30% in pregnant ewes during 1 degree C increases in core temperature. The decreases were accompanied by 60 and 100% increases in serum oxytocin and antidiuretic hormone, respectively. These effects were mimicked by salt loading or injections of antidiuretic hormone or oxytocin, suggesting a role for either or both hormones in regulating uterine blood flow during pregnancy. Chronically heat-stressed pregnant ewes were delivered by Caesarean section. Their fetuses were approximately 20% smaller than thermoneutral controls. Within each pair of heat-stressed twins, one fetus weighted one-third less than its litter mate. No difference in weights were observed within the control twins. The livers and brains of the heat-stressed fetuses were disproportionate in size. The livers from the small heat-stressed twins contained only one-half the protein of the controls and one-fourth the protein of their litter mates. Muscle protein was decreased in the heat-stressed fetuses, and liver and muscle glycogen were elevated as were liver arginase, glutamate-pyruvate transaminase and muscle creatinine. These results are consistent with the following hypothesis: heat stress stimulates the release of maternal antidiuretic hormone or oxytocin, which reduces uterine blood flow and causes a shift in fetal metabolism from anabolic to catabolic pathways; one fetus of heat-stressed twins is more severely affected than its litter mate.

The effect of a low-dose infusion of ritodrine on cardiac output distribution and uteroplacental blood flow in unanaesthetized pregnant guinea pigs

Cardiac output and its distribution were measured using the microsphere technique before and at 2 and 6 h during the infusion of ritodrine, 1.5 microgram X min-1 X kg-1 i.v., into near-term pregnant guinea pigs. Average cardiac output increased 16 and 25%, and total peripheral resistance decreased 16 and 19% after 2 and 6 h of ritodrine infusion. The carcass and gastrointestinal tract together received two-thirds of the extra cardiac output. Uteroplacental blood flow and uteroplacental vascular resistance were not significantly different from the preinfusion values after 2 h of ritodrine treatment, but were increased 35% and decreased 31%, respectively (p less than 0.05 and less than 0.01), after 6 h of infusion. The increase in uteroplacental blood flow between the 2- and 6-h measurements was positively, and the decrease in uteroplacental vascular resistance negatively correlated with the increase in cardiac output during the same period.

Effect of continuous infusion of fenoterol on maternal pelvic and fetal umbilical blood flow in pregnant sheep

The results from studies on the reactions of the uterine vascular bed upon intravenous administrations of beta-adrenergic drugs to the ewe are not all identical. This can be partly explained by different reactions of the pelvic vasculature on beta-adrenergic receptor stimulation. In order to assess whether any differences in flow reactions existed between the vascular beds of two maternal pelvic vessels upon beta-adrenergic receptor stimulation, we studied the effect of continuous maternal intravenous infusion with fenoterol on the blood flow in the maternal internal iliac and the median uterine artery in seven chronically instrumented pregnant sheep between 104 and 142 days gestation. Furthermore, the effects on umbilical venous blood flow, fetal heart rate, blood pressure and acid-base balance were analyzed. Maternal and fetal blood flows were measured with electromagnetic flow transducers. Fenoterol was administered to the ewe via a continuous intravenous infusion in two sequential periods of 30 minutes duration in a dose of 2 respectively 4 micrograms per minute. The blood flow in the internal iliac artery showed an increase of 10.5% (p less than 0.05) at the end of the infusion period and was still but not significantly elevated during the postinfusion period. No significant changes in median uterine artery blood flow were found during the fenoterol infusion, although an incremental trend was present. Fenoterol infusion to the mother had no effect on umbilical venous blood flow. Fetal pH and PO2 did not change, while fetal PCO2 was reduced (p less than 0.005) at the end of the infusion and recovery period, probably as a result of the concomitant maternal hyperventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of prolonged infusion of ritodrine on uteroplacental blood flow and cardiac output distribution in unanesthetized pregnant guinea pigs

