Dynamics of uterine circulation in pregnant and nonpregnant sheep

Transfer of maternal psychosocial stress to the fetus

Psychosocial maternal stress experienced during different vulnerable periods throughout gestation is thought to increase the individual's risk to develop neuropsychiatric, cardiovascular and metabolic disease in later life. Cortisol has generally been identified as the major mediator of maternal stress transfer to the fetus. Its lipophilic nature allows a trans-placental passage and thus excessive maternal cortisol could persistently impair the development of the fetal hypothalamic-pituitary-adrenal axis (HPAA). However, cortisol alone cannot fully explain all effects of maternal stress especially during early to mid pregnancy before maturation of the fetal HPAA has even begun and expression of fetal glucocorticoid receptors is limited. This review focuses on mediators of maternal fetal stress transfer that in addition to cortisol have been proposed as transmitters of maternal stress: catecholamines, cytokines, serotonin/tryptophan, reactive-oxygen-species and the maternal microbiota. We propose that the effects of psychosocial maternal stress on fetal development and health and disease in later life are not a consequence of a single pathway but are mediated by multiple stress-transfer mechanisms acting together in a synergistic manner.

Stress-induced decrease of uterine blood flow in sheep is mediated by alpha 1-adrenergic receptors

Prenatal maternal stress can be transferred to the fetus via a catecholamine-dependent decrease of uterine blood flow (UBF). However, it is unclear which group of adrenergic receptors mediates this mechanism of maternal-fetal stress transfer. We hypothesized that in sheep, alpha 1-adrenergic receptors may play a key role in catecholamine mediated UBF decrease, as these receptors are mainly involved in peripheral vasoconstriction and are present in significant number in the uterine vasculature. After chronic instrumentation at 125 ± 1 days of gestation (dGA; term 150 dGA), nine pregnant sheep were exposed at 130 ± 1 dGA to acute isolation stress for one hour without visual, tactile, or auditory contact with their flockmates. UBF, blood pressure (BP), heart rate (HR), stress hormones, and blood gases were determined before and during this isolation challenge. Twenty-four hours later, experiments were repeated during alpha 1-adrenergic receptor blockage induced by a continuous intravenous infusion of urapidil. In both experiments, ewes reacted to isolation with an increase in serum norepinephrine, cortisol, BP, and HR as typical signs of activation of sympatho-adrenal and the hypothalamic-pituitary-adrenal axis. Stress-induced UBF decrease was prevented by alpha 1-adrenergic receptor blockage. We conclude that UBF decrease induced by maternal stress in sheep is mediated by alpha 1-adrenergic receptors. Future studies investigating prevention strategies of impact of prenatal maternal stress on fetal health should consider selective blockage of alpha 1-receptors to interrupt maternal-fetal stress transfer mediated by utero-placental malperfusion.

Labetalol, a combined alpha- and beta-blocker, in hypertension of pregnancy

Labetalol was given to women with hypertension of pregnancy in their last trimester to study its acute effect on circulation and metabolism. Seven women were given 50 mg labetalol i v. There was a significant decrease of blood pressure from a mean of 143/101 +/- 4/2 (SEM) to 127/88 +/- 5/2 mm Hg. Maternal heart rate fell significantly from 77 +/- 5 to 68 +/- 3 beats per min. These changes persisted during a three-hour observation period. The hypotensive response was accompanied by a significant increase in plasma noradrenaline from 1.54 +/- 0.16 to 2.37 +/- 0.41 nmol/l, suggesting sympathetic activation. Plasma cyclic AMP, which is increased by beta 2-adrenoceptor stimulation, was significantly elevated after labetalol. This supports the hypothesis of partial beta-agonist activity of labetalol. Lipid metabolism, as judged from measurements of plasma FFA, glycerol and 3-hydroxybuturic acid, showed little change. The acute effect of labetalol on uteroplacental blood flow was determined in eight women with pregnancy hypertension using a gammacamera on line with a computer. 0.5 mCi indium-113m was given i v before and 30 min after labetalol was administered i v in a dose of 1 mg per kg body weight. After the injections of indium-113m, serial scintigrams were recorded during 10 s periods for 240 s. By computerized summation of the scintigrams, an image was obtained in which the placenta could be outlined for time-activity analysis of the isotope accumulation curve. From this curve a uteroplacental blood flow index could be calculated. Labetalol induced a significant drop of mean arterial blood pressure from 114 +/- mm Hg to 100 +/- 3 mm Hg after 30 min in this group of women. However, the uteroplacental blood flow index did not change. As we have earlier shown with this technique that uteroplacental blood flow can be severely impaired in hypertension of pregnancy, the finding of substained uteroplacental blood flow simultaneously with a decrease in blood pressure should be of clinical importance. Taken together with other studies of clinical effects, these results indicate that labetalol is useful in the treatment of hypertension of pregnancy.

Anaphylactic shock in pregnancy: a case study and review of the literature

We describe a 22-year-old previously healthy primigravida who presented with spontaneous rupture of membranes at 40 weeks of gestation. Because of her history of inadequate prenatal care, a chemoprophylaxis regimen against group B streptococcal infection was prescribed upon admission. Within a few minutes after initiation of an i.v. infusion of penicillin G, the patient developed generalized erythema and severe hypotension, which was essentially unresponsive to intravenous boluses of ephedrine. Following stabilization of maternal blood pressure with incremental doses of epinephrine, emergency cesarean section was performed with delivery of a severely depressed neonate. Postoperative recovery of the mother was uneventful, although the baby was diagnosed to have suffered significant neurological damage. This unfortunate event highlights the therapeutic dilemma in anaphylaxis during pregnancy, a relatively rare but potentially life-threatening event. A critical review of the scientific literature reveals several etiological agents for anaphylaxis during the perioperative period, with penicillin as the leading cause of anaphylaxis-related mortality. Although epinephrine is the vasopressor of choice during hemodynamic resuscitation in the non-pregnant patient, during pregnancy it may pose a risk to the placental-fetal circulation. Additionally, upon review of the various published reports to date, timing and mode of delivery of the neonate in the face of anaphylactic shock remains controversial.

