Tolerance with low dose intravenous nitroglycerin therapy in acute myocardial infarction

Serelaxin Treatment Reduces Oxidative Stress and Increases Aldehyde Dehydrogenase-2 to Attenuate Nitrate Tolerance

Background: Glyceryl trinitrate (GTN) is a commonly prescribed treatment for acute heart failure patients. However, prolonged GTN treatment induces tolerance, largely due to increased oxidative stress and reduced aldehyde dehydrogenase-2 (ALDH-2) expression. Serelaxin has several vasoprotective properties, which include reducing oxidative stress and augmenting endothelial function. We therefore tested the hypothesis in rodents that serelaxin treatment could attenuate low-dose GTN-induced tolerance.

Methods and Results: Co-incubation of mouse aortic rings ex vivo with GTN (10 μM) and serelaxin (10 nM) for 1 h, restored GTN responses, suggesting that serelaxin prevented the development of GTN tolerance. Male Wistar rats were subcutaneously infused with ethanol (control), low-dose GTN+placebo or low-dose GTN+serelaxin via osmotic minipumps for 3 days. Aortic vascular function and superoxide levels were assessed using wire myography and lucigenin-enhanced chemiluminescence assay respectively. Changes in aortic ALDH-2 expression were measured by qPCR and Western blot respectively. GTN+placebo infusion significantly increased superoxide levels, decreased ALDH-2 and attenuated GTN-mediated vascular relaxation. Serelaxin co-treatment with GTN significantly enhanced GTN-mediated vascular relaxation, reduced superoxide levels and increased ALDH-2 expression compared to GTN+placebo-treated rats.

Conclusion: Our data demonstrate that a combination of serelaxin treatment with low dose GTN attenuates the development of GTN-induced tolerance by reducing superoxide production and increasing ALDH-2 expression in the rat aorta. We suggest that serelaxin may improve nitrate efficacy in a clinical setting.

Clinical effectiveness of telmisartan alone or in combination therapy for controlling blood pressure and vascular risk in the elderly

Elderly patients (age ≥ 65 years) with hypertension are at high risk for vascular complications, especially when diabetes is present. Antihypertensive drugs that inhibit the renin-angiotensin system have been shown to be effective for controlling blood pressure in adult and elderly patients. Importantly, renin-angiotensin system inhibitors were shown to have benefits beyond their classic cardioprotective and vasculoprotective effects, including reducing the risk of new-onset diabetes and associated cardiovascular effects. The discovery that the renin-angiotensin system inhibitor and angiotensin II type 1 (AT(1)) receptor blocker (ARB), telmisartan, can selectively activate the peroxisome proliferator-activated receptor-γ (PPARγ, an established antidiabetic drug target) provides the unique opportunity to prevent and treat cardiovascular complications in high-risk elderly patients with hypertension and new-onset diabetes. Two large clinical trials, ONTARGET (Ongoing Telmisartan Alone in combination with Ramipril Global Endpoint Trial) and TRANSCEND (Telmisartan Randomized AssessmeNt Study in ACE-I iNtolerant subjects with cardiovascular disease) have assessed the cardioprotective and antidiabetic effects of telmisartan. The collective data suggest that telmisartan is a promising drug for controlling hypertension and reducing vascular risk in high-risk elderly patients with new-onset diabetes.

Valsartan in the treatment of heart attack survivors

Survivors of myocardial infarction (MI) are at high risk of disability and death. This is due to infarct-related complications such as heart failure, cardiac remodeling with progressive ventricular dilation, dysfunction, and hypertrophy, and arrhythmias including ventricular and atrial fibrillation. Angiotensin (Ang) II, the major effector molecule of the renin–angiotensin–aldosterone system (RAAS) is a major contributor to these complications. RAAS inhibition, with angiotensin-converting enzyme (ACE) inhibitors were first shown to reduce mortality and morbidity after MI. Subsequently, angiotensin receptor blockers (ARBs), that produce more complete blockade of the effects of Ang II at the Ang II type 1 (AT₁) receptor, were introduced and the ARB valsartan was shown to be as effective as an ACE inhibitor in reducing mortality and morbidity in high-risk post-MI suvivors with left ventricular (LV) systolic dysfunction and and/or heart failure and in heart failure patients, respectively, in two major trials (VALIANT and Val-HeFT). Both these trials used an ACE inhibitor as comparator on top of background therapy. Evidence favoring the use of valsartan for secondary prevention in post-MI survivors is reviewed.

