In vivo platelet function in acute myocardial infarction, acute cerebrovascular accidents and following surgery

Occlusive vascular diseases in oral contraceptive users. Epidemiology, pathology and mechanisms

Despite being an unprecedented departure from normal physiology, the combined oral contraceptive is not only highly effective, but it also has a remarkably good safety record. Concerns over safety persist, though, particularly with regard to venous thromboembolism (VTE), stroke and myocardial infarction (MI). Epidemiological studies consistently show an increase in risk of VTE, but the results are more contentious with regard to arterial diseases. Despite 40 years of research, the mechanisms behind these adverse effects are not understood. In this review, we integrate information from published studies of the epidemiology and pathology of the occlusive vascular diseases and their risk factors to identify likely explanations for pathogenesis in oral contraceptive users. Oral contraceptives induce both prothrombotic and fibrinolytic changes in haemostatic factors and an imbalance in haemostasis is likely to be important in oral contraceptive-induced VTE. The complexity of the changes involved and the difficulty of ascribing clinical significance has meant that uncertainty persists. A seriously under-researched area concerns vascular changes in oral contraceptive users. Histologically, endothelial and intimal proliferation have been identified in women exposed to high plasma estrogen concentrations and these lesions are associated with thrombotic occlusion. Other structural changes may result in increased vascular permeability, loss of vascular tone and venous stasis. With regard to arterial disease risk, epidemiological information relating to dose effects and joint effects with other risk factors, and studies of pathology and changes in risk factors, suggests that oral contraceptive use per se does not cause arterial disease. It can, nevertheless, synergise very powerfully with subclinical endothelial damage to promote arterial occlusion. Accordingly, the prothrombotic effects of the oral contraceptive estrogen intervene in a cycle of endothelial damage and repair which would otherwise remain clinically silent or would ultimately progress - in, for example, the presence of cigarette smoking or hypertension - to atherosclerosis. Future work in this area should focus on modification of the effects of established risk factors by oral contraceptive use rather than modification of the supposed risk of oral contraceptive use by established risk factors. Attempts to understand vascular occlusion in oral contraceptive users in terms of the general features of VTE or with reference to atherosclerosis may be limiting, and future work needs to acknowledge that such occlusions may have unique features. Unequivocal identification of the mechanisms involved would contribute considerably to the alleviation of fears over vascular disease and to the development of even safer formulations.

Platelet activation in mental stress

Platelets are known to become activated in vivo by different stressful stimuli such as surgery and dynamic exercise. Mental stress has been shown to increase platelet aggregability. Platelet activation is thought to be of major importance in atherogenesis and cardiac fatalities. In order to clarify further stress-induced platelet activation with special reference to the period after the stress, we studied eight young, healthy volunteers during and for 1 h after a mental stress test (Stroops Colour Word Conflict Test). Using highly standardized techniques, we have measured platelet aggregability ex vivo and the platelet release products beta-thromboglobulin, platelet factor 4 and serotonin in plasma. As markers of the stress response we measured cyclic-AMP in plasma, heart rate, cardiac output and blood pressure. The stress test induced a significant cardiovascular response with increases in heart rate, blood pressure, and cardiac output and as a measure of adrenergic activity an increase in cyclic AMP in plasma during the test. Platelet aggregability was unaffected during the test but decreased following the stress. During the first hour following the test and release products beta-thromboglobulin and serotonin increased significantly in plasma. We conclude that platelets are activated during mental stress and that this activation involves a post-stress release of vasoactive compounds from platelets.

Detection of reversible platelet aggregates in the blood of smokers and ex-smokers with peripheral vascular disease

OBJECTIVE

To demonstrate that smoking increases platelet aggregation in vivo, that smoking cessation reverses platelet aggregation and that this explains, in part, why smoking perpetuates the development of peripheral vascular disease.

DESIGN

Prospective case-control study involving three groups of patients: smokers with peripheral vascular disease, ex-smokers with peripheral vascular disease and smokers with peripheral vascular disease who quit smoking during the study.

SETTING/PARTICIPANTS

Fourteen smokers and seven ex-smokers, new patients with confirmed peripheral vascular disease, attending the vascular clinic at Fremantle Hospital between February and November, 1988.

INTERVENTIONS

Blood samples taken weekly from all subjects for five weeks. Week 1 was taken as the baseline before smoking cessation in the six smokers who were assigned to stop smoking during the study.

