Effect of low-dose aspirin on vascular refractoriness inangiotensin-sensitive primigravid women

Henk C.S. Wallenburg, MD, PhD, Guus A. Dekker, MD, PhD, jolande W. Makovitz, MD,and Nel Rotmans, BSc

Rotterdam, The Netherlands

The hypothesis that an enhanced vasopressor response to angiotensin II in pregnancy may be correctedby suppressing production of platelet thromboxane A2 with low-dose aspirin was tested in a randomized,placebo-controlled, double-blind trial. We studied 36 normotensive primigravid women with an elevated

blood pressure response to intravenously infused angiotensin II at 28 weeks' gestation; 18 women

received 60 mg of aspirin daily and the same number received matched placebo until 34 weeks' gestation,when angiotensin-sensitivity was again determined. In women taking aspirin, values of thrombin-induced

platelet malondialdehyde production were approximately 10% of those determined in the placebo group,indicating marked suppression of thromboxane A2 synthesis. In the aspirin group vascular refractoriness toangiotensin II was restored in 14 of 17 treated women, by comparison with 5 of 15 women in the

placebo group who had remained normotensive. These results support the hypothesis thatprostacyclin/thromboxane imbalance is an important pathophysiologic factor in the development of the

enhanced angiotensin-sensitivity associated with pregnancy-induced hypertensive disorders. (AM J OasTET

GVNECOL 1991;164:1169-73.)

Key words: Angiotensin 11, pressor response, low-dose aspirin, prostaglandins

Pregnancy is associated with profound physiologicalterations in vascular behavior, including vasodilata­tion and a marked refractoriness to the vasopressoreffect of intravenously infused angiotensin 11.i The vas­cular adaptations to pregnancy are thought to dependto a large extent on delicate changes in eicosanoid bio­synthesis, in particular involving prostacyclin andthromboxane A2• There is increasing evidence that innormal pregnancy these changes lead to a dominanceof the biologic effects of prostacyclin, a potent vaso­dilator and inhibitor of platelet aggregation producedby vascular endothelium and placenta, over the op­posing effects of thromboxane A2 , a potent vasocon­strictor and stimulator of platelet aggregation, mainlyproduced by platelets.':"

For reasons that are not yet well understood, pros­tacyclin synthesis may be insufficient in some, pri­marily primigravid, women, and the prostacyclin­thromboxane A2 balance may become tilted in favor ofthromboxane A2, i, 3 thus leading to the vasoconstriction

and platelet activation characteristic of pregnancy­induced hypertensive disorders.r" Enhanced sensitivityto the vasopressor effect of infused angiotensin 11 can

From the Department of Obstetrics and Gynecology, Erasmus Uni­versity School of Medicine and Health Sciences.Received for publication August 16, 1990; revised November 21,1990; accepted November 28,1990.Reprint requests: Henk C.S. Wallenburg, MD, PhD, Professor, De­partment of Obstetrics and Gynecology, Erasmus University School ofMedicine and Health Sciences,EE 2283, P.O. Box 1738, 3000 DRRotterdam, The Netherlands.6/1/27044

be demonstrated long before the clinical manifestationof high blood pressure" and has been postulated as animportant pathophysiologic mechanism in the devel­opment of pregnancy-induced hypertensive disordersand an early, if not the first, sign of a prostacyclin­thromboxane A2 imbalance. t, 8 We have shown that adaily dose of 60 mg aspirin (acetylsalicylic acid) pre­vents the development of preeclampsia in primigravidwomen with enhanced angiotensin-sensitivity at 28weeks' gestation," probably through selective inhibitionof platelet cyclooxygenase and synthesis of thrombox­ane A2 , resulting in correction of an imbalance betweenthe biologic effects of prostacyclin and thromboxaneA2 . 1O· u If enhanced angiotensin sensitivity is indeeda pathophysiologic factor in the development ofpregnancy-induced hypertensive disorders caused byprostacyclin-thromboxane A2 imbalance, low-dose as­pirin would also be expected to restore, or at least im­prove, refractoriness to the pressor effects of angio­tensin 11 in angiotensin-sensitive pregnant women.To test this hypothesis we conducted a randomized,placebo-controlled, double-blind trial to investigatewhether a low dose of aspirin, taken daily from 28 to34 weeks' gestation, would reduce the vasopressor re­sponse to intravenously infused angiotensin 11 in nor­motensive, angiotensin-sensitive primigravid women.

