Platelet Behaviour in Normal Pregnancy, Pregnancy Complicated by Essential Hypertension and Pregnancy-Induced Hypertension

VTE Risk Assessment and Prevention in Pregnancy

Venous thromboembolism (VTE) remains the leading cause of maternal mortality in pregnancy and the postpartum period. In addition to the higher pregnancy-associated baseline VTE risk, there are several well-established risk factors that can further increase the risk of VTE. At present, a thorough interrogation of these risk factors remains our only tool for estimating which pregnant people may be at an increased risk of VTE, and thus potentially benefit from thromboprophylaxis. However, an important knowledge gap still exists surrounding the duration of increased risk and the interaction of risk factors with each other. Furthermore, up to now, once significant risk has been established, prevention strategies have been largely based on expert opinion rather than high-quality data. Recent trials have successfully bridged a proportion of this knowledge gap; however, the challenge of conducting high-quality clinical trials with pregnant people remains. In this article, we provide an update on the recent evidence surrounding VTE risk factors in pregnancy while concurrently outlining knowledge gaps and current approaches to VTE prevention.

VTE risk assessment, prevention and diagnosis in pregnancy

Venous thromboembolism (VTE) is still reported as the leading cause of direct maternal death in pregnancy in serial international reports in developed countries. VTE risk is higher during pregnancy but is further increased by additional well-characterized risk factors. International guidelines recommend that formal VTE risk assessment should be conducted at least in early pregnancy, at delivery and when risk factors change. High quality data supporting optimal VTE prevention strategies are lacking, outside the setting of prevention of VTE recurrence. Moreover, recent high-quality studies have provided much-needed data on diagnostic strategies for pulmonary embolism (PE) in pregnancy. In this review, we summarize knowledge gaps and recently published data in the prevention and diagnosis of VTE in pregnancy. Moreover, we describe ongoing high-quality randomised trials and prospective clinical management studies in this area. High quality clinical studies and trials in pregnancy can be done and must be prioritised, through international network efforts and national funding advocacy. Ultimately, translation of study results to impact upon guidelines and policy will deliver better care to and will protect the lives and health of pregnant people and those contemplating pregnancy throughout the world.

Platelet aggregation in normal pregnancy

It was recently shown that abnormal platelet aggregation (PA) had played a critical role in some adverse pregnancies. Till now reference range for PA in normal pregnancy has not been determined. Furthermore, few study has been conducted to explore the factors related to PA. Our study was performed to assess the reference range of PA in normal pregnancy (150 participants in the second trimester), and to determine whether it differs from that of the controls (38 nonpregnant participants). In addition, this study explored the factors related to PA. The results showed that PA was significantly higher in normal pregnancy than that in the controls (84.40% vs. 80.7%, respectively, P = 0.013). The reference interval for PA in normal pregnancy was 74.75%-94.77%. Hemoglobin (Hb), platelet counts (PLT) and albumin (Alb) were significant lower in normal pregnancy than those in the control group. Moreover, it was found that PA was positively correlated with PLT (r = 0.263, P < 0.001), and negatively correlated with platelet distribution width (PDW) (r = -0.342, P < 0.001) and mean platelet volume (r = -0.296, P < 0.001). Linear correlations between PA and Alb, PDW were proved by linear regression model (LRM). In conclusion, PA was enhanced in normal pregnancy, and Alb and PDW might be the possible contributing factors to PA.

Arachidonic acid-induced platelet aggregation and acetylsalicylic acid treatment during pregnancy in women with recurrent miscarriage, a post hoc study

In this post hoc study, arachidonic acid (AA)-induced platelet aggregation during pregnancy with and without acetylsalicylic acid (ASA) treatment was studied in 323 women with unexplained recurrent first-trimester miscarriage and in 59 healthy women with normal pregnancies. All women had normal AA-induced platelet aggregation in the non-pregnant state. Women with recurrent miscarriage were treated with 75 mg ASA or placebo daily. AA-induced platelet aggregation was measured with multiple electrode impedance aggregometry and presented in units (U), where 1 U = 10 aggregation units x minutes. There were no significant differences in platelet aggregation between placebo-treated women with recurrent miscarriage and healthy women. The mean differences were-0.7 (95%CI; -7.0; 5.6) U in the non-pregnant state, 3.8 (95%CI; -4.6; 12.2) U during the late first trimester and 1.7 (95%CI; -6.7; 10.3) U and 4.1 (95%CI; -3.9; 12.0) U during the early and late third trimester, respectively. ASA reduced platelet aggregation by median -84.0% (Q1; Q3; -89.8; -76.3), -79.9% (-84.7; -69.2) and -75.7% (-83.5; -49.5), respectively, during pregnancy. The degree of inhibition by ASA decreased during the third trimester (p < .0001). There were two (1.9%) complete non-responders to ASA and 32.1% with a partial response. The rate of subsequent miscarriage was not affected by ASA, which did not seem to influence the rate of early miscarriage if treatment was initiated when a viable pregnancy was detectable by ultrasound.

VTE risk assessment in pregnancy

A State of the Art lecture, "VTE Risk Assessment in Pregnancy," was presented at the ISTH congress in Melbourne, Australia, in 2019. Venous thromboembolism (VTE) remains a leading cause of death in pregnancy and in the postpartum period. Moreover, VTE can result in lifelong disability. The elevated baseline pregnancy-associated VTE risk is further increased by additional maternal, pregnancy, and delivery characteristics, highlighting the importance of VTE risk assessment in early pregnancy, at delivery, and if risk factors change. This review will provide an overview of the impact and epidemiology of VTE in pregnancy (including reported risk factors for pregnancy-associated VTE), will address VTE risk-reduction strategies (including ongoing studies), and will provide a summary of critical knowledge gaps. Finally, throughout this review, relevant new data presented during the 2019 ISTH annual congress in Melbourne will be summarized.