MAIN OUTCOME CRITERIA

Platelet aggregate ratio, an indicator of in-vivo platelet aggregability where an increase in platelet aggregate ratio suggests a decrease in platelet function.

RESULTS

Only three of six smokers stopped smoking for the duration of the study. Median platelet aggregate ratios were: smokers = 0.85 (range, 0.79-0.92) v. non-smokers = 0.93 (range, 0.91-1.00). The difference was statistically significant P less than 0.0002. The difference in platelet aggregate ratios between smokers and quitters was not statistically significant.

CONCLUSIONS

This study demonstrated an increase in platelet aggregability in smokers compared to ex-smokers but there was no clear evidence that platelet function was fully reversed after only four weeks cessation of smoking. The data suggested that platelet function of the ex-smokers had fully reversed to normal over a longer period. This could explain the decreased incidence of complications of peripheral vascular disease in ex-smokers. The small number of patients able to quit smoking impeded this study.

Hemostatic changes following surgery

Measurements of hemostatic variables were performed prior to and up to the fifth day following general surgery in patients of Arab origin (N = 53). There was a significant postoperative elevation in the levels of plasma fibrinogen, clotting factors VIII and reduction of factor VII, ATIII, plasminogen, packed cell volume and platelet count. No significant changes were noted in PT, PTT, TT, RT, alpha-2-antiplasmin and factor X. Platelet aggregation responses to ADP, adrenaline, collagen, arachidonic acid and ristocetin was likewise unaffected by surgery. It was concluded that although the changes in plasmatic coagulation parameters are similar to that reported in Caucasians, lack of evidence of enhanced aggregation following surgery may explain the presumed low incidence of deep vein thrombosis in Arabs.

Platelet aggregability in sleep-related stroke

We examined platelet aggregability during nocturnal sleep and daytime wakefulness in patients with a history of sleep-related stroke onset (SOS) and compared it to that of matched awake-onset stroke (AOS) patients and controls without evidence of vascular disease. Aggregability was evaluated in-vitro at least seven weeks following stroke onset. Platelets were more aggregable to ADP, collagen and arachidonic acid (AA) during both sleep and wakefulness in patients with AOS (p less than 0.01). No significant difference in the mean aggregation thresholds during sleeping or waking periods were found between SOS and control groups. However, platelets were significantly more responsive to AA during sleep than during wakefulness in the SOS patients (p less than 0.01). This difference was confined to the subgroup of SOS patients who had experienced nocturnal as opposed to daytime sleep-related stroke onset, suggesting that the observed difference in platelet responsiveness to AA may be related to a circadian fluctuation in platelet aggregability rather than to a sleep-related fluctuation. Significant sleep-related changes in platelet aggregability were not identified in the other two groups.

Circulating platelet aggregate size in ischemic heart disease

Platelet aggregate size was measured in 178 patients with ischemic heart disease, among whom 56 had stable angina, 42 suffered from unstable angina, and 80 had had uncomplicated acute myocardial infarction. A group of 50 healthy volunteers and 20 hospitalized noncardiac patients served as controls. Venous blood (0.5 cc) was introduced into a solution containing 11.7 mM EDTA and 1.0 g formaldehyde. Platelet aggregate size was determined by microscopic reading as the number of platelets forming aggregates (per 1000 counted platelets) divided by the number of aggregates. Mean aggregate size was found not significantly different in both control groups, as well as in patients with stable angina and acute myocardial infarction (2.21 +/- 0.36 platelets, 2.20 +/- 0.58 platelets, 2.28 +/- 0.19 platelets, 2.76 +/- 1.07 platelets, respectively, p = NS). The highest value was found in the unstable angina group: 4.00 +/- 1.40 platelets (p less than 0.001 vs other studied groups). Platelet aggregate size was found not to be related to sex, age, medication, or coronary risk factors. Unstable angina may thus be a unique entity in ischemic heart disease concerning its platelet behavior, demonstrated in this study by the increased size of peripheral platelet aggregates, which may have pathogenetic, diagnostic, and eventual therapeutic implications.

