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Cardiology

Cardiology (from Greek word kardia and meaning heart or inner self) is the branch of medicine dealing with disorders of the heart and blood vessels. The field is commonly divided in the branches of congenital heart defects, coronary artery disease, heart failure, valvular heart disease and electrophysiology. Physicians specializing in this field of medicine are called cardiologists.

Structure of the heart

Epicardium

Epicardium describes the outer layer of heart tissue (from Greek; epi- outer, cardium heart). When considered as a part of the pericardium, it is the inner layer, or visceral pericardium.

Its largest constituent is connective tissue and functions as a protective layer. The visceral pericardium apparently produces the pericardial fluid, which lubricates motion between the inner and outer layers of the pericardium.

During ventricular contraction, the wave of depolarization moves from endocardial to epicardial surface.

Pericardium

The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.

Layers

There are two layers to this sac: the fibrous pericardium and the serous pericardium. The serous pericardium, in turn, is divided into two layers; in between these two layers there is a potential space called the pericardial cavity.

The fibrous pericardium is the most superficial layer. It is a dense connective tissue, protecting the heart, anchoring it to the surrounding walls, and preventing it from overfilling with blood. It is continuous with the outer adventitial layer of the neighboring great blood vessels.

The serous pericardium is deep to the fibrous pericardium. It contains two layers, both of which function in lubricating the heart to prevent friction from occurring during heart activity.

* The layer next to the fibrous pericardium is the parietal layer.
* The layer next to the heart is the visceral layer, also known as the epicardium.

Together these two layers form a continuous uninterrupted membrane. Between these two layers exists a small cavity called the pericardial cavity, which contains a supply of serous fluid.The serous fluid that is found in this space is known as the pericardial fluid.

Innervation

The fibrous pericardium and the parietal layer of the serous pericardium are sensory innervated by the phrenic nerves (C3-C5). The epicardium receives autonomic innervation from the underlying heart.

Diseases

Pericarditis is inflammation of the pericardium. It can cause fluid to build up in the sac (pericardial effusion). Excessive amounts of fluid may lead to cardiac tamponade by physically blocking the heart from beating properly or compression of the great vessels of the heart.

Myocardium

Myocardium is the muscular tissue of the heart. Other tissues are the endocardium (inner lining, effectively a specialised endothelium) and the pericardium (a connective tissue layer around the heart).

The myocardium is composed of specialized cardiac muscle cells with an ability not possessed by muscle tissue elsewhere in the body. Cardiac muscle, like other muscles, can contract, but it can also conduct electricity, like nerves.

The blood supply of the myocardium is by the coronary arteries. If these arteries are occluded by atherosclerosis and/or thrombosis, this can lead to angina pectoris or myocardial infarction.

Certain viruses lead to inflammation of the myocardium, or myocarditis.

Failure of the heart to contract properly (for various reasons) is termed heart failure, generally leading to fluid retention, edema, pulmonary oedema, renal insufficiency, hepatomegaly, a shortened life expectancy and decreased quality of life.

Papillary muscle

In anatomy, the papillary muscles of the heart serve to limit the movements of the mitral and tricuspid valves and prevent them from being inverted. They do not close or open the valves, which close passively in response to pressure gradients. Instead they brace the valves against the high pressure.

The U wave in an ECG represents papillary muscle repolarization. It usually does not appear unless a patient's electrolytes are imbalanced.

Endocardium

In the heart, the endocardium is the innermost layer of cells, embryologically and biologically similar to the endothelium that lines blood vessels.

The endocardium overlies the much more voluminous myocardium, the muscular tissue responsible for the contraction of the heart. The outer layer of the heart is termed epicardium and the heart is surrounded by a small amount of fluid enclosed by a fibrous sac called the pericardium.

Function

Recently, it has become evident that the endocardium, which is primarily made up of endothelial cells, controls myocardial function. This modulating role is separate from the homeometric and heterometric regulatory mechanisms that control myocardial contractility. Moreover, the endothelium of the myocardial capillaries, which is also closely appositioned to the cardiomyocytes is involved in this modulatory role. Thus, the cardiac endothelium (endocardial endothelium and the endothelium of the myocardial capillaries) controls the development of the heart in the embryo, as well as in the adult, i.e. during hypertrophy. Additionally, the contractility and electrophysiological environment of the cardiomyocyte are regulated by the cardiac endothelium.

The endocardial endothelium may also act as a kind of blood-heart barrier (analogous to the blood-brain barrier), thus controlling the ionic composition of the extracellular fluid in which the cardiomyocytes bathe.

Role in disease

In myocardial infarction, ischemia of the myocardium can extend to the endocardium, disrupting the inner lining of the heart ("Transmural" infarction). Less extensive infarctions are often "subendocardial" and do not affect the endocardium. Subendocardial infarction's are much more dangerous than Transmural infarctions because they create an area of dead tissue surrounded by a boundary region of damaged myocytes. This damaged region will conduct impulses more slowly, resulting in irregular rhythms. The damaged region may enlarge or extend and become more life-threatening.

