Effects of nitroglycerin at therapeutic doses on platelet aggregation in unstable angina pectoris and acute myocardial infarction

Antianginal Therapy for Stable Ischemic Heart Disease

Chronic angina pectoris is associated with considerable morbidity and mortality, especially if treated suboptimally. For many patients, aggressive pharmacologic intervention is necessary in order to alleviate anginal symptoms. The optimal treatment of stable ischemic heart disease (SIHD) should be the prevention of angina and ischemia, with the goal of maximizing both quality and quantity of life. In addition to effective risk factor modification with lifestyle changes, intensive pharmacologic secondary prevention is the therapeutic cornerstone in managing patients with SIHD. Current guidelines recommend a multifaceted therapeutic approach with β-blockers as first-line treatment. Another important pharmacologic intervention for managing SIHD is nitrates. Nitrates can provide both relief of acute angina and can be used prophylactically before exposure to known triggers of myocardial ischemia to prevent angina. Additional therapeutic options include calcium channel blockers and ranolazine, an inhibitor of the late inward sodium current, that can be used alone or in addition to nitrates or β-blockers when these agents fail to alleviate symptoms. Ranolazine appears to be particularly effective for patients with microvascular angina and endothelial dysfunction. In addition, certain antianginal therapies are approved in Europe and have been shown to improve symptoms, including ivabradine, nicorandil, and trimetazidine; however, these have yet to be approved in the United States. Ultimately, there are several different medications available to the physician for managing the patient with SIHD having chronic angina, when either used alone or in combination. The purpose of this review is to highlight the most important therapeutic approaches to optimizing contemporary treatment in response to individual patient needs.

Nitrates for the prevention of cardiac morbidity and mortality in patients undergoing non-cardiac surgery

BACKGROUND

Cardiac complications are not uncommon in patients undergoing non-cardiac surgery, especially in patients with coronary artery disease (CAD) or at high risk of CAD. Perioperative cardiac complications can lead to mortality and morbidity, as well as higher costs for patient care. Nitrates, which are among the most commonly used cardiovascular drugs, perform the function of decreasing cardiac preload while improving cardiac blood perfusion. Sometimes, nitrates are administered to patients undergoing non-cardiac surgery to reduce the incidence of cardiac complications, especially for patients with CAD. However, their effects on patients' relevant outcomes remain controversial.

OBJECTIVES

• To assess effects of nitrates as compared with other interventions or placebo in reducing cardiac risk (such as death caused by cardiac factors, angina pectoris, acute myocardial infarction, acute heart failure and cardiac arrhythmia) in patients undergoing non-cardiac surgery.• To identify the influence of different routes and dosages of nitrates on patient outcomes.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and the Chinese BioMedical Database until June 2014. We also searched relevant conference abstracts of important anaesthesiology or cardiology scientific meetings, the database of ongoing trials and Google Scholar.We reran the search in January 2016. We added three potential new studies of interest to the list of 'Studies awaiting classification' and will incorporate them into our formal review findings for the review update.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) comparing nitrates versus no treatment, placebo or other pharmacological interventions in participants (15 years of age and older) undergoing non-cardiac surgery under any type of anaesthesia.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures as expected by Cochrane. Two review authors selected trials, extracted data from included studies and assessed risk of bias. We resolved differences by discussion and, when necessary, sought help and suggestions from a third review author. We used a random-effects model for data analysis.

MAIN RESULTS

We included 27 randomized controlled trials (RCTs) (8244 participants analysed). Investigators reported 12 different comparisons of three different nitrates (nitroglycerin, isosorbide dinitrate and nicorandil) versus no treatment, placebo or other pharmacological interventions. All participants were older than 15 years of age. More than half of the trials used general anaesthesia. Surgical procedures in most trials were at low to moderate risk for perioperative cardiac complications. Only two comparisons including three studies reported the primary outcome - all-cause mortality up to 30 days post operation. Researchers reported other morbidity outcomes and adverse events in a variable and heterogeneous way, resulting in limited available data for inclusion in the meta-analysis. We determined that the overall methodological quality of included studies was fair to low, in accordance with risk of bias in most domains.In summary, we found no difference in the primary outcome - all-cause mortality up to 30 days post operation - when nitroglycerin was compared with no treatment (one study, 60 participants, 0/30 vs 1/30; (risk ratio (RR) 0.33, 95% confidence interval (CI) 0.01 to 7.87, very low-quality evidence based on GRADE criteria) or with placebo (two studies, 89 participants, 1/45 vs 0/44; RR 2.81, 95% CI 0.12 to 63.83, very low-quality evidence). Regarding our secondary outcomes, we noted no statistically significant differences in angina pectoris, acute myocardial infarction, acute heart failure, cardiac arrhythmia or cardiac arrest in any comparisons. In comparisons versus nitroglycerin, although more events of cardiac ischaemia were observed in participants receiving no treatment or placebo, we found no statistically significant differences in any comparisons, except the comparison of nicorandil versus placebo. One study revealed a potential dose-dependent protective effect of nicorandil for cardiac ischaemia.Adverse events were reported in a heterogeneous way among the comparisons. In general, more participants treated with nitrates had hypotension, tachycardia and headache, but investigators reported no statistically significant differences between groups in any comparisons.

AUTHORS' CONCLUSIONS

This systematic review suggests that nitroglycerin or isosorbide dinitrate is not associated with improvement in mortality and cardiac complications among patients undergoing non-cardiac surgery. Limited evidence suggests that nicorandil may reduce the risk of cardiac ischaemia in participants undergoing non-cardiac surgery. Additional studies are needed to consolidate the evidence.However, the data included in many of the analyses in this review are sparse - that is, adequate data are few - resulting in very low power to detect differences between nitrates and comparators. Thus, a more objective conclusion would state that available evidence is insufficient to show whether nitrates are associated with improvement in mortality and cardiac complications among patients undergoing non-cardiac surgery.Over the past decade, no high-quality studies have focused on association of cardiac mortality and morbidity with use of nitrates during non-cardiac surgery. This review underlines the need for well-designed trials in this field.

Vasodilator Therapy: Nitrates and Nicorandil

Nitrates have been used to treat symptoms of chronic stable angina for over 135 years. These drugs are known to activate nitric oxide (NO)-cyclic guanosine-3',-5'-monophasphate (cGMP) signaling pathways underlying vascular smooth muscle cell relaxation, albeit many questions relating to how nitrates work at the cellular level remain unanswered. Physiologically, the anti-angina effects of nitrates are mostly due to peripheral venous dilatation leading to reduction in preload and therefore left ventricular wall stress, and, to a lesser extent, epicardial coronary artery dilatation and lowering of systemic blood pressure. By counteracting ischemic mechanisms, short-acting nitrates offer rapid relief following an angina attack. Long-acting nitrates, used commonly for angina prophylaxis are recommended second-line, after beta-blockers and calcium channel antagonists. Nicorandil is a balanced vasodilator that acts as both NO donor and arterial K(+) ATP channel opener. Nicorandil might also exhibit cardioprotective properties via mitochondrial ischemic preconditioning. While nitrates and nicorandil are effective pharmacological agents for prevention of angina symptoms, when prescribing these drugs it is important to consider that unwanted and poorly tolerated hemodynamic side-effects such as headache and orthostatic hypotension can often occur owing to systemic vasodilatation. It is also necessary to ensure that a dosing regime is followed that avoids nitrate tolerance, which not only results in loss of drug efficacy, but might also cause endothelial dysfunction and increase long-term cardiovascular risk. Here we provide an update on the pharmacological management of chronic stable angina using nitrates and nicorandil.

