Although work on class III antiarrhythmics remains at an early stage, these agents still appear to possess greater efficacy and less proarrhythmia than conventional class I agents in those experimental arrhythmia models considered to be most representative of the clinical situation. Although prolongation of repolarization carries with its own tendency for pause-dependent arrhythmogenesis (i.e., torsade de pointes), available data suggest that this may be a function of nonspecificity in potassium channel block rather than a general characteristic of class III activity. The availability of new and more selective blockers of specific cardiac potassium channels under development as class III agents have already helped to clarify basic questions about the ionic mechanism of repolarization in the heart, and one hopes that a growing clinical data base will eventually determine the relative safety and efficacy of these agents in preventing symptomatic and life-threatening arrhythmias.

The evidence gained from both human and animal studies of chronic chagasic cardiomyopathy suggests that the disease occurs as a consequence of several discrete and progressive pathophysiological processes occurring after infection, the ultimate expression of which depends on a host of unidentified factors. Collectively, the infection-associated events compromise microvasculature function and result in hypoperfusion, with consequences indistinguishable from those observed in other, nonparasitological cardiomyopathic diseases secondary to hypoperfusion. Therefore, chronic chagasic cardiomyopathy may share similar pathophysiological abnormalities with other chronic congestive cardiomyopathic states.

With the limitations of currently available modalities for treating clinically important tachycardias, the role of implanted antitachycardia devices will continue to expand. The challenge of the future will not only involve continued technological advances but the socioeconomic impact of this efficacious but expensive mode of therapy in an era of increasing financial restraints. Further studies to definitively prove the efficacy of more widespread use of antitachycardia device therapy will be needed.

In the era of comparative and adjunctive trials in reperfusion therapy, the need to develop alternative end points for mortality reduction is clear. Left ventricular ejection fraction, which has been commonly used as a surrogate, is problematic due to missing values, technically inadequate studies, and lack of correlation with mortality results in controlled reperfusion trials performed to date. In this paper, we present a composite clinical end point that includes, in order, severity of adverse outcome death, hemorrhagic stroke, nonhemorrhagic stroke, poor ejection fraction (less than 30%), reinfarction, heart failure, and pulmonary edema. Such a composite index may be useful to detect true therapeutic benefit in reperfusion trials without necessitating greater than 20-30,000 patient enrollment.

Exercise trials in cardiology are often hindered by inconsistent approaches to exercise testing. These inconsistencies include the choice of exercise protocol, exercise end points, points of analysis, and absence or misuse of gas exchange data. Gas exchange techniques greatly enhance the accuracy with which cardiopulmonary function is assessed by exercise. Commonly used protocols are not always appropriate for all patients or all studies. Both cardiovascular disease and the exercise protocol can have an important impact on the relation between changes in work rate and oxygen uptake. Ramp protocols appear to offer the greatest promise for assessing cardiopulmonary function. Analyzing hemodynamic and gas exchange responses at several points submaximally, in addition to those at peak exercise, can add important information concerning the efficacy of a drug. A great deal of confusion continues to hinder the application of the gas exchange anaerobic threshold, and many of the commonly used testing end points are not reliable.

For the past 5 years, investigators from many different laboratories have contributed to a greatly increased understanding of two very important lipid-carrying proteins in plasma--apo B-100 and apo B-48. Apo B-100, an extremely large protein composed of 4,536 amino acids, is synthesized by the liver and is crucial for the assembly of triglyceride-rich VLDL particles. Apo B-100 is virtually the only protein of LDL, a cholesteryl ester-enriched class of lipoproteins that are metabolic products of VLDL. The apo B-100 of LDL serves as a ligand for the LDL receptor-mediated uptake of LDL particles by the liver and extrahepatic tissues. The LDL receptor-binding region of apo B-100 is located in the carboxyterminal portion of the molecule, whereas its lipid-binding regions appear to be broadly dispersed throughout its length. Apo B-48 contains the amino-terminal 2,152 amino acids of apo B-100 and is produced by the intestine as a result of editing of a single nucleotide of the apo B mRNA, which changes the codon specifying apo B-100 amino acid 2,153 to a premature stop codon. Apo B-48 has an obligatory structural role in the formation of chylomicrons; therefore, its synthesis is essential for absorption of dietary fats and fat-soluble vitamins. Both apo B-48 and apo B-100 are encoded on chromosome 2 by a single gene that contains 29 exons and 28 introns. An elevated level of apo B-100 in the plasma is a potent risk factor for developing premature atherosclerotic disease. In the past 3 years, many different apo B gene mutations that affect the concentrations of both apo B and cholesterol in the plasma have been characterized. A missense mutation in the codon for apo B-100 amino aid 3,500 is associated with hypercholesterolemia. This mutation results in poor binding of apo B-100 to the LDL receptor, thereby causing the cholesteryl ester-enriched LDL particles to accumulate in the plasma. This disorder is called familial defective apo B-100, and it is probably a cause of premature atherosclerotic disease. Familial hypobetalipoproteinemia is a condition associated with abnormally low levels of apo B and cholesterol; affected individuals may actually have a reduced risk of atherosclerotic disease.(ABSTRACT TRUNCATED AT 400 WORDS)