In infective endocarditis, the endocardium (especially the endocardium lining the heart valves) is affected by bacteria.

Coronary circulation

The coronary circulation consists of the blood vessels that supply blood to, and remove blood from, the heart muscle itself. Although blood fills the chambers of the heart, the muscle tissue of the heart, or myocardium, is so thick that it requires coronary blood vessels to deliver blood deep into the myocardium. The vessels that supply blood high in oxygen to the myocardium are known as coronary arteries. The vessels that remove the deoxygenated blood from the heart muscle are known as cardiac veins.

The coronary arteries that run on the surface of the heart are called epicardial coronary arteries. These arteries, when healthy, are capable of autoregulation to maintain coronary blood flow at levels appropriate to the needs of the heart muscle. These relatively narrow vessels are commonly affected by atherosclerosis and can become blocked, causing angina or a heart attack. (See also: circulatory system.)

The coronary arteries are classified as "end circulation", since they represent the only source of blood supply to the myocardium: there is very little redundant blood supply, which is why blockage of these vessels can be so critical.

Coronary anatomy

The exact anatomy of the myocardial blood supply varies considerably from person to person. A full evaluation of the coronary arteries requires cardiac catheterization.

In general there are two main coronary arteries, the left and right.

* Right coronary artery * Left coronary artery

Both of these arteries originate from the beginning (root) of the aorta, immediately above the aortic valve. As discussed below, the left coronary artery originates from the left aortic sinus, while the right coronary artery originates from the right aortic sinus.

Variations

Four percent of people have a third, the posterior coronary artery. In rare cases, a patient will have one coronary artery that runs around the root of the aorta.

Occasionally, a coronary artery will exist as a double structure (ie there are two arteries, parallel to each other, where ordinarily there is one). Dana Carvey has this variation, which led to a mishap during his CABG operation.

Coronary artery dominance

The artery that supplies the posterior descending artery (PDA) and the posterolateral artery (PLA) determines the coronary dominance.

* If the right coronary artery (RCA) supplies both these arteries, the circulation can be classified as "right-dominant".
* If the left circumflex artery (LCX) supplies both these arteries, the circulation can be classified as "left-dominant".
* If the RCA supplies the PDA and the LCX supplies the PLA, the circulation is known as "co-dominant".

Approximately 70% of the general population are right-dominant, 20% are co-dominant, and 10% are left-dominant. [1]

Blood supply of the papillary muscles

The papillary muscles tether the mitral valve (the valve between the left atrium and the left ventricle) and the tricuspid valve (the valve between the right atrium and the right ventricle) to the wall of the heart. If the papillary muscles are not functioning properly, the mitral valve leaks during contraction of the left ventricule. This causes some of the blood to travel "in reverse", from the left ventricle to the left atrium, instead of forward to the aorta and the rest of the body. This leaking of blood to the left atrium is known as mitral regurgitation.

The anterolateral papillary muscle receives two blood supplies: the LAD and LCX, and is therefore somewhat resistant to coronary ischemia. On the other hand, the posteromedial papillary muscle is supplied only by the PDA. This makes the posteromedial papillary muscle significantly more susceptible to ischemia. The clinical significance of this is that a myocardial infarction involving the PDA is more likely to cause mitral regurgitation.

Coronary flow

During contraction of the ventricular myocardium (systole), the subendocardial coronary vessels (the vessels that enter the myocardium) are compressed due to the high intraventricular pressures. However the epicardial coronary vessels (the vessels that run along the outer surface of the heart) remain patent. Because of this, blood flow in the subendocardium stops. As a result most myocardial perfusion occurs during heart relaxation (diastole) when the subendocardial coronary vessels are patent and under low pressure. This contributes to the filling difficulties of the coronary arteries.

The primary determinant of coronary blood flow is the level of myocardial/cardiac oxygen consumption. As the heart beats more vigorously, ATP is consumed at a greater rate due to the increased force and/or frequency of contraction and the depolarization and repolarization of the cardiac membrane potential. The increase in oxygen consumption results in the release of a vasodilator substance, the identity of which remains unknown. The vasodilator reduces vascular resistance and allows more blood to flow through the heart during each diastole. Systolic compression remains the same. Failure of oxygen delivery via increases in blood flow to meet the increased oxygen demand of the heart results in tissue ischemia, a condition of oxygen debt. Brief ischemia is associated with intense chest pain, known as angina. Severe ischemia can cause the heart muscle to die of oxygen starvation, called a myocardial infarction. Chronic moderate ischemia causes contraction of the heart to weaken, known as myocardial hibernation.

In addition to metabolism, the coronary circulation possesses unique pharmacologic characteristics. Prominent among these is its reactivity to adrenergic stimulation. The majority of circulation in the body constrict to norepinephrine, a sympathetic neurotransmitter the body uses to increases blood pressure. In the coronary circulation, norepinephrine elicits vasodilation, due to the predominance of beta-adrenergic receptors in the coronary circulation. Agonists of alpha-receptors, such as phenylephrine, elicit very little constriction in the coronary circulation.

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