Platelets and Cardiovascular Disease

Platelets play an important, but often under-recognized role in cardiovascular disease. For example, the normal response of the platelet can be altered, either by increased pro-aggregatory stimuli or by diminished anti-aggregatory substances to produce conditions of increased platelet activation/aggregation and occur in active cardiovascular disease states both on a chronic (e.g. stable angina pectoris) and acute basis (e.g. acute myocardial infarction). In addition, platelet hyperaggregability is also associated with the risk factors for coronary artery disease (e.g. smoking, hypertension, and hypercholesterolaemia). Finally, the utility of an increasing range of anti-platelet therapies in the management of the above disease states further emphasizes the pivotal role platelets play in the pathogenesis of cardiovascular disease. This paper provides a comprehensive overview of the normal physiologic role of platelets in maintain homeostasis, the pathophysiologic processes that contribute to platelet dysfunction in cardiovascular disease and the associated role and benefits of anti-platelet therapies.

Role of short-acting nitroglycerin in the management of ischemic heart disease

Nitroglycerin is the oldest and most commonly prescribed short-acting anti-anginal agent; however, despite its long history of therapeutic usage, patient and health care provider education regarding the clinical benefits of the short-acting formulations in patients with angina remains under-appreciated. Nitrates predominantly induce vasodilation in large capacitance blood vessels, increase epicardial coronary arterial diameter and coronary collateral blood flow, and impair platelet aggregation. The potential for the prophylactic effect of short-acting nitrates remains an under-appreciated part of optimal medical therapy to reduce angina and decrease myocardial ischemia, thereby enhancing the quality of life. Short-acting nitroglycerin, administered either as a sublingual tablet or spray, can complement anti-anginal therapy as part of optimal medical therapy in patients with refractory and recurrent angina either with or without myocardial revascularization, and is most commonly used to provide rapid therapeutic relief of acute recurrent angina attacks. When administered prophylactically, both formulations increase angina-free walking time on treadmill testing, abolish or delay ST segment depression, and increase exercise tolerance. The sublingual spray formulation provides several clinical advantages compared to tablet formulations, including a lower incidence of headache and superiority to the sublingual tablet in terms of therapeutic action and time to onset, while the magnitude and duration of vasodilatory action appears to be comparable. Furthermore, the sublingual spray formulation may be advantageous to tablet preparations in patients with dry mouth. This review discusses the efficacy and utility of short-acting nitroglycerin (sublingual spray and tablet) therapy for both preventing and aborting an acute angina attack, thereby leading to an improved quality of life.

Nitroglycerin and sodium nitroprusside: potential contributors to postoperative bleeding?

Postoperative bleeding is common in patients undergoing cardiac surgery with cardiopulmonary bypass. Most cases of severe postoperative bleeding not due to incomplete surgical hemostasis are related to acquired transient platelet dysfunction mediated by platelet activation during contact with the synthetic surfaces of the cardiopulmonary bypass equipment. Antihypertensive agents nitroglycerin and sodium nitroprusside have been shown to have platelet inhibitory properties, yet the clinical consequence in terms of postoperative bleeding has been little studied. Knowing that cardiopulmonary bypass causes platelet dysfunction, it is prudent for physicians to be aware of the additional platelet inhibition caused by these commonly used antihypertensive agents.

Neuro- and cardioprotective effects of blockade of nitric oxide action by administration of methylene blue

Methylene blue (MB), generic name methylthioninium (C(16)H(18)ClN(3) S . 3H(2)O), is a blue dye synthesized in 1876 by Heinrich Caro for use as a textile dye and used in the laboratory and clinically since the 1890s, with well-known toxicity and pharmacokinetics. It has experimentally proven neuroprotective and cardioprotective effects in a porcine model of global ischemia-reperfusion in experimental cardiac arrest. This effect has been attributed to MB's blocking effect on nitric oxide synthase and guanylyl cyclase, the latter blocking the synthesis of the second messenger of nitric oxide. The physiological effects during reperfusion include stabilization of the systemic circulation without significantly increased total peripheral resistance, moderately increased cerebral cortical blood flow, a decrease of lipid peroxidation and inflammation, and less anoxic tissue injury in the brain and the heart. The last two effects are recorded as less increase in plasma concentrations of astroglial protein S-100beta, as well as troponin I and creatine kinase isoenzyme MB, respectively.

Redox state, oxidative stress and endothelial dysfunction in heart failure: the puzzle of nitrate–thiol interaction

Endothelial dysfunction, a critical component in the progression of heart failure, may result from increased oxidative stress, secondary to activation of the adrenergic and the renin-angiotensin systems and to the production of inflammatory cytokines, which in turn contribute to reduced bioavailability of nitric oxide (NO). Oxidative stress, determined by excess production of reactive oxygen species and impairment in the antioxidant defence, is responsible for both the decline of diffusible NO and the decrease in the concentration of essential co-factors of NO synthases. Reactive oxygen species are formed from NO in the presence of oxidants and are involved in the nitration of protein tyrosine residue that can alter protein function. Recent studies re-addressed the impact of nitrate treatment in heart failure in view of the beneficial vascular and cellular effects of NO, and of the discovery of abnormalities in NO pathways in this disease. Concerns exist, however, on the safety of nitrates in this setting. Nitrates stimulate vascular superoxide anion production via activation of NADPH oxidase, and induction of uncoupling of NO synthase. Furthermore, by using donors of sulfhydryl groups, such as cysteine and glutathione, for NO production, nitrates may favour depletion of the intracellular thiol pool, thus impairing the antioxidant defence mechanisms.

Impaired tissue responsiveness to organic nitrates and nitric oxide: a new therapeutic frontier?

Nitric oxide (NO) is a physiologically important modulator of both vasomotor tone and platelet aggregability. These effects of NO are predominantly mediated by cyclic guanosine-3,'5'-monophosphate (cGMP) via activation of soluble guanylate cyclase. However, in patients with ischemic heart disease, platelets and coronary/peripheral arteries are hyporesponsive to the antiaggregatory and vasodilator effects of NO donors. NO resistance is also associated with a number of coronary risk factors and presents in different disease states. It correlates with conventional measures of "endothelial dysfunction," and represents a multifaceted disorder, in which smooth muscle and platelet NO resistance are equally important, as sites of abnormal NO-driven physiology. NO resistance results largely from a combination of "scavenging" of NO by superoxide anion radical (O(2)(-)) and of (reversible) inactivation of soluble guanylate cyclase. It constitutes an impaired physiological response to endogenous NO (endothelium-derived relaxing factor, EDRF) and, as such, may contribute to the increased risk of ischemic events. Impairment in responsiveness to NO in ischemic patients implies a potential problem that those patients, in greatest need of nitrate therapy, may be least likely to respond. The prognostic impact of NO resistance at vascular and platelet levels has been demonstrated in patients with ischemic heart disease, and it has been shown that a number of agents (angiotensin-converting enzyme [ACE] inhibitors, perhexiline, insulin, and possibly statins) ameliorate this anomaly. The current review examines different aspects of the "NO resistance" phenomenon and discusses some related methodological issues.

Aspirin resistance: moving forward with multiple definitions, different assays, and a clinical imperative

Many patients are treated with aspirin to prevent a serious vascular event, most notably myocardial infarction and stroke. A growing number of studies have been appearing in the literature that indicate a significant fraction of aspirin-treated patients may be resistant to the antiplatelet effects of the drug. Resistance to aspirin may be related to the concomitant ingestion of aspirin and nonsteroidal anti-inflammatory drugs, which impairs the aspirin effect, or to more complex situations, such as metabolic defects that diminish the therapeutic effect of aspirin. The incidence of aspirin resistance is unknown, but it may approach 20% to 30%. The diagnosis of aspirin resistance has been evaluated using multiple assays, and because there are multiple assays, this has resulted in multiple definitions for aspirin resistance. This review considers aspirin resistance at a time when there is confusion about the definition, and the clinical assay to best assess it, because there is a clinical imperative to know now which patients taking aspirin are not receiving a therapeutic effect.