Material and methods

The trial was designed to include 36 angiotensin 11­sensitive primigravid women in the early third trimes­ter of pregnancy, 18 in the treatment group and 18 in

1169

1170 Wallenburg et al.

the placebo group. We chose this number to demon­strate statistical significance with an a-error and al3-error of 0.05, assuming the possibility to restoreangiotensin-refractoriness in 85% of treated and in40% of untreated women. The assumptions were basedon the results of our previous trial with low-dose as­pirin," on the premise that pregnancy-induced hyper­tensive disorders and angiotensin II sensitivity are dif­ferent manifestations of the same pathophysiologic dis­turbance. The end point of the study was the bloodpressure response to infused angiotensin II following6 weeks of active or placebo treatment.

To recruit the required number of angiotensin­sensitive pregnant women, healthy primigravid womenwith an uncomplicated pregnancy of 26 weeks' dura­tion attending the Antenatal Clinic of the UniversityHospital Rotterdam-Dijkzigt were given written andoral information about the study protocol as approvedby the Hospital and University Ethics Committee.

Angiotensin sensitivity test. The blood pressure re­sponse to intravenously infused angiotensin II was de­termined at 28 weeks' gestation in 142 women who gaveinformed oral consent. None of the women had a his­tory of hypertension or cardiovascular or renal disease.The course of pregnancy had been uncomplicated inall cases, with a maximum diastolic blood pressure of~85 mm Hg; none of the women used any drugs exceptoral iron supplements, and diets were unrestricted.

The angiotensin II sensitivity test was performed asdescribed previously." 12 Briefly, an intravenous cathe­ter was introduced into the right cubital vein for ad­ministration of angiotensin II-amide (Hypertensin,Ciba Geigy, Arnhem, The Netherlands) in 5% dextroseby means of an infusion pump. Blood pressure wasmeasured every 3 to 5 minutes in the left upper armwith a standard mercury sphygmomanometer with thewomen in the left lateral recumbent position. The pointof muffling of the Korotkoff sounds (phase IV) wasrecorded as the diastolic pressure. After a stable bloodpressure and heart rate had been recorded for at least15 minutes, infusion of angiotensin II was started ata rate of 3 ng/kg/min and increased stepwise by1 ng/kg/min at 5-minute intervals. Three blood pres­sures were taken at the end of each incremental dose,and the median values were recorded. The minimumamount of angiotensin II (per kilogram per minute),which caused a rise in diastolic blood pressure of 20mm Hg above the mean diastolic blood pressure levelbefore the start of the infusion, was defined as theeffective pressor dose.

The test was stopped after the effective pressor dosehad been determined in triplicate or when infusion of12 ng/kg/min had not caused a rise in diastolic blood

May 1991Am J Obstet Gyneco1

pressure of 20 mm Hg. A positive angiotensin II sen­sitivity test was defined as a test with a median effectivepressor dose of 10 ng/kg/min or less." All tests werecarried out by one of us (J.W.M.).

Study protocol. The 36 women with a positive an­giotensin II sensitivity test were enrolled in the studyat 28 weeks' gestation, immediately after the test. Eachwoman received a coded package with 50 tablets con­taining 60 mg of aspirin or matched placebo, accordingto a randomization list. The women were instructed totake one tablet daily at breakfast over a 6-week period,until angiotensin sensitivity was again tested; at eachantenatal visit they were asked how many tablets re­mained. The women were asked to report the use ofany other drugs and were explicitly told not to useaspirin. All women involved in the study received stan­dard antenatal care, in which the investigators were notinvolved.

Angiotensin sensitivity was again determined at 34weeks' gestation in all women who had remained nor­motensive (diastolic blood pressure ~85 mm Hg);the same procedure applied at 28 weeks' pregnancywas used. After the introduction of the venous cath­eter, a blood sample was drawn for determination ofthe thrombin-induced production of malondialdehydeby platelets. Malondialdehyde is a stable by-productof platelet thromboxane synthesis": its thrombin­induced production is approximately 5 nmol/ 109 plate­lets in the third trimester of uncomplicated pregnancy."Daily ingestion of 60 mg of aspirin causes about90% inhibition of platelet malondialdehyde synthesis,"which can thus be used as a sensitive index of the useof aspirin. The investigators remained unaware of theresults until the end of the study when the code wasbroken.

The study was terminated after the second angio­tensin II sensitivity test at 34 weeks' gestation. Some ofthe women then started taking 60 mg aspirin in anattempt to reduce the risk of developing pregnancy­induced hypertensive disorders."

Statistical analysis. Differences in relative frequen­cies between placebo and treatment groups were ana­lyzed with the use of Fisher's exact test. The Wilcoxontest for paired and unpaired data was used as appro­priate to analyze differences in continuous variablesbetween the two groups. A probability value of <0.05was considered significant. Where appropriate, 95%confidence intervals were calculated.