The primary role of cardiac rate in adapting cardiac output to changing physiological needs has been more clearly recognized in recent years. Previously, the rate of cardiac stimulation had been determined either at pacemaker manufacture, by programming a single rate, or by sensing the atrium. More recently, sensing another physiological or nonphysiological function that changes in response to body need has become possible. Exercise changes blood oxygen saturation, central venous pH, central venous temperature, minute ventilation and respiratory rate, stroke volume, circulating catecholamines, QT interval, evoked endocardial response to a stimulus, and the mechanics of myocardial contraction. Some sensors respond to muscle work but not to intellectual effort or emotion. Pacemaker-based sensors of physiological function or activity allow a change in cardiac stimulation rate in response to need. Whichever sensor is used, increases in ventricular rate during exercise regularly produce a cardiac output response. Single-chamber, rate-modulated pacemakers in atrium or ventricle and dual-chamber devices are now implanted on a widespread basis. These drive the atrium, the ventricle, or both, sensing or pacing the atrium at a rate determined by the sensor.

Of 2,122 consecutive patients undergoing elective coronary angioplasty from 1982 to 1985, 62% had stable angina pectoris (SAP), and 38% had unstable angina pectoris (UAP). There were differences between the two groups in clinical and morphological factors and in initial and late results of angioplasty. UAP patients were more likely than SAP patients to be smokers and to have had prior myocardial infarctions. Lesions in UAP patients were more severe, longer, more eccentric, more irregular, and more likely to have intracoronary thrombi than were lesions in SAP patients. Coronary angioplasty success was achieved in 84% of UAP and in 88% of SAP patients (p less than 0.05), and complications occurred in 6.7% of UAP and in 4.7% of SAP patients (p less than 0.05). Hospital death rates were low and similar, 0.2% for both groups. Follow-up (mean, 37 months) showed recurrent Canadian Cardiovascular Society (CCVS) class III/IV angina in 30.1% of UAP and in 25.2% of SAP patients (p less than 0.05). There was a return to work in 86% of UAP and in 91% of SAP patients (p less than 0.05). When UAP patients' durations of symptoms were further fractionated, it was found that the earlier angioplasty was performed after onset of angina, the lower was the success rate and the higher the complication rate and incidence of late follow-up untoward events. When coronary angioplasty was performed within 1 week of onset of angina ("early"), success was 79.1%; when angioplasty was performed 2 weeks or more after onset of angina ("later"), success was 86.3%. Major cardiac events occurred in 11.5% in the early group and in 4.8% in the later group (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Rupture of an atherosclerotic plaque associated with partial or complete thrombotic vessel occlusion is fundamental to the development of ischemic coronary syndromes. Plaques that produce only mild-to-moderate angiographic luminal stenosis are frequently those that undergo abrupt disruption, leading to unstable angina or acute myocardial infarction. Plaques with increased lipid content appear more prone to rupture, particularly when the lipid pool is localized eccentrically within the intima. Macrophages appear to play an important role in atherogenesis, perhaps by participating in the uptake and metabolism of lipoproteins, secretion of growth factors, and production of enzymes and toxic metabolites that may facilitate plaque rupture. In addition, the particular composition or configuration of a plaque and the hemodynamic forces to which it is exposed may determine its susceptibility to disruption. Exposure of collagen, lipids, and smooth muscle cells after plaque rupture leads to the activation of platelets and the coagulation cascade system. The resulting thrombus may lead to marked reduction in myocardial perfusion and the development of an unstable coronary syndrome, or it may become organized and incorporated into the diseased vessel, thus contributing to the progression of atherosclerosis. In unstable angina, plaque disruption leads to thrombosis, which is usually labile and results in only a transient reduction in myocardial perfusion. Release of vasoactive substances, arterial spasm, or increases in myocardial oxygen demand may contribute to ischemia. In acute myocardial infarction, plaque disruption results in a more persistent thrombotic vessel occlusion; the extent of necrosis depends on the size of the artery, the duration of occlusion, the presence of collateral flow, and the integrity of the fibrinolytic system. Thrombi that undergo lysis expose a highly thrombogenic surface to the circulating blood, which has the capacity of activating platelets and the coagulation cascade system and may lead to thrombotic reocclusion. Measurements aimed at reversing the process of atherosclerosis via cholesterol reduction and enhanced high density lipoprotein activity are encouraging. Active research is being focused on the development of new antithrombotic tools, such as inhibitors of thrombin, thromboxane, and serotonin receptor antagonists, and monoclonal antibodies aimed at blocking platelet membrane receptors or adhesive proteins. These compounds may prove useful when immediate and potent inhibition of the hemostatic system is desired. Intensive research is still needed in the areas of pathogenesis and therapeutic intervention in atherosclerosis.