Methylthioninium chloride: pharmacology and clinical applications with special emphasis on nitric oxide mediated vasodilatory shock during cardiopulmonary bypass

Vasodilatory shock after cardiopulmonary bypass is a common complication requiring treatment with high doses of inotropes and prolonged stays in the intensive care unit. The vasodilatory shock is initiated by an inflammatory response to the extracorporeal circuit. The inflammatory response results in endothelial synthesis and release of nitric oxide resembling the clinical features observed in vasodilatory shock caused by septicaemia. During vasodilatory shock, the inhibition of nitric oxide synthase and the nitric oxide/cyclic guanylyl monophosphate pathway is an attractive adjunct to therapy with traditional inotropes. Methylthioninium chloride inhibits nitric oxide/cyclic guanylyl monophosphate mediated vasodilation and can successfully be used as a supplement in the treatment of vasodilatory shock associated with cardiopulmonary bypass. The application of methylthioninium chloride in septicaemia has not produced comparable positive clinical results.

The effect of hypercholesterolemia on platelet soluble guanylyl cyclase

We sought to determine whether hypercholesterolemia impacts on the NO-stimulated activity of platelet soluble guanylyl cyclase (sGC). We investigated two groups of nine New Zealand white rabbits receiving either a standard (NC) or a cholesterol chow (HC, 0.75%) for 15 weeks. The plasma content of cGMP and the specific activity of sGC in intact platelets were measured by a cGMP-specific radioimmunoassay. In HC, 47.9+/-3.1% of the aortic intimal area was covered with atherosclerotic lesions, and plasma cGMP levels (pmol/ml) were increased from 12.6+/-1.2 to 27.9+/-3.5 (P<.0001). In striking contrast, hypercholesterolemia had no effect on sGC activity stimulated by the NO donor SNAP. At 100 microM SNAP, the specific activities of sGC (pmol/10(9) platelets/min) were 81.8+/-14.5 in NC and 86.2+/-8.1 in HC. Basal sGC activity (pmol/10(9) platelets/min) was also similar in NC (0.21+/-0.04) and HC (0.460+/-0.11, P=.7813). In accordance, washed platelets from both groups showed a similar SNAP-induced inhibition of aggregation. These data suggest that an impaired response of platelets to NO is most likely not involved in platelet hyperreactivity in hypercholesterolemia.

Nitric oxide, platelet function, myocardial infarction and reperfusion therapies

Platelets play an important role in physiologic hemostasis and pathologic thrombosis that complicate the course of vascular disorders. A number of platelet functions including adhesion, aggregation and recruitment are controlled by nitric oxide (NO) generated by platelets and the endothelial cells. Derangements in this generation may contribute to the pathogenesis of thrombotic complications of vascular disorders. The pharmacologic supplementation of the diseased vasculature with drugs releasing NO may help to restore the hemostatic balance.

Long-term smoking causes nitroglycerin resistance in platelets by depletion of intraplatelet glutathione

We investigated whether platelet responsiveness to nitroglycerin (NTG) is maintained in long-term smokers and if not, the mechanism. In the absence or presence of NTG, intraplatelet reduced glutathione (GSH) levels and ADP-induced platelet aggregation and intraplatelet cGMP levels were measured in 10 long-term smokers and 10 age-matched nonsmokers. The intraplatelet GSH level was significantly lower in smokers than in nonsmokers (P<0.05). Platelet aggregation was dose-dependently inhibited by NTG in both groups; however, inhibition was significantly weaker in smokers. N-acetylcysteine (1 mmol/L), an exogenous thiol agent, significantly potentiated NTG-induced platelet inhibition in nonsmokers but not in smokers. The ADP-induced intraplatelet cGMP level was significantly greater in the presence of NTG in nonsmokers but not so in smokers. Because the effects of long-term smoking are multifactorial, a rabbit model was made by chronic administration of buthionine sulfoximine (BSO, n=6) to decrease intraplatelet GSH. The intraplatelet GSH level was significantly lower in BSO-treated rabbits than in saline-treated rabbits (P<0.001). The NTG-induced inhibition of platelet aggregation was significantly weaker in BSO rabbits. N-acetylcysteine-induced potentiation was not observed in BSO rabbits, whereas significant potentiation was found in saline rabbits. These findings were similar to those of long-term smokers. In contrast, the intraplatelet GSH-to-oxidized glutathione ratio, which represents the redox state of glutathione, was significantly lower in smokers than in nonsmokers, whereas no difference was found between saline rabbits and BSO rabbits. In conclusion, long-term smoking causes NTG resistance to aggregation in platelets, possibly through the depletion of intraplatelet GSH.

Studies on inhibition of human platelet function by sodium nitroprusside. Kinetic evaluation of the effect on aggregation and cyclic nucleotide content

In this study, we explored the ability of sodium nitroprusside to inhibit the aggregation of human platelets in platelet-rich plasma (PRP) and whole blood and its effects on intracellular levels of guanosine 3',5'-cyclic monophosphate (cGMP) and adenosine 3',5'-cyclic monophosphate (cAMP). The experiments investigated dose-dependent effects of nitroprusside starting from concentrations in the range of circulating levels achievable in vivo during drug administration in humans. Furthermore, we investigated the time-course of both antiaggregating action and the influence on cyclic nucleotide synthesis. Results showed that sodium nitroprusside inhibited the aggregation induced by adenosine 5-diphosphate (ADP) and collagen starting from concentration as low as 2 micromol/l. The IC(50) value for ADP-induced aggregation in PRP was 18.7+/-2.4 micromol/l. The inhibition of platelet aggregation showed a time-dependent behaviour and was not reversible within 90 min. The accumulation of intraplatelet cGMP in the presence of sodium nitroprusside exhibited a comparable time-course characterized by an early increase, a steady state and a late further increase. The time-course of cAMP synthesis was very similar to that of cGMP. Our data evidenced a long-lasting inhibition of platelet responses by sodium nitroprusside and excluded a desensitization of platelet guanylyl cyclase after 3-h exposure to nitric oxide (NO). Furthermore, they indicated a role of cAMP accumulation in the antiaggregating effects of nitroso donor: the simultaneous increase of intracellular content of cAMP and cGMP can synergize in the reduction of the platelet responses.

Predicting blood transfusion factors in coronary artery bypass surgery

OBJECTIVE

Blood conservation has become one of the most important issues in cardiac surgery. We clarified preoperative predictors of the need for blood transfusions during coronary artery bypass graft surgery.

METHODS

Subjects were 89 patients--66 men (74%) and 23 women (26%) 40 to 84 years old (mean: 66.2 +/- 8.3 years)--undergoing isolated coronary artery bypass surgery from September 1997 to December 1999. Of these, 66 patients (74%) received transfusion during hospitalization and 23 (26%) did not. Nine risk factors detected by univariate study were entered in a multivariate logistic regression model of the relationship between preoperative variables and blood transfusion.

RESULTS

Independent predictors were emergency surgery (P = .0023), lower hematocrit (P = .0027), older age (P = .0043), and the presence of peripheral vascular disease (P = .0070). Optimal cutoff of hematocrit for blood transfusion was 39% and age 64 years via receiver-operating characteristics curves based on the relation between sensitivity and specificity.

CONCLUSION

Patients older than 64 years with hematocrit less than 39% and/or peripheral vascular disease should be treated routinely using preoperative storage of autologous blood whenever the patient's condition permits. For patients undergoing emergency surgery, further studies are required, including lowering transfusion threshold and using determinants other than hematocrit.