The effects of ritodrine, 15 micrograms X min-1 X kg-1 administered intravenously, on the maternal circulation and uteroplacental blood flow were investigated in eight chronically catheterized pregnant guinea pigs near term. Cardiac output and its distribution were measured by means of the radionuclide-labelled microsphere technique before, and after 2 and 6 h of infusion of ritodrine. Ritodrine produced a sustained elevation of cardiac output to 135 and 137% of the control value after 2 and 6 h, respectively. The carcass and gastrointestinal tract together accounted for the greatest portion of the extra blood flow, 77 and 84% at the times of the two flow measurements during treatment. After 2 h of ritodrine, mean placental blood flow was reduced to 85% of the control level (n.s.), uteroplacental vascular resistance was 127% of its initial value (n.s.), and the placental fraction of cardiac output had decreased from 18 to 12% (P less than 0.02). Continuation of the infusion to 6 h resulted in an increase in mean placental blood flow to 107% of the control value and return of uteroplacental vascular resistance to its initial level. The placental fraction of cardiac output increased to 14%, significantly (P less than 0.05) different from both the initial and the 2-h values. Blood flow to the skin, lungs, spleen and mammary glands decreased between the 2- and 6-h flow measurements. These observations are compatible with the hypothesis that the changes in uteroplacental blood flow during administration of ritodrine are passive, resulting from the balance between the increase in cardiac output and vasodilatation in extrauterine vascular beds.

Effect of ritodrine on uteroplacental blood flow and cardiac output distribution in unanesthetized pregnant guinea pigs

The effects of ritodrine, 15 micrograms X min-1 X kg-1 infused intravenously for 2 hours, on cardiac output distribution and uteroplacental blood flow were investigated in 10 chronically catheterized guinea pigs between 57 and 63 days of pregnancy. Ritodrine produced an average 29% increase in cardiac output, three quarters of which was distributed to the carcass and myocardium. Absolute placental blood flow decreased by 10%, and the placental fraction of cardiac output decreased by one third. Uteroplacental vascular resistance increased by 41% during ritodrine infusion. The proportional changes in placental blood flow were positively correlated and those in placental vascular resistance negatively correlated with the change in cardiac output.

Effect of isoproterenol on uterine blood flow and cardiac output distribution in pregnant guinea pigs

The effects of isoproterenol, 0.05 micrograms X min-1 X kg-1 infused intravenously for 2 hours, on cardiac output distribution and uteroplacental blood flow were studied in six chronically catheterized guinea pigs between 60 and 68 days of pregnancy. Isoproterenol caused marked cardiac stimulation: Cardiac output was increased by 41%, of which 70% was distributed to the carcass and gastrointestinal tract. Absolute placental blood flow remained essentially unchanged, but the placental fraction of cardiac output decreased from 16% to 11%. Myometrial blood flow increased by 72%. Uteroplacental vascular resistance did not change significantly. In the guinea pig in late pregnancy beta-adrenergic receptors are present in the myoendometrial vessels but could not be demonstrated in the maternal vessels supplying the placenta.

Maternal and fetal cardiovascular indices during fetal hypoxia due to cord compression in chronically cannulated sheep. I. Responses to timolol

Timolol, one of the newer beta-adrenergic antagonists, has less depressive effects on the heart than propranolol, so that it has the potential for use by pregnant women. In chronically cannulated ewes, timolol at 0.01 and 0.1 mg/kg of body weight induced maternal and fetal bradycardia; the higher dose also depressed uterine blood flow and fetal PaO2. When this higher dose preceded brief compression of the umbilical cord, uterine flow was further depressed and the usual posthypoxia rebound tachycardia did not develop. Both the blocking of fetal responses to hypoxia (precluding detection of fetal distress) and the reduction in uterine flow led us to conclude that timolol taken by a mother could pose particular hazards for her fetus in hypoxic conditions, such as during cord compression at parturition.