Central role of mitochondrial aldehyde dehydrogenase and reactive oxygen species in nitroglycerin tolerance and cross-tolerance

Recent studies suggest that mitochondrial aldehyde dehydrogenase (ALDH-2) plays a central role in the process of nitroglycerin (glyceryl trinitrate, GTN) biotransformation in vivo and that its inhibition accounts for mechanism-based tolerance in vitro. The extent to which ALDH-2 contributes to GTN tolerance (impaired relaxation to GTN) and cross-tolerance (impaired endothelium-dependent relaxation) in vivo remain to be elucidated. Rats were treated for three days with GTN. Infusions were accompanied by decreases in vascular ALDH-2 activity, GTN biotransformation, and cGMP-dependent kinase (cGK-I) activity. Further, whereas in control vessels, multiple inhibitors and substrates of ALDH-2 reduced both GTN-stimulation of cGKI and GTN-induced vasodilation, these agents had little effect on tolerant vessels. A state of functional tolerance (in the GTN/cGMP pathway) was recapitulated in cultured endothelial cells by knocking down mitochondrial DNA (rho(0) cells). In addition, GTN increased the production of reactive oxygen species (ROS) by mitochondria, and these increases were associated with impaired relaxation to acetylcholine. Finally, antioxidants/reductants decreased mitochondrial ROS production and restored ALDH-2 activity. These observations suggest that nitrate tolerance is mediated, at least in significant part, by inhibition of vascular ALDH-2 and that mitochondrial ROS contribute to this inhibition. Thus, GTN tolerance may be viewed as a metabolic syndrome characterized by mitochondrial dysfunction.

Nitric oxide in heart failure: friend or foe

Nitric oxide (NO) is a controversial molecule. It is either beneficial or deleterious. As with NO donors, one reason for this duality is related to the dose. Small doses are highly beneficial, maintaining blood flow in vessels and blood pressure, and protecting against foreign invaders. In high doses, it results in hypotension, forms peroxynitrite which is cytotoxic, and contributes to heart failure.

Functional and biochemical analysis of endothelial (dys)function and NO/cGMP signaling in human blood vessels with and without nitroglycerin pretreatment

BACKGROUND

In experimental animal models, long-term in vivo treatment with nitroglycerin (NTG) induces both endothelial dysfunction and tolerance to nitrates. However, it is still controversial whether nitrate tolerance in humans is associated with both endothelial dysfunction and impaired vascular response to nitrovasodilator-derived NO.

METHODS AND RESULTS

Patients undergoing elective bypass surgery were randomized to receive 48 hours of continuous NTG infusion (NTG group) or no nitrate therapy (control group). Segments of surgically removed arteria mammaria, vena saphena, and arteria radialis not required for the bypass procedure were used to examine (1) the vascular responsiveness to NTG and the endothelium-dependent vasodilator acetylcholine; (2) the expression of the NO target, the soluble guanylyl cyclase; (3) the expression of the soluble guanylyl cyclase/cGMP effector target, the cGMP-dependent protein kinase (cGK); and (4) the cGK activity as assessed by the phosphorylation state of its vascular substrate, the vasodilator-stimulated phosphoprotein at serine(239) (P-VASP). NTG treatment caused a marked degree of nitrate tolerance in all 3 vessel types studied and a significant cross-tolerance to the endothelium-dependent vasodilator acetylcholine in A. mammaria and A. radialis. Although soluble guanylyl cyclase, cGK-I, and VASP expression levels were not modified by NTG treatment, a marked decrease of P-VASP, a surrogate parameter for in-vivo cGK-I activity, was observed.

CONCLUSIONS

We conclude that long-term NTG treatment induces endothelial dysfunction and impaired vascular NO/cGMP signaling in humans, which can be monitored by measuring P-VASP levels.

Effects of in vivo nitroglycerin treatment on activity and expression of the guanylyl cyclase and cGMP-dependent protein kinase and their downstream target vasodilator-stimulated phosphoprotein in aorta

BACKGROUND

Chronic in vivo treatment with nitroglycerin (NTG) induces tolerance to nitrates and cross-tolerance to nitrovasodilators and endothelium-derived nitric oxide (NO). We previously identified increased vascular superoxide formation and reduced NO bioavailability as one causal mechanism. It is still controversial whether intracellular downstream signaling to nitrovasodilator-derived NO is affected as well.