Treatment of Perioperative Myocardial Ischemia

Prevention and treatment of myocardial ischemia re mains a central focus of perioperative care. Myocardial ischemia is best understood in terms of myocardial oxygen supply and demand ratios. Conventional ther apy includes nitrates, β-blockers, and calcium channel blockers. In all 3 drug classes, ischemia is reduced by either improving supply, decreasing demand, or both. More recent investigation evaluates these medications either as prophylactic therapy or as a component of long-term risk reduction for cardiac morbid events. Newer therapies, including anticoagulation, pain ther apy, normothermia, central neuroaxial techniques, and other therapies, are reviewed.

Nitroglycerin rebound associated with vascular, rather than platelet, hypersensitivity

OBJECTIVES

The purpose of this study was to determine whether acute withdrawal of nitroglycerin (NTG) during hemodynamic tolerance is associated with platelet hypersensitivity.

BACKGROUND

Nitroglycerin is an effective antianginal medication but its use is limited by the development of tolerance and rebound. We have previously demonstrated a sustained inhibition of platelet function during continued use of NTG, but whether cessation of NTG is associated with an increase in platelet function that may contribute to rebound is unknown.

METHODS

Normal porcine aortic media were exposed to flowing arterial blood from pigs (n = 8) treated continuously with NTG patches (Nitrodur 0.8 mg/h) for 48 h. Platelet function, blood pressure and the responses to angiotensin II infusion were evaluated before, during and after NTG treatment.

RESULTS

Mean arterial pressure fell by 15% after 3 h of treatment compared with control, returned to baseline by 48 h and increased significantly 2 h after drug removal. Autologous 51Cr-labelled platelet deposition on the aortic media was reduced by 30% after 48 h of continuous NTG administration compared with baseline (p = 0.02) and remained decreased 2 h after cessation of NTG therapy. Platelet aggregation to thrombin decreased in parallel to the decrease in platelet deposition. Blood pressure increase after intravenous injection of 10 microg of angiotensin II was blunted during treatment with NTG but increased significantly 2 h after cessation of nitrate therapy when compared with baseline.

CONCLUSIONS

Supersensitivity of the vessel wall to vasoconstrictors such as angiotensin 11, but not platelet hyperactivity, may contribute to the rebound phenomenon after acute nitrate withdrawal.

Nitric oxide donors

Nitric oxide (NO) donors are pharmacologically active substances that release NO in vivo or in vitro. NO has a variety of functions such as the release of prostanoids, inhibition of platelet aggregation, effect on angiogenesis, and production of oxygen free radicals. This report discusses the chemical and pharmacological characteristics of NO donors, their effect on platelet function and cyclooxygenase, their cardiac action including myocardial infarction, and release of superoxide anions. This review stresses NO tolerance and the effect of NO donors on angiogenesis in myocardial infarction and in solid tumors.

Isosorbide dinitrate inhibits platelet adhesion and aggregation in nonthrombolyzed patients with acute myocardial infarction

BACKGROUND

Apart from their vasodilatatory properties, nitrates have been shown to inhibit platelet aggregation. The effects of nitrates on platelet adhesion have not been studied. Nonselected patients with acute myocardial infarction (AMI) have been suggested to gain no benefit from administration of nitrates. However, the importance of nitrates may be greater in a subgroup of nonthrombolyzed patients with AMI.

HYPOTHESIS

Isosorbide dinitrate (ISDN) decreases platelet adhesion and aggregation in nonthrombolyzed patients with AMI.

METHODS

Consecutive 48 men with AMI, not eligible for thrombolytic therapy because of late presentation (> 12 h), were prospectively randomized 2:1 to double-blind ISDN (mean dose 2.4 +/- 0.9 mg/h) (n = 33) or placebo (0.9% sodium chloride) (n = 15) infusion. All patients received aspirin. Blood samples were taken at baseline (no study medication) and 3 h into ISDN or placebo infusion. Platelet adhesion to collagen was measured in the ethylene diamine tetraacetic acid (EDTA)-platelet rich plasma by recording changes in light transmission with an optical aggregometer. Platelet aggregation was measured using the Born's method.

RESULTS

Isosorbide dinitrate significantly decreased both platelet adhesion and aggregation. No effect was seen in the placebo group.

CONCLUSIONS

In patients with AMI who do not receive thrombolytic therapy, ISDN effectively inhibits platelet adhesion and aggregation. These effects of nitrates may be of therapeutic and prognostic significance in this group of patients.

Platelet Activation Determined by Flow Cytometry Persists Despite Antithrombotic Therapy in Patients with Unstable Angina and Non-Q-Wave Myocardial Infarction

Background: Current strategies in the treatment of patients with acute coronary syndromes include antiplatelet agents and thrombin antagonists, most commonly aspirin and heparin, respectively. Cardiac events, however, occur despite what is considered to be maximal medical treatment. Methods: We determined the percentage of activated platelets in whole blood samples taken from 22 patients with unstable angina and non-Q-wave myocardial infarection participating in the TIMI III B trial. Platelet activation was assessed using a monoclonal antibody to the surface-expressed alpha-granule protein, P-selectin, and flow cytometry. All patients received a full complement of antiischemic medications as well as intravenous heparin and oral aspirin, and were then randomized to tissue plasminogen activator or placebo. Results: Platelet activation prior to randomization was increased threefold to fourfold compared with healthy volunteers (11.4 +/- 11.4% vs. 2.0%; p < 0.01). Serial measurements performed 12, 24, 48, and 96 hours after treatment initiation revealed that platelet activation persisted. No differences in patients experiencing recurrent ischomic events (n = 9) or those randomized to a 90-minute, accelerated infusion of tissue plasminogen activator (n = 12) were observed. Conclusions: A modest degree of platelet activation is seen for at least 96 hours and possibly longer in patients with unstable angina and non-Q-wave myocardial infarction, despite being treated with intravenous heparin and oral aspirin. These findings support current efforts to identify more potent and selective antithrombotic treatment strategies.

Intravenous aspirin at reperfusion does not reduce infarct size in the dog with a residual critical stenosis

Platelet-related events being associated with the increment of infarct size at reperfusion in the presence of a residual stenosis, we tested in dogs whether intravenous aspirin (ASA) could limit infarct size. The left anterior descending coronary artery was occluded for 90 min and reperfused for 6 h in the presence of a residual critical stenosis. Controls received saline, and treated groups were given 2, 6, or 12 mg/kg ASA, i.v., 5 min before reperfusion. Infarct size did not differ significantly between groups (control, 43.80+/-6.28%; ASA, 2 mg/kg: 41.07+/-7.78%; ASA, 6 mg/kg: 37.55+/-3.44%; ASA, 12 mg/kg: 29.40+/-5.41%), as well as transmural collateral blood flow and [111In]-platelet accumulation in the infarcted myocardium (2.5-3.6 x 10(5) platelets/g). However, myocardial neutrophil accumulation was significantly reduced (p < 0.05) in groups given 6 (15.0+/-2.6 x 10(6)/g tissue) and 12 mg/kg (18.4 +/-3.8) ASA, but not in the 2-mg/kg group (21.0+/-5.2), as compared with control group (32.0+/-7.2). Ex vivo platelet aggregation to collagen was abolished during reperfusion in all treated groups (p < 0.05). Transcardiac arteriovenous differences in 6-keto-PGF1alpha were reduced significantly 1 h after reperfusion in groups given 6 or 12 mg/kg ASA (94.7+/-13.1 and 71.7+/-19.2 pg/ml, respectively) but not in the 2-mg/kg group (178.3+/-78.2 pg/ml), as compared with control (405.4+/-171.6 pg/ml). ASA-insensitive platelet activation at the site of stenosis or inhibition by ASA of prostacyclin production by jeopardized myocardium may explain the observed lack of benefit of ASA.