Biochemistry and pharmacology of the myometrium and labor: regulation at the cellular and molecular levels

This review presents the essentials of cell biology, biochemistry, and pharmacology of the myometrium and labor, especially as they relate to premature labor. The aim is to explore the neuroendocrine and pharmacologic regulation of myometrial muscle cells in the context of cell membrane function and contractile processes. In the first section, smooth muscle protein structure and interactions are described, the central roles of myosin light-chain kinase, calcium, and cyclic adenosine monophosphate (cAMP) are reviewed. Subsequently, events relating to hormone/drug receptor function and implications for pharmacologic applications are discussed. Finally, the processes of information transfer via hormones and drugs are incorporated into a model which explains how the hormones and drugs used in clinical practice may mediate myometrial contractility. The reader is introduced to current concepts of the cellular, molecular, and pharmacologic aspects of labor in order to facilitate the understanding of medical augmentation of labor and tocolytic therapy.

Isoxsuprine infusion in the rat: alterations in maternal, fetal and neonatal glucose homeostasis

To determine the mechanism of alteration in glucose homeostasis associated with maternal isoxsuprine administration, isoxsuprine or 0.04 M saline was administered intravenously for 3 hours to term pregnant and age-matched virgin rats. Isoxsuprine infusion significantly increased plasma glucose and insulin concentrations and decreased hepatic glycogen stores in both. Compared to rat pups of saline infused mothers, pups of isoxsuprine infused mothers had significantly elevated plasma glucose concentrations for the first 4 hours of life and plasma insulin concentrations for the first two. Plasma glucose concentrations for the offspring of isoxsuprine treated mothers then decreased significantly and remained so until 16 hours of age. Hepatic glycogen concentrations were significantly less in rat pups of isoxsuprine treated mothers at birth and for the first 4 hours of life. In a limited number of studies, isoxsuprine was present at birth in substantial quantities (80-85% of maternal levels) in the plasma of rat pups of isoxsuprine infused mothers. These data suggest that maternal isoxsuprine therapy mobilizes hepatic glycogen and results in maternal hyperlgycemia. Maternal isoxsuprine infusion may directly deplete fetal hepatic glycogen and result in transient fetal and neonatal hyperglycemia. the in utero depletion of glycogen and possibly, the early stimulation of insulin production may be responsible for the later significant decreases in plasma glucose in the offspring of isoxsuprine treated mothers.

Fetal blood velocity waveforms in normal pregnancy

A combination of pulsed echo and continuous wave Doppler ultrasound was used to obtain blood flow velocity signals from the umbilical arteries of 18 normal patients from the 16th until the 40th week of pregnancy. Audio frequency analysis of these signals yielded fetal blood velocity waveforms. Analysis of these waveforms demonstrated that the placenta is an organ of low vascular resistance and that placental resistance to blood flow declines with advancing gestational age in normal pregnancy.

Myoendothelial and placental blood flow responses to ritodrine infusion in the guinea pig

The effect of ritodrine hydrochloride on uterine blood flow was investigated in near-term guinea pigs. The infusion of ritodrine in doses sufficient to inhibit uterine activity provoked tachycardia, and the cardiac output tended to rise. The percentage of cardiac output reaching the gravid uterus did not alter during the infusion of 12 micrograms per minute of ritodrine but decreased from 12% to 10% when the infusion rate was 120 micrograms per minute. There was an increase in the myoendothelial fraction of cardiac output in both instances, whereas the placental fraction decreased at the higher rate of infusion. Perfusion of the myoendothelial tissue improved during the infusion of 12 or 120 micrograms per minute of ritodrine, increasing by 25% and 18% respectively. No significant alteration occurred in the perfusion of the placental tissue.

Double blind trial of ritodrine and placebo in twin pregnancy

In a double blind trial, 25 patients with twin pregnancy were given 40 mg of ritodrine hydrochloride by mouth daily and 24 similar patients received a placebo. The ritodrine group had no significant prolongation of pregnancy nor increase in birth weight, and a high incidence of side effects occurred.