Cardiopulmonary effects of xylazine and acepromazine in pregnant cows in late gestation

OBJECTIVE

To determine effects of sedation achieved by xylazine (XYL) or acepromazine (ACE) on cardiopulmonary function and uterine blood flow in cows in late gestation.

ANIMALS

8 cows between 219 and 241 days of gestation.

PROCEDURE

Doses of ACE (0.02 mg/kg) or XYL (0.04 mg/kg) were administered IV. Measurements were obtained to determine cardiopulmonary effects and oxygen delivery to the uterus.

RESULTS

Heart rate was not significantly affected by administration of ACE, but it decreased markedly after administration of XYL. Uterine artery flow was decreased at all times by XYL and was always less than for ACE. Xylazine increased uterine vascular resistance through 30 minutes and caused reduced PaO2 and increased PaCO2 at all time periods. Acepromazine caused a 5% decrease in PaO2 only at 5 minutes. Xylazine reduced oxygen delivery by 59% at 5 minutes and 32% at 45 minutes. In contrast, ACE caused a nonsignificant reduction of oxygen delivery by 16% at 15 minutes and a return to baseline values by 45 minutes

CONCLUSIONS AND CLINICAL RELEVANCE

Xylazine markedly reduces flow and availability of oxygenated blood to the uterus, which may critically impair delivery of oxygen to the fetus at a stressful and important time of development or delivery. Acepromazine was associated with slight reductions of much shorter duration. When XYL is used to sedate pregnant cows, it could impose physiologic distress on the fetus and potentially increase fetal morbidity and mortality. When sedation of the dam is desirable, ACE could be an alternative to XYL.

Mechanisms regulating angiotensin II responsiveness by the uteroplacental circulation

Pregnancy is associated with increases in cardiac output and uterine blood flow (UBF) and a fall in systemic vascular resistance. In ovine pregnancy, UBF rises from approximately 3% of cardiac output to approximately 25% at term gestation, reflecting a >30-fold rise in UBF by term. This increase in UBF supports exponential fetal growth during the last trimester and maintains fetal well-being by providing excess oxygen and nutrient delivery. These hemodynamic changes are associated with numerous hormonal changes, including increases in placental steroid hormones and enhanced activation of the renin-angiotensin and sympathetic nervous systems, all of which are believed to modulate systemic and uterine vascular adaptation and vascular reactivity. Systemic pressor responses to infused ANG II are attenuated in normotensive pregnancies and the uteroplacental vasculature is even less sensitive, suggesting development of mechanisms to maintain basal UBF and permit the rise in UBF necessary for fetal growth and well-being. The effects of ANG II on the uteroplacental vasculature are reviewed, and the mechanisms that may account for attenuated vascular sensitivity are examined, including ANG II metabolism, vascular production of antagonists, ANG II-receptor subtype expression, and the role of indirect mechanisms.

Mechanisms of relaxation induced by angiotensin II in isolated canine and human uterine arteries

The present study aimed to determine the action of angiotensin II and to pharmacologically analyze mechanisms of their action in isolated uterine arteries. Canine and human uterine artery strips were suspended in Ringer-Locke solutions for isometric tension recording. Canine and human uterine arteries responded to angiotensin II with transient contraction followed by relaxation, which were abolished by losartan, an AT1 receptor subtype antagonist. Cyclooxygenase inhibitors augmented the contraction and abolished the relaxation. The relaxation was also abolished or suppressed by tranylcypromine, a prostaglandin I2 synthesis inhibitor. The relaxant response of dog uterine arteries to angiotensin II was partially suppressed by endothelium denudation but was not influenced by nitric oxide synthase inhibitor. Conversely, the response of human uterine arteries to the peptide was unaffected by endothelium denudation. The antagonists used and endothelium denudation did not inhibit the relaxation caused by a prostaglandin I2 analogue. It appears that the angiotensin II-induced relaxation is mediated by vasodilator prostaglandins, possibly prostaglandin I2, released from both endothelium and subendothelial tissues in dog uterine arteries. In human uterine arteries, the vasodilator prostaglandin is released from subendothelial tissues due to AT1 receptor stimulation by the peptide.

Regional conductance changes during hemorrhage in pregnant and nonpregnant conscious rabbits

Late pregnant (P) conscious rabbits are less able to maintain arterial pressure during hemorrhage than nonpregnant (NP) animals. This study tested the hypothesis that the difference is due in part to less reflex vasoconstriction when the rabbits are P. Rabbits (n = 14) were instrumented with arterial and venous catheters as well as ultrasonic flow probes around the superior mesenteric, renal, and/or terminal aortic arteries. Pregnancy increased (P < 0.05) blood volume [235 +/- 5 (P) vs. 171 +/- 3 (NP) ml], terminal aortic conductance [1.88 +/- 0.11 (P) vs. 0.98 +/- 0.06 (NP) ml. min(-1). mmHg(-1)], mesenteric conductance [1.20 +/- 0.19 (P) vs. 0.80 +/- 0. 05 (NP) ml. min(-1). mmHg(-1)], and heart rate [191 +/- 4 (P) vs. 162 +/- 3 (NP) beats/min] and decreased arterial pressure [59 +/- 1 (P) vs. 67 +/- 2 (NP) mmHg; P < 0.05]. Renal conductance was unaltered. The rabbits were bled in both the NP and P states at 2% of the initial blood volume per minute until arterial pressure fell below 45 mmHg. Arterial pressure fell with less blood loss in P rabbits [28 +/- 2% (P) vs. 39 +/- 2% (NP) of initial blood volume; P < 0.001]. Terminal aortic conductance decreased (P < 0.001) before the pressure fall in both groups, but the response was reduced in P rabbits. Mesenteric and renal conductances did not change in either group before the blood pressure fall. During the pressure fall, terminal aortic conductance increased (P < 0.05) only in NP rabbits. Mesenteric conductance increased in both groups. In summary, rabbits in late gestation are less able to maintain arterial pressure during hemorrhage, at least in part because of reduced vasoconstriction in tissues perfused by the terminal aorta.