Nitrate resistance in platelets from patients with stable angina pectoris

BACKGROUND

Hemodynamic resistance to nitrates has been previously documented in congestive heart failure. In patients with stable angina pectoris (SAP), we have observed a similar phenomenon: decreased platelet response to disaggregating effects of nitroglycerin (NTG) and sodium nitroprusside (SNP).

METHODS AND RESULTS

In blood samples from normal subjects (n=32) and patients with SAP (n=56), we studied effects of NO donors (NTG and SNP) on ADP-induced platelet aggregation and on intraplatelet cGMP. NTG and SNP inhibited platelet aggregation in patients to lesser extents than in normal subjects (P<0.01). The cGMP-elevating efficacy of NTG and SNP was diminished in platelets from patients in comparison with those from normals (P<0.001). Inhibition of the anti-aggregatory effects of NTG and SNP by ODQ, a selective inhibitor of NO-stimulated guanylate cyclase, was significantly less pronounced in patients than in normal subjects. Content of O2- was higher in blood samples from patients than in those from normal subjects (P<0. 01). In blood samples from patients with SAP, but not in normal subjects, the O2- scavenger superoxide dismutase (combined with catalase) suppressed platelet aggregation (P<0.01) and increased the extent of anti-aggregatory effect of SNP (P<0.01).

CONCLUSIONS

In patients with SAP, platelets are less responsive to the anti-aggregating and cGMP-stimulating effects of NO donors; this may reflect both reduction in guanylate cyclase sensitivity to NO and inactivation of the released NO by O2-. The implied impairment of anti-platelet efficacy of endogenous NO (in the form of EDRF) may contribute to platelet hyperaggregability associated with angina pectoris.

"Traditional" medical therapy for unstable angina. How important? How to use?

Unstable angina comprises a heterogeneous population of patients who present with a wide spectrum of underlying pathophysiology. The traditional treatment of these patients is based on both evidenced-based medicine as well as clinical experience. Despite the large population of patients admitted with this diagnosis, the scientific literature regarding its treatment is scarce. Therefore, the management of patients with unstable angina relies heavily on the clinical skills of the physician. One of the most important steps in this process involves risk stratification, especially in the current environment of cost containment. Those patients who are at low risk for adverse outcomes can be treated and evaluated safely as outpatients. Patients at high or moderate risk, however, should be treated intensively as inpatients. Although there appear to be many new promising therapies for unstable angina on the horizon, the traditional therapies still have a place. The use of aspirin in this population is well supported by the literature and appears to have a positive effect on mortality and cardiovascular events. The other traditional therapies, however, are not as well supported by the literature. They do appear to benefit the patient in terms of reducing symptoms, but their effects on reducing mortality and cardiovascular events are not clear. Therefore, the goal of medical therapy in this patient population should be to stabilize them so that they can proceed with an appropriate risk stratification procedure as soon as possible. This is especially true with performing coronary angiography or interventions because the risk of procedural complications is higher in patients with unstable angina and ongoing symptoms.

Effect of atherosclerosis on endothelium-dependent inhibition of platelet activation in humans

BACKGROUND

We investigated whether luminal release of nitric oxide (NO) contributes to inhibition of platelet activation and whether these effects are reduced in patients with atherosclerosis.

METHODS AND RESULTS

Femoral blood flow velocity and ex vivo whole blood platelet aggregation by impedance aggregometry were measured in femoral venous blood during femoral arterial infusion of acetylcholine (ACh; 30 microg/min) in 30 patients, 19 of whom had angiographic atherosclerosis. Measurements were repeated with sodium nitroprusside (40 microg/min), L-arginine (160 micromol/min), and N(G)-monomethyl-L-arginine (L-NMMA; 16 micromol/min). There was significant inhibition of collagen-induced platelet aggregation with ACh (45+/-9.5% lower, P<0.001), and this inhibition was greater in patients without atherosclerosis (68.7+/-10.4% reduction) than in those with atherosclerosis (32.5+/-8.1%, P=0.04). The magnitude of inhibition correlated with vasodilation with ACh, indicating an association between the smooth muscle and antiplatelet effects of endothelium-dependent stimulation. Neither L-NMMA nor sodium nitroprusside altered platelet aggregation. L-Arginine inhibited platelet aggregation equally in vitro (34+/-8% reduction, P<0.01) and in vivo (37+/-13% reduction, P<0.01).

CONCLUSIONS

Stimulation of NO release into the vascular lumen with ACh inhibits platelet aggregation, an effect that is attenuated in patients with atherosclerosis and endothelial dysfunction. Basal NO release does not appear to contribute to platelet passivation in vivo. L-Arginine inhibited platelet aggregation by its direct action on platelets. These findings provide a pathophysiological basis for the observed increase in thrombotic events in atherosclerosis. Use of L-arginine and other strategies to improve endothelial NO activity may impact favorably on thrombotic events in atherosclerosis.

Contribution of vascular tissue to the antiplatelet activity of sodium nitroprusside

We tested whether or not platelet inhibition by sodium nitroprusside (SNP) was enhanced by vascular tissue production of nitric oxide (NO) and calcitonin gene-related peptide (CGRP) release. Platelet aggregation was determined with whole blood impedance aggregometry after incubations of SNP in the presence or absence of rat aortic tissue (AT) or AT + CGRPS(8-37) (a specific CGRP antagonist). SNP alone had no effect on platelet aggregation until 100 microM was used (2.3 + 1.5 omega vs. control aggregation of 9.9 +/- 2.0 omega; p < 0.001). Co-incubation of AT with SNP significantly enhanced platelet inhibition at 1 (1.6 +/- 1.3 omega; p < 0.001), 10 (0.7 +/- 0.4 omega; p < 0.001), and 100 microM (0.3 +/- 0.3 omega; p < 0.001). CGRP(8-37) did not significantly antagonize aggregation by SNP + AT (p > 0.05). The inhibition of platelet aggregation by 10 microM SNP was inhibited by methylene blue (MB) (9.0 +/- 1.7 omega at 10 microM; 11.7 +/- 2.4 omega at 100 microM; p < 0.001) but not by 30 microM L-N(upsilon)-monomethyl-L-arginine (L-NMMA; 2.9 +/- 1.8 omega; p > 0.05). These results indicate that vascular tissue significantly contributes to the ability of SNP to inhibit platelet aggregation, probably through greater vascular enzymatic production of NO, but not by releasing CGRP, in contrast to nitroglycerin.

Assessment of platelet function assays

Optical aggregometry, traditionally used to assess platelet function, is highly dependent on sample preparation and technical procedure; as a result, data from various laboratories can be quite variable. In a study designed to assess the sources of variation, it was determined that the total standard deviation ranged from 3.6% to 7.7%. The assay variation by one analyst on one aggregometer on a single day ranged from 1.7 to 4.6. Day-to-day variation contributed 42% to 63% of the total variation, between-operator variation contributed 1% to 33% of the total variation, and within [between repeat measurements for a given sample by a given operator on a single day] variation contributed 22% to 36% of the total variation. Because of the disadvantages associated with optical aggregometry, alternate platelet function assays were considered and their correspondence to optical aggregometry was evaluated: activated clotting time, whole blood aggregometry, platelet count ratio, Platelet Function Analyzer (PFA-100, Dade), and Rapid Platelet function Assay (Accumetrics). Of those assays evaluated, activated clotting time and PFA-100 are assays that measure aspects of coagulation and hemostasis, whereas whole blood aggregometry, platelet count ratio, and RPFA are more closely related to platelet function. Each assay has value in monitoring various aspects of the coagulation process. The best method for monitoring safety and efficacy of various inhibitors of platelet function will ultimately be determined by clinical trials.