In vivo platelet aggregation and plasma catecholamines in acute myocardial infarction

In vivo platelet aggregation assessed with the Filtragometer and potential correlates were compared among (1) patients with acute myocardial infarction (AMI), (2) normal controls, (3) patients with acute chest pain in whom AMI was eventually ruled out (ROMI), and (4) chronic outpatients (Cardiac Clinic group) with a history of myocardial infarction and/or angina pectoris. The measure was independent of sex, age, platelet count, immediate food intake, serum cholesterol, and triglyceride levels. The AMI group showed higher in vivo platelet aggregation than any of the other three groups (p less than 0.01). Least in vivo aggregation was seen in the normal group. Despite lack of correlation with the platelet aggregation measure, plasma epinephrine and norepinephrine showed statistically significant differences between the AMI and each of the other three groups. Our data support an association between platelet function and AMI, although not necessarily a cause and effect relationship.

Hemodynamic effects of prostaglandin E1 infusion in patients with acute myocardial infarction and left ventricular failure

Prostaglandin E1 (PGE1) has been shown to limit infarct size, improve coronary blood flow, inhibit platelet aggregation, and reduce both left ventricular (LV) preload and afterload in experimental animals. Its use in the therapy of patients with acute myocardial infarction (AMI) and congestive heart failure (CHF) has not, however, been reported. Five patients with AMI of less than 12 hours' duration and LV dysfunction were studied to assess the hemodynamic effects of IV infusion of PGE1. PGE1 in the concentration of 0.4 microgram/ml was infused at a rate of 0.003 microgram2kg/min (3 ng . kg-1 . min-1) to a maximum rate of 0.021 microgram/kg/min (21 ng . kg-1 . min-1) for a total time of up to 90 minutes. There was an insignificant increase in heart rate, with significant decreases in mean arterial blood pressure and systemic vascular resistance. Pulmonary capillary wedge pressure declined from 21 +/- 3 to 15 +/ 1 mm Hg (p less than 0.05), mean pulmonary artery pressure and pulmonary vascular resistance decreased (p less than 0.05), mean pulmonary artery pressure and pulmonary vascular resistance decreased (p less than 0.05), with increases in cardiac index from 2.38 +/- 0.08 to 2.89 +/- 0.58 L/min/m2 (p less than 0.01) and stroke volume from 51 +/- 17 to 59 +/- 20 ml/beat (p less than 0.05). No major cardiac or extracardiac side effects were encountered during PGE1 infusion. One patient had transient nausea which did not require discontinuation of the drug. PGE1 is an effective vasodilator and deserves further application in therapy for AMI patients with CHF.

Determination of the formation of thromboxane B2 (TxB2), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT) and 12L-hydroxy-5,8,10,14 eicosatrienoic acid (HETE) from arachidonic acid and of the TxB2 :HHT, TxB2 :HETE and (TxB2 +HHT) :HETE ratio in human platelets. Possible use in diagnostic purposes

The formation of thromboxane B2 (TxB2), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT) and 12L-hydroxy-5,8,10,14 eicosatrienoic acid (HETE) was determined in the platelets of normal human males and of patients with disorders in which an abnormal platelet aggregation occurs. Platelets were labelled with [1-14C] arachidonic acid. After Aggregation and extraction the metabolites wee separated by TLC and determined. In the platelets of normal males, TxB2 values were in the range 4.4-12.4%, expressed as a percentage of total radioactivity. Curves were constructed for the following ratios: TxB2 :HHT, TxB2 :HETE and (TxB2 + HHT) :HETE. These ratios were fairly contant. A comparison was made with the ratios obtained in the platelets of a small number of patients with either an enhanced or a diminished aggregation.