METHODS AND RESULTS

We therefore studied the effects of 3-day NTG treatment of rats and rabbits on activity and expression of the immediate NO target soluble guanylyl cyclase (sGC) and on the cGMP-activated protein kinase I (cGK-I). Tolerance was induced either by chronic NTG infusion via osmotic minipumps (rats) or by NTG patches (rabbits). Western blot analysis, semiquantitative reverse transcription-polymerase chain reaction, and Northern blot analysis revealed significant and comparable increases in the expression of sGC alpha(1) and beta(1) subunit protein and mRNA. Studies with the oxidative fluorescent dye hydroethidine revealed an increase in superoxide in the endothelium and smooth muscle. Stimulation with NADH increased superoxide signals in both layers. Although cGK-I expression in response to low-dose NTG was not changed, a strong reduction in vasodilator-stimulated phosphoprotein (VASP) serine239 phosphorylation (specific substrate of cGK-I) was observed in tolerant tissue from rats and rabbits. Concomitant in vivo and in vitro treatment with vitamin C improved tolerance, reduced oxidative stress, and improved P-VASP.

CONCLUSIONS

We therefore conclude that increased expression of sGC in the setting of tolerance reflects a chronic inhibition rather than an induction of the sGC-cGK-I pathway and may be mediated at least in part by increased vascular superoxide.

Review article: a drug therapy for the prevention of variceal haemorrhage

The development of varices is a major complication of cirrhosis, and variceal haemorrhage has a high mortality. There have been major advances in the primary and secondary prevention of variceal haemorrhage over the last 20 years involving endoscopic, radiological and pharmacological approaches. This review concentrates principally on drug therapy, particularly on the numerous haemodynamic studies. Many of these drugs have not been studied in clinical trials, but provide data about the underlying pathogenesis of portal hypertension. Also covered in this review are the randomized controlled trials and meta-analyses that involve a large number of patients. These trials involve relatively few drugs such as non-selective beta-blockers and nitrates. Correlations between haemodynamic and clinical parameters are discussed. Despite the recent increase in the use of alternative endoscopic therapies, an effective and well tolerated drug remains a clinically important research goal.

Long-term nitrate use may be deleterious in ischemic heart disease: A study using the databases from two large-scale postinfarction studies. Multicenter Myocardial Ischemia Research Group

BACKGROUND

Secondary coronary prevention studies have generally focused on specific medications, often to the exclusion of commonly used therapies. To date, long-term nitrate use has not been investigated in large-scale clinical trials.

METHODS AND RESULTS

We examined the relation between long-acting nitrates given during the chronic phase of the disease and the outcome. We analyzed data prospectively acquired in a large, observational study involving 1042 patients enrolled for the Multicenter Study of Myocardial Ischemia (MSMI) in North America, Israel, and Japan as well as 1779 patients enrolled for the Multicenter Diltiazem Post Infarction Trial (MDPIT). The Cox analyses with all the variables retained revealed that nitrates were associated with a significantly increased mortality risk (MSMI: hazard ratio 3.78, P =.011; MDPIT: hazard ratio 1.61, P =.019) in patients who had recovered from an acute coronary event. The analyses with the propensity score method on the MSMI and the MSMI databases also showed that the risk for cardiac death with use of nitrates was increased in most of the 5 subclasses according to the score.

CONCLUSION

These analyses raise concern about the potential adverse effects of long-acting nitrate therapy in chronic coronary disease.

Pharmacokinetics of ITF 296 (Sinitrodil) a novel organic nitrate, in healthy volunteers

ITF 296 is a new orally active nitrate acting selectively on large arterial vessels over a wide range of doses. In healthy volunteers it causes less reduction in vascular resistance and less venodilatation than classic nitrates. Its pharmacokinetic profile was evaluated after intravenous infusion and oral (solution and immediate-release tablet) administration in a randomised cross-over design on 11 healthy volunteers. The plasma levels of ITF 296 and its metabolite ITF 1124 were determined by a HPLC method. The drug is rapidly distributed (mean steady-state distribution volume 53+/-17 liters) and eliminated (half-life of about 25 minutes) both after intravenous and oral administration. The total clearance is 2. 31+/-0.46 l/min. The oral solution of ITF 296 is well absorbed (Cmax=0.057 microg/ml, tmax=30 min) but it undergoes a first-pass effect (F=25%). The tablet, developed only for Phase 1 clinical studies, is characterised by an immediate release (Cmax=0.057 microg/ml, tmax=30 min). The extent of its absolute and relative bioavailability is about 14% and 53% respectively.