Intravenous nitroglycerin reduces ischaemia in unstable angina pectoris: a double-blind placebo-controlled study

OBJECTIVE

To study whether intravenous nitroglycerin (NTG) reduces the incidence of ischaemic events and leucocyte activation, as well as inhibiting platelet aggregation in patients with unstable angina pectoris.

DESIGN

Randomized double-blind placebo-controlled study.

SUBJECTS

One hundred and sixty-two patients with a history and electrocardiographic changes suggesting unstable angina pectoris.

INTERVENTIONS

A 48-hour titrated intravenous infusion of NTG or placebo.

RESULTS

Of the 162 randomized patients, 19 were excluded because of an acute myocardial infarction on randomization (11 patients) or proven presence of a non-ischaemic cause of the pain (6 patients). Other causes (2 patients). No differences in the clinical findings were detected between the groups on randomization. In the comparison of NTG and placebo, fewer patients in the former group had more than two new attacks of chest pain lasting for < 20 min or one new attack of chest pain lasting > 20 min, despite sublingual NTG (13/25, P < 0.03). In addition, the attacks of pain lasting > 20 min in the NTG group were delayed compared to those in the placebo group (P < 0.05), suggesting a beneficial effect on these more severe episodes. Fewer patients in the NTG group required more than two sublingual NTG tablets (P < 0.005). NTG also reduced the rate-pressure product (P < 0.05), compared to placebo after 2 h but not after 24 h. Compared to baseline, platelet aggregation was inhibited in the patients who had received an NTG infusion for 2 h (P < 0.05). In both groups, leucocytes were activated at baseline, but remained unchanged thereafter.

CONCLUSIONS

Intravenous NTG seems to reduce myocardial ischaemia in patients with unstable angina pectoris more than the placebo.

Nitroglycerin-inhibited whole blood aggregation is partially mediated by calcitonin gene-related peptide -- a neurogenic mechanism

1. The role of the vasculature and calcitonin gene-related peptide (CGRP) in nitroglycerin (NTG)-mediated platelet inhibition was studied. 2. In vitro incubations of CGRP in whole blood induced a dose-dependent inhibition of platelet aggregation with an IC50 of 62.1 nM. 3. The platelet inhibition induced by CGRP was blocked by co-incubation of 0.53 microM CGRP8-37, as well as 30 microM N(G)-nitro-monomethyl-L-arginine (L-NMMA). 4. In a separate group of experiments, 100 nM NTG in rat whole blood (WB) induced platelet inhibition of 6.0 +/- 1.3% (mean +/- s.d.), which was enhanced to 77.6+/-3.5% by the addition of rat aortic tissue (AT) (P<0.001). The inclusion of CGRP8-37 with NTG and AT in WB reduced platelet inhibition to 31.6+6.8% (P<0.01). Incubation of WB and AT with 30 microM L-NMMA reduced NTG-induced inhibition of platelet aggregation to 26.4+/-4.2% (P<0.001). 5. It is concluded that vascular tissue contributes to the antiplatelet mechanism of action of NTG. Furthermore, NTG apparently evokes the release of CGRP from vascular tissue and this neuropeptide contributes to the antiplatelet actions of NTG. 6. The antiplatelet activity of CGRP in whole blood is mediated primarily through the activation of nitric oxide synthase.

Persistent inhibition of platelets during continuous nitroglycerin therapy despite hemodynamic tolerance

BACKGROUND

Nitroglycerin has been shown to possess antiplatelet properties in both animals and humans. Tolerance to the hemodynamic effects of nitroglycerin develops with continuous therapy, but it is unclear whether there is tolerance to its antiplatelet effect.

METHODS AND RESULTS

Tolerance to nitroglycerin was studied by exposing porcine aortic media to flowing arterial blood from control pigs (n = 9) or pigs treated with continuous nitroglycerin patches (Nitro-dur, 0.8 mg/h; n = 11) at a shear rate of 3397 s-1 for 3 minutes. Relative to baseline, mean arterial pressure fell by approximately 10% at 3 and 24 hours (P < .05) but returned to baseline at 48 hours of continuous nitroglycerin treatment, whereas no significant changes were observed in control animals. Autologous 51Cr-labeled platelet deposition (x 10(6)/cm2) on the aortic media at baseline and 3, 24, and 48 hours remained stable in control animals, with mean values of 94.8 +/- 5.9, 89.4 +/- 8.3, 89.3 +/- 8.8, and 84.3 +/- 5.7, respectively. However, in pigs treated continuously with nitroglycerin for 48 hours, platelet deposition was reduced significantly at 3 (65.9 +/- 4.8), 24 (63.8 +/- 6.4), and 48 hours (56.5 +/- 7.3) of nitroglycerin treatment compared with baseline (93.1 +/- 3.6). Platelet aggregation induced by thrombin also decreased at 3 (12.4 +/- 1.3), 24 (12.6 +/- 1.7), and 48 hours (10.8 +/- 1.6) of nitroglycerin treatment compared with baseline (16.3 +/- 1.4) but remained unchanged in the control group. Also, nitroglycerin treatment increased intraplatelet cGMP at 3, 24, and 48 hours compared with baseline.

CONCLUSIONS

This study demonstrates the persistent inhibition of platelet function and platelet deposition on an injured arterial wall by continuous nitroglycerin therapy despite hemodynamic tolerance.

Dietary L-arginine supplementation normalizes platelet aggregation in hypercholesterolemic humans

OBJECTIVES

The present study was designed to test the hypothesis that long-term dietary supplementation with the nitric oxide precursor L-arginine would enhance vascular or platelet-derived nitric oxide activity, or both, and thereby inhibit platelet reactivity in hypercholesterolemic humans.

BACKGROUND

We have shown that reduced vascular activity of nitric oxide in hypercholesterolemic rabbits can be restored by L-arginine supplementation. The improvement in nitric oxide activity is associated with an inhibition of platelet aggregation ex vivo. This effect is most likely due to increased elaboration of endothelium- or platelet-derived nitric oxide, or both, because the inhibition of platelet reactivity was associated with elevation of intraplatelet cyclic guanosine monophosphate and was reversed by the nitric oxide synthase antagonist N-methyl-arginine.

METHODS

In a double-blinded, randomized, placebo-controlled trial, hypercholesterolemic patients were assigned to L-arginine hydrochloride, 8.4 g/day orally, or placebo for 2 weeks. Platelet-rich plasma was obtained for aggregometry induced by collagen (1 to 10 micrograms/ml) at four points: baseline, after 2 weeks of treatment, after a 2-week washout and after a long-term washout of 16 weeks on average. Aggregation was quantified by light transmittance and expressed as a percent transmittance observed with platelet-poor plasma.

RESULTS

Compared with normocholesterolemic control subjects, platelets from hypercholesterolemic subjects stimulated with 5 micrograms/ml of collagen showed increased aggregability (68.6% in hypercholesterolemic patients vs. 54.5% in normocholesterolemic control subjects, p < or = 0.02). After 2 weeks of treatment with L-arginine (but not placebo), platelet reactivity was modestly reduced; this effect persisted for 2 weeks after discontinuation of arginine (52.6% in arginine-treated patients vs. 65.1% in normocholesterolemic control subjects, p = 0.07). After 18 weeks (i.e., 16 weeks after discontinuing arginine treatment), the platelets of hypercholesterolemic patients once again became hyperaggregable, and the extent of platelet aggregation was significantly increased compared with the 4-week point (73.6% after vs. 52.6% during arginine treatment, p < 0.01). No significant change in platelet reactivity was seen in placebo-treated hypercholesterolemic patients throughout the study. L-Arginine treatment was well tolerated without side effects.

