INTRODUCTION

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Asthmatic pregnancies are associated with low-birthweight babies at term (1). The mechanisms that cause this outcome are presently unknown; however, asthma severity and the use of glucocorticoids for the treatment of asthma may play a role. We are the first group to address this issue and to propose some potential alterations in placental function that may contribute to the low-birthweight outcome. Fetal growth can be affected by many different factors; however, abnormal vascular function is a known etiological mechanism for intrauterine growth restriction. Animal models of reduced placental blood flow reliably produce fetal growth restriction (6) and in humans, defects in oxygen transfer, placental villous development, and changes in fetal-placental vascular morphology and function are associated with abnormal fetal growth (7).

Asthma or its treatment could contribute to altered placental vascular function. In asthmatic pregnancies the risks of maternal and fetal hypoxia are significantly increased (11) especially when asthma management is inadequate (12) and could potentially have effects upon placental and fetal development (17). Furthermore, glucocorticoids are known to alter vascular function (18) and are used widely for the treatment of asthma. For these reasons we have sought to identify placental vascular anomalies in pregnant women with asthma. Before delivery, changes in placental vascular function can be examined in vivo using Doppler ultrasound measurements of the umbilical artery end-diastolic flow velocity (19, 20) and after delivery, in vitro using the placental dual perfusion method (21, 22). In this study, we have used both of these techniques to explore the effect of asthma and its treatment on the placental vasculature in an effort to determine the causes of the observed reduction in birthweight in this group of women.

	METHODS

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Subjects

The following experiments were formally approved by the Hunter Area Health Service Research Ethics Committee and the University of Newcastle Human Research Ethics Committee. Both asthmatic women and control women without asthma were recruited to the study from the antenatal clinic at the John Hunter Hospital during the first trimester of their pregnancy. Written, informed consent was obtained from women for participation in the study.

Assessment of Asthma Severity

Clinical asthma severity was rated as mild, moderate, or severe using the integrated severity score described in the Australian Asthma Management Guidelines (23), which closely approximate the National Heart, Lung, and Blood Institute Guidelines (24). Measurements of current daytime symptoms, nocturnal and morning symptoms due to asthma, bronchodilator use, forced expiratory volume at one second (FEV₁), peak expiratory flow (PEF), and hospitalizations were used to assign the patient a severity rating of either mild, moderate, or severe. The patient was assigned to the most severe grade in which any feature occurred during her pregnancy. Subjects with mild persistent asthma exhibited any of the following characteristics: FEV₁ > 80% predicted, no nighttime asthma symptoms, no asthma symptoms on wakening, infrequent short-acting bronchodilator use, no severe attacks in the past year, and only occasional daytime symptoms (< 4 times per week).

Moderate asthma was defined by symptoms on most days, short-acting ₂-agonist requirements most days, night symptoms up to once/ week, FEV₁ > 60% and < 80% predicted, or PEF variability < 25%. In severe asthma, there were daily symptoms, limited physical activity, frequent nighttime symptoms (more than once per week), asthma on awakening and FEV₁ < 60% predicted, or peak flow variability > 25%. Subjects were managed according to a standard treatment protocol (25) in a combined antenatal/asthma management clinic.

Control subjects had spirometry monitored at 18 wk gestation whereas asthmatic women received 2 to 8 visits at the asthma management service (AMS) depending on severity of asthma and individual needs. At the first visit, a history of asthma control was recorded, including number of emergency presentations, hospital admissions, and oral glucocorticoid use in the previous 2 yr before pregnancy. Each asthmatic study subject received instruction with an asthma educator in asthma control, management skills, and a crisis plan. All subjects were asked to perform self-monitoring of peak flows and to record these in a diary for 2 wk after each AMS visit or after an exacerbation. Follow-up visits were guided by asthma severity with subjects being reviewed at ultrasound appointments at 18 and 30 wk gestation. During these visits there was reinforcement of asthma knowledge and management skills, inhaler technique, compliance with medications, and discussion of individual issues. Inhaled glucocorticoid therapy and oral prednisolone intake were recorded for each trimester during the pregnancy. Subjects with uncontrolled asthma or experiencing an exacerbation were reviewed by a respiratory physician.