Safety, tolerability, and antiischaemic efficacy of ITF-296, a nitric oxide donor, in patients with chronic stable angina

A double-blind, placebo-controlled, parallel-group, dose-escalation study was performed to evaluate ITF-296, a new nitric oxide donor, in chronic stable angina. Twenty-four male patients underwent exercise electrocardiography in conjunction with echocardiography on three separate occasions within 2 weeks. After two reproducibility tests, the third exercise test was immediately preceded by a 30-min intravenous infusion of either placebo or 0.3, 1.0, or 3.0 microg/kg/min of ITF-296, with six patients in each treatment group. During ITF-296 infusion, systolic blood pressure decreased by a mean of 12 mm Hg compared with a 2 mm Hg increase in the placebo group [difference, -14; 95% confidence interval (CI), -23 to -6 mm Hg]. ITF-296 administration was followed by consistent increases in total exercise time, time to angina threshold, and time to 1-mm ST-segment depression. The mean change in total exercise time was 39 s on placebo and 94 s on ITF-296 (mean difference, 54 s; 95% CI, -48 to 157 mm Hg). The 1.0- and 3.0-microg/kg/min doses increased exercise time by >2 min with no important change in peak rate-pressure product compared with baseline. The trends exhibited in this small preliminary study were consistently pharmacologically plausible and indicate that ITF-296 shows potential antiischaemic efficacy in patients with chronic stable angina.

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.

Absence of vascular tolerance in conductance vessels after 48 hours of intravenous nitroglycerin in patients with coronary artery disease

OBJECTIVES

We examined whether reflex neurohormonal constrictor forces attenuate the vasodilator action of nitroglycerin on large peripheral conductance vessels.

BACKGROUND

Continuous nitroglycerin therapy is associated with the development of early tolerance with respect to its hemodynamic effects. It remains to be demonstrated whether vascular tolerance of large conductance vessels is an important contributory factor.

METHODS

Radial artery diameter and forearm blood flow velocity were measured before and 24 and 48 h after continuous intravenous nitroglycerin infusion (0.5 microgram/kg body weight per min) in 10 patients with coronary artery disease (mean age +/- SEM 59 +/- 4 years) by using a high resolution ultrasound device. Blood flow (ml/min) was calculated from mean blood flow velocity and cross-sectional area.

RESULTS

Increasing concentrations of nitroglycerin led to a dose-dependent increase in radial artery diameter (maximal +24 +/- 2%) and heart rate. Forearm vascular resistance and forearm blood flow were unchanged. After 24 and 48 h of treatment, additional nitroglycerin did not further increase radial artery diameter, indicating that the nitroglycerin-induced dilation of the radial artery was maintained and was still maximal. In addition, radial artery diameter measured before and after 48 h of nitroglycerin infusion and after withdrawal of nitroglycerin in five additional patients showed that, after withdrawal, arterial diameter returned to baseline values within 35 min. Plasma renin activity and serum aldosterone and vasopressin levels increased significantly at 24 and 48 h, accompanied by a decrease in hematocrit.

CONCLUSIONS

Continuous intravenous administration of nitroglycerin exerts a sustained vasodilator effect for 48 h in large conductance vessels. Neurohormonal activation and compensatory intravascular volume expansion do not attenuate the vasodilator effects of nitroglycerin on peripheral conductance vessels during the 1st 48 h of treatment.