Maternal Platelets—Friend or Foe of the Human Placenta?

Human pregnancy relies on hemochorial placentation, including implantation of the blastocyst and deep invasion of fetal trophoblast cells into maternal uterine blood vessels, enabling direct contact of maternal blood with placental villi. Hemochorial placentation requires fast and reliable hemostasis to guarantee survival of the mother, but also for the neonates. During human pregnancy, maternal platelet count decreases gradually from first, to second, and third trimester. In addition to hemodilution, accelerated platelet sequestration and consumption in the placental circulation may contribute to a decline of platelet count throughout gestation. Local stasis, turbulences, or damage of the syncytiotrophoblast layer can activate maternal platelets within the placental intervillous space and result in formation of fibrin-type fibrinoid. Perivillous fibrinoid is a regular constituent of the normal placenta which is considered to be an important regulator of intervillous hemodynamics, as well as having a role in shaping the developing villous trees. However, exaggerated activation of platelets at the maternal-fetal interface can provoke inflammasome activation in the placental trophoblast, and enhance formation of circulating platelet-monocyte aggregates, resulting in sterile inflammation of the placenta and a systemic inflammatory response in the mother. Hence, the degree of activation determines whether maternal platelets are a friend or foe of the human placenta. Exaggerated activation of maternal platelets can either directly cause or propagate the disease process in placenta-associated pregnancy pathologies, such as preeclampsia.

Platelet behaviour on von Willebrand Factor changes in pregnancy: Consequences of haemodilution and intrinsic changes in platelet function

Platelet function in pregnancy is poorly understood. Previous studies of platelet function in pregnancy have used non-physiological assays of platelet function with conflicting results. This study using a physiological assay of platelet function investigated platelet interactions with von Willebrand Factor (VWF) in blood from healthy pregnant women and healthy non-pregnant controls. Blood samples (200 µl) from third-trimester pregnancies (n = 21) and non-pregnant controls (n = 21) were perfused through custom-made parallel-plate flow chambers coated with VWF under arterial shear (1,500 s⁻¹). Multi-parameter measurements of platelet interactions with the immobilized VWF surface were recorded by digital-image microscopy and analysed using custom-designed platelet-tracking software. Platelet interactions with VWF decreased in healthy third-trimester pregnant participants relative to controls. This effect is most likely due to haemodilution which occurs physiologically during pregnancy. Interestingly, platelets in blood from pregnant participants translocated more slowly on VWF under arterial-shear conditions. These decreases in platelet translocation speed were independent of haemodilution, suggesting intrinsic changes in platelet function with pregnancy.

Placental hemostasis and sterile inflammation: New insights into gestational vascular disease

Activation of the coagulation and inflammatory systems are physiologically occurring during pregnancy. However, excess activation of either system is well documented in gestational vascular diseases (GVD). GVD are placenta-mediated pregnancy complications and a major cause of feto-maternal morbidity and mortality. The causal relevance of excess coagulation and inflammatory responses for GVD remains largely unknown. Deciphering the causal relationship of excess coagulation and inflammation in GVD may allow conceptualizing new therapeutic approaches to combat GVD. Platelet activation and procoagulant extracellular vesicles (EVs) provide a link between coagulation and inflammation and their activation or generation in GVD is well established. As recently shown EVs cause sterile placental inflammation by activating maternal platelets that release ATP and activate purinergic receptor signaling and NLRP3 inflammasome in the embryonic trophoblast. This thrombo-inflammatory mechanism suggests a novel link between coagulation activation and sterile inflammation in GVD. These findings highlight a role of anti-platelet therapies in GVD. In addition, targeting the inflammasome alone or in combination with platelet inhibition may provide a new therapeutic strategy in GVD.

Platelet Adhesion and Thrombus Formation in Whole Blood at Arterial Shear Rate at the End of Pregnancy

PROBLEM

Platelet reactivity has not been evaluated in integrated functional testing during normal pregnancy. Here, we analysed platelet functions under arterial shear rate in comparison with static conditions.

METHOD OF STUDY

Thirty pregnant women with uncomplicated pregnancies and 30 healthy non-pregnant women were enrolled in this study. Platelet adhesion to collagen and fibrinogen and subsequent thrombus formation were measured at arterial shear rate in whole blood using a microfluidic and imaging system. Standard light transmission aggregometry, flow cytometry of activation markers in washed platelets and impedance aggregometry in whole blood were also used to assess platelet responsiveness in static conditions.

RESULTS

Compared to non-pregnant controls, thrombus formation on collagen fibres and firm platelet adhesion on fibrinogen under arterial shear rate were significantly reduced in pregnant women. Platelet aggregometry assays in suspension showed a slight increase in platelet reactivity in pregnant women.

CONCLUSION

While platelet aggregometry and platelet activation markers in static conditions show little changes in platelet reactivity, monitoring of platelet adhesion and thrombus growth on collagen or fibrinogen under flow condition in whole blood indicates a significant decrease in pregnant women compared to controls. This decrease might contribute to counteract a hypercoagulable state and to reduce the risk of arterial thrombosis.