Assessment of Glucocorticoid Use

Cumulative, inhaled glucocorticoid dose was calculated for each trimester, and summarized as the mean daily dose of beclomethasone dipropionate (BDP) or equivalent used during the pregnancy, where 1 µg BDP was considered equal to 1 µg budesonide and to 0.5 µg fluticasone propionate. Subjects were then grouped into four categories based upon glucocorticoid dosage: no glucocorticoid use and ₂-agonist use only (nil); low-dose glucocorticoid (low), where the average daily dose during pregnancy was < 400 µg/d; moderate use where the average daily dose during pregnancy was between 400 and 1,500 µg/d (moderate); and high dose, where the average daily dose during pregnancy was greater than 1,500 µg/d (high). Oral glucocorticoid intake was noted for each woman.

Doppler Ultrasound

All women participating in the study were screened by Doppler ultrasound of the umbilical arteries as part of the routine fetal assessment at 18 and 30 wk of gestation. Abdominal circumference was recorded as a measure of fetal growth. Umbilical artery flow velocity waveforms were assessed by pulsed Doppler ultrasound (Acuson XP4; Acuson Corporation, Mountain View, CA). Abnormal umbilical artery blood flow was defined as a systolic to diastolic flow ratio (SD ratio) greater than the 95th centile for gestational age (19). Data collected from asthmatic women who smoked cigarettes during their pregnancies were analyzed separately.

Placental Perfusion Protocol

Placental lobules were perfused by the technique originally described by Penfold and coworkers (21) as modified by Mak and coworkers (22). A suitable paired artery and vein, typically third or fourth branches of the chorionic plate vessels, to a peripheral placental lobule were chosen. The artery was cannulated with polyethylene tubing and the vein cut at a convenient point to allow blood and perfusate to escape. The cannula, which was inserted to the point where the artery disappeared below the surface of the chorionic plate, was connected to a Gilson Minipuls 3 (Gilson Medical Electronics, Villiers-le Bel, France) peristaltic pump and the lobule perfused at 5 ml/min with Krebs' solution containing (mmol/L) NaCl, 97.0; NaHCO₃, 24.3; KCl, 3.0; KH₂PO₄, 1.2; CaCl₂, 1.89; MgSO₄, 1.0; D-glucose, 5.5; pH 7.3 (all chemicals from BDH, Vic, Australia), maintained at 37° C and equilibrated with 95% O₂; 5% CO₂. The corresponding maternal sinus to the lobule was also perfused with Krebs' solution, under identical conditions to those used for perfusion of the fetal circulation, except that perfusate was delivered through a cannula inserted into a remnant of one of the spiral arterioles of the basal plate. The effluent was allowed to drain from the remaining vessels.

Experimental Design

Changes in fetal-placental vascular resistance were monitored by recording the inflow pressure to the lobule, using a Gould Statham P23D transducer (Cleveland OH), connected by a T-junction to the fetal arterial perfusion line. Signal conditioning and amplification were performed by a J-RAK (Melbourne, Vic, Australia) PA-2 module and displayed on a Kontron W+W 330 flat-bed recorder (Basel, Switzerland). Inflow pressure at the commencement of perfusion was 80 to 100 mm Hg, declining to a stable baseline pressure between 20 and 40 mm Hg within a period of 1 h. Preparations having baseline pressures greater than 60 mm Hg were discarded. Effects of vasoactive agents were measured after the baseline perfusion pressure had become constant.