Effectiveness of enalapril versus nifedipine to antagonize blood pressure and the renal response to endothelin in humans

Endothelin-1 infusion into humans to obtain pathophysiological plasma levels causes mild hypertension, strong renal vasoconstriction, and sodium retention. We studied whether oral use of the angiotensin-converting enzyme inhibitor enalapril (20 mg BID) or the calcium channel blocker nifedipine (60 mg OD) could attenuate these effects of endothelin-1 (2.5 ng/kg per minute for 90 minutes) in six healthy volunteers. Endothelin infusion alone increased plasma endothelin from 3.0 +/- 0.3 to 8.8 +/- 1.0 pmol/L (P < .05). Blood pressure rose by approximately 6 mm Hg (P < .05). Renal function changes were relatively large: Renal blood flow decreased from 941 +/- 76 to 729 +/- 118 mL/min (P < .05) and glomerular filtration rate from 105 +/- 9 to 92 +/- 10 mL/min (P < .05); renal vascular resistance increased from 101 +/- 7 to 152 +/- 20 mm Hg.min/L (P < .05); and sodium excretion decreased from 158 +/- 54 to 86 +/- 27 mumol/min (P < .05). Enalapril treatment reduced blood pressure from 94 +/- 2 to 87 +/- 3 mm Hg (P < .05) and prevented the hypertensive response to endothelin. By contrast, despite renal predilatation, endothelin reduced renal blood flow strongly (from 1063 +/- 127 to 763 +/- 100 mL/min, P < .05), although maximal renal vascular resistance was numerically lower (124 +/- 11 mm Hg.min/L) than during endothelin alone (P < .05). Glomerular filtration rate fell from 118 +/- 11 to 108 +/- 11 mL/min (P < .05). Enalapril did not alter the antinatriuretic effect of endothelin.(ABSTRACT TRUNCATED AT 250 WORDS)

Portal and systemic haemodynamic response to acute and chronic administration of low and high dose isosorbide-5-mononitrate in patients with cirrhosis

Oral isosorbide-5-mononitrate (Is-5-Mn) was given in doses of 10 and 40 mg acutely and chronically (twice daily for four weeks), allowing a nitrate free interval to 25 patients with cirrhosis. Both 10 mg and 40 mg Is-5-Mn reduced the hepatic venous pressure gradient acutely and chronically, without evidence of tolerance. This was achieved by a reduction in the wedged hepatic venous pressure. The effect on mean azygos blood flow was variable with no significant mean change seen acutely or after chronic use with either dose. The variability was dependent not on the dose used but on the initial azygos flow; the flow in patients with initially low values increased and those with high azygos flows decreased after nitrate challenge. The development of the porto-collateral flow seems an important parameter in predicting haemodynamic response to Is-5-Mn.

Nitrates in myocardial infarction

Until two decades ago nitroglycerin was contraindicated in acute myocardial infarction (MI). Studies in the canine model demonstrated that low-dose intravenous (i.v.) infusion, carefully titrated to decrease mean blood pressure by 10% but not below 80 mmHg, during early stages of acute MI produced marked reduction of left ventricular (LV) preload, improvement in regional perfusion, and limitation of infarct size and remodeling. However, more i.v. nitroglycerin to decrease blood pressure further resulted in a paradoxical J-curve effect, with hypoperfusion and increased infarct size. Clinical studies have confirmed that low-dose i.v. nitroglycerin infusion for the first 48 hours after acute MI is safe, not only for improving performance in LV failure, but also for limiting ischemic injury, infarct size, remodeling, and infarct-related complications, including deaths in-hospital and up to 1 year. Recent studies suggest that more prolonged therapy with nitrates spanning the healing phase of acute anterior Q-wave MI can further limit LV remodeling and preserve function. Preliminary results of the recently completed ISIS-4 megatrial suggest, however, that long-term nitrate in patients with suspected MI in the 1990s does not improve survival significantly.

Use of nitroglycerin for the treatment of acute myocardial infarction

After a decade of warnings against the use of nitrates in acute myocardial infarction (MI), they are becoming recognized for their potential to salvage left ventricular (LV) myocardium, geometry and function. Low-dose intravenous (IV) nitroglycerin (NTG) infusion for the first 48 h after acute MI, titrated to lower mean blood pressure by 10% to 30%, but not below 80 mmHg, has been shown to be safe, to improve hemodynamics, and to decrease infarct size, infarct expansion, complications, and deaths in a prospective, randomized, single-blind study of 310 patients. In addition, low-dose NTG infusion for the first 48 h, followed by prolonged buccal NTG given during healing after acute MI in an eccentric dose schedule to minimize tolerance, was found to limit further progressive remodeling and preserve LV function. Meta-analysis of nitrate studies in acute MI indicate that they improve survival. Preliminary and ongoing studies suggest that prolonged NTG therapy post MI can produce further benefit.

