Relative effects of alpha 1-adrenoceptor blockade, converting enzyme inhibitor therapy, and angiotensin II subtype 1 receptor blockade on ventricular remodeling in the dog

The renin-angiotensin system in left ventricular remodeling

Left ventricular remodeling is a dynamic process that occurs in reaction to an insult to the myocardium. The response to either loss of cells, as may occur following myocardial infarction, or to hemodynamic overload, as may occur in aortic stenosis, is an attempt to maintain cardiac output and normalize wall tension. This is accomplished through the activation of the renin-angiotensin system and hypertrophy of noninfarcted segments of the myocardium. in the case of moderate or large infarctions these mechanisms fail to normalize wall stress. The stimulus to further remodeling remains, viable myocytes hypertrophy (with greater increases in cell length than width), the mass-to-volume ratio increases, and an exponential increase in wall stress results. This increase in myocyte tension has been associated with premature myocyte cell death. Thus, a vicious cycle is established wherein overstretch of the myocardium while sustaining cardiac output leads to progressive myocyte loss and left ventricular dilation. The renin-angiotensin system plays an integral role in this process. Its inhibition by angiotensin-converting enzyme (ACE) inhibitors both chronically and immediately after myocardial infarction has been shown to decrease left ventricular volumes and reduce mortality. Controversy exists regarding the mechanism through which ACE inhibitors exert their effects. ACE inhibitors reduce afterload/preload, circulating angiotensin II levels, and raise circulating levels of bradykinin. It is not yet clear which mechanism is responsible for the greatest impact on left ventricular dilation and mortality. inhibition of the renin-angiotensin system is clearly beneficial to cardiac performance as well as morbidity and mortality when myocardium is lost and heart failure ensues. Specific modes of action require further definition, including local and systemic effects. Possible benefits of angiotensin receptor blockade versus augmentation of bradykinin requires definition, setting the stage for further study, while the beneficial therapeutic use of these agents continues.

Left ventricular dysfunction and heart failure. Mechanistic and therapeutic distinctions

Dysfunction of the left ventricle may result from a variety of insults, all of which may initiate a self-perpetuating process of ventricular remodeling which may progress to end-stage heart disease. Symptoms of heart failure may or may not co-exist with this ventricular remodeling. Treatment and prevention of these two largely distinct entities differ. Symptoms may respond to diuretics, vasodilators and digoxin. Progressive ventricular remodeling may be slowed by angiotensin converting enzyme inhibitors, hydralazine + isosorbide dinitrate and beta blockers. Prevention of symptomatic heart failure is dependent on early recognition of ventricular dysfunction and aggressive treatment to slow its progression. Development of more effective and targeted therapies will be dependent on expanded insight into the cellular and molecular mechanisms contributing to the remodeling process.

Critical review of heart failure: the role of left ventricular remodeling in the therapeutic response

Recent clinical investigations have provided new insights into physiologic alterations taking place as heart failure progresses. As a result, new end points may be more relevant to monitor in patients, rather than the classical assessment of severity based on subjective evaluation of functional capacity. Structural changes of the myocardium, referred to as remodeling, have profound effects on the performance of the left ventricle and on long-term prognosis. Left ventricular ejection fraction may serve as a clinical marker for remodeling changes. Remodeling changes involve hypertrophy of the muscle mass, dilation of the left ventricle, and a change in the shape of the ventricle. Histologically, these changes are produced by myocyte lengthening, interstitial growth, and myocyte slippage. The consequences of remodeling include an abnormal bio-energetic state because of the augmented wall stress and accentuated myocardial oxygen consumption. The elevated wall stress promotes further hypertrophy. Superimposed on the remodeling changes is neurohormonal activation. Both remodeling and neurohormonal activation are directly related to mortality. An experimental canine model has been used successfully to investigate the potential impact of pharmacologic intervention on remodeling. Angiotensin-converting enzyme inhibitors and the combination of hydralazine and isosorbide dinitrate have been shown in clinical trials to reduce long-term mortality and increase ejection fraction. The effects of these drugs appear to be mediated in part by alterations in remodeling and neurohormonal activation. Thus, left ventricular ejection fraction can be used as a clinical marker for remodeling processes, and plasma norepinephrine levels can be used as a marker for neurohormonal activation.