Erythrocyte-platelet interaction in uncomplicated pregnancy

Maternal and fetal requirements during uncomplicated pregnancy are associated with changes in the hematopoietic system. Platelets and erythrocytes [red blood cells (RBCs)], and especially their membranes, are involved in coagulation, and their interactions may provide reasons for the changed hematopoietic system during uncomplicated pregnancy. We review literature regarding RBC and platelet membrane structure and interactions during hypercoagulability and hormonal changes. We then study interactions between RBCs and platelets in uncomplicated pregnancy, as their interactions may be one of the reasons for increased hypercoagulability during uncomplicated pregnancy. Scanning electron microscopy was used to study whole blood smears from 90 pregnant females in different phases of pregnancy. Pregnancy-specific interaction was seen between RBCs and platelets. Typically, one or more platelets interacted through platelet spreading and pseudopodia formation with a single RBC. However, multiple interactions with RBCs were also shown for a single platelet. Specific RBC-platelet interaction seen during uncomplicated pregnancy may be caused by increased estrogen and/or increased fibrinogen concentrations. This interaction may contribute to the hypercoagulable state associated with healthy and uncomplicated pregnancy and may also play a fundamental role in gestational thrombocytopenia.

Platelet reactivity changes significantly throughout all trimesters of pregnancy compared with the nonpregnant state: a prospective study

OBJECTIVE

Platelets play an important role in the pathophysiology of uteroplacental disease and platelet reactivity may be an important marker of uteroplacental disease activity. However, platelet reactivity has not been evaluated comprehensively in normal pregnancy. We sought to evaluate platelet reactivity using a number of agonists at defined time points in pregnancy using a novel platelet assay and compare these with a nonpregnant cohort.

DESIGN

Prospective longitudinal study.

SETTING

Outpatient department of a large tertiary referral centre.

SAMPLE

Eighty participants with 30 nonpregnant women and 50 pregnant women assessed longitudinally.

METHODS

This was a prospective cohort study performed longitudinally throughout uncomplicated singleton pregnancies with participants recruited before 15 weeks of gestation. They were controlled for a number of factors known to affect platelet reactivity. Blood samples were obtained in each trimester. Thirty nonpregnant healthy female volunteers also had a platelet assay performed. A modification of standard light transmission aggregometry was used to assess platelet function, with light absorbance measured following the addition of five different agonists at submaximal concentrations. Dose-response curves were plotted for each agonist for the nonpregnant cohort and in each trimester for the pregnant cohort.

MAIN OUTCOME MEASURES

Dose-response curves and median effective concentration.

RESULTS

When compared with the nonpregnant controls a significant reduction was demonstrated in platelet reactivity to collagen during the first trimester of pregnancy (P < 0.0001). Platelet aggregation increased significantly from the first to third trimesters in response to collagen and arachidonic acid.

CONCLUSION

Platelet reactivity varies according to pregnancy state, gestational age and agonist. The finding that platelet reactivity is reduced in the first trimester of pregnancy may be useful for the interpretation of further studies examining the role of platelet reactivity in the first trimester of pregnancies that develop uteroplacental disease.

Pregnancy in type 2B VWD: a case series

Type 2B von Willebrand disease (VWD) is a rare, inherited bleeding disorder resulting from a qualitative defect in von Willebrand factor (VWF). There is very little published information on how to quantify bleeding risk and manage haemostasis in type 2B VWD patients during pregnancy. This article presents the changes in VWF parameters and details of patient management and delivery outcomes for four pregnancies in three women with two different mutations causing type 2B VWD. We report an unexpected rise in the VWF:Ag at 37 weeks gestation in two sisters with R1306W associated with significant thrombocytopenia. These patients were supported with platelet transfusions as well as intermediate purity VWF-FVIII plasma concentrates during the peri- and postpartum periods. No thrombocytopenia was observed in our third case with a mutation encoding an R1308C substitution; haemostatic support was with intermediate purity VWF-FVIII plasma concentrates alone. No adverse bleeding events occurred and in all cases a live healthy infant was delivered. One patient was readmitted post partum with bleeding symptoms due to retained placenta; no further haemostatic support was given at this time. This case series is the first to detail the progression of laboratory parameters, management and outcomes of pregnancy in patients with type 2B VWD. The cases illustrate some of the challenges posed by the increased production of a VWF variant with a gain-of-function effect. The rapid coagulation changes observed in this series illustrate the need for continual monitoring of VWF parameters and platelet count throughout pregnancy in women with type 2B VWD.

Physiologic and pathologic changes of platelets in pregnancy

Platelets are key players in haemostasis and thrombus formation. Defects affecting platelets during pregnancy can lead to heterogeneous complications, such as thrombosis, first trimester miscarriage and postpartum haemorrhage. The incidence of complications is increased in women who have heritable platelet function disorders. Modifications of platelet count or platelet functions during normal pregnancy and preeclampsia will be summarized and the management of pregnant women with heritable platelet function disorders will be discussed.

[Is a biological assessment necessary to realize an obstetrical epidural anaesthesia in a patient whose interrogation and clinical examination are strictly normal?]

In France, coagulation blood tests are usually ordered before performing an epidural anaesthesia. This French habit obeys to the fear of triggering an epidural haematoma induced by neuraxial anaesthesia. This analysis of literature shows that these practices do not protect anaesthesiologists against this clinical risk or its medico-legal consequences. As shown by epidemiological studies, epidural haematoma in pregnancy is associated to the occurrence of HELLP syndrome. On the opposite, gestational thrombocytopenia is not associated to any bleeding risk. According to the recommendations of the French Society of Anaesthesia (Sfar), only a clinical examination and an interrogatory must be done to diagnose coagulation defects before general or loco regional anaesthesia. Normal pregnancy is not an exception to this rule except for the platelet number which must be evaluated during the 3rd trimester of pregnancy. This platelet numeration might detect a rare idiopathic thrombopenic purpura. This recommendation is valid only for normal pregnancy. The clinician must ensure that pregnancy is still normal by seeking for symptoms of pregnancy-induced pathology such as preeclampsia or HELLP syndrome before setting an epidural anaesthesia. The possibly late occurrence of these complications during the per- or post-partum explain why a coagulation test performed even a few days before anaesthesia may not allow to detect any coagulation defect favouring the risk of epidural haematoma.

