Causes and Symptoms
Although varied in origin and effect on the body, heart attacks (or myocardial
infarctions) occur when there are interruptions in the delicately synchronized
system either supplying blood to the heart or pumping blood from the
heart to other vital organs. The heart is a highly
specialized muscle whose function is to pump life-sustaining blood to all parts of
the body. The heart’s action involves the development of pressure to propel blood
through arriving and departing channels—veins and arteries—that must maintain that
pressure at critical levels throughout the system.
The highest level of pressure in the total cardiovascular system is to be found
closest to the two “pumping” chambers on the right and left lower sections of the
heart, called ventricles. Dark blood, emptied of its oxygen content and
laden with carbon dioxide waste instead of the oxygen in fresh blood, flows into
the upper portion of the heart via the superior and inferior venae cavae.
It then passes from the right atrium chamber into the right ventricle. Once in the
ventricle, this blood cannot flow back because of one-way valves separating the
“receiving” from the “pumping” sections of the total heart organ.
After this valve closes following a vitally synchronized timing system,
constriction of the right ventricle by the myocardium muscle in the surrounding
walls of the heart forces the blood from the heart, propelling it toward the
oxygen-filled tissue of the lungs. Following reoxygenation, blood that is still
under pressure from the thrust of the right ventricle flows into the left atrium.
Once channeled into the left ventricle, the pumping process that began in the
right ventricle is then repeated on the left by muscular constriction, and
oxygenated blood flows out of the aortic valve under pressure throughout the
cardiovascular system to nourish the body’s cells. Because the force needed to
supply blood under pressure from the left ventricle for the entire body is greater
than the first-phase pumping force needed to move blood into the lungs, the
myocardium surrounding the left ventricle constitutes the thickest muscular layer
in the heart’s wall.
The efficiency of this process, as well as the origins of problems of fatigue in
the heart that can lead to heart attacks and eventual heart
failure, is tied to the maintenance of a reasonably constant
level of blood
pressure. If pulmonary problems (blockage caused by the
effects of smoking or environmental pollution, for example) make it harder for the
right ventricle to push blood through the lungs, the heart must expend more energy
in the first stage of the cardiovascular process. Similarly, and often in addition
to the added work for the heart because of pulmonary complications, the efficiency
of the left ventricle in handling blood flow may be reduced by the presence of
excessive plaque in the circulatory system, causing this ventricle to expend more
energy to propel oxygenated blood into vital tissues.
Although factors such as these may be responsible for overworking the heart and
thus contributing to eventual heart failure, other causes of heart attacks are to
be found much closer to the working apparatus of the heart, particularly in the
coronary arteries. The coronary arteries begin at the top of the heart and fan out
along its sides. They are responsible for providing large quantities of blood to
the myocardium muscle, which needs continual nourishment to carry out the pumping
that forces blood forward from the ventricles. The passageways inside these and
other key arteries are vulnerable to the process known as atherosclerosis, which can affect the blood supply to other
organs as well as to the heart. In the heart, atherosclerosis involves the
accumulation, inside the coronary arteries, of fatty deposits called atheromas. If
these deposits continue to collect, less blood can flow through the arteries. A
narrowed artery also increases the possibility of a variant form of heart attack,
in which a sudden and total blockage of blood flow follows the lodging of a blood
clot in one of these vital passageways.
A symptomatic condition called angina pectoris, characterized by intermittent
chest pains, may develop if atherosclerosis reduces blood (and therefore oxygen)
supply to the heart. These danger signs can continue over a number of years. If
diagnosis reveals a problem that might be resolved by preventive medication,
exercise, or recommendations for heart surgery, then this condition, known as
myocardial ischemia, may not necessarily end in a full heart attack.
A full heart attack occurs when—for one of several possible reasons, including a
vascular spasm suddenly constricting an already clogged artery or a blockage
caused by a clot—the heart muscles suddenly cease to receive the necessary supply
of blood. This brings almost immediate deterioration in some of the heart’s tissue
and causes the organ’s consequent inability to perform its vital functions
effectively.
Another form of attack and disruption of the heart’s ability to deliver blood can
come either independently of or in conjunction with an arterially induced heart
attack. This form of attack involves a sustained interruption in the rate of
heartbeats. The necessary pace or rate of myocardial contractions, which can vary
depending on the person’s rate of physical exertion or age, is regulated in the
sinoatrial node in the right atrium, which generates its own electrical impulses.
The ultimate sources for the commands to the sinoatrial node are to be found in
the network of nerves coming directly from the brain. There are, however, other
so-called local pacemakers located in the atria and ventricles. If these sources
of electrical charges begin giving commands to the myocardium that are not in
rhythm with those coming from the sinoatrial node, then dysrhythmic or premature
beats may confuse the heart muscle, causing it to beat wildly. In fact, the
concentrated pattern of muscle contractions will not be coordinated and instead
will be dispersed in different areas of the heart. The result is atrial
fibrillation, a series of uncoordinated contractions that
cannot combine to propel blood out of the ventricles. This condition may occur
either as the aftershock of an arterially induced heart attack or suddenly and on
its own, caused by the deterioration of the electrical impulse system commanding
the heart rate. In patients whose potential vulnerability to this form of heart
attack has been diagnosed in advance, a heart physician may decide to surgically
implant an
electronic pacemaker to ensure coordination of the necessary
electrical commands to the myocardium.