Atherosclerotic plaques are either concentric, producing a fixed degree of obstruction, or eccentric, with retention of an arc of normal vessel wall that allows changes in medial muscle tone to vary the degree of stenosis. Plaques may also either be solid and fibrous or may contain, in addition to fibrous thickening, a pool of extracellular cholesterol. Most subjects with ischemic heart disease have mixtures of all plaque types. The endothelium over established human plaques often shows focal denudation injury, with adhesion of a platelet monolayer not detectable by angiography. Larger thrombi are due either to superficial intimal injury, which is the progression of the endothelial denudation seen over otherwise static and intact plaques, or to deep intimal injury caused by plaque fissuring (rupture). Both forms of intimal injury expose collagen and von Willebrand factor to platelets. In deep injury, tears extend from the lumen into the depths of the intima and often enter a lipid pool; in consequence, thrombus initially forms within the plaque, thereby altering its configuration and expanding its volume. Many fissures will reseal at this stage, but the plaque is larger, and the process is an important cause of episodic sudden plaque growth. A proportion of plaque fissures are associated with the additional formation of a luminal thrombus, which may be either mural or occlusive. In life, transitions between mural and occlusive thrombi and vice versa occur rapidly and frequently. Mural thrombus is associated with distal embolization of platelet masses and, in some cases, is associated with cholesterol from the plaque.(ABSTRACT TRUNCATED AT 250 WORDS)

Prognostic data on the importance of silent ischemia is lacking. Preliminary reports suggest that ambulatory electrocardiographic monitoring provides additional information not contained in the exercise test, but it is still unclear whether silent ischemia is a marker of subsequent events or is responsible for their development. Recent investigations also suggest that intermittent episodes of ischemia may cause myocardial necrosis. Drugs that are effective in the treatment of angina are also effective in the treatment of silent ischemia, but their value in terms of long-term morbidity and mortality is unclear. Recently, the circadian distribution of silent ischemia has been reported in 150 patients off therapy, in 33 receiving nifedipine, and in 41 receiving atenolol. Most ischemic episodes off therapy occurred between 7:30 AM and 7:30 PM with a peak in the morning and a lesser peak in the evening. Nifedipine did not alter the circadian pattern of ischemic episodes; atenolol abolished the morning peak, the peak incidence of ischemia then occurring in the evening. This circadian distribution of ischemic episodes and the observed changes with treatment resemble the reported circadian variations of acute myocardial infarction and sudden death. Large multicenter studies are now being performed to determine the effects of treatment on silent ischemia and how this treatment may influence outcome. Until such studies have been completed, it is not possible to clearly define the indication for drug therapy in the management of silent ischemia.

In the United States, there may be as many as two million totally asymptomatic men with silent myocardial ischemia due to coronary artery disease. Because detection of such patients is often fortuitous, a rigorous screening protocol has been suggested for high-risk subgroups (i.e., persons with multiple coronary risk factors and/or family histories of premature coronary artery disease). At present, the initial procedure of choice for a screening protocol is the exercise test, followed by radionuclide procedures to differentiate true-positive from false-positive responses if coronary angiography is being considered. Holter monitoring is useful in documenting out-of-hospital ischemic events in asymptomatic patients with documented coronary artery disease. Prognostic studies in patients with and without coronary angiographic data indicate that asymptomatic patients with an ischemic ST-segment depression on exercise tests comprise a high-risk subgroup that has a well-defined morbidity and mortality due to future cardiac events. The angiographic surveys show that the greatest risk is in those individuals with more extensive disease. Treatment in asymptomatic persons requires the use of objective measurements as end points. Several reports have demonstrated the efficacy of beta-blockers, calcium channel blockers, and coronary angioplasty in this population, at least so far as reducing total ischemic activity is concerned. How nonsurgical therapy will affect ultimate prognosis is still unclear. However, surgical therapy in asymptomatic patients with left main coronary artery disease and triple-vessel disease has been reported to improve long-term survival.