Lack of positive correlation between placental blood flow and mean arterial pressure in the non-anaesthetised guinea pig near term; an indication for autoregulation?

The objective of the present study was to evaluate whether placental blood flow (PBF) decreases or increases, in response to a fall or rise in mean arterial pressure (MAP), respectively, in an unanaesthetised animal model with a haemochorial placenta. Such a response would support the absence of autoregulation of placental blood flow for changes in mean arterial pressure. To this end, 31 late-pregnant guinea pigs were subjected to isovolemic haemodilution, a sham procedure or isovolemic haemoconcentration, after adequate recovery from instrumentation. The percentage change from baseline in PBF (delta PBF, %), was compared between animals with a fall in MAP, in response to haemodilution or the sham procedure (MAP-fall, n = 8, mean delta MAP -11 +/- 7%) and those with a rise in MAP, following haemoconcentration (MAP-rise, n = 10, mean delta MAP +15 +/- 8%). In addition, the percentage change in the blood flow to the myocardium, gastrointestinal tract, adrenals, kidneys, brain, skin, carcass and myometrium was compared between the MAP-fall and the MAP-rise group. The direction of the change in PBF did not differ between the MAP-fall and the MAP-rise group. In this respect, the distribution of delta PBF over the two groups was comparable with that observed for the myocardium, gastrointestinal tract, adrenals, skin, carcass and myometrium. The blood flow to the kidneys and the brain varied in response to the change in haematocrit. The results of this study show that PBF varies independently from MAP. This observation supports the idea that the blood flow to the labyrinthine placenta of the awake and healthy guinea pig near term may be autoregulated for hydrostatic inflow pressure, at least over the spectrum of arterial pressures observed in the present study.

Effects of labetalol on uterine blood flow and cardiovascular hemodynamics in the hypertensive gravid baboon

OBJECTIVE

The purpose of this study was to investigate the effects of labetalol on uterine blood flow and cardiovascular parameters in acutely instrumented, hypertensive gravid baboons.

STUDY DESIGN

During the latter half of pregnancy six gravid baboons were acutely instrumented, with ultrasonic flow probes placed on ipsilateral, external iliac, and uterine arteries and a flow-directed pulmonary artery catheter in the pulmonary artery. After a stable arterial pressure baseline was obtained, norepinephrine was infused to increase mean arterial pressure by at least 20%. A 20-minute hypertensive steady state was obtained. Labetalol at 0.5 and 1.0 mg/kg was randomly infused, followed by a 2.0 mg/kg dose, each over 1 minute. A 20-minute recovery period followed every labetalol infusion, allowing the hypertensive steady state to reestablish. External iliac and uterine blood flow measurements were continuously recorded during the baseline and experimental trials. Mean arterial blood pressure, heart rate, pulmonary artery and capillary wedge pressure, central venous pressure, and cardiac output were obtained at 5, 10, and 15 minutes during each steady state and after each labetalol infusion.

RESULTS

Labetalol at all dosages significantly reduced the mean arterial pressure and the systemic vascular resistance at 1.0 and 2.0 mg/kg. External iliac blood flow was not consistently significantly reduced; however, uterine blood flow was significantly reduced after the 1.0 and 2.0 mg/kg labetalol dosages (p < 0.05). Although uterine vascular resistance tended to increase after the 1.0 and 2.0 mg/kg doses, statistical significance was not achieved.

CONCLUSION

Low-dose labetalol (0.5 mg/kg) significantly reduces the pharmacologic hypertensive gravid baboon's mean arterial blood pressure without adversely affecting uterine blood flow.

Effects of felodipine on utero-placental blood flow in normotensive rabbits

The utero-placental blood flow was investigated in anaesthetized normotensive pregnant rabbits after repeated administration of a high dose of the antihypertensive calcium antagonist felodipine and in untreated controls. By means of the microsphere technique blood flow was also determined in the lungs, skin, intestine, skeletal muscle and kidneys during chloralose anaesthesia. Felodipine reduced mean arterial blood pressure, which was associated with a marked reduction in vascular resistance in the skeletal muscle vascular bed, where blood flow was increased seven-fold. In contrast, blood flow to placentae and kidneys were reduced. The pronounced reduction in placental blood flow may limit foetal nutrition and hence explain reported foetal digital defects after administration of high doses of felodipine to pregnant normotensive rabbits.

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.

On the pathogenesis of abortion in acute nitrite toxicosis of pregnant dairy cows

Six cows, 220 to 260 d pregnant, were used to study possible abortifacient effects of absorbed nitrite. Maternal and fetal bloodvessels were catheterized. Nitrite was infused intravenously (i.v.) for 30 minutes on Days 1, 3 and 5, commencing at least 5 d postcatheteration. The dose used was 7, 9.5 and 12 mg NO(2)(-)/kg bodyweight, respectively. Nitrite caused a dose-related (P < 0.05) conversion of maternal hemoglobin (Hb) into methemoglobin (MHb); a 30 to 50% decrease of mean arterial bloodpressure; an increase in heart rate, with dose-related recovery periods; and a decrease in partial oxygen tension (PO(2)) of maternal blood. Maternal partial carbon dioxide tension (PCO(2)), pH and electromyographic activity of the uterus were unchanged. Fetal changes included a small increase in MHb content, variable changes in heart rate (tachycardia and bradycardia), and decreases in fetal PO(2), with considerable differences between animals. All calves were born alive. Three cows calved early, 2 to 3 d after the highest nitrite dose. The hematological and cardiovascular data suggest that these three fetuses experienced a more serious hypoxemic stress than the other fetuses.