Upon obtaining a stable baseline pressure between 20 and 40 mm Hg, constrictive agents KCl (19.3 to 504 mM) or prostaglandin F₂ (PGF₂) (0.4 to 151 pM) (Pharmacia-Upjohn, NSW Australia) were infused by means of a peristaltic pump (Gilson Minipuls 3) in a series of semilog doses of increasing concentrations until a maximum increase in perfusion pressure was obtained. The infusion rate of KCl and PGF₂ was not increased until an equilibrium response had occurred to the previous concentration. Each agonist was examined in a single placental lobule only. Experiments with different agonists were often examined in individual lobules of the same placenta. In these cases, lobules were selected to be located as far apart as possible.

To study the vasodilator action of corticotropin-releasing hormone (CRH) (Auspep, Vic, Australia), vasoconstriction was induced with KCl (50 to 100 mmol/L) (BDH Chemicals, Vic, Australia) to a submaximal arterial pressure between 80 and 120 mm Hg, as under normal conditions placental basal arterial pressure in vitro is too low to observe responses to dilator agents. KCl was infused continuously into the artery using a peristaltic pump (Gilson Minipuls 3) for the duration of the experiment. CRH was added in a semilog series of increasing concentrations. Intermediate concentrations in the response curve were not increased until the perfusion pressure had reached an equilibrium. Vasodilatation was expressed as a percentage of the induced vasoconstriction induced by KCl.

Patients may have received one or more of the following drugs during labor; oxytocin (2 international units [IU] over 6 to 8 h), pethidine hydrochloride (100 mg, intramuscularly), promethazine maleate (12.5 to 25 mg, intramuscularly), or inhaled 70% N₂O and 30% O₂. These drugs have no apparent effects on responses of the fetal vascular tissues under the conditions used (22). Twenty-two percent of asthmatic women were cigarette smokers and their placental vascular responses in vitro were reported separately.

Statistical Analyses

The maternal, fetal, and neonatal data were analyzed using Stata statistical software (Version 6; Stata Corporation, College Station, TX). Placental blood flow data were analyzed using Graphpad Instat Software (Graphpad Software 1990-1993, Version 2.04a, San Diego, CA). The tests used include one-way analysis of variance (ANOVA), regression analysis, and Student's t test (26). All values are expressed as means ± SEM unless otherwise stated. A value of p < 0.05 was considered significant.

	RESULTS

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Both control and asthmatic women were not significantly different in age, gravidity, parity, height, or weight gain during pregnancy (Table 1). However, maternal weight at the first antenatal visit around 12 wk gestation was significantly higher in women using high-dose glucocorticoid (Table 1). FEV₁ adjusted for height and weight was significantly reduced in severe asthmatic women when compared with control groups (Table 1). Fetal abdominal circumference, neonatal birthweight, birthweight centile, head circumference and length were not significantly different between the groups (Table 2). However, there was a trend for reduced birthweight centile and head circumference centile of neonates from women who did not use glucocorticoids to treat their asthma (Table 2).

Umbilical Artery Waveforms

Umbilical artery waveforms as demonstrated by Doppler ultrasound and expressed as SD ratio, did not differ significantly between control (n = 21) and asthmatic women (mild: n = 14, moderate: n = 17, severe: n = 22) at 30 wk gestation (ANOVA, p > 0.05) (Figure 1A). However, SD ratios at 18 wk gestation were significantly reduced in moderate (n = 4) and severe (n = 11) asthmatic groups when compared with mild (n = 14) asthmatics and control (n = 13) subjects (Figure 1A, ANOVA, p < 0.05). When asthmatic women were grouped on the basis of inhaled glucocorticoid intake, there was a significant reduction in SD ratio at 18 wk gestation in women using high-dose glucocorticoid (n = 4) (Figure 1B, ANOVA, p < 0.05).