Asymptomatic ischemia after myocardial infarction is a common clinical problem. As much as 50% of the postinfarction patient population who have ischemia on exercise testing may have no symptoms at all. In these patients, ischemia detected by either exercise testing or ambulatory electrocardiographic monitoring, with or without symptoms, confers a worse prognosis. The mechanism of silent ischemia in this group of patients may be due to the deprivation of afferent innervations created by critically placed infarctions. The management of these postinfarction patients must be centered on the treatment of ischemia, regardless of symptoms.

Silent myocardial ischemia is increasingly recognized as a common phenomenon in a variety of populations with coronary artery disease. In patients with angina pectoris, this condition has been recognized during either exercise stress testing or ambulatory monitoring when ischemic-type ST-segment shifts occur without symptoms. Although more information is clearly needed, available data suggest an important independent relation between ambulant ischemia, most of which is silent, and adverse outcome. Other studies show that currently available antianginal agents can prevent or modify symptomatic as well as silent ischemic episodes. Several reports indicate that when these agents are prescribed to control symptomatic ischemic episodes, silent ischemia may continue to occur. Yet, when antianginal drugs are used either alone or in combination in those with recurrent silent ischemia, both the frequency and duration of the silent ischemic episodes are greatly reduced. To date, however, no data are yet available on the impact on outcome of a strategy directed at detecting and treating silent ischemia. Until this information is available, more attention should be focused on identifying and adequately controlling ischemia rather than on simply managing chest pain.

During myocardial ischemia, malignant arrhythmias and acceleration of cell damage may be induced by sympathetic overstimulation of the heart. This stimulation is due to excessive concentrations of catecholamines within the underperfused myocardium, in combination with enhanced myocyte sensitivity to adrenergic stimuli. Various mechanisms may account for local accumulation of catecholamines in the extracellular space of the ischemic but still viable myocardium. In early myocardial infarction, plasma noradrenaline and adrenaline concentrations are enhanced, reflecting increased activity of the whole sympathetic nervous system, rather than local activity in the heart. In uncomplicated infarction, these concentrations are only five times the normal levels at rest, and there are no convincing data that these mildly increased levels of plasma catecholamines directly induce a major deterioration of myocardial function during the ischemic process. Of more importance is the reflex increase in cardiac sympathetic nerve activity that is induced by pain, anxiety, and a fall in cardiac output or arterial blood pressure and that is accompanied by local exocytotic release of noradrenaline from sympathetic nerve endings of the heart. Excessive accumulation of the neurotransmitter, however, is prevented by at least three mechanisms: 1) Released noradrenaline is rapidly removed so long as neuronal catecholamine reuptake is functional. 2) Adenosine accumulating in the ischemic myocardium effectively suppresses exocytotic noradrenaline release by stimulating presynaptic A1-adenosine receptors. 3) Exocytotic catecholamine release ceases when the sympathetic neurons become depleted of adenosine triphosphate since this release mechanism requires high-energy phosphates. However, with progression of ischemia (i.e., greater than 10 minutes), the myocardium is no longer protected against excess adrenergic stimulation since local metabolic release mechanisms become increasingly important. This release, which is independent of both central sympathetic activation and extracellular calcium, occurs in two steps. First, catecholamines escape from their storage vesicles and accumulate in the cytoplasm of the neuron. In the second, rate-limiting step, noradrenaline is transported across the axolemma from the cytoplasm to the interstitial space via the neuronal uptake carrier in reverse of its normal transport direction. As a consequence of this nonexocytotic local metabolic release, extracellular noradrenaline reaches 100-1,000 times its normal plasma concentrations within 30 minutes of ischemia. Concentrations of this magnitude are capable of producing myocardial necrosis, even in the nonischemic heart, and may play an important role in the pathogenesis of ventricular fibrillation in early ischemia.

Prompt reperfusion of the ischemic myocardium by the successful use of thrombolytic agents in the setting of acute myocardial infarction has substantially reduced mortality among patients judged eligible for their use. This approach has revolutionized the management of acute infarction patients worldwide. Such therapy, however, is associated with increased rates of reinfarction and postinfarction angina that have been attributed to the underlying residual atherosclerotic lesion. The expectation that coronary angioplasty would provide the logical solution to this clinical problem has been considerably lowered by the results of a number of prospective randomized clinical trials. These findings, coupled with the realization that as many as 50% of patients suffering acute infarctions will not safely qualify for the use of thrombolytic agents, present compelling reasons to more accurately investigate the role of primary angioplasty and bypass surgery as a means of effecting reperfusion therapy in the setting of acute infarction.