Platelet activation in pregnancy-induced hypertension

BACKGROUND

Although excess platelet activation, as indicated by increased plasma beta thromboglobulin (beta-TG), has been shown in pregnancy-induced hypertension (PIH), platelet adhesion, platelet morphology and a comparison of platelet and soluble (plasma) levels of the adhesion molecules P-selectin (pPsel and sPsel, respectively) have not been studied.

METHODS

We conducted a cross-sectional study of 35 consecutive women with PIH (age 31+/-6 years), 31 consecutive women with normotensive pregnancies (age 29+/-5 years) and 30 normotensive non pregnant women (age 30+/-5 years). Platelet adhesion was studied in vitro by binding to fibrinogen-coated microwells, platelet morphology [mass and volume by flow cytometry], whole-platelet P-selectin (pPsel) by ELISA of the lysate of 2 x 10(8) cells, and the plasma markers soluble P-selectin (sP-sel) and beta-TG, by ELISA.

RESULTS

The women with PIH had significantly raised sPsel, pPsel and (as expected) beta-TG (all p<0.05), when compared to the normotensive pregnant women and controls. However, in PIH platelet adhesion was similar to that in the normotensive pregnancy, but still higher than the normal controls (p<0.001). There was no difference among the three groups with respect to platelet mass and volume. pPsel and platelet adhesion correlated with gestational age and with systolic and diastolic blood pressure (all p<0.05).

CONCLUSIONS

Increased platelet activation and adhesion develop during normal pregnancy, with some indices being further altered in PIH.

Platelet activation in the hypertensive disorders of pregnancy

The hypertensive disorders of pregnancy, including gestational hypertension, pre-eclampsia and eclampsia, continue to be an important cause of maternal morbidity and mortality. Abnormal placentation is considered to be the main instigating factor, which then leads to widespread maternal endothelial activation and dysfunction. This endothelial perturbation leads to the release of many substances into the circulation, many of which result in platelet activation. For example, there is an imbalance between the levels of prostacyclin (a vasodilator and platelet inhibitor) and thromboxane (a platelet activator and vasoconstrictor), which then results in the maintenance of high blood pressure and complications. It is also likely that platelets play an important part in the pathogenesis of hypertension in pregnancy. The use of antiplatelet drugs has been shown to be effective in reducing the incidence of gestational hypertension in women at high risk and in preventing the complications associated with it. In addition, some antihypertensive agents are effective in reversing platelet activation in essential hypertension and, therefore, their use in pregnancy-induced hypertension may be beneficial in more ways than simply blood pressure reduction.

Thrombocytopenia in the parturient

Thrombocytopenia in pregnant women can be associated with substantial maternal and neonatal morbidity. It may result from a range of conditions and early implementation of some specific treatment may improve both maternal and neonatal outcome. In this review we discuss the clinical features of the more common causes of thrombocytopenia associated with pregnancy, and provide an overview of the anaesthetic considerations.

Platelet activation in the hypertensive disorders of pregnancy

OBJECTIVE

The purpose of this study was to determine whether platelet activation occurs only in preeclampsia or also in normal pregnancy.

STUDY DESIGN

Thirty women with preeclampsia, 30 women with gestational hypertension, 20 women with essential hypertension, 30 pregnant women with normotension, and 30 nonpregnant women were recruited at St George Hospital, Sydney, Australia. Platelet activation was determined by flow cytometry on whole blood samples.

RESULTS

Platelet activation was similar in all groups, except the group with preeclampsia. Compared with normal pregnant women, women with preeclampsia had significantly greater CD62 expression (1.35% vs 0.61%; P =.002), CD63 expression (1.73% vs 0.95%; P <.0001) and annexin V binding (1.03% vs 0.66%;P =.03) and significantly fewer circulating platelet microparticles (33 vs 49 x10(9)/L; P =.001). This was unrelated to other parameters that included platelet counts. Women with gestational hypertension in whom preeclampsia developed did not have enhanced platelet activation profiles.

CONCLUSION

Platelet activation is increased in preeclampsia but not in other hypertensive disorders or in normal pregnancy. This may be part of the pathophysiologic factors of preeclampsia complications but is not predictable by the platelet count and is not apparent in all women with preeclampsia.

Mechanisms involved in agonist-induced hyperaggregability of platelets from normal pregnancy

There is substantial evidence of increased platelet reactivity in vivo and in vitro during pregnancy. Platelet activation occurs in pregnancy with a risk of the development of preeclampsia. In this study, platelet behavior was studied during 28-40 weeks of gestation in a group of women who remained normotensive and a group of nonpregnant female controls. Platelet aggregation and ATP release stimulated by agonists (i.e. collagen and adenosine 5'-diphosphate) were markedly enhanced in washed platelets from pregnant subjects. Furthermore, the collagen-evoked increase in intracellular Ca(2+) ([Ca(2+)](i)) mobilization in fura-2-AM-loaded platelets was also enhanced in pregnant subjects. Moreover, the binding activity of fluorescein isothiocyanate-triflavin toward the platelet glycoprotein IIb/IIIa complex did not significantly differ between the nonpregnant and pregnant groups. In addition, the amount of thromboxane A(2) (TxA(2)) formation from pregnant subjects was significantly greater than that from nonpregnant subjects in both resting and collagen-activated platelets. On the other hand, prostaglandin E(2) formation in the presence of imidazole in either resting or arachidonic acid (100 microM)-treated platelets did not significantly differ between these two groups. The levels of cyclic AMP formation in both resting and prostaglandin E(1) (10 microM)-treated platelets from pregnant subjects were significantly lower than those in nonpregnant subjects. Nitric oxide production was measured by a chemiluminescence detection method in this study. The extent of nitrate production in either resting or collagen-stimulated platelets from pregnant subjects did not significantly differ from that of platelets from the nonpregnant group. We conclude that the agonist-induced hyperaggregability of platelets from normal pregnancy may be due, at least partly, to an increase in TxA(2) formation and a lowering of the level of cyclic AMP formation, which leads to increased [Ca(2+)](i) mobilization and finally to enhanced platelet aggregation and ATP release.

