Cardiology
Definition
Cardiology is a medical specialty dealing with the diagnosis and treatment of diseases and disorders of the heart.History
Cardiology first became a specialized field of study when Jean Baptiste de Sénac in 1749 published a summary of contemporary knowledge of the heart. This was followed 12 years later by Leopold Auenbrugger's discovery that the condition of the heart can be estimated by the sound returned from tapping on the chest (percussion). Listening to heart sounds became a major part of medical diagnosis after René Laënnec's invention of the stethoscope in 1816.
Much of the development of cardiology during the 19th century consisted of improved diagnostic methods. An important diagnostic advance was Willem Einthoven's invention in 1903 of the electrocardiograph, which measures the heart's electrical activity; by 1915 the basic methods of diagnosis of heart disease, including fluoroscopic studies of the beating heart, were in place. Various advances in diagnostic technology opened up the possibility of surgical correction of many heart problems.
Cardiology itself remains a medical, not a surgical, specialty although cardiologists work closely with surgeons in cases of heart surgery. Cardiologists provide the continuing care of heart patients, performing basic studies of heart function and supervising all aspects of therapy, including the administration of drugs to modify heart functions.
Much of the development of cardiac medicine in the second half of the 20th century has been in the field of heart surgery. Major advances in this field have included the routine repair of coronary artery disease, one of the major causes of heart attacks; the first human heart transplant, performed by Christiaan Barnard of South Africa in 1967; and the development of a permanently functioning, surgically implanted artificial heart by a research team at the University of Utah, first used in 1982.
Cardiovascular system
HistoryThe heart has been regarded throughout recorded history as an important organ, but the structure and function of the cardiovascular system have been misunderstood for much of that time. The Greek physician Galen in the second century AD is credited with first recognizing that the heart and vessels contained blood rather than air, but he erroneously thought that blood moved from one chamber of the heart to another by seeping through perforations in the muscular divisions, and that blood flowed to the periphery through the effects of a mysterious "attractive force". Galen's views persisted for over 1,500 years until Dr. William Harvey, through careful experimentation and deductive reasoning established the modern view of the cardiovascular system, which has since been refined, but not significantly altered.
Description of cardiovascular system
The cardiovascular system consists of the heart, a muscular hydrodynamic pump, and its conduit vessels, the arteries and veins. Through this hydraulic circuit flows blood carrying oxygen and nutrients to all parts of the body, and transporting accumulated toxic metabolites to the kidneys, liver and lungs for elimination. The heart in fact is two pumps, the right and left heart, which are connected in series and which independently circulate blood to the lungs or to the rest of the body, respectively. Each of these pumping systems, right and left, consists of a small, thin walled atrium which collects blood and forces it into a larger thick walled ventricle which generates enough pressure to move the blood to the lungs or the rest of the body. Since the heart muscle alternately contracts to expel blood and relaxes to take in blood, the pressure in arterial conduits varies in a pulsatile fashion.Reflecting this, blood pressure in the body (the left heart or systemic circulation) is measured as the peak systolic pressure and the trough diastolic pressure, typically 120 and 80 mm of mercury, respectively. The heart maintains these perfusion pressures by pumping 2 to 3 ounces of blood per contraction, approximately 72 times per minute. Thus, the heart pumps over 500,000 gallons of blood per year, and will beat over two and a half billion times in a lifetime.
Ventricular contraction
Unidirectional flow of blood through the heart is maintained by flexible valves at the entrance and exit points of both ventricles. As the valve leaflets snap closed they produce an audible sound which can be heard with a stethoscope, or even an ear pressed against the chest wall. The cardiac entrance valves (mitral on the left and tricuspid on the right) close at the beginning of ventricular contraction to prevent blood from being expelled backward into the venous system and produce the first heart sound. Likewise, the cardiac exit valves (aortic on the left and pulmonary on the right) are open during ventricular contraction to allow blood to flow into the arteries, but snap shut as the ventricles relax to prevent reverseCardiovascular irregularities
flow of arterial blood into the heart, and produce the second heart sound. Closure of the cardiac valves thus produces the "lub-dub....lub-dub...." sounds one hears when listening to the heart. Under normal circumstances blood flow through the heart, valves and vessels is smooth and silent because there is no turbulence. However, when the cardiac valves malfunction, either by leaking (a condition termed regurgitation) or by not fully opening (termed stenosis), turbulent blood flowing across the diseased valve produces a rushing sound or "heart murmer" by which a Cardiologist can frequently diagnose a particular valvular condition. Valvular disease may be congenital, such as being born with misshapen leaflets, or acquired such as rheumatic heart disease or bacterial and fungal valvular infections, termed infective endocarditis. Although the heart muscle can adapt to the extra work demands imposed by valvular regurgitation or stenosis by increasing the amount of heart muscle (cardiac hypertrophy), over many years this compensatory process is ultimately overwhelmed and the heart abnormally dilates and fails. Therefore, severe valvular disease generally requires surgical replacement with a mechanical or biological prosthesis to prevent the development of heart failure.
Electrocardiography
Cardiac muscle
The functional unit of the heart is the cardiac muscle cell or cardiomyocyte. Each cardiomyocyte maintains an electrical charge or potential across its cell membrane, and contracts when this potential is discharged. In order for all of the cardiomyocytes to contract at the same time and thus produce an effective muscular contraction, the heart also maintains its own electrical conducting system which coordinates the electrical activity of the heart.
The sum total of the simultaneous electrical discharging and re-charging of all the cardiomyocytes in the heart is sufficient to be detected by sensing probes placed on the exterior of the body at various positions around the heart. This is the principle behind the electrocardiograph or ECG which can be used to monitor the rhythm of the heart.
Also, since injured cardiomyocytes such as those suffering from lack of oxygen during a heart attack leak electrical current rather than discharge it in a coordinated manner, the altered electrical signal of the injured heart results in a characteristic ECG pattern which can lead to the diagnosis of acute myocardial infarction.
In contrast, dead cardiomyocytes or scarred cardiac muscle does not carry or maintain an electrical charge, and this absence of electrical activity is also detectable by ECG. Thus, a previously unrecognized or "silent" heart attack can be diagnosed by electrocardiogram, and even localized to a particular area of the heart by using multiple sensing probes or ECG leads.