Results

At the beginning of the study the two groups werecomparable with regard to age, body weight, baselineblood pressure, and heart rate (Table I). In the aspirin

Volume 164Number 5. Part I

Low-dose aspirin and angiotensin refractoriness 1171

Table I. Clinical characteristics of 36 angiotensin II-sensitive primigravid women at 28 weeks' gestation

Aspirin (n = 18) Placebo (n = 18)

Age (yr)Weight (kg)Systolic blood pressure (mm Hg)Diastolic blood pressure (mm Hg)Heart rate (beats/min)

Median I Range

23 18-3673 55-96

115 103-13670 61-8080 72-104

Median I Range

24 19-3472 46-88

111 100-12874 66-8384 76-108

*Effeetive pressor dose reached in 13 women.tEffective pressor dose reached in 15 women.

the outcome of pregnancy was not analyzed. All infantswere live-born, and serious neonatal problems were notencountered.

Table II. Angiotensin II sensitivity before thestart of treatment with aspirin or placebo at 28weeks and at the end of treatment at 34weeks' gestation

CommentThe values of thrombin-induced production of

malondialdehyde by platelets in both groups ofwomen indicate that they had complied with the pre­scribed regimen. The observation that three of 18women in the placebo group had already become hy­pertensive before 34 weeks' gestation is in accordancewith the results of previous studies on the predictivevalue of the angiotensin II sensitivity test, a level of 10ng/kg/min used as the threshold dividing normaland abnormal angiotensin sensitivity. 12 The resultsof our trial indicate that daily ingestion of 60 mg ofaspirin from 28 to 34 weeks' gestation significantly re­duces the vascular responsiveness to angiotensin II inangiotensin-sensitive primigravid women. In 82% (95%confidence interval, 57% to 95%) of 17 treated women

o5

Placebo(n = 15)

7.5 (5.5-9)8 (3.5-11)

74 (60-96)76 (48-98)t

o14

Aspirin(n = 17)

7.5 (6-9)11 (7->12)

72 (60-92)74 (60-84)*

Effective pressor dose(median and range)At 28 wkAt 34 wk

Heart rate during ef­fective pressor dose(beats/ min)(median and range)At 28 wkAt 34 wk

No. of women witheffective pressordose> 10ng/kg/minAt 28 wkAt 34 wk

group four women smoked 10 to 20 cigarettes per day,in comparison with three women in the placebo group.The dose of aspirin at 28 weeks' gestation was 0.9 ± 0.2mg/day/kg body weight (mean ± SD).

One woman in the aspirin group withdrew from thestudy immediately after randomization. The angio­tensin II sensitivity test was not repeated in threewomen in the placebo group, because they had becomehypertensive (diastolic blood pressures of95 to 110 mmHg) before 34 weeks' gestation; in the aspirin groupall women were normotensive at 34 weeks. In theplacebo group the mean (± SD) value of thrombin­induced malondialdehyde production per 109 plateletsat 34 weeks' gestation was 4.5 (± 1.0) nmol, in com­parison with 0.48 (± 0.2) nmol in the aspirin group(P < 0.001); the highest value determined in the aspiringroup was 1.0 and the lowest value in the placebo groupwas 2.8 nmol/Jf)" platelets. The mean (± SD) numberof days during which medication was taken in the as­pirin group (40 ± 3) was not different from that in theplacebo group (41 ± 4).

The median effective pressor dose at 28 weeks' ges­tation was 7.5 ng/kg/min in both groups (Table II).The effective pressor dose at 34 weeks was significantlyhigher than at 28 weeks in the aspirin group (p < 0.01)but not in the placebo group (Fig. 1). Seven women inthe placebo group but none in the aspirin groupshowed a decrease in the effective pressor dose between28 and 34 weeks' gestation (p < 0.01). At 34 weeks'pregnancy 14 women in the aspirin group, as com­pared with five in the placebo group, showed an ef­fective pressor dose of >10 ng/kg/min (p = 0.02); allthese women had effective pressor doses of 11 or 12ng/kg/min, except one woman in the aspirin groupwith an effective pressor dose of > 12 ng/kg/min inwhom the actual value was not determined. When theeffective pressor dose was reached during angiotensininfusion, maternal heart rate showed a significant fallof 8 to 10 beats/min before and after treatment(p < 0.01); the fall in maternal heart rate was not dif­ferent between groups (Table II).

