Changes in fetal ovine metabolism and oxygen delivery with fetal bypass.

Since the 1980s, attempts at experimental fetal cardiac bypass for the purpose of correcting severe congenital heart defects in the womb have been hampered by deterioration of placental function. This placental pathophysiology in turn affects transplacental transport of nutrients and gas exchange. To date, the effects of bypass on fetal metabolism and oxygen delivery have not been studied. Nine Suffolk sheep fetuses from 109-121 days gestation were instrumented and placed on fetal bypass for 30 min and followed postbypass for 2 h. Blood gases, glucose, and lactate were serially measured in the fetal arterial and umbilical venous circulations throughout the procedure. Insulin and glucagon levels were serially measured by immunoassay in fetal plasma. Fetal-placental hemodynamics were measured continuously. The expression of glycogen content was examined in fetal liver. Oxygen delivery to the fetus and fetal oxygen consumption were significantly deranged after the conduct of bypass (in-group ANOVA (P = 0.001) and overall contrast (P = 0.072) with planned contrast (P < 0.05) for delivery and consumption, respectively). There were significant alterations in fetal glucose metabolism in the postbypass period; however, insulin and glucagon levels did not change. Fetal liver glycogen content appeared lower after bypass. This is the first report documenting fetal metabolic dysregulation that occurs in response to the conduct of fetal bypass. The significant alterations in fetal oxygen and glucose delivery coupled with hepatic glycogen depletion complicate and impede fetal recovery. These initial findings warrant further investigation of interventions to restore metabolic and hemodynamic homeostasis after fetal bypass.

Fetal stress response to fetal cardiac surgery.

BACKGROUND: A deleterious fetal stress response, although not fully elucidated, may account for poor outcomes after experimental fetal cardiac surgery. We set out to characterize this fetal stress response and its potential role in placental dysfunction. METHODS: Fifteen ovine fetuses at gestational day 100 to 114 were placed on extracorporeal support for 30 minutes and were then followed 2 hours after cardiopulmonary bypass. Fetal plasma samples were analyzed for vasopressin, cortisol, and beta-endorphin levels, and correlated to fetal hemodynamics and placental gas exchange. RESULTS: Unique temporal patterns of response were seen in release of the three stress hormones. Vasopressin demonstrated the most profound and early response followed by cortisol and beta-endorphin, the latter continuing to rise in the post-bypass period. A sharp rise in fetal mean arterial pressure and placental vascular resistance strongly correlated with rising vasopressin levels. Post-bypass deterioration of fetal gas exchange and hemodynamics correlated with the ensuing rise in cortisol and beta-endorphin. Rising fetal lactate levels correlated with elevations in all three stress hormones. CONCLUSIONS: Fetal cardiopulmonary bypass leads to a profound, early rise in vasopressin concentrations that strongly correlates with placental dysfunction after fetal bypass. Vasopressin may play an important mechanistic role in pathogenesis of this placental dysfunction.

Umbilical blood flow during pregnancy: evidence for decreasing placental perfusion.

OBJECTIVE: The present study was designed to determine the relation of umbilical venous blood flow (UmbBF) to fetal weight (FW) at different times in late pregnancy, and to assess fetal O2 supply near term. STUDY DESIGN: In 46 pregnant women, UmbBF was calculated just before delivery using the product of flow velocity and the cross section of the umbilical vein determined by pulsed Doppler technique and measuring of the diameter of the vessel, respectively. Based on the gestational age at delivery (range, 29-42 weeks), infants were divided into a preterm group (PT; < or = 36 weeks, n = 13) and a full-term group (FT; > 36 weeks, n = 33). Blood gas, pH, and hemoglobin analysis was performed in specimens of umbilical venous and arterial blood obtained after delivery. RESULTS: UmbBF was higher in FT infants (515 +/- 125 mL/min, mean +/- standard deviation) than in PT infants (423 +/- 120 mL/min; P < .05). This was associated with a larger increase in umbilical vein diameter: FT 8.8 +/- .7 mm, PT 8.1 +/- .6 mm (P < .01). Partial pressure of O2 (pO2) did not differ significantly between FT and PT; the correlation of pO2 with gestational age showed a weak decrease (P < .05). Hemoglobin was elevated in FT (P < .01), whereas O2 content remained constant in PT and FT. The ratio UmbBF/FW was considerably reduced in FT [154 +/- 37 (mL/min)/kg], relative to PT [221 +/- 37 (mL/min)/kg; P < .001], and was accompanied by a marked reduction of O2 transport capacity: FT 17.6 +/- 6.7 and PT 26.6 +/- 9.2 (mL/min)/kg (P < .01). CONCLUSION: Due to the growth of the umbilical vein, UmbBF increases over the last weeks of gestation. The ratio UmbBF/FW is reduced in FT. Despite a constant O2 content, the continuous weight-related decrease in UmbBF results in a reduction of the fetal O2 transport capacity per unit that may contribute to an adverse intrauterine environment at the end of gestation, especially in postterm pregnancies.