Platelet responsiveness to L-arginine in hypertensive disorders of pregnancy

OBJECTIVE

In chronically hypertensive (CH), preeclamptic (PE), and normotensive pregnant women (N), we investigated ex vivo platelet aggregation in response to L-arginine (L-Arg) and sodium nitroprusside (SN), which are respectively the substrate and donor of nitric oxide (NO).

METHODS

Platelet aggregation was determined with a dual-channel aggregometer by measuring transmittance of light through the sample in comparison to platelet poor plasma, as a reference. Aggregation induced by adenosine diphosphate was continuously recorded for 3 min and measured before and after preincubation with L-Arg and SN.

RESULTS

Preincubation with L-Arg significantly reduced platelet aggregation in N and CH patients (p < 0.05) but not in PE women. Preincubation with SN affected aggregation in PE women also (p < 0.001). No correlation was found between platelet response to L-Arg or SN stimuli and the severity of hypertensive disorders expressed as week of gestation at delivery or birth weight.

CONCLUSIONS

The present study demonstrates that a decreased platelet sensitivity to L-Arg characterizes PE women, whereas SN maintains its antithrombotic power. This impairment seems to be specific for PE, because platelets of CH patients utilize L-Arg normally. This finding supports the involvement of the L-Arg-NO pathway in the pathogenesis of the procoagulative features of PE and probably in the onset of the disease. The maintained response to SN in PE patients suggests a possible therapeutical use of NO donors in the disease.

Platelets from eclampsia patients have reduced membrane microviscosity and lower activities of the signalling enzymes

It has been shown that platelets from patients suffering from eclampsia are hyporesponsive to stimulation by agonists like thrombin and ADP. Although platelet hyporeactivity contributes to the pathogenesis of the disease process, the cause for this is still not known. Platelet aggregation and secretion are membrane-based phenomena initiated by the processes of cell signalling. Hence, to understand the mechanisms underlying platelet hyporeactivity in eclampsia, membrane microviscosity and activities of the signalling enzymes were measured in human platelets stimulated with thrombin. Membrane fluidity was determined from the steady-state fluorescence anisotropy of diphenylhexatriene incorporated in cell membranes. Activities of phospholipase C and protein kinase C in stimulated platelets were assessed from the extents of phosphatidic acid generation and pleckstrin phosphorylation, respectively. Platelet membrane microviscosity in eclampsia (2.3 +/- 0.2 SEM, n = 5) was significantly lower (P < 0.05) than that in the matched gravid control subjects (3.1 +/- 0.2, n = 4). In eclampsia, generation of phosphatidic acid and phosphorylation of pleckstrin were decreased by 25% (P < 0.05, n = 3) and 35% (P < 0.05, n = 3), respectively, after 60 sec of platelet stimulation. It was concluded that the hyporeactive platelets obtained from eclampsia have more fluid membranes and diminished activities of phospholipase C and protein kinase C. In summary, this study shows that alterations in membrane fluidity and activities of the signalling enzymes (phospholipase C and protein kinase C) may contribute to the diminished platelet responsiveness observed in the eclamptic condition.

Nifedipine reduces thromboxane A2 production by platelets without changing aggregation in hypertensive pregnancy

Platelet aggregation and thromboxane A2 generation were studied in hypertensive pregnant women using normotensive non-pregnant and pregnant controls. In hypertensive pregnancy, adrenaline- and adenosine diphosphate-induced platelet aggregation was at the non-pregnant level and lower than in normotensive pregnancy. Collagen-induced aggregation was at a lower level in hypertensive pregnancy than in both control groups. Thromboxane generation during spontaneous clotting and in platelet-rich plasma did not differ between the three groups. However, thromboxane generation was low during aggregation induced by small collagen concentrations in hypertensive pregnancy, but at higher collagen concentrations the difference between the groups disappeared. During nifedipine treatment (10 mg t.i.d.), aggregation and thromboxane production in platelet-rich plasma induced by the three stimuli remained unaltered in hypertensive pregnancy, while thromboxane synthesis during spontaneous clotting was reduced. In nifedipine-treated non-pregnant controls, only EC80 for adrenaline-induced aggregation decreased. In vitro, pharmacological concentrations of nifedipine inhibited platelet aggregation and thromboxane production. In conclusion, nifedipine reduces thromboxane generation in spontaneous clotting, without inhibiting platelet aggregation and thromboxane production in platelet-rich plasma in hypertensive pregnancy. Reduced aggregability of platelet ex vivo may reflect their continuous activation and desensitization in vivo in hypertensive pregnancy.

Platelet sensitivity to prostaglandin E1 inhibition is reduced in pre-eclampsia but not in nonproteinuric gestational hypertension

OBJECTIVE

Platelet aggregometry was used to discriminate platelet sensitivity to prostaglandin E1 (PGE1) inhibition, to evaluate whether platelet behaviour in pre-eclamptic women was different in this respect than from that in nonproteinuric hypertensive women.

METHODS

The amount of PGE1 required to inhibit in vitro platelet aggregation induced by arachidonic acid was determined in samples from 60 women: 20 nonpregnant controls, 20 women with normal pregnancies, 10 women with gestational hypertension and 10 with pre-eclampsia.