Mechanisms for attenuated pressor responses to alpha-agonists in ovine pregnancy

Attenuated pressor responses to angiotensin II and alpha-agonists normally occur during ovine pregnancy; however, for alpha-agonists it is not known to what extent this reflects alterations in cardiac output. We therefore compared peripheral and cardiac responses to alpha-agonists in pregnant (n = 6) and nonpregnant (n = 6) sheep, measuring mean arterial pressure, heart rate, and cardiac output before and during steady-state dose responses to norepinephrine (0.458 to 45.84 micrograms/min) and phenylephrine (1.29 to 129 micrograms/min). Both alpha-agonists caused dose-dependent increases in mean arterial pressure and systemic vascular resistance, more so in nonpregnant than pregnant sheep (p less than 0.01), and decreases in cardiac output and heart rate (p less than 0.01). In both nonpregnant and pregnant sheep the percent change in systemic vascular resistance generally exceeded the percent change in mean arterial pressure, reflecting decreases in cardiac output; however, at equivalent increases in percent change in mean arterial pressure and percent change in systemic vascular resistance, the fall in the percent change in cardiac output was greatest in pregnant sheep (p less than 0.05). Also, at similar increases in mean arterial pressure the percent change in heart rate was greatest in pregnant sheep (p less than 0.01). In contrast to angiotensin II, the attenuated pressor responses to alpha-agonists observed during ovine pregnancy are partly a result of reflex decreases in cardiac output mediated mainly through falls in heart rate.

Fetal and maternal Doppler flow velocity waveforms in normal sheep pregnancy

Abnormalities in placental vascular resistance have been shown to have an important association with intrauterine growth retardation in both human and sheep pregnancies. In this study we have assessed impedance to blood flow through the ovine placental circulation by Doppler flow velocity waveform analysis. As gestation advanced from 66 to 136 days, a significant decrease in impedance to flow was observed in both umbilical and maternal placental arteries. The greatest decrease in impedance was observed between 66 and 109 days' gestation in umbilical arteries and between 66 and 80 days' gestation in maternal placental arteries. In these normal pregnancies, no significant correlations could be found between Doppler flow velocity waveform analyses and fetal or placental growth. We conclude that Doppler flow velocity waveform analysis is a valuable and non-invasive method to assess impedance to blood flow through the placental circulation in pregnant sheep.

Effects of acute haemorrhagic hypotension during pregnancy and lactation in conscious goats

Effects of acute, short-lasting haemorrhage have been studied in six goats. The experiments were made 55 +/- 3 days (mid-pregnancy = MP), and 24 +/- 2 days (late pregnancy = LP) before parturition, in lactation weeks 3-8, and in the non-pregnant, non-lactating (= control) period. The room temperature was kept at 20 +/- 1 degrees C, except during one experimental series in lactating goats, which was performed at +30 degrees C. Approximately the same volume of blood had been removed in all periods, when the mean arterial blood pressure (MAP) fell. At this moment bleeding was stopped. The volume loss, calculated as per cent of total blood volume, was significantly smaller during pregnancy and lactation than in the control period. The fastest heart rate, both at rest and during haemorrhage, was observed in LP goats. The respiratory frequency of the LP goats increased markedly in response to haemorrhage, but the most rapid respiratory rate was observed in lactating goats bled at the room temperature +30 degrees C. The increase in plasma vasopressin (AVP) concentration coincided with the fall in MAP. The AVP rise seen during late pregnancy was significantly higher than in all the other periods. The plasma noradrenaline (NA) concentration increased by about five times in LP goats, but did not change significantly in the other periods. Plasma renin activity did not rise, with the exception of a small increase in lactating goats bled at +30 degrees C. The plasma aldosterone concentration rose, but there was no difference between periods.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of pregnancy on the changes in hind-limb vascular resistance following haemorrhage in the rabbit

The change in vascular resistance of the skinned hind limb perfused at constant flow has been measured following rapid removal of 10% of the blood volume in anaesthetized pregnant rabbits (27-29 days gestation) and compared with that of non-pregnant rabbits. 5 s after haemorrhage was completed vascular resistance had risen in all rabbits, but in the pregnant ones this increase was only one-fifth of that in the non-pregnant ones. After section of either the carotid sinus and aortic nerves or the sympathetic innervation to the hind limb the increases of vascular resistance at this time were abolished. It is concluded that the previously described diminution during pregnancy of the protective function of the baroreflexes during haemorrhage results, to some extent, from their diminished role in a vascular bed not associated with the reproductive process. The blood withdrawn was not re-infused for 10 min, throughout which, in all rabbits with sinus, aortic and sympathetic nerves intact, the increase in vascular resistance was sustained. After section of the carotid sinus and aortic nerves and the sympathetic innervation there was, during this 10 min, a slow increase in vascular resistance comparable in final size to that which occurred immediately in the respective innervated preparations, i.e. it was smaller in the pregnant than in the non-pregnant rabbits. A possible difference in action during pregnancy of a vasoconstrictor substance released during haemorrhage is discussed briefly.