Hemodynamic effects of acute and repeated exposure to raloxifene in ovariectomized sheep.

We hypothesize that administration of acute and daily doses of raloxifene will have significant effects on ovine coronary and uterine hemodynamics and that these changes are estrogen receptor dependent. Eleven ovariectomized sheep were instrumented to measure mean arterial pressure, heart rate (HR), cardiac output (CO), and coronary (CBF) and uterine artery blood flows (UBF). A dose-response curve was generated for raloxifene (1, 3, and 10 microg/kg) and compared with a standard dose of estradiol-17beta (1 microg/kg) given intravenously. In a second group of animals, raloxifene (10 microg.kg-1.day-1) was administered intravenously for 14 consecutive days, and cardiovascular responses were compared with a group of animals administered estradiol-17beta (10 microg/kg) daily for the same period. To determine whether raloxifene-related vascular responses were estrogen receptor (ER) mediated, the animals were pretreated with estrogen antagonist ICI-182,780 given intravenously. Finally, RT-PCR was preformed to determine the presence of ERalpha and ERbeta mRNA in ovine coronary and uterine vessels. Raloxifene increased CBF and UBF dose dependently with a parallel decrease in the associated vascular resistances. Acute cardiovascular responses to daily doses of raloxifene and estradiol-17beta were sustainable. In contrast to estradiol-17beta, which significantly increases CO by increasing HR but not stroke volume, raloxifene significantly increased stroke volume without a significant parallel increase in HR. ICI-182,780 abolished raloxifene-induced hemodynamic responses, and ERalpha and ERbeta mRNA are present in both ovine coronary and uterine vessels. Hence, the hemodynamic effects of raloxifene are dose dependent, sustainable, and estrogen receptor mediated.

Vacuum-assisted venous drainage during fetal cardiopulmonary bypass.

Fetal bypass presents several perfusion challenges, including the need for high arterial flow rates using flexible arterial and small venous cannulae. We hypothesized that vacuum-assisted venous drainage (VAVD) would improve drainage and allow perfusion at higher flow rates which are thought to prevent placental dysfunction induced by fetal bypass. We conducted bypass for 60 minutes in 14 fetal lambs (90-105 days gestation; approximately 1-1.5 kg) using a roller pump and various angled venous cannulae (8-12 Fr). VAVD at -20 mm Hg or -40 mm Hg was compared with gravity drainage. Average flow using gravity drainage was 139 ml/kg/min; after VAVD, we achieved average flows of 285 ml/kg/min (range, 109-481 ml/kg/min). VAVD at -40 mm Hg caused right atrial trauma in four fetuses; no injury was seen at -20 mm Hg. Venous air entrainment during repair of the injuries did not result in any apparent air embolism. Spontaneous pulmonary hemorrhage occurred in two fetuses at the highest flows (> or = 400 ml/kg/min). In all but one case, termination of bypass was followed by placental dysfunction within 120 minutes. VAVD can be safely applied during fetal bypass provided pressures are kept < or = -20 mm Hg. However, the achieved higher flow rates do not prevent postbypass placental dysfunction and may indeed be detrimental to the fetus.

Expression of phosphodiesterase 5 in maternal and fetal sheep.

OBJECTIVE: Ovine pregnancy is associated with elevated levels of nitric oxide. Nitric oxide stimulates cyclic guanosine monophosphate (cGMP), which results in vascular smooth muscle relaxation/vasodilation. Phosphodiesterase type 5 regulates cGMP in the urogenital track. The present study was designed to determine message expression of phosphodiesterase type 5 in the myometrium, uterine vessels, and placentome of the sheep and phosphodiesterase type 5 protein expression in the maternal and fetal placentome. STUDY DESIGN: Tissue was collected from 5 nonpregnant and 5 pregnant anesthetized animals (gestational day = 134 +/- 4) and frozen at -80 degrees C. Optimized reverse transcription-polymerase chain reaction was performed on all tissues, and immunohistochemistry was performed on the placentome only. RESULTS: Phosphodiesterase type 5 messenger ribonucleic acid levels were significantly higher in the myometrium and placentome, compared with the maternal blood vessels. Phosphodiesterase type 5 protein was immuno-localized to the vascular smooth muscle of the maternal vessels, stroma, and placental capsule only. CONCLUSION: Although phosphodiesterase type 5 messenger ribonucleic acid was present in the fetal placenta, phosphodiesterase type 5 protein was expressed only in maternally derived placental tissue. This suggests that regulation of cGMP levels and vascular tone in the umbilical circulation differs from the uterine circulation.