RESULTS

The response to arachidonic acid was similar among the four groups. Amounts of PGE1 necessary to inhibit platelet aggregation were significantly higher in normal pregnant women compared with nonpregnant controls (P<0.001). Platelets from pre-eclamptic women required significantly higher concentrations of PGE1 to inhibit aggregation than the other groups studied (P<0.001). However, there was no significant difference between normal and nonproteinuric hypertensive pregnant women.

CONCLUSIONS

Our findings support the notion that increased platelet reactivity during late pregnancy is exacerbated in pre-eclamptic women but not in nonproteinuric hypertensive women. This is in agreement with the hypothesis that pre-eclampsia and gestational hypertension are different conditions. Prospective studies are required to confirm if this simple test may be useful in the early identification of pregnant women at risk for pre-eclampsia.

Whole blood platelet aggregation in moderate and severe pre-eclampsia

OBJECTIVE

To compare whole blood platelet aggregation in moderate and severe pre-eclampsia with normal pregnancy.

DESIGN

Whole blood platelet aggregation in response to collagen, ADP, PAF, adrenalin and arachidonic acid was measured in the pre-eclampsia group at 36 weeks gestation and at 1, 24 and 48 h and at five days and six weeks post delivery. The normal pregnancy group were studied serially at 12, 20, 28, 32, and 36 weeks gestation and at 1, 24, 48 h and six weeks post delivery.

SETTING

Trinity College Medical School, St James's Hospital, Dublin.

SUBJECTS

Thirty women with diagnosed pre-eclampsia were recruited for the study. Fifteen of these women had severe pre-eclampsia and the remaining 15 had moderate disease. The pre-eclampsia group were compared with 20 healthy primigravid women with uncomplicated pregnancies and deliveries.

RESULTS

In women with severe pre-eclampsia, platelet aggregation in response to collagen, ADP, adrenalin and arachidonic acid was significantly lower at 36 weeks gestation compared with normal pregnancy. Lower levels of collagen induced aggregation were also found at 1 h post delivery when compared with normal pregnancy. Women with moderate pre-eclampsia showed a decreased response to aggregating agents at 36 weeks gestation but this was not significant. ADP, collagen and PAF induced aggregation was higher in women with moderate pre-eclampsia at 36 weeks gestation and during the early puerperium compared with severe pre-eclampsia.

CONCLUSIONS

The clinical signs of pre-eclampsia are accompanied by a reduction in platelet responsiveness, the extent of which is related to the severity of the disease. This suggests that an abnormal platelet activation occurs early in pregnancies destined to be complicated by pre-eclampsia. This activation may be involved in the pathogenesis of pre-eclampsia since its inhibition using low dose aspirin has been shown to modify the disease in high risk pregnancies.

Changes in platelet reactivity following aspirin treatment for pre-eclampsia

OBJECTIVE

To determine whether aspirin therapy in women at risk of pre-eclampsia alters in vitro platelet aggregation.

DESIGN

Longitudinal observational study.

SETTING

RPMS Institute of Obstetrics and Gynaecology, Hammersmith Hospital, London.

SUBJECTS

Six pregnant women at-risk or with pre-eclampsia.

METHODS

Women considered to be at risk of pre-eclampsia or with the disorder were treated with aspirin. In vitro platelet aggregation in response to platelet-activating factor (PAF) and adenosine diphosphate (ADP) before and after aspirin therapy were analysed.

RESULTS

Prior to aspirin therapy platelet aggregation was decreased compared with normal pregnant women. After aspirin therapy platelet aggregation in response to PAF and ADP was greater, but did not return to levels associated with normal pregnancy.

CONCLUSIONS

In women with pre-existing decreased platelet aggregation suggestive of platelet exhaustion, aspirin increased aggregation. This suggests that aspirin decreases in vivo platelet activation and thereby decreases the platelet exhaustion revealed by the in vitro studies.

Retrospective analysis of platelet numbers and volumes in normal pregnancy and in pre-eclampsia

OBJECTIVE

To determine the distribution of platelet volumes and numbers through pregnancy, and to compare these to changes in platelet volumes and numbers in women with pre-eclampsia.

SUBJECTS

Four hundred twenty-eight women with normal pregnancy from whom four or more platelet measurements were available were identified. 74 women with pre-eclampsia (blood pressure > or = 140/90 mmHg, at least 0.5 g protein/24 h urine collection) from whom platelet measurements were available between 27 and 30 weeks of gestation were identified.

RESULTS

Mean platelet volume and platelet number remained constant in normal pregnancies between the first trimester and the end of pregnancy. A persistent increase of > or = 0.8 fl (> or = 90th centile) in mean platelet volume was found in 14 out of 15 pre-eclamptic patients between 24 weeks and 38 weeks of gestation and in only 13 of 428 normal pregnant individuals. Platelet numbers were decreased by > or = 50 x 10(9)/l (i.e. to less than the 10th centile) in 12 of the 15 patients with pre-eclampsia. 10% of the normal pregnant population showed a similar decline in platelet numbers showing that changes in platelet numbers may be a less accurate assessment of the development of pre-eclampsia.

CONCLUSION

We suggest that longitudinal determination of platelet volumes may be of use in identifying those women at risk of pre-eclampsia.

A randomized placebo-controlled study of the effect of low dose aspirin on platelet reactivity and serum thromboxane B2 production in non-pregnant women, in normal pregnancy, and in gestational hypertension

OBJECTIVE

To investigate the effect of 60 mg aspirin daily on platelet reactivity and prostaglandin production in various groups of patients. Similar regimens, which are thought to act through inhibition of platelet thromboxane production, are currently undergoing clinical assessment for the prevention of pre-eclampsia and intrauterine growth retardation.