View larger version (26K): [in this window] [in a new window]   Figure 1.   (A) Comparison of SD ratio in human umbilical artery at 18 and 30 wk gestation as measured by Doppler ultrasound in control subjects (n = 13 at 18 wk, n = 21 at 30 wk) and women with mild (n = 14 at 18 wk, n = 25 at 30 wks), moderate (n = 4 at 18 wk, n = 17 at 30 wk), or severe asthma (n = 11 at 18 wk, n = 22 at 30 wk). (B) Comparison of SD ratio in human umbilical artery at 18 and 30 wk gestation as measured by Doppler ultrasound in control subjects (n = 13 at 18 wk, n = 21 at 30 wk) and women using no inhaled glucocorticoid (n = 9 at 18 wk, n = 13 at 30 wk), low (n = 3 at 18 wk, n = 10 at 30 wk), moderate (n = 10 at 18 wk, n = 17 at 30 wk) or high dose of inhaled steroid during pregnancy (n = 4 at 18 wk, n = 11 at 30 wk).

Placental Perfusion Studies

Placentae from women with normal pregnancies used in this study had a mean basal fetal-placental arterial perfusion pressure of 18.9 ± 2.9 mm Hg (n = 20) during in vitro studies. Basal fetal-placental arterial perfusion pressures in placentae collected from mild, moderate, and severe asthmatics were 19.2 ± 2.9 mm Hg (n = 23), 19.4 ± 2.8 mm Hg (n = 13), and 21.9 ± 2.5 mm Hg (n = 15), respectively. There were no significant differences in basal arterial pressure between the control and asthmatic groups (p > 0.05, ANOVA).

Responses to Vasodilators In Vitro

The responses to CRH (17 to 5,300 pmol/L) during submaximal vasoconstriction with KCl (50 to 200 mmol/L) in placentae collected from control subjects (n = 7), mild (n = 7), moderate (n = 6), and severe asthmatics (n = 5) are shown in Figure 2. A concentration-dependent vasodilatory response to CRH was observed in all groups. However, the vasodilatory response to CRH was significantly reduced in placentae collected from moderate and severe asthmatics (Figure 2) when compared with control and mild groups (p < 0.05, ANOVA, regression analysis). When the data were examined in relation to inhaled glucocorticoid intake, CRH-induced dilation was significantly reduced in women using moderate (n = 6) and high (n = 4) doses of steroid when compared with control (n = 7) and no glucocorticoid treatment groups (n = 5) (p < 0.05, ANOVA, regression analysis, data not shown).

View larger version (21K): [in this window] [in a new window]   Figure 2.   The vasodilator effects of CRH (17 to 5,300 pmol/L) compared in normal placentae (n = 7) (closed diamonds) and those placentae collected from women with mild (n = 7) (closed circles), moderate (n = 6) (closed triangles), or severe (n = 8) (closed squares) asthma during pregnancy. Placental vessels were submaximally constricted with KCl (50 to 100 mmol/L). All values are expressed as mean ± SEM.

Responses to Vasoconstrictors In Vitro

Dose-dependent vasoconstriction to KCl (19 to 504 mM) was examined in placentae collected from control subjects (n = 7) and mild (n = 4), moderate (n = 5), and severe (n = 6) asthmatics (Figure 3A). The linear portions of the curves were significantly different from parallel in the placentae collected from moderate and severe asthmatics (regression analysis, p < 0.05) and the maximal response to KCl-induced vasoconstriction at 171 mM was significantly reduced in moderate and severe groups (Figure 3B, ANOVA, p < 0.05). Maximal responses to KCl were reduced in all placentae collected from asthmatic women who used no glucocorticoid (n = 3) or moderate (n = 4) and high (n = 5) doses of inhaled glucocorticoid (p < 0.05, ANOVA, regression analysis, data not shown). A similar response was observed with PGF₂-induced constriction (0.4 to 151 pM) in vitro in placentae collected from moderate and severe asthmatics (n = 9, Figures 4A and 4B). The linear portions of the PGF₂ curves were not significantly different (regression analysis, p > 0.05, Figure 4A) but the maximal response to PGF₂ at 50.4 pM was reduced (Figure 4B) in the combined moderate and severe groups (Student's t test, p < 0.05) when compared with control placentae. The numbers in the PGF₂ groups were too small to analyze in relation to glucocorticoid intake.