CONCLUSIONS

This double-blinded, placebo-controlled study demonstrates that dietary supplementation with L-arginine can modestly attenuate the increased platelet reactivity seen in hypercholesterolemic patients. The data are consistent with our previous studies in hypercholesterolemic animals, demonstrating that L-arginine restores endogenous nitric oxide activity and inhibits platelet aggregation. Enhancement of endogenous nitric oxide activity is a potential novel therapeutic strategy worthy of further study.

Different effects of calcium antagonists, nitrates, and beta-blockers on platelet function. Possible importance for the treatment of unstable angina

BACKGROUND

The three major classes of antianginal drug all inhibit platelet aggregation at high concentrations in vitro, but detecting clinically relevant effects has proved to be more difficult. We used whole-blood flow cytometry, a sensitive method that allows direct measurement of activation antigens on the surface of individual platelets in whole unfixed blood, to evaluate the effect of representative antianginal drugs on platelet function in vivo in healthy volunteers.

METHODS AND RESULTS

The effects of glyceryl trinitrate (GTN), amlodipine, and atenolol were studied in nine normal volunteers. Fibrinogen binding to activated GP IIb/IIIa and expression of P-selectin, GP Ib, and GP IIb/IIIa on the platelet surface were measured. In addition, fibrinogen binding and P-selectin expression were measured in response to ex vivo stimulation with the agonists ADP and thrombin. The three drugs had very different effects on platelets. GTN inhibited platelet fibrinogen binding and expression of P-selectin at rest and in response to agonist stimulation, whereas amlodipine enhanced P-selectin expression and atenolol increased fibrinogen binding in response to agonists. Atenolol did not block the stimulatory effects of epinephrine on ADP-induced platelet activation. GTN neutralized the proactivatory effects of amlodipine, whereas the effects of atenolol and amlodipine were not additive.

CONCLUSIONS

The three main classes of antianginal medication have different and possible clinically relevant effects on platelet behavior in vivo, nitrates causing inhibition of aggregation (fibrinogen binding) and degranulation (P-selectin expression), calcium antagonists enhancing degranulation, and beta-blockers enhancing aggregation.

Nitrate tolerance, rebound, and their clinical relevance in stable angina pectoris, unstable angina, and heart failure

Vascular tolerance develops rapidly in isolated vascular strips exposed to millimolar concentrations of nitroglycerin. Several mechanisms, including depletion of sulfhydryl groups, reduced biotransformation of nitrates to NO or nitrosothiols, oxygen free radical injury, and downregulation of a membrane-bound enzyme or a nitrate receptor, have been proposed, but the exact mechanism responsible for in-vitro tolerance remains unknown. In-vivo tolerance of the beneficial effects of nitrates on hemodynamics, myocardial ischemia, and exercise performance develops rapidly. It has been suggested, but remains to be proven, that development of venous tolerance and not arterial tolerance is responsible for the attenuation of nitrate effects during long-term nitrate therapy. Several mechanisms, including neurohormonal activation, depletion of sulfhdryl groups, and the shift of fluid from the extravascular to intravascular compartment have been implicated. However, the use of agents to counteract these mechanisms (ACE inhibitors, sulfhydryl donors, diuretics) has produced conflicting results. Thus, at present the mechanism responsible for in vivo tolerance to nitrates remains unknown. Both in vitro and in vivo vascular tolerance to nitrates can be prevented or minimized by providing nitrate-free or low-nitrate intervals. However, during nitrate-free periods, rebound phenomena (rest angina in patients with ischemic heart disease or a deterioration in exercise performance prior to the renewal of the morning dose in patients with stable angina) remain a clinical concern. When treating patients with stable angina pectoris, it must be recognized that none of the nitrate preparations or formulations can provide round-the-clock antianginal or antiischemic prophylaxis. In these patients, beneficial antianginal and antiischemic effects of nitrates for 10-14 hours during the daytime can be maintained by using formulations and dosing regimens that avoid or minimize the development of tolerance (standard formulation of isosorbide-5-mononitrate, 20 mg in the morning and 7 hours later; slow-release formulation of isosorbide-5-mononitrate, 120-240 mg once a day; or nitroglycerin patch delivering 0.6 nitroglycerin per hour for 10-12 hours each day). Only the patch on and off treatment is associated with nitrate rebound. Although intermittent nitrate therapy is not associated with the development of tolerance, this strategy cannot be recommended for treating unstable angina because rebound angina during nitrate-free periods complicates clinical decision making. In the acute phase of unstable angina, continuous treatment with intravenous nitroglycerin is recommended because it permits rapid up- or down-titration. Tolerance towards antianginal and antiischemic effects does develop in a substantial number of patients with 24 hours, but this can be overridden by dose escalation and restoration of the therapeutic effectiveness of nitroglycerin. Tolerance towards the beneficial effects of nitrates on hemodynamics and on exercise performance also develops rapidly during continuous or long-term nitrate therapy, and for these reasons nitrates are not used as first-line therapy to treat chronic heart failure. In combination with hydralazine, high-dose isosorbide dinitrate (30-40 mg four times a day) improves survival, but this combination therapy is inferior to ACE inhibitors.

Sustained antiplatelet properties of nitroglycerin during hemodynamic tolerance in rats

Organic nitrates possess important antiplatelet actions that are useful in the treatment of unstable angina and myocardial infarction, but the susceptibility of platelets to nitrate tolerance has not been extensively studied. In normal conscious rats, we showed that continuous infusion of nitroglycerin (NTG) at 10 micrograms/min had no significant effect on mean arterial pressure (MAP) as compared with control, but hemodynamic tolerance could be demonstrated by MAP response to a bolus intravenous (i.v.) NTG challenge. By this criterion, continuous 8-h NTG infusion produced hemodynamic tolerance (a decrease in MAP response of 45.7 +/- 19.9%, p < 0.05), whereas D5W control and S-nitroso-N-acetylpenicillamine (SNAP) infusions did not. During NTG infusion, platelet-rich plasma (PRP) cyclic GMP was increased by 41.4 +/- 13.6% as compared with control and remained increased throughout the infusion (p < 0.05). Bleeding time during a 2-h infusion of NTG was 8.9 +/- 1.2 min as compared to 3.8 +/- 0.4 min in controls (p < 0.05). After 8-h of NTG infusion, the bleeding time was 10.2 +/- 1.4 min versus 4.4 +/- 0.4 min in controls (p < 0.05). NTG also decreased the PRP platelet concentration by 30% in 8 h, whereas D5W had no effect. In vitro experiments showed that platelets in themselves do not produce significant amounts of cyclic GMP. These data indicate that the biochemical and antiaggregation effects of NTG on platelets are not diminished during hemodynamic tolerance and that these effects may be dependent on extraplatelet production of nitric oxide (NO).

