Failure of thromboxane A2 blockade to prevent attacks of vasospastic angina

Analytic Review: Treatment of Coronary Artery Spasm and Variant Angina

Coronary artery spasm is an abnormal constriction of the conductive arteries that produces myocardial ischemia in the absence of any marked increase in heart rate or blood pressure. Ischemia is a transient phenomenon and is promptly reversed spontaneously or by nitroglycerine. The so-called "hallmark of spasm"—ST segment elevation—is evidence for severe myocardial ischemia rather than conclusive evidence for coronary artery spasm. Any process that transiently or permanently restricts coronary blood flow (e.g., transient occlusion by thrombus) will produce similar electrocardiogram (EKG) abnormalities.

Most patients with symptoms and EKG changes related to coronary artery spasm respond to sublingual nitrates. Thus, their use for relief of the acute ischemic episode remains the initial treatment of choice. When symptoms are moderate in severity or unacceptably controlled in frequency using nitrates alone, other pharmacologic measures are needed.

When spasm is superimposed upon hemodynamically important atherosclerotic obstruction, the favorable response to calcium antagonist may not be as great as that seen when spasm occurs alone. One possible explanation is that spasm is not really occurring. The ST segment elevation may be related to transient total occlusion from platelet aggregation or thrombosis. Thus, aspirin may be the drug of choice. However, if spasm is the culprit, recommendations for surgery and angioplasty require proof that spasm is occurring in and around the area of fixed atherosclerotic obstruction and not in other vessels or over the entire course of the distal vessel.

Persistent angina pectoris, cardiac mortality and myocardial infarction during a 12 year follow-up in 273 variant angina patients without significant fixed coronary stenosis

The incidence of cardiac events in patients with variant angina pectoris without significant coronary stenosis and ST-segment elevation was analyzed during a 12-year follow-up period in 273 consecutive patients (82% men) admitted from 1986 through 2010. Among the 252 patients who underwent electrocardiography during pain, 205 had ST-segment elevation (82%) and 45 had ST-segment depression (18%). During index hospitalization, angina occurred in 179 patients (66%), ventricular tachycardia or fibrillation in 28 (10%), and complete atrioventricular block in 3 (1%), but there were no deaths or myocardial infarctions (MIs). At 140 months, angina was still present in 129 patients (47%), but frequent angina (>10 episodes/year) occurred in only 6%. Total mortality, cardiac mortality, and MI rates were 24%, 7.0%, and 6%, respectively. Cardiac death or MI occurred in 28 patients (10%), associated with tobacco smoking (p = 0.004), antecedent "first-wind" angina (p = 0.020), and angina during hospitalization (p = 0.044) and with continued smoking (p = 0.056) and recurrent angina during follow-up (p <0.001). Multivariate analysis identified age (p = 0.001), antecedent infarction (p = 0.005), first-wind angina (p = 0.009), and smoking at index hospitalization (p = 0.027) as predictors of total mortality and treatment with calcium antagonists (p = 0.047) and smoking during follow-up (p = 0.110) for cardiac mortality and MI. In conclusion, during 12-year follow-up, patients with variant angina pectoris, mostly with ST-segment elevation during pain, had a reduced incidence of cardiac mortality and MI, associated with first-wind angina, persistent angina, and continued smoking.

Pharmacological treatment of Raynaud's phenomenon with special reference to calcium-entry blockers

The development of new vasoactive drugs has increased the possibilities to treat patients with digital vasospasm, known as Raynaud's phenomenon. The purpose of this paper is to give a review of recent clinical trials and also a short review of the pathophysiological reasons for each type of pharmacological treatment.

Role of coronary vasoconstriction in ischemic heart disease and search for novel therapeutic targets

Atherothrombosis has long been recognized as an important mechanism of cardiac events in ischemic heart disease, and large multicenter clinical studies have shown the benefit of antiplatelet agents, statins, beta-blockers and angiotensin converting enzyme inhibitors in preventing these events. However, more recent studies have been less successful at showing incremental gains in targeting these mechanisms, suggesting that the limits of this strategy have been exploited. Coronary vasoconstriction is another important mechanism in ischemic heart disease but has received little attention and yet is a potential therapeutic target. In the current review, the reasons why coronary vasconstriction has received insufficient consideration are explored. In particular, we need to change our approach from lumping heterogeneous clinical entities together to focusing on clinically-discrete homogeneous groups with a common mechanism and thus therapeutic target. The role of coronary vasoconstriction is examined in the various ischemic syndromes (variant angina, chronic stable angina, acute coronary syndromes and syndrome X) and the underlying mechanisms discussed. Finally, in order to advance studies in this field, an innovative research strategy is proposed, including: (1) selection of paradigmatic cases for the various ischemic syndromes; (2) candidate therapeutic targets; and (3) approaches in assessing the clinical efficacy of these potential therapies.

