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GRADED EXERCISE STRESS TEST A graded exercise stress test involves exercising on a treadmill or bike at increased degrees of difficulty while the heart’s response is recorded on an electrocardiogram (EKG). The level of exercise tolerance is measured by changes on the EKG, reported symptoms, as well as blood pressure and heart rate response. It also allows the physician to evaluate the effectiveness of your medical treatment. Patients recovering from a heart attack are usually given a low level exercise stress test prior to discharge.

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HEART The heart is a muscular cone-shaped organ responsible for pumping blood throughout the body. It lies between the two lungs and is divided vertically into a right and left side. Both sides are also divided horizontally into an atrium and a ventricle. Oxygen-poor blood enters the right atrium and then flows through the tricuspid valve into the right ventricle. From the right ventricle the blood is pumped through the pulmonic valve to the lungs where it absorbs oxygen. Freshly oxygenated blood then travels from the lungs into left atrium. The blood collects in the left atrium before flowing through the mitral valve into the left ventricle. From the left ventricle the blood is then pumped out the aortic valve through the aorta to the body. Once the oxygen and other nutrients have been absorbed from the bloodstream, the blood returns to the right atrium and the process repeats. The walls of the heart are composed of three layers: (1) the endocardium, the delicate inner lining of the heart that also covers the arteries and forms the folds of the valves; (2) the myocardium, the thick muscular middle layer of the heart responsible for pumping; and (3) the epicardium, the outer layer of the heart that also forms the inner layer of the pericardium, the sac in which the heart rests.

HEART ATTACK A heart attack is a common term which applies to both cardiac arrest and myocardial infarction, or death of heart muscle.

HEART BLOCK Heart block is when the heart beats too slowly because of a problem with the conduction of electrical impulses (from the sinoatrial node) that tell the heart when to contract. There are varying degrees of heart block. In complete heart block (or third degree heart block) the electrical impulses stop completely and the ventricles beat at their slow base rate of 20-40 beats per minute (versus a healthy average of 70 beats per minute). Causes of heart block include congenital heart disease, death of heart muscle (myocardial infarction), myocarditis, diseased valves, and scarring of the electrical conductive tissue of the heart. An artificial pacemaker can restore the heart's rhythm.

HEART FAILURE In heart failure, the ventricle cannot pump the blood hard enough, so blood begins to stagnate in the tissues, causing back pressure (blood flow in the wrong direction). This back pressure results in breathlessness, swelling of the tissues (edema), congestion in the lungs and the liver, and the swelling of veins in the neck. In severe cases, not enough blood may reach the vital organs of the body (cardiogenic shock). Causes of heart failure include multiple myocardial infarctions, hypertension, coronary thrombosis, diseased valves, and arrhythmias.

HEART-LUNG MACHINE See Cardiopulmonary Bypass.

HIGH DENSITY LIPOPROTEINS (HDL) High Density Lipoprotein (HDL) is a type of cholesterol carrier which helps remove cholesterol from the bloodstream. (See also Low Density Lipoprotein)

HOLTER MONITORING Holter monitoring is 24-hour monitoring of heart rhythms on an ambulatory basis. The patient's heart is monitored during their normal activities of daily living in order to discover arrhythmias. The information is stored by the Holter monitor and analyzed at a later date.

HYPERTENSION Hypertension is when an individual's blood pressure is above their healthy range. Causes of hypertension range from diseased or clogged arteries, to kidney disease and unknown causes (essential hypertension). Severe cases of hypertension can lead to heart failure. Normally hypertension can be controlled with careful monitoring of diet and drug therapy. Other forms of hypertension, such as the narrowing of the descending aorta or narrowing of the arteries to the kidneys require more serious treatments, such as surgery.

HYPOTENSION Hypotension is when an individual's blood pressure is below their healthy range. Symptoms include lightheadedness, fainting, sweats, a weak pulse, and loss of circulation in the limbs. It can be caused by a severe, sudden loss of blood, death of heart muscle (myocardial infarction), pulmonary embolism, arrhythmias, and a severe infection or allergic reaction. Treatments include oxygen, medicines, intravenous fluids and blood.

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ILIAC ARTERIES The iliac arteries branch off the abdominal aorta and supply most of the blood to the pelvic region and the lower limbs.

INTERNAL MAMMARY ARTERIES The internal mammary arteries originate from the subclavian artery (that run beneath the shoulders and provide blood to the arms). The mammary arteries run parallel down each side of the front of the chest and connect with small arteries under each rib. They may be taken for use in coronary artery bypass grafting and minimally invasive direct coronary artery bypass procedures since the small arteries beneath the ribs provide a secondary source of blood for the area.

ISOTONIC EXERCISE Isotonic exercise is accomplished by an alternate contraction and relaxation of large muscles. This form of exercise promotes cardiovascular fitness by strengthening the heart muscle. Excellent examples of this form of exercise are walking, biking, cross-country skiing, and swimming.

ISOMETRIC EXERCISE Isometric exercise is a sustained contraction of large muscles, such as weight lifting. This places a disproportional workload on the heart and limits the amount of oxygen delivered to the heart.