Since some of the women had been taking a low doseof aspirin after 34 weeks' gestation and others had not,

1172 Wallanburg at al.

A B12 12C CE 'E...III ...~ 10 ~... 10III ...s III

IIs

III II0 II

" 0.. 8 " 80 ..II 0II IIII II.. l!!a- a-

~ 6

~6

u

ffi !w4 4

T~ L...--J T~28 34wks L...--J28 34wks

May 1991Am J Obstet Gynecol

Fig. 1. Minimum amount of angiotensin II (nanograms per kilogram per minute) infused intra­venously causing a rise in diastolic blood pressure of 20 mm Hg (effective pressor dose). A, In 17primigravid women at 28 weeks' gestation before and at 34 weeks after 6 weeks of daily oral ingestionof 60 mg of aspirin. B, In 15 primigravid women at 28 weeks' gestation before and at 34 weeksafter 6 weeks of daily oral ingestion of matched placebo.

but in only 33% (95% confidence interval, 12% to 62%)of 15 women in the placebo group, the effective pressordose returned to values> 10 ng/kg/min. The differ­ence may in fact be more pronounced because of theexclusion ofthe three women in the placebo group whohad become hypertensive at 34 weeks' gestation and,for that reason, most likely had enhanced angiotensinII sensitivity at that time. The fall in heart rate duringthe effective pressure dose of angiotensin II has beenobserved by other investigators and may be explainedby baroreflex function. 14 Since the fall in heart rate thatoccurred when the effective pressor dose was reachedwas equal in both groups, baroreceptor response wasapparently not influenced by low doses of aspirin.

Our results are consistent with those of a recent trialin which 22 angiotensin-sensitive women were treatedat about 33 weeks' gestation with 81 mg of aspirin dur­ing a 1 week period." In that study the mean effectivepressor dose increased from 5.9 to 10.2 ng/kg/min,and values of ~1O ng/kg/min were obtained in 53%of 17 women studied before and after 1 week of lowdoses of aspirin. However, the study was not placebocontrolled or blinded. In an earlier, also uncontrolledstudy conducted in 13 primigravid women with a phys­iologic angiotensin refractoriness, the effective pressordose 2 hours after a single oral dose of 80 mg of aspirinwas found to be twice that before treatment. 15 In con­trast, oral ingestion of 650 mg of aspirin twice with aninterval of 6 hours between doses has been demon­strated to lead to a marked increase in angiotensin sen­sitivity in pregnant women." The contrasting effect oflow and high doses of aspirin may be explained by thefinding in various controlled studies in pregnancy 10, 11

and in nonpregnant" subjects that long-term admin­istration of aspirin in a daily dose between about 40and 80 mg markedly reduces the production of plateletthromboxane A2 while sparing the synthesis of vascularprostacyclin; cumulative higher doses result in pro­gressive inhibition of vascular prostacyclin biosyn­thesis. 17 The demonstrated selective inhibition of plate­let thromboxane A2 by low doses of aspirin is thoughtto be due to cumulative presystemic inhibition of cy­clooxygenase in platelets passing through gut capillar­ies with a relatively high concentration of aspirin afteroral ingestion, whereas the concentration in the sys­temic circulation remains too low to affect the enzymein vascular endothelium." In addition, there are ap­proximately 1000 times as many vascular endothelialcells capable of forming prostacyclin as platelets pro­ducing thromboxane A2 •

19 Although we did not mea­sure serum concentrations or urinary excretion of ei­cosanoid metabolites, the inhibition of the thrombin­induced formation of malondialdehyde by platelets toapproximately 10% of the placebo values indicatessuppression of thromboxane A2 dominance and is con­sistent with the results of other studies in which thesemetabolites were deterrnined.!" 11. 14.20

Several randomized, placebo-controlled and blindedstudies have now confirmed the original report" thatlow doses of aspirin reduce the incidence of pregnancy­induced hypertensive disorders in women with variousrisk factors, 10. 11.22 including increased angiotensin sen­sitivity," most likely because of suppression of plateletthromboxane synthesis and restoration of relative pros­tacyclin dominance. 10. 11 The finding that a low dose ofaspirin improves vascular refractoriness in angiotensin-

Volume 164Number 5, Part I

sensmve primigravid women supports the hypothesisthat enhanced vasopressor sensitivity to angiotensin IIis a pathophysiologic mechanism in the developmentof pregnancy-induced hypertensive disorders associ­ated with an imbalance in prostacyclin-thromboxaneA. synthesis. The mechanism through which such animbalance affects vascular reactivity is not fully under­stood. The vasoconstrictor action of angiotensin II maybe enhanced because of an insufficient, direct antag­onizing effect of vasodilator prostacyclin. However, theresults of a recent study on platelet angiotensin II bind­ing sites" suggest that a lack of down regulation ofangiotensin II receptors also could be involved.

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