Effects of combined use of sildenafil citrate (Viagra) and 17beta-estradiol on ovine coronary and uterine hemodynamics.

OBJECTIVE: This study was undertaken to determine whether Viagra (Pfizer, New York, NY), a type 5 phosphodiesterase (PDE-5) inhibitor, increases baseline coronary and uterine blood flow or potentiates estrogen-induced increases in blood flow. STUDY DESIGN: Seven nonpregnant ovariectomized ewes were chronically instrumented to measure blood pressure, heart rate, cardiac output, and coronary and uterine blood flow. Estradiol-17beta (E2, 0.3 microg/kg) (Sigma Chemical Co, St Louis, Mo) and Viagra (50 mg) were intravenously administered separately or together and hemodynamic responses monitored. RESULTS: Viagra alone rapidly decreased mean arterial pressure and cardiac output while increasing heart rate and uterine blood flow. Viagra tended to decrease coronary blood flow but not significantly. E2 by itself increased coronary and uterine blood flow over a 2-hour time course, whereas mean arterial pressure, cardiac output, and heart rate remained unchanged. Administration of E2 to animals pretreated with Viagra reversed Viagra-related decrease in cardiac output and coronary blood flow, resulting in a time-related increase in both parameters over baseline values. Viagra significantly augmented estradiol-related increases in uterine blood flow. CONCLUSION: In our surgically menopausal ovine model, Viagra alone has a tendency to decrease baseline coronary blood flow, but the reduction does not reach statistical significance. The presence of estradiol reverses this Viagra-related negative trend and results in a net increase in coronary blood flow and cardiac output. Furthermore, Viagra significantly augments baseline and estrogen-induced increases in uterine blood flow suggesting the presence of PDE-5 in the ovine uterine vasculature.

Differential effects of selective estrogen receptor modulators and estrogens on mammary blood flow in the ovine.

OBJECTIVE: Hormone replacement therapy has been implicated in the increased incidence of breast cancer, although selective estrogen receptor modulators have been shown to be effective in the prevention of breast cancer. Breast cancers are associated with increased mammary blood flow compared to benign breast lesions. However, few studies have examined the hemodynamic effects of hormonal agents on the mammary circulation that promote or reduce the risk of breast cancers. Although estradiol-17beta has been shown to increase mammary blood flow, the effect of selective estrogen receptor modulators remains undetermined. We therefore compared the vascular effects of selective estrogen receptor modulators and estrogens on mammary blood flow. STUDY DESIGN: Fourteen nonpregnant ovariectomized ewes were instrumented to measure mean arterial pressure, heart rate, and uterine and mammary blood flows. Compounds were administered intravenously on separate days, and responses were monitored up to 4 hours. Compounds that were studied included estradiol-17beta (1 microg/kg), conjugated equine estrogens (0.625 and 1.25 mg), tibolone (2.5 and 5 mg), raloxifene (10 microg/kg), and tamoxifen (300 microg/kg). RESULTS: None of these compounds significantly affected mean arterial pressure or heart rate, but all of the compounds significantly increased uterine blood flow. Estradiol-17beta increased mammary blood flow by 98% +/- 25%; conjugated equine estrogen increased mammary blood flow by 46% +/- 6% and 68% +/- 13% at the 0.625 and 1.25 mg doses, respectively. Tibolone increased mammary blood flow by 37% +/- 13% at the 2.5-mg dose and by only 14% +/- 4% at the 5-mg dose. Neither raloxifene nor tamoxifen significantly altered mammary blood flow. CONCLUSION: Although estrogens and selective estrogen receptor modulators induced similar increases in uterine blood flow, they had differential effects on mammary blood flow.

Estrogen increases iNOS expression in the ovine coronary artery.