Intracoronary administration of a thromboxane A2 synthase inhibitor relieves acetylcholine-induced coronary spasm

This study sought to clarify the effectiveness of intracoronary administration of a thromboxane (TX) A2 synthase inhibitor, Ozagrel Na, to relieve coronary spasms induced by intracoronary injection of acetylcholine (ACh). An ACh spasm provocation test was performed in 92 consecutive patients with coronary spastic angina using incremental doses of 20, 50, and 80 microg into the right coronary artery, and 20, 50, and 100 microg into the left coronary artery within 20s. A coronary spasm was defined as TIMI 0 or 1 flow and an intracoronary injection of 20 mg Ozagrel Na was administered when it was provoked. Within 2 min of the administration of the TXA2 synthase inhibitor, ACh-induced coronary spasms were relieved (TIMI 3 flow) in 88.1% of procedures without complications. In only 4 cases (4.3%), it took more than 3 min to relieve the coronary spasms. Intracoronary administration of 20mg Ozagrel Na when ACh-induced spasms occurred, shortened the spasm relief time in all 7 patients (200 +/- 59s vs 111 +/- 23s, p < 0.01), improved the maximal ST segment elevation in 5 of them (3.9 +/- 3.7 mm vs 0.7 +/- 1.5 mm, p < 0.05), and stopped chest pain in 4 patients. In 4 patients who had ACh-induced coronary spasm of the left anterior descending artery, the TXB2 concentration in the coronary sinus decreased after intracoronary administration of Ozagrel Na into the left coronary artery (463 +/- 562 vs 96 +/- 45, p < 0.01). In conclusion, intracoronary administration of a TXA2 synthase inhibitor can relieve ACh-induced coronary spasms by inhibiting TXA2 synthesis in the local coronary circulation.

Coronary Artery Spasm

Attacks of variant angina usually respond promptly to sublingual administration of short-acting nitrates (nitroglycerin, 0.3 to 0.4 mg, or isosorbide dinitrate, 5 to 10 mg), which may be repeated after 3 to 5 minutes if pain persists. In the rare cases resistant to sublingual nitrates, sublingual nifedipine (5 to 10 mg) or, when readily available, intravenous nitrates (nitroglycerin or isosorbide dinitrate, 2 to 10 mg) or calcium antagonists (verapamil, 5 to 10 mg, or diltiazem, 0.15 mg/kg) can be given. All attempts to prevent ischemic attacks by means of specific receptor blockade in patients with vasospastic angina have been unsatisfactory. This may be either because the doses of the blockers used were insufficient or, more likely, because the blockade of a single receptor-agonist interaction leaves receptors for other vasoconstrictor stimuli unopposed and therefore capable of eliciting spasm. Thus, for instance, alpha-adrenergic, serotoninergic, and thromboxane A(2) antagonists all failed to reduce significantly the number of anginal attacks, although they appeared to be effective in some patients. Until the actual causes of the coronary smooth muscle hyperreactivity to constrictor stimuli are known, treatment of vasospastic angina is based on the use of nonspecific vasodilators. Indeed, the mainstay of pharmacologic treatment of coronary artery spasm is calcium channel blocking agents together with nitrates to cover the periods in which spasm is most likely to occur. These powerful vasodilating agents, at their usual doses, are able immediately and completely to control the recurrences of ischemic attacks in as many as 80% of patients. Moreover, some studies have shown that use of calcium antagonists significantly improves clinical outcome in patients with variant angina.

Stent placement for recurrent vasospastic angina resistant to medical treatment

The successful stent placement for treatment of recurrent vasospastic angina in a patient with nonstenotic coronary arteries is described. Use of the Palmaz-Schatz stent resulted in successful vasodilation that completely prevented anginal attacks. This procedure represents an alternative treatment for patients with vasospastic angina refractory to aggressive medical therapy.

Control of coronary blood flow by autacoids

Coronary flow and thus myocardial perfusion is regulated by myogenic, metabolic, humoral and neuro-hormonal factors which closely interact with local autacoids released from the endothelial lining of the coronary bed. In a number of disease states an impaired synthesis and release of autacoids decisively limit the overall capacity of coronary regulation and adaptation of myocardial perfusion to increased metabolic demands. The important factors for these control mechanisms are analyzed and reviewed in this article.

Silent myocardial ischemia

The documentation of abnormalities related to myocardial ischemia, whether symptomatic or silent, is of central importance. Whenever this information is available, it should be used in the overall assessment of the patient at risk for adverse outcome. The level of concern for treatment of CAD should be based on the risk implications associated with the ischemia-related abnormalities detected during objective testing rather than on the presence or absence of pain. The exercise stress test is still the single most useful test to begin the evaluation of a patient with an analyzable ST segment. In persons suspected of having CAD, the detection of ischemic-type ST-segment depression, at a low workload (e.g., < 120 beats/min or < 6.5 METS) of > 2 mm magnitude or persisting for more than 6 min implies high risk for adverse outcome. Asymptomatic ischemia during everyday activities, detected by Holter monitoring, in the high-risk patient, most probably adds additional risk beyond the risk of an abnormal stress test alone. Left ventricular imaging by two-dimensional echocardiography, RNA, angiogram, vest, etc, showing an ejection fraction > or = 40%, reversible wall motion abnormalities in multiple regions and redistribution defects or a failure to increase ejection fraction during exercise even if the patient remains asymptomatic, also imply high risk. The presence of any of these abnormal findings, regardless of symptoms, should therefore prompt as high a degree of concern as with ischemia-related signals associated with pain. Thus any therapy chosen should be directed toward elimination of transient ischemia, not just relief of symptoms that may or may not be ischemia related. If this course is chosen, the efficacy of the therapeutic regimen and possible progression of CAD should be assessed with follow-up testing for ischemia. We believe that risk factor modification and aspirin should be considered for most, if not all, patients in whom ischemia, silent or symptomatic, is suspected or detected. If symptoms or ischemia suggesting low risk is present, anti-ischemic medical therapy may be considered, but follow-up is advised. If a high-risk ischemic signal, even without symptoms, is detected, medical therapy should be used to attempt to modify the signal. If the ischemic signal suggesting high risk persists despite medical therapy, revascularization should be considered. Until additional data from large clinical trials are available, this approach appears to have the greatest likelihood of modifying the adverse outcome of CAD.