Prostaglandins, platelets, and atherosclerosis

Metabolism of arachidonic acid (AA) in blood platelets and in vascular endothelium does not lead to prostaglandins, but thromboxane A2 and prostacyclin are generated. These labile metabolites of AA antagonize each other: thromboxane A2 is a vasoconstrictor and proaggregatory agent, whereas prostacyclin dilates arteries, prevents platelets from aggregation, and dissipates the preformed platelet clumps. Prostacyclin is a powerful stimulator of adenylate cyclase in platelets and therefore its antiplatelet action is potentiated by phosphodiesterase inhibitors such as theophylline or dipyridamole. Cyclo-oxygenase of AA is inhibited by aspirin, thromboxane synthetase by analogues of prostaglandin endoperoxides, and prostacyclin synthetase by linear lipid peroxides. A hypothesis is put forward that atherosclerosis develops because of pathological, nonenzymic lipid peroxides. A hypothesis is put forward that atherosclerosis develops because of pathological, nonenzymic lipid peroxydation in the body and the subsequent molecular damage to prostacyclin synthetase in the rheologically determined areas of arterial walls. Endothelium deprived of prostacyclin is the basis for microthrombi formation, and follows a sequence of events described by Rokitansky and later by Ross. Prostacyclin is also a circulating hormone which is generated by the lungs. Thereby a damage of this "endocrine gland" by respiratory disorders, air pollution, or tobacco smoking are likely to contribute to pathogenesis of atherosclerosis, myocardial infarction, and arterial thromboembolism. Pharmacological treatment and prevention of these diseases should logically include antioxydants, prostacyclin and its analogues, thromboxane synthetase inhibitors and perhaps cyclooxygenase inhibitors (aspirin ?). Prostacyclin was already infused intravenously to men and its powerful antiaggregatory and deaggregatory actions were demonstrated. These properties of prostacyclin along with its vasodilator and positive inotropic actions destine this hormone to be a new type of antithrombotic drug in acute myocardial infarction.

Increased platelet aggregation and decreased high-density lipoprotein cholesterol in women on oral contraceptives

In vivo platelet function, serum total cholesterol (TC), high-density lipoprotein cholesterol (HDL), and serum triglycerides (TG) were determined in 26 women starting oral contraceptives (OCA). Studies were run prior to, and after one and two months of therapy. Platelet aggregation time decreased by 50 percent or more in eight of the 26 women from a pre-experimental mean value of 387 +/- 136 seconds. TC did not vary but after the women were on medication for two months, HDL decreased from 56 +/- 14.4 mg/dl to 52 +/- 12.3 mg/dl (p less than 0.05). There was a progressive increase in TG under treatment, 74 +/- 23.3 mg/dl on the pre-experimental measurement to 93 +/- 31.0 mg/dl after two months of therapy (p less than 0.005). Women planning to use OCA might best be advised to have aggregation and serum lipid studies done before and early in usage.

Influence of vasodilators used in the therapy of heart failure on platelet aggregation

Of the vasodilators used at present in the treatment of heart failure, only nitroprusside and phentolamine inhibit platelet aggregation at therapeutic dose levels. The other vasodilators studied, viz. nitroglycerin, isosorbide dinitrate, hydrallazine, dihydrallazine and prazosin, only inhibit platelet aggregation at relatively high concentrations, well above those reached in vivo. The exact nature of the platelet receptor, stimulation and blockade of which respectively initiate and inhibit aggregation, is not yet know, but it would appear to resemble the presynaptic alpha-receptors of other tissues.

Platelet function studies in coronary artery disease. V. Evidence for enhanced platelet microthrombus formation activity in acute myocardial infarction

Circulating platelet microthrombi were evaluated during the acute and convalescent phases of illness in 44 patients admitted to the hospital for chest pain. Similar studies were performed in 10 healthy volunteers and 6 patients with infection. Circulating platelet microthrombi were significantly increased during the acute phase in 22 patients with transmural myocardial infarction compared with values in the other 22 patients without myocardial infarction, the healthy volunteers and the patients with infection alone. This increase in circulating platelet microthrombi declined to normal levels by the 7th hospital day in all but two patients who had evidence of extension of myocardial infarction and died. In contrast, circulating platelet microthrombi were similar in acute and convalescent phases of patients with chest pain but without myocardial infarction and were comparable with values in healthy volunteers. This study suggests that increased circulating platelet microthrombi may be related to tissue necrosis associated with transmural myocardial infarction.

Acute myocardial infarction with normal and near normal coronary arteries. Documentation with coronary arteriography within 12 1/2 hours of the onset of symptoms in two cases (three episodes)

Three instances (in two patients) of acute myocardial infarction associated with arteriographically normal or near normal coronary arteries are reported. One patient with a lateral infarction had a normal coronary arteriogram and hypokinesia of the lateral wall. Another patient had two infarctions: (1) a transmural inferior-lateral infarction associated with occlusion of the most distal segment of the posterior descending branch of the right coronary artery, and (2) a transmural anterior-lateral-superior infarction associated with occlusion of the most distal segment of the left anterior descending coronary artery. Neither occlusion was consistent with the extent of infarction. Although coronary arteriography was performed as early as 12 1/2, 3 3/4 and 11 2/3 hours, respectively, after the onset of symptoms of infarction in these three instances, the pathophysiologic features of the infarctions are obscure. Temporary occlusion of an epicardial coronary artery by spasm or platelet aggregates, or both, is suggested as a possible mechanism of the acute event.