Estrogen is believed to protect postmenopausal women from coronary vascular disease, in part by increasing production of nitric oxide (NO). In this study, we investigated the possibility that transcriptional activation of inducible NO synthase (iNOS) is responsible for a component of the estrogen-induced increase in coronary blood flow. Twenty-two ewes were instrumented with Doppler flow probes on their left circumflex coronary and pulmonary arteries. Nine ewes received 17beta-estradiol (1 microg/kg), and the coronary vascular response was followed for 16 h. Estradiol significantly increased coronary blood flow by 22 +/- 4% over baseline and the peak response occurred at 2 h (P < 0.01). To examine the effect of estrogen on NOS expression in the ovine coronary artery, 17 noninstrumented animals were killed 2 h after administration of estradiol or vehicle. Coronary arteries were analyzed for ovine iNOS and endothelial NOS (eNOS) expression by semiquantitative RT-PCR. PCR primers were based on partial cDNA clones for ovine eNOS and iNOS isolated as part of this study. The expression of iNOS was significantly increased (27-fold) by the administration of estradiol, whereas the expression of eNOS was much weaker (2-fold). To confirm these effects in vivo, additional instrumented animals received either the estrogen receptor (ER) antagonist ICI-182,780 (n = 5), the iNOS antagonist dexamethasone (n = 5), or pyrrolidine dithiocarbamic acid, an inhibitor of nuclear factor-kappaB (n = 5). All three antagonists inhibited estrogen-induced increases in coronary blood flow and increases in cardiac output by over 85%. These results strongly support the hypothesis that 17beta-estradiol increases coronary blood flow in the unanesthetized nonpregnant ewe via an ER-dependent mechanism that results in an increase in both eNOS and iNOS expression.

Fetal umbilical vascular response to chronic reductions in uteroplacental blood flow in late-term sheep.

OBJECTIVE: The present study was designed to determine the effects of chronic reduction in uterine blood flow (UBF) on umbilical blood flow (UmbBF) and fetal cardiovascular hemodynamics and oxygenation. STUDY DESIGN: Sixteen sheep with singleton pregnancies were instrumented on gestational day (GD) 110; an externally adjustable vascular occluder was placed on the common internal iliac artery. UBF in control animals rose from 867 +/- 61 mL/min on GD 115 to 1520 +/- 158 mL/min (n = 8) on GD 138, whereas UBF in restricted animals was maintained at 750 +/- 50 mL/min (n = 8). RESULTS: UmbBF in control animals increased from 472 +/- 25 mL/min to 744 +/- 58 mL/min over the study period from GD 115 to GD 138. This was associated with normal gestational increases in fetal arterial pressure and significant reductions in calculated umbilical vascular resistance. Although restricted animals initially had a similar UmbBF on GD 115, UmbBF rose only to 545 +/- 43 mL/min over the study period (control vs restricted, P <.008). Although fetal arterial pressure showed normal gestational changes, umbilical vascular resistance failed to decrease over gestation in restricted animals as it did in control animals. Fetal heart rate and oxygenation showed normal changes in both groups. CONCLUSION: Chronic reduction in UBF prevents umbilical vascular resistance from undergoing normal gestational decreases, leading to significantly lower UmbBF. This altered umbilical perfusion pattern would be expected to significantly affect fetal delivery of oxygen and nutrients and ultimately fetal growth.

Effect of Ro 61-1790, a selective endothelin-A receptor antagonist, on systemic and uterine hemodynamics and fetal oxygenation in sheep.

OBJECTIVE: Endothelin-1 is reportedly elevated in preeclampsia, and studies in our laboratory have shown that infusion of endothelin-1 produces increased mean arterial pressure, hemoconcentration, and proteinuria, while decreasing uterine blood flow. If a role in preeclampsia is confirmed for endothelin-1, therapeutic intervention may involve selective endothelin-A receptor blockers. Thus, this study was designed to determine the effects of Ro 61-1790, a selective endothelin-A receptor inhibitor, on mean arterial pressure, heart rate, and uteroplacental blood flow in pregnant and nonpregnant sheep. STUDY DESIGN: Seven pregnant and seven nonpregnant sheep were instrumented with indwelling femoral artery and vein catheters and bilateral uterine artery flow probes, allowing determination of mean arterial pressure, heart rate, and uteroplacental blood flow. After baseline administration, animals received intravenous Ro 61-1790 either 1 mg/kg or 3 mg/kg, and hemodynamic responses were monitored for 120 minutes. RESULTS: Intravenous administration of Ro 61-1790 at 1 mg/kg had no effect on the parameters measured in either group studied. However, at 3 mg/kg, Ro 61-1790 caused an increase in total uterine blood flow in nonpregnant sheep from 22 +/- 6 mL/min to 51 +/- 15 mL/min. This occurred in the absence of significant changes in mean arterial pressure. In pregnant animals, administration of 3 mg/kg Ro 61-1790 decreased mean (+/- SEM) uteroplacental blood flow by 20% (from 771 +/- 130 mL/min to 621 +/- 110 mL/min) and increased uterine vascular resistance transiently. Administration of Ro 61-1790 had no significant effect on fetal mean arterial pressure, heart rate, or oxygenation. CONCLUSIONS: These data suggest that endogenous endothelin-1 may play an important role in regulating uterine vascular tone in pregnant and nonpregnant sheep and that inhibition of endogenous endothelin-1 may lead to reductions in uteroplacental perfusion in pregnant animals.