Treatment strategies for daily life silent myocardial ischemia: A correlation with potential pathogenic mechanisms

Recent investigations of SMI occurring during daily life have advanced our understanding of the pathophysiology of myocardial ischemia. These contributions have directed our attention away from "chest pain" alone and physical exertion as the central provoking factor toward transient myocardial ischemia and its broader triggers and consequences. Transient myocardial ischemic episodes, the majority of which are silent, are found in a subset of patients with any clinical manifestations of CAD (eg, stable angina, unstable angina, myocardial infarction, and sudden death), as well as in those patients with CAD who are and have been totally asymptomatic. These episodes are an independent predictor of increased risk for future cardiac events. Most medical therapy and revascularization therapies have the potential to prevent or relieve these silent episodes; however, we do not yet know which method is superior in reducing SMI episodes or preventing future cardiac events. Furthermore, the benefit of reducing SMI versus the cost and potential morbidity of these chosen therapies is not known. At least three trials are now underway to examine some of these concerns (Table 2). Focus on pain relief alone does not appear to be an adequate approach to alter outcome in patients with CAD and may prove insufficient to control SMI. Until these issues are resolved, we believe a conservative approach to the management of patients with CAD is warranted. Documentation of ischemia (painful or painless) is essential. Three general principles should be kept in mind. First, the presence of detectable ischemia is of central importance. This information should be used in the overall risk assessment of the patient. Second, the level of concern or aggressiveness of treatment should be based on the risk associated with the ischemic abnormalities documented (Table 3). The exercise stress test is the most useful to begin this process. The detection of ischemic-type ST-segment depression, either silent or painful, at a low workload (eg, less than or equal to 120 beats per minute or less than or equal to 6.5 metabolic equivalents [METS]) implies high risk for adverse outcome. Likewise, these ST-segment changes occurring in leads that reflect multiple coronary artery distribution, of greater than 2 mm in magnitude and persisting for greater than 6 minutes, are all markers for high risk. Thallium redistribution defects occurring at low work loads, in multiple areas, associated with increased lung uptake and enlargement of the cardiac pool all imply high risk.(ABSTRACT TRUNCATED AT 400 WORDS)

Case report: coronary vasospasm--relation to the hyperthyroid state

While angina is not uncommonly seen in association with hyperthyroidism, only rare case reports have suggested that myocardial ischemia in this state may be due to coronary artery spasm. The authors review the literature and describe a case in which the repetitive occurrence of episodes of myocardial ischemia due to coronary spasm correlated with repeated transient elevations in thyroid hormone levels, thus clarifying this relationship. The importance of defining thyroid status in patients presenting with coronary vasospasm is emphasized and the effects of thyroid hormone on the heart are reviewed.

Failure of adjuvant heparin to reduce myocardial ischemia in the early treatment of patients with unstable angina

Although the efficacy of long-term administration of antithrombotic agents in unstable angina has been established, short-term effects on myocardial ischemia are unknown. A retrospective analysis was performed in 47 patients undergoing three-channel continuous ST segment monitoring as part of a multicenter trial using esmolol in unstable angina, in which 20 patients received a continuous heparin infusion during the initial assessment of chest pain. Concomitant medications included calcium channel blockers, beta-adrenergic blockers, nitrates, and aspirin in the majority of patients. Clinical variables between the heparin and no heparin groups were similar, except for fewer males and fewer total artery occlusions in the heparin group. No significant differences in the incidence or duration of ischemia were found in a 36 +/- 16 hour monitoring period. Forty percent of the heparin group had 35 episodes of ischemia with a mean of 11 +/- 10 minutes per episode and a total ischemic time of 48 +/- 39 minutes per patient with ischemia. Forty-four percent of the no heparin group had 47 episodes of ischemia with a mean of 13 +/- 13 minutes per episode and a total ischemic time of 58 +/- 47 minutes per patient with ischemia. Multiple linear regression analysis to adjust for intergroup differences did not alter the results. Eighty-five percent of all episodes were asymptomatic. Clinical events, such as episodes of chest pain, emergency coronary arteriography, or coronary revascularization, were also similar between groups. Thus the short-term administration of heparin did not alter the incidence or duration of ischemia in patients with unstable angina.(ABSTRACT TRUNCATED AT 250 WORDS)

A prospective study of the diagnostic value of urinary thromboxane in patients presenting with acute chest pain

In chance single-case observations thromboxane excretion has been reported to increase several days prior to myocardial infarction. To test its frequency and potential diagnostic value we prospectively measured thromboxane excretion in 166 consecutive patients who had presented to the emergency unit with acute chest pain indicative of ischaemia. Thromboxane excretion at presentation was increased, sometimes dramatically, in 17 of 33 (52%) patients with unstable angina, in 42 of 73 (57%) patients with definite myocardial infarction, but in only two of 14 (14%) patients with stable angina. Nineteen of 29 patients undergoing early angiography had detectable intracoronary thrombi, and these patients excreted significantly more thromboxane than patients without thrombi. Ongoing platelet activation may be detected by increased thromboxane excretion in more than 50% of the patients presenting with unstable angina and myocardial infarction, particularly in those with intracoronary thrombi, but it is not a general phenomenon that can be used in diagnosis.