View larger version (13K): [in this window] [in a new window]   Figure 3.   (A) The vasoconstrictor effects of KCl (19 to 504 mmol/L) compared in normal placentae (n = 7) (closed diamonds) and those placentae collected from women with mild (n = 4) (closed circles), moderate (n = 5) (closed triangles), or severe (n = 6) (closed squares) asthma during pregnancy. Vasoconstriction is expressed as cumulative increase in placental arterial pressure (mm Hg). (B) Comparison of maximal arterial pressure induced by KCl in placentae collected from control subjects and women with mild, moderate, or severe asthma. All values are expressed as mean ± SEM.

View larger version (12K): [in this window] [in a new window]   Figure 4.   (A) The vasoconstrictor effects of PGF2 (0.4 to 151 pmol/L) compared in normal placentae (n = 5) (closed squares) and those placentae collected from women with moderate and severe (n = 9) (closed triangles) asthma during pregnancy. Vasoconstriction is expressed as cumulative increase in arterial pressure (mm Hg). (B) Comparison of maximal arterial pressure induced by PGF2 at 50.4 pM in placentae collected from control subjects and women with moderate and severe asthma. All values are expressed as mean ± SEM.

Asthma and Smoking

Twenty-two percent of asthmatic women smoked during their pregnancies in our study and the birthweight centile was significantly reduced in this group of asthmatic women (38.8 ± 6.5, n = 18) compared with nonsmoking control subjects (57.8 ± 4.8, n = 26) (Student's t test, p < 0.05). Umbilical artery waveforms as demonstrated by Doppler ultrasound were recorded at 18 (n = 12) and 30 wk (n = 17) gestation of asthmatic women who smoked, and the SD ratios were not significantly different from the nonsmoking control subjects (n = 13 at 18 wk and n = 21 at 30 wk) (Figure 5A, ANOVA, p < 0.05). However, in vitro placental perfusion experiments revealed that vascular responses to CRH (n = 9) and KCl (n = 4) were further reduced by the added complication of smoking when compared with the vascular responses of nonsmoking severe asthmatics (Figures 5B and 5C).

View larger version (16K): [in this window] [in a new window]   Figure 5.   (A) Comparison of SD ratio in human umbilical artery at 18 and 30 wk gestation as measured by Doppler ultrasound in control subjects (n = 13 at 18 wk, n = 21 at 30 wk) and asthmatic women who smoked throughout pregnancy (n = 12 at 18 wk, n = 17 at 30 wk). (B) The vasodilator effects of CRH (17 to 5,300 pmol/L) compared in placentae collected from nonsmoking control subjects (n = 7) (closed circles), nonsmoking, severe asthmatics (n = 8) (closed squares), and asthmatics who smoked during pregnancy (n = 9) (closed diamonds). (C) The vasoconstrictor effects of KCl (19 to 504 mM) compared in placentae collected from nonsmoking control subjects (n = 7), (closed circles), nonsmoking, severe asthmatics (n = 6) (closed squares), and asthmatics who smoked during pregnancy (n = 4) (closed diamonds). Vasoconstriction is expressed as cumulative increase in arterial pressure (mm Hg). All values are expressed as mean ± SEM.

	DISCUSSION

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This study has demonstrated that there are a number of alterations in the placentae of pregnancies associated with moderate and severe asthma that may contribute to the adverse outcome of decreased birthweight. Because glucocorticoids are prescribed in relation to the severity of asthma, it is difficult to determine whether these alterations in vascular function are the result of asthma or the glucocorticoid therapy. However, the consequences of abnormal placental vascular function may be considerable, as low birthweight at term has been associated with the adult development of Syndrome X (27, 28).