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

The effect of continuous maternal intravenous infusion with norepinephrine on the blood flow in the maternal internal iliac and the median uterine artery was studied in ten chronically instrumented pregnant sheep between 104 and 146 days gestation. Furthermore the effects on umbilical venous blood flow, fetal heart rate and acid-base balance were analyzed. Maternal and fetal blood flows were measured with electromagnetic flow transducers. Norepinephrine was administered to the ewe via a continuous intravenous infusion in increasing sequential doses of 15 minutes duration from 4 to 40 micrograms per minute. Variations of often considerable magnitude associated with e. g. micturition, defecation and fear of the ewe occurred in the maternal pelvic blood flow during the steady state period. The blood flow in the maternal vessels substantially decreased immediately following the onset of the norepinephrine infusion but gradually returned towards the preinfusion level despite the continued drug infusion and except one no significant changes in blood flow were found at the end of each sequential infusion period. Umbilical venous blood flow did not change. No significant changes in fetal arterial blood pressure, heart rate and acid base balance were found. It is concluded that the decrease in maternal pelvic blood flow associated with continuous norepinephrine administration gradually abates with time, possibly by the involvement of local factors such as prostaglandin formation and/or by the phenomenon of down-regulation.

Arterial pressure maintenance after haemorrhage in the pregnant rabbit

The changes in arterial blood pressure and cardiac output following rapid removal of 10% of the blood volume have been compared in non-pregnant and pregnant anaesthetized rabbits close to term (28 or 29 days gestation). Immediately after the end of haemorrhage the fall in arterial pressure was greater in the pregnant group (27.7 vs. 17.8 mmHg), but as recovery proceeded the difference diminished and by the tenth minute the reduction from pre-haemorrhage levels was similar in pregnant and non-pregnant rabbits. Cardiac output fell by about 30% in both groups and increased only very slightly during the ensuing 10 min. 1 h after section of the carotid sinus and aortic nerves the haemorrhage was repeated. The fall in arterial pressure in the non-pregnant rabbits was now more than doubled (47.2 vs. 17.8 mmHg). By contrast it was little changed in the pregnant rabbits (32.0 vs. 27.7 mmHg). It is concluded that pregnancy diminishes the role of the baroreceptor reflexes in the response of the rabbit to haemorrhage. Other mechanisms, such as an increased compliance of the resistance vessels and/or a more effective chemically mediated vasoconstriction, probably become of more importance to the maintenance of arterial pressure.

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.

Reflex response of the rabbit hind-limb muscle vascular bed to baroreceptor stimulation and its modification by pregnancy

1. The pressure--flow relationship of the blood-perfused skinned hind limb has been compared in primigravid pregnant and virgin non-pregnant rabbits while pressure within the vascularly isolated carotid sinus was held at a series of non-pulsatile values. 2. At each level of sinus pressure the slope of the pressure--flow relationship was steeper in the pregnant animals. Thus, at all but the very lowest flows, the same flow was achieved at a lower perfusion pressure in the pregnant rabbits. 3. When hind-limb vascular resistance at constant flow was plotted against carotid sinus pressure it could be seen that at each level of sinus pressure vascular resistance was less in the pregnant rabbits. 4. The reflex responses of the hind-limb bed to changes in sinus pressure were compared in pregnant and non-pregnant animals; there was little difference in the threshold and saturation values of carotid sinus pressure. There seemed to be a small reduction in the range and gain of the reflex in the pregnant group but this was not statistically significant. 5. We have previously shown total peripheral resistance in the pregnant rabbit to be less than that in the non-pregnant rabbit at the same level of sinus pressure. The lower hind-limb vascular resistance of the pregnant rabbit at any given sinus pressure must contribute to this reduced total peripheral resistance.

Acute effect of an antihypertensive drug, labetalol, on uteroplacental blood flow

The effect of a new antihypertensive drug, labetalol, on uteroplacental blood flow was determined in eight pre-eclamptic women. After injection of 0.5 mCi of 113mIn the radioactivity in the placenta was recorded by a gamma camera linked to a computer and the placental blood-flow index was calculated from the ratio between the maximum radioactivity of the isotope-accumulation curve and the rise time of the curve. Labetalol, a combined alpha- and beta-adrenoceptor antagonist was given intravenously and after 30 min a second uteroplacental blood-flow index was calculated. There was a significant mean decrease of blood pressure from 147/98 to 128/83 mmHg, but no change in uteroplacental blood-flow index, so that uteroplacental vascular resistance tended to decrease.

Experimental renal hypertension in pregnant sheep. II. Response to a one-kidney model

Pelvic blood flow was serially determined in 19 pregnant ewes before and after the induction of one-kidney renal hypertension. The six twin gestations and 13 singleton gestations behaved similarly for all parameters, except renal blood flow. After the renal preparatory surgical procedure, there was a 19% increase in mean arterial pressure and an 18% increase in pelvic blood flow, with pelvic vascular resistance being unchanged. After right renal artery constriction, there was a 14% further increase in mean arterial pressure, whereas pelvic blood flow increased only 3.5% and pelvic vascular resistance increased nearly 9%. Thereafter, although the arterial pressure remained elevated, the pelvic blood flow decreased and pelvic vascular resistance increased. Statistical analysis by means of a general linear model and covariance before and after renal artery constriction revealed significant changes in arterial pressures, pelvic blood flow, and pelvic vascular resistance.

Responses of the pelvic vascular bed to intra-arterial stimulation of beta-adrenergic and cholinergic receptors in pregnant and nonpregnant sheep

The responses of different parts of the pelvic vascular bed to beta-adrenergic and cholinergic receptor stimulation were studied in chronically instrumented, unanesthetized pregnant and nonpregnant sheep. Isoproterenol and acetylcholine were administered intra-arterially in progressively increasing bolus injections, and dose-response curves were constructed for changes in blood flows, arterial pressure, and heart rate. Response to beta-adrenergic and cholinergic stimulation was measured before and after the agents reached the central circulation. In addition, data were obtained from the same animals before and after pharmacologic cardiac denervation. Results show: (1) Stimulation of beta-adrenergic and cholinergic receptors produced active vasodilatation in extrauterine vascular beds with minor changes in uterine blood flow of the pregnant horn; (2) the changes in uterine flow observed during beta-adrenergic and cholinergic stimulation were probably secondary and related to a shift of blood from the uterus to other vascular beds that were actively dilated; (3) the response of the pregnant animal to any given dose of the autonomic agonist was strikingly smaller to that of the nonpregnant animal. A number of physiologic factors peculiar to pregnancy may contribute to this difference; among these are: (1) dilution factor related to the greater volume flow; (2) near-maximal dilatation of some blood vessels and the presence of the two resistance system of placental circulation, and (3) the effect of progesterone on the vascular smooth muscle.