DESIGN

A prospective randomized placebo controlled study.

SETTING

University Hospital, Nottingham.

SUBJECTS

12 non-pregnant female volunteers, 18 normal primigravidae before 16 weeks gestation and 16 pregnant women admitted with gestational hypertension (GH) at a mean gestation of 38 weeks.

INTERVENTIONS

In the non-pregnant women blood samples were taken before and after a 10-day course of 60 mg aspirin daily. The primigravidae had blood samples taken at 16 weeks and then they were randomized to receive either 60 mg aspirin daily or a matched placebo. Further blood samples were obtained at 28, 32 and 36 weeks.

MAIN OUTCOME MEASURES

Changes in platelet reactivity and release reaction, and serum thromboxane production, were estimated in whole blood.

RESULTS

60 mg aspirin daily significantly inhibited cyclo-oxygenase dependent platelet aggregation, release reaction and serum thromboxane production in non-pregnant and pregnant women, and in women with GH (P less than 0.01). When adrenaline was used as the aggregating agent, the cyclo-oxygenase pathway was recruited in the increased reactivity seen in the third trimester of normal pregnancy, and was sensitive to inhibition by low dose aspirin.

CONCLUSION

Low dose aspirin would appear to be an appropriate agent for the inhibition of platelet reactivity associated with hypertensive pregnancy.

Endothelium-derived relaxing factor and indomethacin-sensitive contracting factor alter arterial contractile responses to thromboxane during pregnancy

OBJECTIVES

Pregnancy reduces uterine artery contractile responses to norepinephrine and angiotensin II in many species, including the human and the guinea pig, by release of endothelium-derived relaxing substances. We hypothesized that vascular reactivity to thromboxane during pregnancy would also be reduced by a similar mechanism.

STUDY DESIGN

Isolated ring segments of uterine and carotid arteries from nonpregnant and near-term pregnant guinea pigs were suspended in a myograph for the measurement of isometric tension.

RESULTS

Uterine but not carotid artery sensitivity to cumulative addition of the thromboxane analog U46619 was decreased during pregnancy. The maximal contractile responses of both vessels were unaltered by pregnancy. N omega-nitro-L-arginine (10(-4) mol/L), an inhibitor of nitric oxide endothelium-derived relaxing factor synthesis, increased the sensitivity of uterine and carotid arteries to U46619 in both pregnant and nonpregnant animals. The maximal contractile response of uterine arteries from pregnant guinea pigs was also increased, but that of nonpregnant ones was not. The maximal U46619 contractile response of the carotid artery was not significantly altered by N omega-nitro-L-arginine. Indomethacin (10(-5) mol/L), a cyclooxygenase inhibitor, reduced both the sensitivity and the maximal response of U46619 in each vessel group. Removal of the endothelium from uterine artery of pregnant animals enhanced both sensitivity and maximal response to U46619. Pretreatment of the denuded segments with indomethacin reduced the sensitivity to U46619. However, indomethacin-treated denuded segments were still more sensitive to U46619 than controls.

CONCLUSION

The sensitivity of guinea pig uterine artery but not carotid artery to thromboxane is reduced during pregnancy. Although the precise mechanism remains unclear, both endothelium-derived relaxing factor and an indomethacin-sensitive contracting factor are involved. If indomethacin-sensitive contracting factor is released by humans and disease alters that release, it is possible that any enhanced contractile response to thromboxane resulting from the loss of endothelium-derived relaxing agents such as prostacyclin and endothelium-derived relaxing factor would be offset by the loss of indomethacin-sensitive contracting factor.

Increased whole blood platelet aggregation in normal pregnancy can be prevented in vitro by aspirin and dazmegrel (UK38485)

OBJECTIVE

To determine the effect of normal pregnancy and the early puerperium on whole blood platelet aggregation and to assess the role of thromboxane A2 (TXA2) in platelet aggregation in pregnancy.

DESIGN

A prospective descriptive study.

SETTING

TCD Medical School, St James's Hospital, Dublin.

SUBJECTS

Twenty healthy primigravidae who remained normotensive during pregnancy and the puerperium.

INTERVENTIONS

20 ml blood samples were obtained serially at 12, 20, 28, 32 and 36 weeks gestation, during established labour and at 1 h, 24 h, 48 h and 6 weeks after delivery.

MAIN OUTCOME MEASURES

Whole blood platelet aggregation in response to aggregating agents ADP, PAF (platelet aggregating factor) collagen, adrenaline and arachidonic acid (AA) at each stage of pregnancy and peuerperium was measured using a particle counting technique. The in vitro effect of aspirin and dazmegrel (thromboxane synthetase inhibitor UK38485) on platelet aggregation in pregnancy was also investigated.

RESULTS

Platelet aggregation in response to collagen, adrenaline, ADP and AA were increased in the last trimester, during labour and at 1 h after delivery but decreased 24-48 h after delivery. Platelet aggregation in response to AA, collagen and adrenalin was reduced by both aspirin and dazmegrel.

CONCLUSIONS

The earliest and most marked increases in platelet aggregation during normal pregnancy were found in response to AA and collagen. These platelet changes were prevented when whole blood was pre-incubated with either aspirin or dazmegrel. This suggests that enhanced production of TXA2 is responsible for increased platelet reactivity in normal pregnancy.

Changes in platelet function in pregnancies complicated by fetal growth retardation

Platelet function was investigated in three patients with severely decreased fetal growth rates detected by ultrasound scanning. Only one patient had hypertension, which was mild and developed after decreased fetal growth and altered platelet responses had been detected. Much higher concentrations of platelet-activating factor (PAF) (20-500 nM) were required to stimulate maximal platelet aggregation in all three patients compared with the concentrations of PAF (5-10 nM) required in control pregnancies of similar gestational age. In a fourth patient, platelet desensitisation was observed 5 weeks before the detection of decreased fetal growth. These results are similar to those observed in women with hypertensive disorders of pregnancy, and indicate that there may be a similar change in platelet function in gestational hypertension and in fetal growth retardation, although the clinical manifestations are different.