In normal pregnancy, the SD ratio of the umbilical artery flow velocity waveforms as demonstrated by Doppler ultrasound, is high in early gestation and decreases as diastolic flow increases with gestational age (19, 20). A high SD ratio is indicative of increased placental vascular resistance which creates a low-oxygen environment that promotes placental development and growth during early gestation (17). Our study has found that the SD ratio of umbilical artery flow velocity was significantly reduced at early gestation in moderate and severe asthmatics and women using high-dose inhaled glucocorticoid for the treatment of their asthma, suggesting vascular resistance may be prematurely decreased. Reductions of vascular resistance early in gestation may not be beneficial to placental development and fetal growth as it may create hyperoxic conditions that are inhibitory upon terminal villous development and angiogenesis (29). However, the SD ratios of umbilical artery flow at 30 wk gestation were not significantly different between the groups, indicating that there were no abnormalities of placental vascular function in utero. Hitschold and coworkers (7) have described an association between reduced umbilical artery vascular resistance as demonstrated by Doppler ultrasound and intrauterine growth restriction (IUGR), which suggests there may be changes in placental vascular function that are not detectable in utero using this technique. The use of the in vitro placental perfusion method may be a more sensitive tool to distinguish differences in vascular function by comparing responses to dilators and constrictors.

Our study found no significant differences in basal arterial pressure of placentae from control and asthmatic pregnancies using the in vitro placental perfusion method. However, there were significant alterations in vascular responses to dilation and constriction in placentae collected from moderate and severe asthmatic women or women using moderate and high doses of inhaled glucocorticoid. We observed both a decreased dilator response to CRH and decreased maximal constriction to KCl and PGF₂. These responses were further reduced in placentae collected from asthmatic women who smoked cigarettes during their pregnancies. Similar findings have been reported in placentae collected from pregnancies complicated by preeclampsia (30, 31), which are known to be associated with low-birthweight outcomes (32).

Placental perfusion studies have shown that CRH is a potent vasodilator of the fetal-placental circulation (30, 33) and the action of CRH is mediated by the endothelial cell product, nitric oxide (NO) (30). We have examined responses to CRH in the placental circulation to determine if there are changes in endogenous NO-mediated dilation because NO is an important regulator of placental vascular tone (34). Previous studies have demonstrated that those pregnancies identified to have abnormal placental blood flow in utero using Doppler ultrasound had a significantly reduced response to CRH in the fetal-placental vasculature in vitro (30) even though circulating CRH concentrations were increased in these fetuses (35). This reduced response to high circulating concentrations of CRH was possibly due to decreased placental NO synthase activity (36). Our present findings indicate that umbilical artery flow velocity waveforms were within the normal range in late gestation (20) in pregnancies associated with asthma. However, there were significant reductions in the vasodilator response to CRH, suggesting that there are changes within the CRH-NO signaling pathway.

In women with moderate and severe asthma, maximal vasoconstrictor responses to KCl and PGF₂ were decreased in the placental circulation. Similar vascular responses to the thromboxane agonist, U46619, have been reported in placentae collected from preeclamptic (31) and diabetic (37) pregnancies. In placentae from patients with diabetes mellitus, Wilkes and coworkers (37) demonstrated that the reduced vasoconstrictive response to U46619 was a result of decreased thromboxane receptor affinity and numbers. Reduced maximal constrictive responses to KCl have been reported in the fetal sheep coronary artery from pregnancies complicated by high-altitude hypoxia (38). Garcia and coworkers (38) proposed that there are alterations of calcium responsiveness in the smooth muscle myofilaments that result in reduced maximal constriction. These changes in response to vasoconstrictors may be either compensatory adjustments to the complications of the disease state, inhibitory effects of glucocorticoid treatment on prostaglandin pathways, or overall dysfunction in both vascular endothelium and smooth muscle of the placental circulation.