Circulatory and metabolic effects of a combined alpha- and beta-adrenoceptor blocker (labetalol) in hypertension of pregnancy

1 Seven women with hypertension of pregnancy were given the combined alpha- and beta-adrenoceptor blocking drug labetalol (50 mg i.v.) in their last trimester. Acute effects were studied for 3 h after administration. 2 Systolic and diastolic blood pressures were significantly reduced from 143 +/- 4 (s.e. mean) to 127 +/- 5 mmHg and from 101 +/- 2 to 88 +/- 2 mmHg, respectively. Maternal heart rate fell significantly from 77 +/- 5 to 68 +/- 3 beats/min. The changes remained during the 3 h of observation. Foetal heart rate was not affected. No side-effects were encountered. 3 Plasma noradrenaline increased significantly from 1.54 +/- 0.16 to a peak value of 2.37 +/- 0.41 nmol/l suggesting sympathetic activation following labetalol. Plasma adrenaline levels were essentially unchanged. Plasma glucose, insulin and C-peptide showed only minor changes. No major effects on lipid metabolism were seen except a significant fall of nonesterified fatty acids at 60 min. Plasma cyclic AMP increased significantly throughout the observation period, perhaps indicating beta-adrenoceptor agonist activity of labetalol. 4 The effectiveness of labetalol as an acute hypertensive agent together with apparent absence of metabolic disturbances and other side-effects makes it an interesting drug for the treatment of hypertension during pregnancy.

Effect of angiotensin II on uterine and systemic vasculature in pregnant sheep

The response of uteroplacental blood flow (UBF) to angiotensin II is controversial. Moreover, the relationship of the uterine and systemic responses to infused angiotensin II is not well understood. Thus, in eight chronically instrumented, near-term pregnant sheep, we have determined the relationships between the dose and duration of constant systemic infusions of angiotensin II ([Val5] ANG II) and changes in UBF, uterine vascular resistance (UVR), mean arterial pressure (MAP), and systemic vascular resistance (SVR). [Val5] ANG II caused dose-dependent increases in UVR and MAP at all doses studied (P less than 0.05). The response in UBF was bidirectional, with increases at doses less than or equal to 1.15 microgram/min and decreases at greater than or equal to 2.29 micrograms/min (P less than 0.05). Increases in UBP occurred when the relative rise (delta) in MAP greater than delta UVR, whereas UBF was unchanged when delta MAP = delta UVR and decreased when delta MAP less than delta UVR. SVR also rose in a dose-dependent fashion (P less than 0.05); delta SVR was greater than delta UVR at doses less than or equal to 2.29 micrograms [Val5] ANG II/min (P less than 0.01). In studies of the effect of duration of [Val5] ANG II infusions, UBF increased at all doses during the 1st min, followed by stabilization at 4--5 min, with eventual decreases at doses greater than or equal to 2.29 micrograms/min and increases at doses less than 2.29 micrograms/min. The relationship between the changes in MAP and UVR to the response of UBF was as noted above. It is evident that (a) [Val5] NAG II is uterine vasoconstrictor, (b) changes in UBF are dependent upon relative changes in perfusion pressure and UVR, which in turn are dependent upon both the dose and duration of a [Val5] ANG II infusion, and (c) the uteroplacental vasculature is relatively refractory to the vasoconstricting effects of low doses of [Val5] ANG II.

Plasma levels and cardiovascular effect of nitroglycerin in pregnant sheep

The cardiovascular effects and blood levels of intravenous nitroglycerin in mother and foetus were studied in normotensive and hypertensive ewes. Although nitroglycerin produced a reduction in uterine blood flow following a decrease in blood pressure, there was little if any incidence of adverse effects in the foetus. Nitroglycerin was detected in all but two samples of foetal arterial blood but the foetal/maternal artery ratio was only 0.04. Nitroglycerin merits further evaluation for the treatment of acute hypertensive episodes in the preeclamptic patient.

The effect of pregnancy on the angiotensin II pressor response in the rabbit

The pressor response to angiotensin II was measured in unanesthetized, chronically catheterized pregnant and nonpregnant rabbits. Angiotensin II was infused intravenously for 10 minutes at a dose of 50 and 124 ng/kg/min. No difference in control mean arterial blood pressure was observed between pregnant and nonpregnant rabbits. The pressure change in response to angiotensin II was significantly greater in nonpregnant rabbits than in pregnant rabbits (P less than 0.002). Plasma samples were analyzed for angiotensin II concentration by radioimmunoassay. The results showed that there was no difference in plasma angiotensin II concentration between pregnant and nonpregnant rabbits following angiotensin II infusions. In a separate series we observed the effect of 5 mg of phenoxybenzamine on the pressor response to angiotensin II. After phenoxybenzamine treatment the control mean arterial blood pressure was significantly greater in nonpregnant rabbits than in pregnant rabbits, but the change in pressure in response to angiotensin II in nonpregnant rabbits was not significantly different from that of pregnant rabbits. These results show (1) that pregnant rabbits have a decreased sensitivity to angiotensin II, (2) that this decreased sensitivity is not due to differences in plasma concentration of angiotensin II, and (3) that this differential sensitivity to angiotensin II can be prevented by alpha-receptor blockade.

Central neural blockade and caesarian section. Part 1: Review and case series

Caesarian Section was performed on 62 patients who chose regional anaesthesia (epidural or subarachnoid) in preference to general anaesthesia. Results are presented and discussed with a review of relevant literature.