Beneficial actions of nitrates in cardiovascular disease

Nitroglycerin and the long-acting nitrates have been used in cardiovascular medicine for >100 years. Nitrates are widely utilized for the various anginal syndromes and are also used in congestive heart failure and patients with left ventricular dysfunction. The potential mechanisms for relief of myocardial ischemia with nitrates are multiple. The nitrovasodilators are a related group of drugs that result in the formation of nitric oxide (NO) within vascular smooth muscle cells. NO stimulates the enzyme guanylate cyclase, which results in increases in cyclic guanosine monophosphate and vasodilation. In the presence of atherosclerosis, endothelial dysfunction is ubiquitous and associated with decreased NO availability, probably due to increased destruction of NO by free radical anions. Nitrovasodilators, including the nitrates, supply exogenous NO to the vascular wall and improve the vasodilator state. When nitrates are administered, endothelial-dependent stimuli cause relaxation rather than constriction in the setting of endothelial dysfunction. Nitrates also have antiplatelet effects, and recent evidence confirms that these drugs decrease platelet aggregation and thrombosis formation. This may play an important role in the therapy of acute unstable myocardial ischemia, including unstable angina and myocardial infarction. Nitrate hemodynamic effects have been long known. They are primarily modulated through a decrease in myocardial work that results from smaller cardiac chambers operating with lower systolic and diastolic pressures. These changes are caused by a redistribution of the circulating blood volume away from the heart to the venous capacitance system, with a fall in venous return to the heart. The afterload or arterial effects of nitrates are also useful in decreasing myocardial oxygen consumption. Considerable evidence confirms a variety of mechanisms whereby nitrates increase coronary blood flow, including epicardial coronary artery dilation, stenosis enlargement, enhanced collateral size and flow, improvement of endothelial dysfunction, and prevention or reversal of coronary artery vasoconstriction. These effects help increase nutrient coronary blood flow to zones of myocardial ischemia. Recent data with the nitroglycerin patch confirm that myocardial ischemia is decreased after nitrate administration. Nitroprusside, another nitrovasodilator, is a commonly used intravenous agent for lowering arterial pressure and left ventricular filling pressure. This drug is highly effective for the treatment of acute or severe hypertension and congestive heart failure. However, there are data suggesting that nitroprusside may be deleterious in the presence of acute myocardial ischemia, perhaps by shunting blood away from zones of jeopardized myocardial blood flow. Therefore, nitroprusside cannot be recommended to treat myocardial ischemia; intravenous nitroglycerin should be used in this context.

Effect of nitrates on myocardial remodeling after acute myocardial infarction

Nitrates are effective for the therapy of acute coronary syndromes, including acute myocardial infarction. Their application in acute infarction has established that vasodilators are beneficial provided hypotension is avoided. Nitrates limit early ventricular remodeling in infarction. New dosing strategies and formulations that permit chronic use after infarction with less tolerance might limit late remodeling. Over the last decade, the demonstrated effectiveness of angiotensin-converting enzyme (ACE) inhibitors in limiting ventricular dilation postinfarction has generated controversy over the usefulness of nitrates for that indication. The uncertainty has been intensified by 2 large mortality trials that tested both agents as adjuncts to conventional therapy. These trials were not designed to test whether nitrates might limit remodeling. Mechanistic experimental and clinical studies that tested whether nitrates or ACE inhibitors could effectively limit ventricular remodeling showed that both improved remodeling endpoints. However, experimental studies raise some concern about the decrease in infarct collagen associated with ACE inhibition and emphasize the fact that final outcome represents a balance of effects. That nitrates do not decrease infarct collagen could be important. Nitrate-induced early recruitment of ventricular function after late reperfusion of acute infarction might also be important. In the mortality trials, >50% of patients received open-label nitrates as per indication. Thus, the trial results to date do not suggest that nitrates are ineffective for remodeling, but rather that ACE inhibitors can confer added benefit. There has been no large clinical trial to test the efficacy of nitrates for remodeling as there has been for ACE inhibitors.

Platelet activation in acute myocardial infarction and unstable angina is inhibited by nitric oxide donors

Platelet activation and thrombus formation within the coronary artery are major factors in acute myocardial infarction (AMI) and unstable angina (UA), and continuing platelet activation is associated with an adverse prognosis. We assessed platelet activation by using flow cytometry to measure platelet surface expression of P-selectin and glycoprotein IIb/IIIa in 20 patients with AMI and 20 with UA, all of whom were treated with aspirin. Platelet studies were repeated after the infusion of a nitric oxide donor (glyceryl trinitrate or S-nitrosoglutathione) that produced a fall in mean arterial pressure of no more than 10 mm Hg. P-selectin was expressed on 2.5% (range, 1.4% to 6.3%) of platelets from AMI and 2.3% (range, 1.6% to 3.3%) from UA subjects compared with 1.0% (range, 0.6% to 1.9%) of platelets from 20 control volunteers without angina (P < .001). Glycoprotein IIb/IIIa expression was 101.6 +/- 2.7 arbitrary units of relative fluorescence in AMI and 100.2 +/- 3.3 in UA compared with 87.8 +/- 2.5 in control subjects (P < .01). In both AMI and UA, S-nitrosoglutathione reduced P-selectin (P < .001) and glycoprotein IIb/IIIa (P < .05) expression, as did glyceryl trinitrate (P < .02 and P < .01, respectively). In 3 of 20 patients receiving glyceryl trinitrate the lowest dose was not tolerated due to headache or hypotension. These findings show that platelet activation persists in AMI and UA despite aspirin treatment and that this can be inhibited by using glyceryl trinitrate or S-nitrosoglutathione. S-nitrosoglutathione is better tolerated at the doses required.

Glyceryl trinitrate enhances the adenosine-induced inhibition of platelet responses: a mechanism potentially involved in the in vivo anti-aggregating effects of organic nitrates

1. The present study investigated the influence of the organic nitrate glyceryl trinitrate (GTN) on the anti-aggregating effects of adenosine. We determined the effects of adenosine, GTN and their combination on platelet responses in platelet-rich plasma and whole blood, and on intracellular levels of 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP). 2. Adenosine inhibited the in vitro platelet aggregation in response to different agonists in a dose-dependent way through an elevation of intraplatelet cAMP levels. Effective adenosine concentrations were higher than those detectable under physiological conditions, but very close to levels achieved during myocardial ischaemia or haemorrhagic shock. 3. GTN was able to decrease platelet responses influencing intraplatelet cGMP levels. Furthermore, the drug increased the inhibitory effects of adenosine and enhanced its effects on intraplatelet cAMP levels. 4. The present data provides further evidence that compounds that increase intraplatelet levels of cGMP and cAMP act synergistically on the inhibition of platelet aggregability through the influence of increased cGMP levels on cAMP accumulation. The interplay between GTN and adenosine in the inhibition of platelet function could be effective during nitrate administration in the treatment of acute myocardial ischaemia when blood adenosine levels are significantly increased.

Contemporary approaches in medical management of patients with stable coronary artery disease

CAD continues to be the principal cause of mortality in the United States, and the largest group of patients with CAD are those with stable angina. Among this group of patients, the most common manifestation of CAD is presence of transient episodes of myocardial ischemia. The presence of transient ischemia and not the severity of angina has been found to be associated with poor clinical outcome in patients with stable CAD. As part of a global treatment strategy for patients with stable CAD, changes in lifestyle and modification of coronary risk factors should be emphasized as an integral part of treatment. Conventional antianginal therapy is quite effective in controlling anginal attacks. Currently, several drugs and therapeutic strategies are available for the treatment of patients with angina (see Table 5). Nitrates are highly effective antianginal drugs with complex beneficial actions in patients with CAD, but their usefulness is limited by development of tolerance during long-term use. When clinically indicated, the use of nitrates should be supplemented with another longer-acting antianginal drug, such as a beta-blocker or a calcium channel blocker. Based on the available data, beta-blockers, when tolerated, seem to be the most effective antianginal drugs for most patients with stable CAD. Beta-blockers are also the most effective anti-ischemic drugs that reduce the magnitude of myocardial ischemia detected during routine daily activities. Calcium channel blockers are also effective vasodilators and good antianginal drugs. The clinician should become familiar with the different actions that this heterogeneous group of drugs has on the heart and vessels. This knowledge allows the clinician to choose the appropriate combination of different antianginal drugs for patients on an individualized basis. It is also critical to develop the treatment strategy by carefully taking into account other associated medical conditions that are frequently encountered in patients with CAD.

Choice of anesthesia and intraoperative monitoring for lower extremity revascularization

The prevalence of significant coronary artery disease re-enforces the importance of careful preoperative and intraoperative management in patients undergoing lower extremity revascularization. This article presents a practical approach toward the evaluation of anesthetic risk and the proper use of anesthetic agents and monitoring devices to minimize morbidity. The role of general and regional anesthetic agents is discussed, and complications of both techniques are presented.