Platelet reactivity and serum thromboxane B2 production in whole blood in gestational hypertension and pre-eclampsia

OBJECTIVE

To determine the nature and extent of changes in platelet reactivity in gestational hypertension and pre-eclampsia (using whole blood techniques which may be more physiological than those previously employed).

DESIGN

Cross-sectional observational study.

SUBJECTS

8 normal primigravidae, 16 women with gestational hypertension and 12 women with pre-eclampsia, studied at around 36 weeks gestation.

RESULTS

Platelet reactivity (aggregation and release reaction) induced by stimulation with adrenaline was decreased in the pre-eclamptic group. Serum thromboxane B2 production was unchanged in both hypertensive groups compared with the control group.

CONCLUSIONS

In the context of evidence of platelet activation in pre-eclampsia, our findings are interpreted as reflecting platelet exhaustion.

A longitudinal study of platelet behaviour and thromboxane production in whole blood in normal pregnancy and the puerperium

A longitudinal study of platelet behaviour (platelet aggregation and release reaction) in whole blood and of serum thromboxane B2 production was performed before, during and after normal pregnancy. The response of platelets to arachidonic acid and to adrenaline was significantly increased in the third trimester. Six weeks after delivery, values were still modestly increased but return to non-pregnant values was complete by 12 weeks. Serum thromboxane B2 production was unchanged throughout pregnancy and the puerperium.

Neutrophil activation in pregnancy-induced hypertension

Human neutrophil elastase may be a major mediator of vascular damage and could contribute to the vascular damage seen in women with pregnancy-induced hypertension (PIH). Elevated plasma levels of this substance will reflect neutrophil activation in vivo. To determine neutrophil activation in PIH, we studied 30 normal non-pregnant women, 32 women with normal pregnancies, 19 with mild/moderate PIH and 16 with severe PIH between 28 and 39 weeks gestation. Plasma neutrophil elastase was measured by radioimmunoassay. There was a significantly higher concentration of plasma neutrophil elastase in both mild/moderate and severe PIH than in normotensive pregnancies and this may contribute to the vascular lesion associated with PIH. Concentrations were also significantly higher in normal pregnancy than in non-pregnant women which suggests that neutrophil activation and degranulation are increased in normal pregnancy.

Neonatal and maternal platelets: activation at time of birth

Determination of the plasma concentrations of beta-thromboglobulin (BTG), thromboxane B2 (TxB2) and platelet factor 4 (PF4) were made at the time of birth in 18 newborns and their respective mothers. Both groups show significant elevation of all these molecular markers, suggesting marked platelet activation. The elevated TxB2 levels in the newborn group, 25 +/- 8 pg/ml, are compatible with a normally functioning and activated platelet prostaglandin pathway. Mode of delivery, vaginal or caesarean section, does not significantly influence the degree of activation in either group. Ultrastructural platelet examination did not reveal any morphologic differences between maternal and newborn platelets. There appears to be marked activation of the newborn and maternal platelet systems at the time of birth, and we postulate that this may explain in part the transient platelet dysfunction observed in newborns.

Beneficial effects of U-63,557A, a thromboxane synthetase inhibitor, in an ovine model of pregnancy-induced hypertension

Pregnancy-induced hypertension was induced in five ewes (gestational day 135; term 150 days) by 72 hours of food deprivation. Maternal arterial pressure, uterine blood flow, platelet function, renal function, and plasma levels of 6-ketoprostaglandin F1 alpha and thromboxane B2 were measured before and during hypertension and after three intravenous injections of U-63,557A; sodium 5-(3'-pyridinylmethyl) benzofuran-2-carboxylate, monohydrate (30 mg/kg every 8 hours). Blood pressure increased (p less than 0.03), and returned to normal after U-63,557A. Left uterine artery blood flow increased after U-63,557A (p less than 0.03). Creatinine clearance decreased during hypertension (p less than 0.03) and increased after U-63,557A. Urine protein increased during hypertension (p less than 0.03) and decreased after treatment. Platelet count dropped during hypertension (p less than 0.03) and was elevated after treatment. Collagen lag phase decreased during hypertension (p less than 0.03) and increased after treatment. After U-63,557A, 6-ketoprostaglandin F1 alpha levels were higher (p less than 0.04) than baseline or hypertensive values. Administration of a thromboxane synthetase inhibitor caused resolution of hemodynamic, renal, and coagulation dysfunctions that occurred in ovine pregnancy-induced hypertension.

Inhibition of platelet aggregation in whole blood by dipyridamole and aspirin

We have examined the effects of dipyridamole on platelet aggregation in whole blood both in vitro and after administration to man. The effects of dipyridamole ex vivo were compared with those of aspirin and a combination of dipyridamole and aspirin. In vitro dipyridamole was most effective as an inhibitor of platelet aggregation induced by platelet activating factor (PAF) and low concentrations of arachidonic acid (AA). Its inhibitory effect was always potentiated by adenosine suggesting that its effect on aggregation may be via inhibition of adenosine uptake into blood cells. Ex vivo, dipyridamole, aspirin and the combination of these drugs inhibited the platelet aggregation induced by PAF and AA. Again, adenosine increased the degree of inhibition. These results stress the importance of measuring platelet aggregation in the natural whole blood environment for detection of the inhibitory effects of dipyridamole and suggest a mode of action for the drug.