Foetal hypoxia--an anaesthetist's approach to classification and prevention

Nearly one third of perinatal deaths are due to hypoxia occurring during pregnancy and labour. Many factors contribute to foetal hypoxia and in order to group these into orderly categories a classification of hypoxia, which is familiar to anaesthetists, has been applied to the maternal and foetal circulations. This theoretical approach forms a logical guide to our understanding the cause, prevention and treatment of foetal hypoxia.

Control of the uteroplacental circulation in health and disease

The various neurohumoral and intrinsic factors that control the uteroplacental hemodynamics in health and disease and in responses to physiologic and pharmacologic stimuli have been reviewed. The following conclusions may be derived: We still need improvement in our methodology of monitoring uterine blood flow. The present methods, which have some reliability, are not easily applicable to human subjects and even in animals their use presents problems of accuracy and sensitivity with which the investigator must become familiar. The marked and progressive increase in uterine blood flow that occurs during pregnancy is caused by complex factors, some of which are hormonal and hemodynamic in nature. The increased vascularity of the pregnant uterus and the opening of the arterioles during the process of formation of the intervillous space are important factors that facilitate the increase in uterine blood flow. The increment seems to be totally derived from the increment in the cardiac output that occurs during pregnancy. There seems to be no redistribution among the regional blood flows of the body. In the anesthetized condition the blood flow to the uterus depends largely on the perfusing pressure; the critical closing pressure seems to be around the 40 mm Hg level. This linear flow-pressure relationship does not, however, apply to the unanesthetized condition. A rise or fall in the perfusing pressure in the conscious state may be accompanied by an increase or decrease in the uterine blood flow, depending on the underlying mechanisms. Factors that lead to alpha-adrenergic stimulation produce an increase in uterine vascular resistance and a decrease in flow, irrespective of the status of the perfusing pressure. beta-adrenergic stimulation may increase uterine blood flow either through their vasodilating action or through their myometrial relaxing effects. Hypertensive diseases are most often accompanied by a decrease in uterine blood flow, whereas hypoxic states may decrease the flow even though the arterial pressure may not change significantly. It is extremely risky to extrapolate from information obtained in the anesthetized animal to the unanesthetized, conscious animal. Likewise, data obtained from normotensive conditions may not hold true for the hypertensive or hypotensive states. This is of particular relevance when one is dealing with the effects of pharmacologic agents that act on the cardiovascular system. Uterine contractions, whether induced through spontaneous or oxytocin-induced labor, produce a decrease in uterine blood flow.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in cardiac output and total peripheral resistance during the carotid sinus baroreceptor reflex in the pregnant rabbit

1. The reflex cardiovascular effects of changes in pressure within the isolated carotid sinus have been examined in sixteen anaesthetized pregnant rabbits. 2. Comparison of the mean results at sinus pressures of 40 and 200 mmHg showed that heart rate fell by 32.1 beats min-1 and arterial pressure was reduced by 48.2 mmHg. Cardiac output, measured by thermal dilution, fell by 25.7 ml. min-1.kg-1 and total peripheral resistance was reduced by 0-292 mmHg. ml.-1 min. kg. 3. The corresponding changes previously reported in a group of seventeen non-pregnant female rabbits were a similar fall in heart rate of 34.5 beats min-1 but significantly greater reductions in arterial pressure of 69-8 mmHg and in total peripheral resistance of 0-432 mmHg. ml.-1 min.kg. However, the fall in cardiac output of 12.6 ml.min-1.kg-1 was significantly smaller. 4. These observations show that the smaller blood pressure response of the pregnant rabbit to alterations in sinus pressure is due to a reduced change in total peripheral resistance.

Differential reactivity of the gravid uterine vasculatures: effects of norepinephrine

Norepinephrine dose-conductance-response relationships were determined for the placental and nonplacental vasculatures of ewes between 95 and 130 days of gestation with the use of radioactive-labeled microspheres injected under flow meter guidance. The regression line for the placental circulation differed significantly from that for the nonplacental circulation (P less than 0.01). The decrease in nonplacental conductance was 30 to 45% of control greater than the decrease in placental conductance. These responses are consistent with the hypothesis that the site of placental vascular resistance is in vessels of larger caliber than those which are usually the primary site of resistance to blood flow.

Circulatory response to systemic infusion of norepinephrine in the pregnant ewe

Seven pregnant ewes from 100 to 137 days of gestation were infused with systemic doses of norepinephrine and uterine arterial flow dose-response curves were determined. A constant infusion of norepinephrine at a mean rate of 0.24 mug per minute per kilogram led to a 39.3 per cent decrease in total uterine arterial blood flow as measured with isotope-labeled microspheres while systemic pressure was unaltered. At this dose the reduction in endometrial blood flow (--64 per cent) was significantly greater than that in either the myometrium (--45 per cent) or placental cotyledons (--31 per cent) (p less than 0.005). Significant decreases in blood flow to small bowel, skeletal muscle, vagina, cervix. Fallopian tubes, kidneys, spleen, pancreas, and mammary gland were documented. There were no significant increases in blood flow. This study demonstrates that during the period or pregnancy studied, the overwhelming response to norepinephrine is vasoconstriction and that the vascular beds of all the tissues of pregnant uterus are sensitive to the alpha-adrenergic effects of norepinephrine.