Spontaneous coronary artery spasm in variant angina is caused by a local hyperreactivity to a generalized constrictor stimulus

To assess whether spontaneous coronary artery spasm in patients with variant angina results from local coronary hyperreactivity to a generalized constrictor stimulus or from a stimulus generated only at the site of the hyperreactive segment, the behavior of spastic and nonspastic coronary segments was studied in six patients with variant angina in whom focal coronary spasm developed spontaneously during cardiac catheterization. None of the patients had critical (greater than 50% luminal diameter reduction) organic coronary stenoses. Coronary diameters were measured by computerized quantitative arteriography during control, spontaneous spasm and ergonovine-induced spasm and after intracoronary nitrates were given. During spontaneous spasm, the luminal diameter of spastic and both proximal and distal nonspastic coronary segments was significantly reduced from control values, 64.2%, 13.2% and 14.8%, respectively. Average diameter reduction of unrelated arteries was 12.3%. Ergonovine, which was also administered to four patients, provoked focal spasm at the same site as spontaneous spasm. During intravenous ergonovine, luminal diameter of spastic segments was reduced by 91.5%, that of nonspastic proximal segments by 17.8% and that of nonspastic distal segments by 11.5%. Luminal diameter of unrelated arteries during ergonovine-induced spasm was reduced by 17.7%. Constriction of spastic segments was greater during ergonovine-induced spasm (p less than 0.05), whereas the extent of diameter reduction of nonspastic segments was not significantly different during spontaneous spasm and ergonovine-induced spasm. Intracoronary isosorbide dinitrate dilated spastic and nonspastic coronary segments to a similar extent from control (20.7%, 18% and 16.5%, respectively; p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Humoral regulation during cold-induced coronary arterial spasm

Previous attempts to define the etiology of coronary arterial spasm have been focused on mechanisms such as autonomic nervous dysfunction and/or enhanced platelet activation. In the present study, humoral regulation was investigated in patients with vasospastic angina and scintigraphically documented transient myocardial perfusion abnormalities after a peripheral cold pressor test. Serial changes in angiotensin II, epi- and norepinephrine as well as thromboxane B2 (the stable derivate of thromboxane A2), and malondialdehyde were determined at baseline (I), immediately after 5 minutes cold water hand immersion (II), and following 10 minutes recovery (III). Angiotensin II and epinephrine remained unchanged during observation (I vs II, II vs III: P = NS). Norepinephrine was elevated after cold (I vs II: P less than 0.001) and normalized after 10 minutes (I vs III: P = ns). Thromboxane B2 and malondialdehyde increased continuously (I vs III: P less than 0.05 and I vs III: P less than 0.002, respectively). Further radiothin-layer chromatography results indicate an activation of platelet function during myocardial ischemia. Our results do not establish a cause-effect relationship but, together with other evidence, they may suggest that thromboxane A2 is unlikely to be the cause of spasm. It might, however, play an important role in the maintenance of vasoconstriction.

Attenuation of the vasoconstrictor action of neuropeptide-Y by calcium-channel blockers

Neuropeptide-Y (NPY) was administered intracoronarily in dogs to see the modification of its vasoconstrictor action by the calcium (Ca)-channel blockers nisoldipine (0.1 microgram/kg) and nifedipine (1 microgram/kg). Dogs were anesthetized and the left circumflex artery was cannulated without opening the chest by using a specially designed cannula perfused at constant pressure. The change in coronary flow due to NPY was determined before and after the systemic administration of the two Ca-channel blockers. With administration of 1 to 2 nmol of NPY, coronary blood flow decreased maximally by 23.4 +/- 7.8% without changes in perfusion pressure or central venous pressure and it became significantly less after nisoldipine: 16.0 +/- 5.7% (p less than 0.02). A similar attenuation in the decrease in coronary flow was observed in the nifedipine study: 23.2 +/- 7.5% to 12.0 +/- 6.7% (p less than 0.02). A fall in systemic arterial blood pressure was observed after administration of both Ca-channel blockers, but it was significant only after nisoldipine (p less than 0.01). Nonsignificant increases in heart rate were observed after both drugs. Nisoldipine seemed to attenuate the NPY-induced vasoconstriction in dogs, and its equimolar potency is about ten times that of nifedipine.

Attenuation of reactive hyperemia caused by aspirin in canine coronary artery

Effects of intracoronary aspirin on coronary blood flow and reactive hyperemia were evaluated in closed-chest, anesthetized dogs. In 18 dogs the left circumflex coronary artery was cannulated and perfused by arterial blood at a constant pressure. Coronary blood flow was measured by an electromagnetic flowmeter. Intracoronary aspirin at doses of 5, 10, and 20 mg reduced coronary blood flow in a dose-dependent manner. Injection of aspirin at doses of 10 to 25 mg also inhibited reactive hyperemia following the coronary occlusion for fifteen seconds. The mean peak flow ratio was reduced from 2.13 +/- 0.42 to 1.75 +/- 0.35 (p less than 0.005). The increment of coronary blood flow provoked by intracoronary arachidonic acid at doses of 150 to 300 micrograms was almost entirely inhibited by the pretreatment of the coronary artery with aspirin. The authors conclude that aspirin increases coronary arterial resistance in a dose-dependent manner and also restricts the maximal dilating capacity, possibly by inhibition of prostacyclin synthesis.