Peripheral venous platelet aggregates in patients with unstable angina pectoris and acute myocardial infarction

In vivo peripheral venous platelet aggregates (PVPA) have been demon strated in patients with myocardial infarction (MI), transient cerebral ischemic attacks (TIAs), and acute arterial insufficiency by the platelet count ratio (PCR) method. But PVPA determinations have not been reported in patients with unstable angina pectoris (UA), controls have not been specified as acutely ill in earlier studies using this technique, and variability of the method has not been presented in detail. Because platelet aggregation has been mentioned as a possible mechanism for perpetuating myocardial ischemia, we studied 21 acutely ill noncardiac control (NC) patients, 17 UA patients, and 13 MI patients for PVPA.

PCR represents ratio of platelet counts obtained when identical aliquots of blood were drawn into two syringes containing formalin-EDTA and EDTA respectively. Free flow was demonstrated before blood was drawn. Mean PCR was 0.90 ± 0.01 (SE) in NC, 0.85 ± 0.02 in UA, and 0.85 ± 0.02 in MI. In the subset of 7 patients with acute transmural MI, PCR was 0.83 ± 0.02. Only this subset was significantly different from NC (P < 0.001). PCR did not vary significantly with age or sex. Aspirin history was positive in 25-30% of the patients in each group and raised PCR by an average of 0.04 units. When PCR was repeated after 45 minutes in 6 patients, moderate variability with a mean absolute change in PCR of 0.08 was noted.

Our data fail to support an etiologic role for circulating platelet aggre gates in initiating UA. Clinical trials of platelet inhibitors and experimen tal models of arterial insufficiency probably offer the greatest chance of further defining the potential etiologic role of platelet aggregation in initiating or perpetuating myocardial ischemic phenomena.

Effects of acute myocardial ischaemia on platelet aggregation in the coronary sinus and aorta in dogs

The effect of an acute myocardial ischaemia on circulating platelet aggregates (CPA) in the coronary sinus and aorta was studied in open chest dogs. Even before induction of myocardial ischaemia significantly more CPA were found in the coronary sinus than in the aorta. Acute myocardial ischaemia produced by a non-thrombotic coronary artery occlusion increased CPA in coronary sinus, but not in aorta. Administration of acetylsalicylic acid (ASA) reduced the extent of the myocardial ischaemia as evidenced by reduced ST-segment elevations in epicardial ECG. Before induction of myocardial ischaemia, ASA significantly reduced CPA in coronary sinus, but no significant effect was observed during myocardial ischaemia. In the aorta no effect of ASA on CPA was found.

Platelet function and plasma free fatty acids during acute myocardial infarction and severe angina pectoris

The relationship between platelet function and plasma free fatty acid concentration has been studied serially during the initial 24 h in 11 patients suffering from acute myocardial infarction and in 5 patients with severe angina pectoris, Similar results were obtained in the 2 groups. Plasma free fatty acid concentration was initially high, and decreased significantly. The distribution of plasma free fatty acids remained unchanged. Platelet concentration was constant, whereas the percentage of reversible venous platelet aggregates initially was higher than in 11 healthy persons matched for age and sex. Platelet aggregates decreased transiently at 16 h. Venous reversible platelet aggregates correlated significantly with concentration of plasma free fatty acid, thus establishing a possible link between a change in lipid metabolism and platelet function. Plasma concentration of platelet factor 4 increased slightly but significantly during the initial hours. This may indicate an increased platelet release reaction.

Thrombosis and intravascular coagulation

Stress, including trauma and sepsis, is associated with a state of hypercoagulability. In these circumstances the patient is at risk of generalized or local thrombotic complications. New laboratory investigative procedures facilitate diagnosis and permit improved assessment of therapy, which at present remains of unproven efficacy both in the general and local situation.

Effect of stress due to anticipated minor surgery upon in vivo platelet aggregation in humans

When measured just prior to hospital admission, platelet aggregation was faster and systolic and diastolic blood pressure were higher compared to measurements just prior to surgery and upon discharge fromthe hospital.