[The experimental examination of the effect on the uterine blood flow of angiotensin II during pregnancy (author's transl)]

In experiments with animals it was investigated the endurance of the myometrial and the blood flow of the renal cortex during endogenous pressure substances. At the same time it was tested, if treatment with sexual hormones or a pregnancy had the tested principles and changes through pressure substances, and that the changes were significant. The investigations were conducted on three groups of female rabbits. The blood changes in myometrial and in the uterine were measured and continually registered with the special heated thermistor, from the principle of the thermoclearance. The success of the blood pressure was intraarterial measured with an electric mechanism. Precisely the same doses (in relativity of the animals weight) of pressure substances were applied with an infusions pump intravenously. And pressure substances Angiotensin II, Norepinephrine and Epinephrine, and their actions on the blood pressure and blood flow through the myometrium and through the renal cortex were examined. Altogether 131 values were registered. The results of the study that were statistically secure were as follows: a) The uterine blood flow pro tissue volume unit stays constant also by pregnancy or pseudopregnancy. b) The blood flow of the kidney is perhaps twice as high as the myometrial. c) The myometrial blood flow is with the arterial systolic blood pressure tightly correlated. Blood pressure increases through Angiotensin-infusion and also recovery of the uterine blood flow. d) An autoregulation of the uterine blood flow is not observed. e) The decrease of the renal blood flow after the giving of pressure substances was not modified through pregnancy. f) In quality the behaviour of the organ blood flow is the same after applied infusion of the pressure substances. Quantity differences exist however between Angiotensin II, Norepinephrine and Epinephrine. The method of measuring the blood flow through the uterus and in the kidney was placed in one view there. The finding of another examination groups for the problem of the organ blood flow in pregnancy was under critical consideration the methods combined and in connection with the proper examinations discussed. Till now in the theory over the cause of EPH-syndrom the predominate recently compiled comprehensive summary was; the proper body pressure substances--especially from the renin Angiotensin system--after chronical invoices it was decides diminished uterus blood flow appeared. After the earlier results were not all secure. The proper examination speech was therefore, that regarding the kidney function relevant alterations, also unter the conditions of pregnancy, are to be observed. The pressure dependant regulation of myometrial blood flow without proving autoregulation required however another test of the predominante gestose theory.

Effect of ritodrine infusion on uterine and umbilical blood flow in pregnant sheep

The effect of ritodrine upon uterine artery blood flow (UtBF) and umbilical vein blood flow (UmBF) was investigated in near-term chronic sheep preparations. During intravenous ritodrine infusions to the ewe in sequential dose rates from 100 to 800 mug per minute, UtBF fell progressively to 43 per cent below control levels and mean maternal arterial pressure (MMAP) declined 20 per cent. During constant infusions of 100, 400, or 800 mug per minute of ritodrine to the ewe for 120 minutes,, UtBF decreased 10, 37, and 31 per cent, respectively, and MMAP decreased 6, 20 and 25 per cent respectively. Dose-related maternal tachycardia and hyperglycemia occurred. There were no significant changes in UmBF, mean fetal arterial pressure, or fetal heart rate. During all infusions, maternal and fetal arterial pH, PCO2, and PO2 remained within normal physiologic limits. Simultaneous infusion of ritodrine (400 mug per minute) and propranolol (100 mug per minute) blocked the maternal tachycardia, but decreases in UtBF, MMAP, and UmBF were observed. Ritodrine infusions to the fetus (25 mug per minute) resulted in fetal tachycardia and a variable increase in UmBF.

Actions of the uterine relaxant, fenoterol, on uteroplacental hemodynamics in human subjects

The action of fenoterol, a uterine relaxant of the adrenergic beta-mimetic group of drugs, has been investigated on placental, myometrial, and cardiac blood pools. 113mIndium was injected intravenously and the blood pools were measured by recording gamma activity externally. In eight cases with oxytocin-induced labor, fenoterol inhibited the typical blood pool changes of labor, i.e., contraction-synchronized decrease of placental and myometrial blood pools with increase in cardiac blood pool. Furthermore, fenoterol produced an increase in placental and myometrial pools with corresponding decrease of cardiac blood pool. These changes were observed consistently. In four other cases, of which three had spontaneous labor, similar effects were observed. The possible mechanisms of hemodynamic action of fenoterol, with reference to uterine relaxation, are discussed.

The effect of changes in perfusion pressure on uteroplacental blood flow in the pregnant rabbit

The effect of perfusion pressure on uteroplacental blood flow was determined in pregnant rabbits utilizing the radioactive microsphere method. Control mean arterial pressure, 93 mm Hg +/- 2.6 SEM, was raised by carotid ligation to 109 +/- 4.1 mm Hg and then reduced with antihypertensive drugs to 74 +/- 1.3 mm Hg. Over this range of pressure there was no significant change in cardiac output, 605 +/- 36, 523 +/- 37, and 540 +/- 39 ml/min; or uteroplacental blood flow, 30 +/- 3.2, 27 +/- 5.2, and 29 +/- 4.5 ml/min, respectively. When prostaglandin synthesis was inhibited with either indomethacin or meclofenamate (2 mg/kg), uterine vascular resistance was higher but maintenance of uteroplacental flow occurred over a perfusion pressure of 89 +/- 6.7-115 +/- 9.3 mm Hg. With more severe hypotension induced with trimethaphan, control arterial pressure fell from 92 +/- 2.4 to 39 +/- 0.9 mm Hg, cardiac output fell from 514 +/- 17 to 407 +/- 22 ml/min (P less than 0.025) and uteroplacental blood flow fell from 6.1 +/- 0.9 to 2.5 +/- 0.9% of cardiac output (P less than 0.05), which represented an absolute fall from 32.4 +/- 5 to 10.6 +/- 3 ml/min (P less than 0.025). There was no significant change in renal blood flow expressed as percentage of cardiac output, 14.9 +/- 2 and 13 +/- 1.5%, or in absolute flow, 75 +/- 7.7 and 54 +/- 7 ml/min with trimethaphan-induced hypotension. These studies indicate that uteroplacental blood flow is maintained relatively constant over a range of perfusion pressure of 60-140 mm Hg in both normal and prostaglandin-inhibited pregnant rabbits. However, with reduction in pressure to 36-42 mm Hg, uteroplacental blood flow falls, expressed as a percentage of cardiac output and in absolute flow.