Neuropeptide-Y. A peptide found in human coronary arteries constricts primarily small coronary arteries to produce myocardial ischemia in dogs

Neuropeptide-Y (NPY), a brain peptide, is located in the walls of human coronary arteries. This study assessed the effects of NPY on the coronary circulation in 40 chloralose-anesthetized, open-chest dogs. Intracoronary NPY (42 nmol over 5.2 min) caused a 39% reduction in coronary blood flow without changing heart rate or aortic pressure. To determine whether this vasoconstriction could produce ischemia, intramyocardial pH was measured in seven dogs (group I) and decreased from 7.45 +/- 0.06 to 7.37 +/- 0.06 pH units after NPY in the subendocardium (P less than 0.0002), and from 7.45 +/- 0.06 to 7.40 +/- 0.05 pH units (P less than 0.04) in the subepicardium of the infused zone. Left ventricular ejection fraction (LVEF), measured by radionuclide angiography, decreased from 0.52 +/- 0.08 to 0.42 +/- 0.12 U (n = 5, P less than 0.01) during NPY. NPY-induced vasoconstriction was also associated with ST-T wave changes on the electrocardiogram (ECG) in eight of nine other animals (group V). In another group of six dogs (group IV), the change in small vessel resistance accounted for 94% of the increase in total resistance, so that the primary vasoconstrictor effect of NPY was exerted on small coronary arteries. Thus, NPY, a peptide found in human coronary arteries, caused constriction of primarily small coronary arteries that was severe enough to produce myocardial ischemia as determined by ECG ST-T wave changes, and decreases in intramyocardial pH and LVEF in dogs.

Aspirin and prevention of myocardial infarction

Determining the cardioprotective effects of aspirin (acetylsalicylic acid) remains a focus for both basic science and clinical investigation. Although other contributors are probably present, the favorable prostacyclin-to-thromboxane ratio induced by low-dose aspirin appears beneficial for reducing cardiovascular mortality associated with unstable angina and myocardial infarction. The precise dosage, frequency and timing of aspirin's administration to reduce the incidence of vaso-occlusive events remains to be determined. This article reviews aspirin's mechanism of action and use for the prevention of myocardial infarction.

Ambulatory (Holter) electrocardiography and myocardial ischemia

Ambulatory (Holter) electrocardiography has evolved over the past two decades to allow accurate assessment of the cardiac rhythm, and more recently, accurate detection and measurement of ST segment changes. These ambulatory ECG ST segment changes that occur with and without symptoms, although thought to be of questionable clinical value for many years, have recently been clearly documented in coronary artery disease patients to represent true myocardial ischemia. Concurrent with these technologic developments has been an evolution of the pathophysiologic understanding of myocardial ischemia, and the relative role and sequential nature that ECG ST segment changes have in its development. This review examines from a clinical perspective the current understanding of the pathophysiologic sequence of development of myocardial ischemia, emphasizes the ECG diagnostic methods that detect this sequential change, examines the criteria that define ambulatory ECG myocardial ischemia, and discusses those nonischemic factors that affect the ECG ST segment and its interpretation. Moreover, an ever increasing number of ambulatory ECG studies of coronary artery disease and normal patients have defined unique characteristics of the ambulatory ECG ST segment changes observed with regard to its diagnostic, prognostic, and therapeutic assessment value in the study of myocardial ischemia.

Aspirin in cardiovascular disease

Although other mechanisms may be contributory, the antithrombotic properties of aspirin derive predominantly from its platelet-inhibitory effects. These are mediated via irreversible acetylation of platelet cyclo-oxygenase with subsequent blockade of platelet thromboxane synthesis. Long term administration of doses of aspirin as low as 20mg daily depresses platelet thromboxane formation by more than 90%; however, higher doses appear to be necessary to prevent thromboxane-dependent platelet activation in vivo. While there is evidence of biochemical selectivity with low doses of aspirin, significant reduction of the platelet-inhibitory eicosenoid, prostacyclin, occurs even at dosages ranging from 20 to 40mg daily. The ability of aspirin to prevent the occurrence or recurrence of vaso-occlusion has been extensively investigated. In the secondary prevention of myocardial infarction 7 placebo-controlled trials involving more than 15,000 patients have been completed. The dose of aspirin varied from 300 to 1500mg daily. Although none of the individual trials produced statistically significant reductions in total or coronary mortality, taken together the results are highly suggestive of a beneficial effect of aspirin. Similarly, 2 recent studies in patients with unstable angina demonstrated a protective effect of aspirin against acute myocardial infarction and death. While each study employed widely different doses of aspirin (324mg and 1250mg daily) similar reductions in mortality were reported. The effects of aspirin on the prevention of coronary artery bypass graft occlusion have been evaluated in 9 trials. Aspirin in doses of 100 to 975mg daily was shown to be of benefit in preventing early (less than 6 months) graft occlusion, particularly when therapy was started within 24 hours of operation. In patients with prosthetic vascular grafts of the lower limbs, aspirin has been shown to reduce platelet deposition, however further controlled trials will be required to establish the patient population most likely to benefit and, as in all these studies, the optimum dose of aspirin to employ. In patients with prosthetic heart valves it is clear that aspirin alone is insufficient to prevent thromboembolic complications and when administered as an adjunct to anticoagulant therapy it is associated with a high incidence of bleeding. In contrast, there is convincing evidence from several studies for the efficacy of aspirin in doses of 990 to 1300mg daily in the prevention of stroke and death in patients with transient ischaemic attacks.(ABSTRACT TRUNCATED AT 400 WORDS)

