Heart failure is routinely classified as either left or right ventricular failure. Left ventricular failure occurs when the left ventricle ceases to function as an adequate pump of the systemic circulation. In right ventricular failure, the right ventricle fails to adequately pump blood to the pulmonary system, thereby interfering with gas exchange in the alveoli and leading to a decreased filling volume of the left ventricle.
There are several etiologies, which may be permanent or temporary, that cause the heart to fail as a forward pump. The most common etiologies of heart failure are myocardial infarction and hypertensive cardiomyopathy. Less common causes include infectious cardiac disease processes, such as acute myocarditis or endocarditis; induced heart failure from drugs/medications (e.g., cocaine, beta-blockers, tricyclic antidepressants); traumatic mechanisms, such as a myocardial contusion; or metabolic derangements that result in cardiac dysrhythmias, such as sustained tachycardias or bradycardias.
Congestive heart failure (CHF) is a common disorder associated with various degrees of ventricular failure. Approximately 4.6 million Americans are being treated for CHF, and 550,000 new cases are diagnosed each year. The prevalence of CHF increases dramatically with age, occurring in 1%-2% of people ages 50-59 years and up to 10% of people older than 75 years. Approximately 80% of all heart failure admissions occur in patients older than 65 years of age. In fact, CHF is the leading hospital discharge diagnosis in individuals aged 65 years or older.
Despite a steady decline in the incidence of coronary artery disease and stroke, both the incidence and prevalence of CHF are on the rise. Between 1985 and 1995, the number of heart failure hospitalizations increased by 51%. Approximately 870,000 hospital discharges for CHF occurred in 1996.
Heart failure has a tremendous economic impact on the U.S. healthcare system because of direct medical costs, disability and loss of employment. Estimated treatment costs in 1994 were $38 billion, of which $23 billion were spent on hospitalizations. The cost of hospitalizations for heart failure is twice that for all forms of cancer and myocardial infarction combined.
The heart itself is comprised of specialized fibers known as cardiac muscle. Myocardial fibers are unique in that they have the ability not only to initiate an impulse that will lead to contraction, but they are able to alter the speed of the conduction as it travels between fibers. The cardiac muscle is divided into three distinct tissue layers: the endocardium, myocardium and pericardium (sometimes referred to as the epicardium).
The endocardium is the innermost layer and is constantly surrounded by the blood that flows through the heart. The myocardium is the middle layer of the heart and tends to be the thickest due to its bulky muscle mass. The myocardial muscle cells are unlike other muscles in the body, because they are very strong and possess the ability to maintain constant stretch, like skeletal muscle, while simultaneously being stimulated by self-generated electrical impulses. The overall strength of contraction comes from within the myocardial muscle. The continuous workload of the myocardium and its ability to constantly contract is largely due to the vast amount of capillary blood flow that is networked within the muscle bed. This provides the required oxygen, glucose and other nutrient delivery, and, equally important, waste removal, which is mandatory in order to sustain the ongoing workload. The pericardium is the layer commonly referred to as the "pericardial sac." It is a tough fibrous sac that surrounds the heart to provide protection and also lubrication to reduce the friction of the heart wall against the sac as it contracts.