Asthma severity is a likely cause of alterations in placental vascular function and the adverse outcome of low birthweight at term. Previous studies show that low-birthweight neonates were associated with pregnancies complicated by severe asthma that required hospitalization versus severe asthmatics who did not require emergency care (1, 39, 40). In our study, 25% of the women classified as moderate or severe asthmatics were medically advised to use daily, inhaled glucocorticoid for the treatment of their asthma and refused to do so for personal reasons, resulting in poor asthma control throughout gestation. We have shown a trend between poor asthma control and decreased birthweight centile in women who did not use glucocorticoid inhalers during pregnancy. In pregnancies associated with deficient asthma control and status asthmaticus there are risks of development of maternal alkalosis (13) and subsequent reductions in uterine blood flow and fetal oxygenation leading to fetal hypoxia, hypercapnia, or acidosis under extreme conditions (11).

Reduced maternal oxygen, known as preplacental hypoxia, occurs in pregnancies complicated by smoking, anemia, hypertension, high altitude (17), and possibly also uncontrolled asthma. The placenta adapts to these conditions by increased capillary growth and branching, increased trophoblast proliferation, and thinning of the placental barrier for rapid transfer of oxygen into the fetal circulation (41). Regardless of the compensatory efforts of the placenta, these pregnancies can still be complicated by IUGR. In our study, we observed no significant reductions in mean birthweight centile of the study population of moderate and severe asthmatic women even though there were significant alterations in placental vascular function in vitro. It is possible that these changes in placental vascular function may not be detrimental to fetal development except in combination with other complications such as poorly controlled asthma or smoking.

Fetal exposure to excess maternal glucocorticoid in the human model and in a number of animal models results in IUGR (45). This is a concentration-dependent effect as multiple doses of betamethasone for fetal lung maturation in women presenting with threatened preterm labor was associated with a 9% decrease of neonatal birthweight and 4% decrease of head circumference when compared with neonates whose mothers only received one betamethasone dose (50). The chronic use of oral glucocorticoids for treatment of asthma is reported to be associated with low-birthweight neonates at term (1, 3, 51). However, there is no scientific evidence to suggest that daily, inhaled glucocorticoid concentrations could alter placental and fetal development during pregnancy, even though it is known that these drugs exert systemic effects in nonpregnant individuals (52). Our study demonstrates that inhaled steroid use is not associated with a dramatic decrease in average neonatal birthweight or birthweight centile and that the most marked effect on birthweight centile was observed in the group of women with poorly controlled asthma who did not use inhaled steroids and in asthmatic women who smoked throughout pregnancy. However, our study was designed to examine vascular function in asthmatic women and the power of the study to detect changes in birthweight is low. Similar trends have been reported by Schatz and coworkers (40) in a prospective controlled analysis of pregnant asthmatic women. Larger epidemiologic studies would clearly define birthweight outcomes of asthmatic pregnancies in relation to severity and glucocorticoid intake.

Glucocorticoids are known to alter vascular function, as administration of cortisol for 5 d to humans induces hypertension (53). The hypertensive effect of cortisol does not appear to be a direct vasoactive action but acts through the downregulation of endothelial NO synthase (54) and the potentiation of responses to vasoconstrictors (55, 56). We have not observed augmentation of vascular responses to the vasoconstrictors, PGF₂ or KCl in the circulation of placentae exposed to inhaled glucocorticoids throughout gestation, although we have observed changes in vasodilator pathways involving NO. To further complicate the interpretation, betamethasone administration for fetal lung maturation in pregnancies associated with absent end umbilical artery diastolic flow caused a return of diastolic flow for a short period (57), suggesting that glucocorticoids, in the short term, act as vasodilators in the placental circulation. Preliminary studies of dexamethasone effects in the placental circulation in vitro confirm that glucocorticoids cause dilation in this vascular bed (58).

This is the first study to address placental vascular function in pregnancies complicated by asthma. We have found that although placental blood flow appears to be normal in late gestation as demonstrated by Doppler ultrasound, there are abnormalities in early gestation in vivo and significant changes in vascular responses to constrictors and dilators in vitro in pregnancies associated with moderate and severe asthma. These changes in vascular function in combination with other complications such as poorly controlled asthma or smoking could result in reduced blood flow to the fetus and low-birthweight neonates.