Platelets and antiplatelet therapy in ischemic heart disease

Blood platelets have been shown to play an important role not only in thrombosis, but also in the pathogenesis of coronary artery disease and its complications. Drugs that affect platelets have been shown to reduce mortality in survivors of acute myocardial infarction, to reduce the risk of myocardial infarction in patients with unstable angina, and to preserve the potency of saphenous venous grafts used to bypass obstructed coronary arteries. The drugs may also play a role in the primary prevention of arteriosclerosis and in preventing thrombotic complications following coronary angioplasty.

Antianginal drug therapy for silent myocardial ischemia

Many of the available nitrate preparations, beta-adrenergic blockers, and calcium antagonists appear to be useful in patients with painful and silent ischemic episodes detected on the ECG (Table 1). More controlled studies need to be done using standardized methodologies for assessing silent myocardial ischemia, to evaluate and compare the different antianginal medications. It is fortunate, however, that the nitrates, beta-blockers, and calcium antagonists, used alone and in combination, appear to have favorable effects not only on painful ischemic episodes but also on those ischemic episodes not associated with pain.

Reduction in myocardial neutrophil accumulation and infarct size following administration of thromboxane inhibitor U-63,557A

We examined the effects of a new selective thromboxane A2 (TXA2) synthetase inhibitor, U-63,557A, on myocardial infarct size 48 hours following left coronary ligation in rats. With a single 8 mg/kg dose of U-63,557A (furegrelate) administered prior to coronary ligation, platelet aggregation and serum TXA2 formation declined significantly (p less than 0.02) for up to 48 hours. Myocardial infarct size, as measured by planimetry of serial left ventricular sections, was decreased from 44 +/- 3% (saline-treated control rats) to 34 +/- 4% (p less than 0.05). Left ventricular creatine kinase (CK) following coronary ligation was also preserved in U-63,557A vs saline-treated control animals (p less than 0.05). These beneficial effects of U-63,557A were not accompanied by reduction in the indices of myocardial oxygen demand (heart rate and arterial pressure). Furthermore, neutrophil accumulation in the infarcted myocardium was significantly decreased by U-63,557A (26 +/- 6 vs 96 +/- 3/high-power field [p less than 0.01]). These data suggest that administration of a single dose of selective TXA2 synthetase inhibitor prior to coronary ligation modulates platelet function for up to 48 hours and reduces the extent of myocardial injury, which may, in part, relate to decrease in neutrophil accumulation.

Coronary vasoconstrictor response to cold pressor test in variant angina: lack of relation to intracoronary thromboxane concentrations

To test the hypothesis that intracoronary concentrations of thromboxane (Tx)A2 could influence the response to cold pressor test (CPT) in variant angina, great cardiac vein blood flow (by thermodilution) and the concentration of TxB2 (the stable metabolite of TxA2) in the great cardiac vein and aorta were measured under control conditions and during CPT in 14 patients with angina at rest associated with transient ST-segment elevation in the anterior leads. In seven patients pretreated with aspirin (intravenous administration of 3.6 mg/kg lysine salt of acetylsalicylic acid, corresponding to 2 mg/kg aspirin), TxB2 baseline concentrations were lower in both the great cardiac vein (47 +/- 19 vs 176 +/- 88 pg/ml; p less than 0.005) and the aorta (45 +/- 16 vs 109 +/- 56 pg/ml, p less than 0.02) than in seven patients who were not taking cyclooxygenase inhibitors. In the two groups, great cardiac vein flow and anterior region coronary resistance were similar under control conditions. During CPT anterior region coronary resistance increased in patients pretreated with aspirin (from 1.97 +/- 0.99 to 2.22 +/- 1.11 mm Hg/ml/min; p less than 0.02) and in patients without aspirin pretreatment (from 1.94 +/- 0.43 to 2.06 +/- 0.34 mm Hg/ml/min; p less than 0.05), and the difference between the two groups was not statistically significant. Therefore the vasoconstrictor response of coronary vessels to CPT in variant angina is not influenced by the intracoronary TxB2 concentrations and is not modified by aspirin pretreatment.

Management of the total ischemic burden in angina pectoris

Recent reports suggest that neither the severity nor control of angina influences prognosis in patients with coronary heart disease. One possible explanation for such findings is that episodes of angina are only a small fraction of the daily ischemic episodes occurring in these patients. Silent episodes represent most of the ischemic burden in many patients with coronary disease who have positive exercise test results despite the absence of pain. Silent episodes also represent most of the ischemic burden in patients with either stable or unstable angina. Since silent episodes may have prognostic significance, a major goal of therapy should be the modification of both silent and painful ischemic episodes. Currently available pharmacotherapy has the potential to reduce the total ischemic burden caused by both painful and painless attacks and, thereby, alter prognosis.