Treatment and Therapy
Extraordinary medical advances have helped reduce the high death rates formerly
associated with heart attacks. Many of these advances have been in the field of
preventive medicine. The most widely
recognized medical findings are related to diet, smoking
cessation, and exercise. Although controversy remains, there
is general agreement that cholesterol absorbed by the body from
the ingestion of saturated fats plays a key role in the dangerous buildup of
platelets inside arterial passageways. It has been accepted that regular, although
not necessarily strenuous, exercise is an essential long-term preventive strategy
that can reduce the risk of heart attacks. Exercise also plays a role in therapy
after a heart attack. In both preventive and postattack contexts, it has been
medically proven that the entire cardiovascular system profits from the natural
muscle-strengthening process (in the heart’s case) and general cleansing effects
(in the case of oxygen intake and stimulated blood flow) that result from
controlled regular exercise.
The actual application of medical scientific knowledge to assist in the campaign
against the deadly effects of heart disease involves multiple fields of
specialization. These may range from the sophisticated use of electrocardiograms (ECGs) to monitor the regularity of
heartbeats, to specialized drug therapies aimed at preventing heart attacks in
people who have been diagnosed as high-risk cases, to coronary bypass
surgery or even heart transplants. In the 1980s, highly
specialized surgeons at several university and private hospitals began performing
operations to implant artificial hearts in human patients.
In the case of ECGs, it has become possible, thanks to the use of portable units
that record the heartbeat patterns of persons over an extended period of time, to
gain a much more accurate impression of the actual functioning of the heart.
Previous dependence on electrocardiographic data gathered during an appointed and
limited examination provided only minimal information to doctors.
The domains of preventive surgery and specialized drug treatment to prevent
dangerous blood clotting are vast. Statistically, the most important and widely
practiced operations that were developed in the later decades of the twentieth
century were replacement of the aortic valve, the coronary bypass operation, and,
with greater or lesser degrees of success, the actual transplantation of donors’
hearts in the place of those belonging to heart disease patients. Coronary bypass
operations involve the attachment to the myocardium of healthy arteries to carry
the blood that can no longer pass through the patient’s clogged arterial
passageways; these healthy arteries are taken by the heart surgeon from other
areas of the patient’s own body.
Another sphere of medical technology, that of angioplasty,
held out a major nonsurgical promise of preventing deterioration of the arteries
leading to the heart. This sophisticated form of treatment involves the careful,
temporary introduction of inflatable devices inside clogged arteries, which are
then stretched to increase the space within the arterial passageway for blood to
flow. By the 1990s, however, doctors recognized one disadvantage of balloon
angioplasty: By stretching the essential blood vessels being treated, this
procedure either stretches the plaque with the artery or breaks loose debris that
remains behind, creating a danger of renewed clogging. Thus, although angioplasty
remains a standard approach to treatment of heart disease, another technique,
called atherectomy, was developed to clear certain coronary arteries, as well as
arteries elsewhere in the body.
Atherectomy involves a motorized catheter device resembling a miniature drill that
is inserted into clogged arteries. As the drill turns, material that is literally
shaved off the interior walls of arteries is retrieved through a tiny collection
receptacle. Early experimentation, especially to treat the large anterior
descending coronary artery on the left side of the heart, showed that atherectomy
was 87 percent effective, whereas, on the average, angioplasty removed only 63 percent of the
blockage.
Perspective and Prospects
The modern conception of cardiology dates from William
Harvey’s seventeenth-century discovery of the relationship
between the heart’s function as a pump and the circulatory “restoration” of blood.
Harvey’s much more scientific views replaced centuries-old conceptions of the
heart as a blood-warming device only.
Although substantial anatomical advances were made over the next two centuries
that helped explain most of the vital functions of the heart, it was not until the
early decades of the twentieth century that science developed therapeutic methods
to deal with problems that frequently cause heart attacks. Drugs that affect the
liver’s production of substances necessary for normal coagulation of blood, for
example, were discovered in the 1930s. A large variety of such anticoagulants have
since been developed to help thin the blood of patients vulnerable to blood
clotting. Other drugs, including certain antibiotics, are used to treat persons
whose susceptibility to infection is known to be high. In these cases, the simple
action of dislodging bacteria from the teeth when brushing and flossing can cause
an invasion of the vital parts of the heart by an infection. This bacterial
endocarditis, the result of the actual destruction of heart
tissue or the sudden release of clots of infectious residue, could lead to a heart
attack in such individuals although they have no other symptoms of identifiable
heart disease.
The most spectacular advance in the scientific treatment of potential heart attack victims, however, has been in the field of cardiac surgery. Many advances in open heart surgery date from the late 1950s, when the development of heart and lung replacement machines made it safe enough to substitute electronic monitors for some of the organism’s normal body functions. Before the 1950s, operations had been limited to surgical treatment of the major blood vessels surrounding the heart.
Various technical methods have also been developed that help identify problems early enough for drug therapy to be attempted before the decision to perform surgery is made. The use of catheters, which are threaded into the coronary organ using the same vessels that transport blood, became the most effective way of locating problematic areas. The process known as angiography, which uses x-rays to trace the course of radiopaque dyes injected through a catheter into local heart areas under study, can actually tell doctors if drug therapy is having the desired effects. In cases where such tests show that preventive drug therapy is not effective, an early decision to perform surgery can be made, preventing the source of coronary trouble from multiplying the patient’s chances of suffering a heart attack.
Early detection of an oncoming heart attack is critical to improving survival
rates. According to the US Centers of Disease Control and Prevention, major
warning signs and symptoms of a heart attack include chest pain; pain or
discomfort in the arms, back, neck, and upper abdomen; shortness of breath;
nausea; light-headedness; or cold sweats. Individuals experiencing these symptoms
should seek emergency medical care immediately. Individuals who are at the highest
risk of suffering a heart attack include those with high blood pressure, high LDL
cholesterol, a history of smoking; are physically inactive; and are overweight or
obese. A diet that is low in salt, fat, and cholesterol and regular exercise can
significantly reduce a person's risk of heart attack.
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