Neonatal hypoglycemia after beta-sympathomimetic tocolytic therapy

The effect of oral beta-sympathomimetic tocolytic therapy on neonatal serum glucose concentrations in the first several hours after delivery was examined in 12 babies. Hypoglycemia was noted in eight babies, and was sustained over at least a 30-minute period in five. The group with sustained hypoglycemia had a higher cord serum insulin concentration, a lower serum glucose nadir, and a more rapid initial rate of serum glucose disappearance than those babies with normoglycemia or transient hypoglycemia. Sustained hypoglycemia was observed in five of six babies delivered within two days of the termination of tocolytic therapy, but was not present in any of six babies delivered five or more days after the end of tocolytic therapy. Speculations as to the interaction between beta-sympathomimetic tocolytic drugs administered to the mother and fetal and neonatal glucose metabolism are made.

Effect of epinephrine infusion on maternal and uterine oxygen uptake in the pregnant ewe

Ten chronically instrumented pregnant ewes were studied to assess the effect of an epinephrine-induced decrease in uterine blood flow on the oxygen uptake of the uteroplacental-fetal tissue mass. Additional measurements of maternal cardiac output, oxygen uptake, arterial pressure, and hematocrit were obtained. Continuous systemic infusion of epinephrine at a rate of 0.3 to 0.4 microgram per kilogram per minute produced a mean decrease in uterine blood flow of 42 per cent (288 +/- 15 to 157 +/- 8 ml.per kilogram per minute). However, the oxygen uptake of the uteroplacental-fetal tissue mass remained at control levels due to an associated 79 per cent increase in the arteriovenous oxygen content difference across the uterine circulation. The calorigenic and cardiovascular systemic effects of epinephrine (a 22 per cent increase in maternal oxygen uptake and a 43 per cent increase in maternal cardiac output) were not related to the changes in either uterine flow or oxygen uptake. The implications of these observations are discussed.

Effect of intravenous infusion of betasympathomimetic agents on plasma renin activity during pregnancy

The plasma renin activity (PRA) was measured in 26 pregnant patients before and during ritodrine infusion (10 patients), during isoxsuprine infusion (12 patients), and during glucose infusion (4 control patients). All patients were in the third trimester of pregnancy, not in labour and not hypertensive. A low dose of ritodrine, a beta2 adrenergic agent, caused a rise of PRA but no simultaneous rise in heart rate; this suggests that beta2 activity is involved in renin release. Normal doses of ritodrine and isoxsuprine increased the maternal heart rate to 110 to 120 beats/minute and also doubled the PRA level. Two hours after the end of the ritodrine infusion the PRA level was still above that before the infusion. The results suggest high reserves of renin during pregnancy.

The relationship between blood flow and oxygen uptake in the uterine and umbilical circulations

Control measurements of uterine blood flow (N = 110) or umbilical blood flow (N = 84) and the oxygen content difference across the circulation in question were analyzed to assess the effect of flow rate, measured by the diffusion equilibrium technique, on the oxygen uptake of either the uteroplacental-fetal tissue mass or the fetus alone. Although the rate of uterine blood flow varied widely (110 to 418 ml. per kilogram per minute, the rate of uterine oxygen uptake did not change significantly at low or high rates of flow. Likewise, the rate of umbilical blood flow varied widely (154 to 444 ml. per kilogram per minute) without a significant change in fetal oxygen uptake at the extremes of flow. In both circulations this was due to the existence of a reciprocal relationship between flow rate and deltaO2.

Vasoconstrictive effects of prostaglandins in sheep placental circulations

Changes in the uterine and umbilical circulations after intra-aortic injection of prostaglandin E2 (PGE2) or protaglandin F2alpha (PGF2alpha) to either the fetus or the mother have been examined in 13 ewes at late gestation. PGF2alpha vien to either the fetus or the mother is vasoconstrictive in both uterine and umbilical circulations. PGE2 given to the fetus results in a dissociated response with vasodilation of the uterine circulation and vasoconstriction of the umbilical circulation. PGE2 given to the mother causes a uterine contraction that precludes specific hemodynamic analysis. Therefore, exposure of either maternal or fetal ovine circulation to exogenous prostaglandins in associated with acute changes in both placental circulations. Similar responses to pertubations in endogenous prostaglandin levels may exist in pregnant sheep.