Remodelling of the heart after myocardial infarction

Ex vivo stretch reveals altered mechanical properties of isolated dystrophin-deficient hearts

Duchenne muscular dystrophy (DMD) is a progressive and fatal disease of muscle wasting caused by loss of the cytoskeletal protein dystrophin. In the heart, DMD results in progressive cardiomyopathy and dilation of the left ventricle through mechanisms that are not fully understood. Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes. To expand upon these findings, here we have subjected the left ventricles of dystrophin-deficient mdx hearts to mechanical stretch. Unexpectedly, isolated mdx hearts showed increased left ventricular (LV) compliance compared to controls during stretch as LV volume was increased above normal end diastolic volume. During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts. This suggests that the mechanical properties of the intact heart cannot be modeled as a simple extrapolation of findings in single cardiac myocytes. To explain these findings, a model is proposed in which disruption of the dystrophin-glycoprotein complex perturbs cell-extracellular matrix contacts and promotes the apparent slippage of myocytes past each other during LV distension. In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan-null and laminin-α₂ mutant mice, but not in dysferlin-null mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes. Collectively, these findings suggest a novel and cell-death independent mechanism for the progressive pathological LV dilation that occurs in DMD.

Clinical perspectives and therapeutics of thrombolysis

Reperfusion therapy with thrombolytic agents has been a significant advancement in the management of patients with acute ST elevation myocardial infarction. The outcome of acute myocardial infarction has significantly improved by early application of thrombolytic therapy. Intracoronary streptokinase has been used for >30 years, but reawakening interest occurred in the early 1980s in the use of thrombolytic therapy to establish rapid reperfusion during an acute myocardial infarction. Initial studies aimed at direct intracoronary thrombolysis, but owing to its cumbersome process and requirement of an active round the clock cardiac catheterization laboratory, it has been replaced by regimens of intravenous thrombolytic therapy which is as efficacious as intracoronary administration. Consideration of thrombolytic therapy has become a standard treatment for patients presenting with acute ST elevation myocardial infarction and various well-controlled trials have demonstrated the importance of both early and full reperfusion in improving clinical outcome in the setting of acute myocardial infarction. The subject of intravenous thrombolysis is perhaps the most rapidly evolving area in the management of acute myocardial infarction patients in the past decade. The current review focuses on the thrombolysis in the treatment of myocardial infarction and other conditions.

Pharmacologic therapies after myocardial infarction

Despite the availability and use of effective methods for limiting infarct size with thrombolytic agents and primary angioplasty, patients experiencing a myocardial infarction (MI) are at increased risk for a second cardiac event in the post-MI period (e.g., reinfarction, heart failure, and sudden death). For this reason, postinfarction risk management is crucial. An extensive data base has firmly established the efficacy of beta blockers in reducing cardiovascular risk following acute MI. The full advantages of angiotensin-converting enzyme (ACE) inhibitors have only recently begun to emerge as the result of a growing understanding of the mechanisms of adverse outcomes following MI. The importance of lipid-lowering agents, in particular the "statins," should be considered in all post-MI patients, especially since recent studies have conclusively shown improved survival and reduced rates of MI and coronary artery bypass surgery in this population with this therapy. Aspirin is now considered a standard part of the early management of the acute infarct patient as well as for secondary prevention in post-MI patients. At present, chronic anticoagulation with warfarin should be reserved for selected patients. The nondihydropyridine calcium antagonists diltiazem and verapamil can be considered for post-MI use only in patients in whom beta blockers are contraindicated and who have preserved systolic function and/or those without clinical heart failure. In contrast, the dihydropyridine calcium antagonists, particularly nifedipine, have no role in secondary prevention. Although long-term benefits are minimal, nitrates continue to be useful in post-MI patients with residual ischemia (angina or silent ischemia), heart failure (systolic or diastolic), or postinfarction hypertension. Antiarrhythmic agents, except amiodarone, are relatively contraindicated in post-MI patients. Recent data show that vitamin E reduces the rate of nonfatal MI. Its role in cardiovascular death and overall mortality remains to be clarified. Despite their demonstrated value, agents used in secondary prevention generally appear to be underutilized. In addition, when pharmacologic therapies are administered for secondary prevention, they are often prescribed at lower doses than those tested and proved in trials. A greater appreciation for the efficacy and safety profiles of these agents could lead to more widespread use and more pronounced reductions in morbidity and mortality among post-MI patients.

Factors affecting outcome after recovery from myocardial infarction

Patients surviving acute myocardial infarction are susceptible to heart failure, recurrence of angina, reinfarction, arrhythmias, and sudden cardiac death. Most deaths occur in the first six months after infarction. Advancing age is the most important nonmodifiable prognostic factor for long-term prognosis, whereas left ventricular function assessed clinically or measured as either ejection fraction or end-systolic volume is the most important modifiable factor. Other significant long-term prognostic factors include: postinfarction angina at rest, inducible ischemia during exercise testing with or without radioisotope imaging, severity and extent of coronary artery disease, patency of the infarct-related artery, late ventricular arrhythmias, decreased heart rate variability, cigarette smoking, hypercholesterolemia, and diabetes mellitus. Identification of these adverse prognostic factors permits risk stratification and enables physicians to determine the most appropriate and cost-effective treatment. Most patients should have a stress test for inducible ischemia and a non-invasive (echo or radionuclide) assessment of left ventricular function. For high-risk patients such as those with prior infarction, heart failure, early postinfarction angina, or frequent late ventricular arrhythmias, coronary angiography and ventriculography prior to discharge are recommended. Assessment of late potentials and heart rate variability will help identify a subgroup of patients at risk for ventricular arrhythmias and cardiac death. However, a more accurate prediction of reinfarction is not possible at present, and no reliable test for atherosclerotic plaque instability has been developed.