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When coronary heart disease (CHD) is present, there is an imbalance between:
   .   the supply of blood from coronary arteries, and
   .   the workload demands of the heart.

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Coronary Artery Blood Supply

Decreased coronary blood flow is most frequently the result of arteriosclerosis and atherosclerosis. Arteriosclerosis is a hardening of the arteries with increased tension and loss of elasticity in the muscular walls. It is associated with aging and also results from hypertension. The decreased elasticity of the arterial walls facilitates the buildup of atherosclerotic deposits.

Atherosclerosis, results from the deposition of substances such as fats, cholesterol, minerals, enzymes and cellular debris within the inner lining of the arteries. Over time, these deposits build up into plaques or lesions which narrow the arterial diameter and decrease blood flow (press to enlarge image).

Cardiac Workload

The workload of the heart involves its pumping action as blood is pumped from the left ventricle (lower heart chamber) into the aorta and systemic arteries. The oxygen demand is in proportion to the energy it must expend. Three important factors determine cardiac workload:

  1. the frequency and strength of muscle contraction,
  2. the volume of blood to be pumped from the left ventricle, and
  3. the resistance of systemic arteries to blood flow.

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Substances such as nicotine, which stimulate heart muscle, increase workload. Likewise, arteriosclerosis, which increases resistance to blood flow in the systemic arteries, forces the heart muscle to pump harder (press to enlarge image).

Risk Factors

The major risk factors for CHD are:
   .   high blood pressure,
   .   high blood cholesterol,
   .   smoking,
   .   diabetes mellitus,
   .   obesity, and
   .   physical inactivity.

High Blood Pressure

Hypertension or high blood pressure has two important effects on CHD. First, the increased blood volume increases the tension in the muscular wall of the left ventricle. With each contraction or heartbeat, the heart must work harder to eject blood into the arterial system. This increased tension within the muscular wall increases oxygen demand for the pumping heart to maintain constant output.

The second effect of hypertension is the structural change it causes within the small arteries. The higher pressure causes stretching of the arterial walls with increasing wall tension and rigidity (arteriosclerosis). This wall tension increases resistance to blood flow, and thus, increases the heart workload. Additionally, the muscular rigidity creates an environment in which deposits are more likely to aggregate; thus, hypertension increases the risk of atherosclerosis. Hypertension doubles a person's risk of developing CHD.

Elevated Cholesterol

High levels of circulating blood cholesterol lead to atherosclerosis or buildup of fatty deposits within the lining of the artery. This eventually becomes a plaque or lesion narrowing the diameter of the artery and reducing coronary blood flow. The higher the level of circulating blood cholesterol the higher the risk of atherosclerosis.

Cholesterol is transported in the bloodstream by large molecules called lipoproteins. There are at least five classes of lipoproteins, but two of major concern include:

• Low-density lipoproteins (LDL) which carry 60 to 75% of the total cholesterol; high LDL levels are associated with the development of CHD -- however, a person can have CHD with normal cholesterol levels, and
• High-density lipoproteins (HDL) which are associated with a lower incidence of CHD; they may play a role in removing cholesterol from tissues -- low levels of HDL are associated with an increased incidence of CHD.

Lowering dietary fat intake, particularly saturated fats and cholesterol, helps to reduce blood cholesterol levels. It also helps decrease weight.

More on Fats and Lipoproteins

Smoking

When inhaled, nicotine enters the bloodstream immediately and is pumped to the brain and other organs. Nicotine acts on nerve cells, both in the brain and along nerve pathways, to stimulate muscle in the heart and blood vessels. Nerve fiber stimulation causes the release of adrenalin-type substances from both the nerve endings in target organs and the adrenal glands situated atop the kidneys. From the adrenal glands, adrenalin is released directly into the bloodstream. The effect of this stimulation is widespread.

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In the smallest arteries, sympathetic nerve stimulation causes muscle constriction which narrows the arterial diameter and increases resistance to blood flow. Thus, it raises the blood pressure and increases the workload of the heart.

In the heart, two important areas are affected. On the right side of the heart, where the origin of heartbeat or "pacemaker" activity is located, increased stimulation increases heart rate. In the left ventricle, increased muscle stimulation increases the strength of contraction. Thus, the volume of blood ejected into the circulation is increased. Smoking doubles a person's risk of developing CHD.

Diabetes Mellitus

Persons with both Type 1 (insulin-dependent) and Type 11 (non-insulin-dependent) diabetes have higher risk of developing CHD. Insulin resistance increases blood glucose and triglyceride levels while lowering HDL levels. Good diabetic control is essential.

Obesity

Obesity places greater demand on the heart workload and on insulin requirements. Weight increase is usually accompanied by an increase in blood pressure and an increased risk of high blood cholesterol. Lowering caloric intake reduces overall stress on the heart and decreases the risk of insulin resistance in the obese person.

Physical Inactivity

While inactivity reduces oxygen requirements in the immediate situation, balanced exercise in a healthy individual makes heart muscle work harder and increases blood supply to the heart. Thus, exercise strengthens the heart muscle and improves arterial circulation. Venous circulation, which carries blood back to the heart, is improved also. Veins, unlike arteries, have thin walls and a very low blood pressure. During exercise, as skeletal muscles contract they provide a squeezing effect on the thin-walled veins. This helps push blood back toward the heart.

Persons with more severe CHD may have to restrict exercise to prevent further imbalance in oxygen demand and supply. However, research has shown that even moderate physical activity can reduce mortality associated with CHD. Exercise improves the chemical profile, particularly in diabetics, and benefits most individuals. Persons with CHD should seek advice on physical activity from their Physician. Regular exercise, good nutrition, and smoking cessation are prime factors in preventing CHD.

Links of interest:

Coronary Heart Disease: Overview
Fats:  What Are They?
Diabetes Mellitus

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