Louis F. Bishop lecture. Role of coronary artery spasm in symptomatic and silent myocardial ischemia

The revival of the concept of coronary spasm has stimulated research into coronary artery disease. Observations in patients with variant angina have substantially contributed to the appreciation of painless myocardial ischemia. However, the presence or absence of pain during ischemic episodes is not related to the cause of ischemia, because painless ischemia can be observed in variant angina (caused by spasm), in effort-induced angina (caused by increased myocardial demand) and in myocardial infarction (caused by thrombosis). Continuous monitoring initially of patients with variant angina and subsequently of patients with unstable and stable angina proved that often painful and painless ischemic episodes are caused by a transient impairment of regional coronary blood flow rather than by an excessive increase of myocardial demand. The transient impairment of coronary flow appears to be caused by dynamic stenosis of epicardial coronary arteries. This most often occurs at the site of atherosclerotic plaques encroaching on the lumen to a variable extent. Dynamic stenosis can be caused by 1) "physiologic" increase of coronary tone, as in stable angina, 2) spasm, as in variant angina, and 3) thrombosis, usually in combination with "physiologic" changes in tone or with spasm, or both, as in unstable angina. The mechanisms of spasm, as typically observed in variant angina, are different from those of "physiologic" increase of tone; they appear to be related to a local alteration that makes a segment of coronary artery hyperreactive to a variety of constrictor stimuli causing only minor degrees of constriction in other coronary arteries. The nature of this abnormality, which may remain stable for months and years, is yet unknown.

Calcium antagonists for Prinzmetal's variant angina, unstable angina and silent myocardial ischemia: therapeutic tool and probe for identification of pathophysiologic mechanisms

The calcium antagonists provide a unique tool to reduce myocardial oxygen demand and prevent increases in coronary vasomotor tone. For patients with Prinzmetal's variant angina, diltiazem, nifedipine and verapamil are extremely effective in preventing episodes of coronary vasospasm and symptoms of ischemia. Unstable angina pectoris is a more complex pathophysiologic syndrome with episodes of ischemia due to increases in coronary vasomotor tone, intermittent platelet aggregation or alterations in the underlying atherosclerotic plaque. Each of the calcium antagonists is effective as monotherapy in decreasing the frequency of angina at rest. Nifedipine is the only calcium antagonist that has been studied in a combination regimen with beta blockers and nitrates for patients with unstable angina, and control of angina is better with the combination regimen than with either form of therapy alone. Although symptoms of myocardial ischemia in unstable angina are reduced by calcium antagonists, these agents do not seem to decrease the incidence of adverse outcomes. Antiplatelet therapy appears to improve morbidity and mortality in patients with unstable angina, suggesting that thrombus formation may play a central role in that disorder. Episodes of silent or asymptomatic myocardial ischemia, identified by ST-segment monitoring, occur in a variety of disorders of coronary disease. Among patients with Prinzmetal's variant angina and unstable angina, episodes of silent ischemia appear to be as frequent as episodes of angina and the calcium antagonists are effective in decreasing episodes of ischemia regardless of the presence or absence of symptoms. Persisting episodes of silent ischemia among patients with unstable angina despite maximal medical therapy identify patients at high risk for an early unfavorable outcome. Among patients with stable exertional angina, episodes of silent ischemia may be up to 5 times as frequent as episodes of angina, and may be due to increases in coronary vasomotor tone, transient platelet aggregation or increases in myocardial oxygen demand. Preliminary experience suggests that calcium antagonists and beta blockers are effective in decreasing episodes of silent ischemia in patients with stable exertional angina and that a combination regimen may be more effective than either form of therapy alone.

Human leucocyte antigen in variant angina

Human leucocyte antigen analysis of 45 patients with variant angina was performed to determine the presence of causative genetic factor(s). A significantly low frequency of human leucocyte antigen DQ omega 3 was found in these patients, as compared with that in 152 normal Japanese adults. There were no differences in frequencies of antigens between patients with normal and those with atherosclerotic coronaries. These data suggest that some genetic factor(s) may contribute to the pathogenesis of coronary spasm.

Effect of the specific thromboxane receptor blocking drug AH23848 in patients with angina pectoris

The effect of the specific thromboxane receptor blocking drug AH23848 was investigated in two double blind placebo controlled studies in male patients with exercise induced angina pectoris and angiographically verified coronary lesions. In the first study cardiac pacing was performed in twenty patients after coronary angiography. Patients were then randomised into two groups and received either AH23848 (70 mg orally) or placebo. One hour later cardiac pacing was repeated. Neither treatment had any significant effect upon time to angina or the rate-pressure product at the onset of chest pain in these patients. In the second study twenty male patients were randomised to seven days' treatment with AH23848 (70 mg three times a day) or placebo followed by a crossover to the other treatment for a further seven days. Clinical assessment was performed before treatment and at the end of each treatment period. There was no significant difference between the placebo and AH23848 treatment periods in exercise tolerance, the rate-pressure product at angina after exercise testing, the number of ischaemic attacks as determined from 24 hour ambulatory electrocardiograms, the number of attacks of pain, or the number of glyceryl trinitrate tablets consumed. This lack of a clinical effect with AH23848 was seen despite a profound inhibition of ex vivo platelet aggregation stimulated by the thromboxane A2-mimetic U-46619. Because in experimental animals in vivo AH23848 blocks vascular thromboxane receptors as well as platelet thromboxane receptors the lack of effect of AH23848 in cardiac pacing and exercise induced angina is unlikely to be the result of inadequate blockade of thromboxane receptors. The lack of effect of the drug is more likely to indicate that thromboxane A2, is not a factor in the aetiology of the pain experienced by these patients during exercise or cardiac pacing.