Effects of nitrate therapy on ventricular remodeling and function

The hypothesis that nitrates might effectively limit left ventricular remodeling and improve function after acute myocardial infarction has been tested in experimental and clinical models, with special attention to the pathophysiologic evolution of remodeling. In 1 clinical study, before the thrombolytic era, the effects of low-dose intravenous nitroglycerin infusion for the first 48 hours during acute myocardial infarction was evaluated in a prospective, randomized, single-blinded, placebo-controlled study of 310 patients (154 nitroglycerin; 156 placebo). Nitroglycerin proved to be safe and produced several benefits compared with placebo: (1) smaller infarct size; (2) less left ventricular dysfunction; (3) less infarct expansion and thinning; (4) better functional status; (5) fewer in-hospital complications such as left ventricular failure, left ventricular thrombus, cardiogenic shock, and infarct extension; and (6) fewer deaths up to 1 year. Two subsequent clinical studies in the thrombolytic era, with low-dose intravenous nitroglycerin infusion during infarction over the first 48 hours followed by buccal nitrate (eccentric dose regimen) or placebo during healing over 6 weeks postinfarction, indicated that prolonged nitrate therapy effectively limited left ventricular remodeling and improved function further compared with placebo.

Rationale for intermittent nitrate therapy

Tolerance to the pharmacologic and therapeutic effects of nitrate therapy is now well established. This phenomenon may be defined as either a decreased response to a given amount of nitrate or the need for an increased amount of nitrate to maintain a constant effect. Tolerance has been demonstrated with all forms of nitrate therapy that maintain continuous blood levels of the drug, including frequent oral dosing, constant intravenous infusion, and continuous transdermal delivery. It can develop rapidly after only a few doses of a nitrate preparation and tends to be partial rather than absolute. Strategies for the prevention of nitrate tolerance include the avoidance of maximum nitrate doses and the use of intermittent nitrate dosing regimens. Providing a relatively brief nitrate-free interval restores vascular responsiveness to nitrates, most likely due to a recovery of the metabolic mechanisms responsible for the therapeutic effect of these drugs. The duration of this period of nitrate abstinence varies, depending on the nitrate preparation used but is generally in the range of 8-12 hours. Such intermittent therapy not only reduces the risk of nitrate tolerance, but also provides a convenient approach to outpatient management.

Role of nitrates after acute myocardial infarction

Apart from their ability to relieve myocardial ischemia, nitrates have an important role to play on preservation of left ventricular (LV) geometry and function after acute myocardial infarction (MI). In the first 48 hours after acute MI, intravenous nitroglycerin infusion titrated to a low-dose regimen produces multiple benefits, including smaller infarct size, better regional and global LV function, less remodeling, fewer in-hospital complications, and fewer deaths in-hospital and up to 1 year. This regimen might be an effective adjunct during reperfusion therapy for salvaging ischemic myocardium, LV geometry, and function. Recent studies indicate that prolonged therapy with nitrates during the healing phase after acute MI can effectively further limit progressive LV remodeling (less LV dilation, expansion, thinning, and aneurysm formation) and preserve LV function. Tolerance with chronic therapy is avoided by an eccentric dose regimen to provide a nitrate-free interval.

Role of nitrates in unstable angina pectoris

Unstable angina pectoris is a clinically heterogeneous process with patient symptoms varying between reduced threshold for exertional angina and the occurrence of multiple episodes of rest pain. The major factors in the pathogenesis of unstable angina appear to be intracoronary platelet aggregation and thrombus formation secondary to fissuring or rupture of atheromatous plaques, with associated coronary vasoconstriction due to release of constrictor materials from aggregating platelets and deficiency of endothelium-related vasodilator activity. The latter factor is of particular interest in view of the similar biochemical mechanisms of action of nitroglycerin (NTG) and endothelium-derived relaxing factor (EDRF). The efficacy of NTG in limiting platelet aggregation is also of particular interest in this condition. Medical therapy in patients with unstable angina usually requires use of multiple agents. In the short term, there is a strong case for the use of intravenous heparin both to relieve pain and to reduce the risk of acute myocardial infarction. Aspirin is perhaps less effective in the short term, but very useful in long-term treatment of such patients. Despite their widespread clinical use, beta-adrenoceptor antagonists are probably only marginally beneficial, whereas dihydropyridine calcium antagonists such as nifedipine are potentially harmful as monotherapy and of questionable use in combination with other drugs. Other agents that are effective in relieving ischemic symptoms are the nondihydropyridine calcium antagonists verapamil and diltiazem and the oxygen-sparing agent perhexiline maleate. Despite a paucity of controlled trial data, nitrates are used in the vast majority of patients with unstable angina.