Coronary artery thrombosis and elevated urine immunoreactive thromboxane B2

Immunoreactive thromboxane B2 (i-TXB2) was measured by radio-immunoassay (RIA) in urines collected over eight hours on the day of admission in 25 patients who were admitted with the diagnosis of myocardial infarction. In 16 of the patients myocardial infarction was confirmed by ECG and plasma enzymes. Another patient presented with pulmonary embolism and the remaining eight patients had angina pectoris. A further eight hour urine collection was obtained 24 hours later from eleven of the sixteen patients with myocardial infarction. In these eleven patients myocardial infarction was associated with five fold higher urine i-TXB2 (2.72 +/- 0.48 ng/ml) at the day of admission when compared to patients admitted under the same diagnosis but found to have angina only (0.51 +/- 0.08 ng/ml, p less than 0.001). In patients with myocardial infarction the urine i-TXB2 values were reduced 24 hours later (1.58 +/- 0.27 ng/ml, p less than 0.01). One patient was followed with urine i-TXB2 from three days prior to diagnosis of myocardial infarction and to one day prior to a second infarction. In this patient i-TXB2 was highest three days prior to infarction. We conclude that this early elevation of urine i-TXB2 three days prior to diagnosis of infarction and the increased i-TXB2 in patients with myocardial infarction when compared to patients with angina suggest thromboxane is probably released from activated platelets prior to infarction. We suggest that urine i-TXB2 may be of value in the differential diagnosis between myocardial infarction and angina.

Clinical aspects of silent myocardial ischemia in patients with angina and other forms of coronary heart disease

Silent myocardial ischemia is defined as objective evidence of myocardial ischemia occurring in the absence of symptoms in a patient who has either coronary artery disease or spasm. The largest number of patients with silent myocardial ischemia identified to date are those with effort angina, in whom silent episodes are at least three to four times as frequent as painful episodes. Since episodes of silent myocardial ischemia are not alarming to the patient and are likely to escape detection, their clinical importance must reside in their association with morbid events of ischemic heart disease. In fact, silent myocardial ischemia can result in silent myocardial infarction, sudden death, or other events. The detection of these episodes has the potential to provide clinicians with a more complete understanding of all the ischemic processes occurring in patients with angina or other forms of coronary disease. Silent myocardial ischemia can be detected by a variety of methods. Transient ischemic-type electrocardiographic abnormalities that occur without symptoms and that have been recorded during exercise testing or by means of ambulatory, coronary care unit, or telemetry monitoring may be used to document silent myocardial ischemia. Reversible left ventricular wall motion abnormalities without symptoms, observed at rest or during exercise radionuclide angiography, 2-D echocardiography, or contrast angiography, may also be used to document silent myocardial ischemia. However, only ambulatory electrocardiographic monitoring provides long-term evaluation that documents repeated episodes of silent myocardial ischemia outside of the laboratory. Controlled studies, directed specifically at preventing silent myocardial ischemia and/or its consequences in patients with angina, are limited. It has been determined, however, that both hourly administration of sublingual nitroglycerin and intravenous administration of isosorbide dinitrate are effective. Other studies examining the effects of beta blockers (propranolol, atenolol, and labetalol) in the treatment of angina have shown that their use also results in a reduced frequency of silent episodes, but that a large number of episodes persist. It has been observed that verapamil alone and nifedipine, either alone or in combination with a beta blocker, appear to significantly decrease silent myocardial ischemia in patients with angina. A major limitation of these studies is that treatment was not specifically titrated to reduce or eliminate silent myocardial ischemia.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective thromboxane A2 synthetase inhibition in vasospastic angina pectoris

To investigate whether thromboxane A2 is responsible for the initiation of vasospastic angina pectoris, thromboxane B2 levels were measured in the great cardiac vein and the arterial blood of 12 patients with clinically and angiographically proved vasospastic angina and therapeutic trials were performed with selective thromboxane A2 synthetase inhibitor OKY-046, an imidazole derivative. During ergonovine-provoked (11 cases) and spontaneous (1 case) anginal attacks, great cardiac vein thromboxane B2 increased from 121 +/- 27 to 430 +/- 382 pg/ml (p less than 0.05, n = 12), arterial thromboxane B2 increased from 93 +/- 18 to 122 +/- 33 pg/ml (NS, n = 12) and thromboxane B2 production increased from 3.18 +/- 1.88 to 25.16 +/- 22.32 ng/min (p less than 0.05, n = 6). Subsequently, OKY-046, 400 mg/day orally, was administered to 7 of the 12 patients, while a continuous electrocardiogram was recorded on a dual channel Holter monitor during a 3 day placebo period and the 3 day OKY-046 regimen. Although peripheral plasma thromboxane B2 levels decreased significantly from 98 +/- 15 to 12 +/- 8 and 28 +/- 10 pg/ml (1 and 6 hours after ingestion, respectively) (p less than 0.05 for both), 6-keto-prostaglandin F1 alpha production in serum increased significantly from 0.48 +/- 0.22 to 2.3 +/- 0.72 (1 hour) and 1.8 +/- 0.46 ng/ml (6 hours) (p less than 0.05 for both) during OKY-046 administration.(ABSTRACT TRUNCATED AT 250 WORDS)