Use of nitrates in ischemic heart disease

The organic nitrates have remarkably diverse actions that are or should be beneficial in patients with ischemic heart disease. These drugs are effective in all the important ischemic syndromes. Preliminary data in patients with acute infarction suggest that the drugs may be truly cardioprotective, resulting in improved mortality. This review has not discussed the role of nitrates in congestive heart failure or LV dysfunction, a subject of great importance. The nitrates are useful adjunctive agents in these syndromes, and the two VeHfT trials support the concept that long-term nitrate administration, in conjunction with hydralazine, may favorably alter the natural history of heart failure. This cardioprotective effect is similar to that suggested for the post-MI patient. The data are not strong enough for definitive conclusions at this time. The clinical benefits of nitrates in decreasing subjective (angina) and objective indices of ischemia in stable and unstable angina, as well as limited data in asymptomatic myocardial ischemia, are unequivocal and are as favorable as those for beta blockers or calcium antagonists. Tolerance is an important problem that unfavorably influences the potential benefits of nitrate therapy. I believe that this problem can be avoided with well-designed dosing regimens. Current research into endothelial biology in health and disease has further supported a physiologic role for the organic nitrates in patients with ischemic heart disease. The nitrate-platelet story, while controversial, is promising and offers another positive rationale for nitrate administration. The concept of nitrates replenishing disordered EDRF release or action is an exciting one. Physicians should feel fortunate to have such a remarkable group of drugs available for their patients.

Intravenous nitroglycerin unloading in acute myocardial infarction

Low-dose intravenous nitroglycerin infusion can be safely administered during acute myocardial infarction to unload the left ventricle and salvage ischemic myocardium and left ventricular geometry and function. In an experimental conscious dog model, low-dose infusion titrated to decrease mean blood pressure by 10% over the first 6 hours after coronary artery ligation resulted in 51% decrease in infarct size, 54% decrease in preload, and more than 50% increase in collateral blood flow. The same benefits were seen when methoxamine was given to counteract that 10% decrease in blood pressure. Similar short-term nitroglycerin infusion also limited remodeling in the dog model. More important, no myocardial salvage was seen with excessive nitroglycerin-induced hypotension to levels less than 80 mm Hg. Clinically, prolonged low-dose nitroglycerin infusion was evaluated in a prospective, randomized, single-blinded, placebo-controlled study of 310 patients with acute infarction: 154 received nitroglycerin and 156 received placebo. Nitroglycerin was titrated to reduce mean blood pressure by 10% in normotensive patients and up to 30% in hypertensive (blood pressure greater than 140/90 mm Hg) patients, but not to less than 80 mm Hg. Nitroglycerin produced several benefits compared with placebo: (1) smaller creatine kinase infarct size; (2) less regional left ventricular dysfunction, better global ejection fraction, and less infarct expansion and thinning; (3) better clinical functional status and hemodynamics; (4) fewer inhospital complications such as acute left ventricular failure and dilation due to marked infarct expansion, left ventricular thrombus, cardiogenic shock, and infarct extension; and (5) fewer deaths up to 1 year in patients with anterior Q-wave infarction.

Adjunctive use of beta-adrenergic blockers, calcium antagonists and other therapies in coronary thrombolysis

The availability of thrombolytic agents for use in the treatment of acute myocardial infarction is an important step in the management of a common, often debilitating, and potentially lethal disorder. However, despite the proven benefits of coronary thrombolysis, the importance of adjunctive treatment modalities is being increasingly recognized. Beta-adrenergic blockers, calcium antagonists, nitrates, magnesium, and angiotensin-converting enzyme inhibitors each exert favorable cardiovascular properties that may offer additional benefits. Clinical trials combining thrombolytic and adjunctive pharmacologic agents offer hope for further advances in the treatment of acute myocardial infarction.