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OMEGA-3 FATTY ACIDS Omega-3 fatty acids are fats found in fish sources which help to lower Low Density Lipoprotein (LDL) cholesterol. (See also Saturated Fat; Polyunsaturated Fat; and Monounsaturated Fat.)

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PACEMAKER A pacemaker is an electrical device that can be implanted in the body and connected to the heart. The pacemaker gives small shocks to the atrium and/or ventricle when the heart fails to beat quickly enough. Pacemakers contain batteries and special computer chips that simulate the heart's normal electrical function (see sinoatrial node and heart block).

PARASYMPATHEIC NERVOUS SYSTEM The parasympathetic nervous system helps regulate those parts of the body, like the heart and the blood vessels, that the brain does not consciously control. Nerves for the parasympathetic nervous system originate in the brain and in the lower spinal cord. As one of the two parts that compose the autonomic nervous system, the parasympathetic nervous system is often responsible for giving the opposite message as its counterpart, the sympathetic nervous system. The parasympathetic nervous system usually has a calming or slowing effect on the systems it enervates.

PERICARDIUM The pericardium is a thin lubricated sac in which the heart rests, protecting the heart from friction.

PERICARDITIS Pericarditis is the inflammation of the pericardium, the lubricated sac in which the heart rests. Among the causes of pericarditis are viral infection and cancer. Results include fever, chest pain, and the build-up of too much fluid in the sac (pericardial effusions). Rarely, the pericardium thickens (chronic constrictive pericarditis), interfering with the functioning of the heart.

PERSANTINE STRESS TEST A persantine stress test is similar to the graded exercise stress test except that a medication called persantine is used to elevate the heart rate instead of exercise.

PHLEBOTHROMBOSIS Phlebothrombosis occurs when a vein is blocked by a blood clot. Phlebothromboses are most commonly found in the leg.

POLYUNSATURATED FAT Polyunsaturated fat comes primarily from vegetable sources which are generally liquid at room temperature. When used in moderation, they tend not to effect blood cholesterol levels. (See also Saturated Fat; Monounsaturated Fat; and Omega-3 Fatty Acids.)

POST INFARCTION VENTRICULAR SEPTAL DEFECT A post infarction ventricular septal defect is a hole that forms in the ventricular septum (wall) due to a weakening of the septum from a myocardial infarction (the death of a section of heart muscle). In other words, a section of the heart muscle in the septum dies from a myocardial infarction. As a result that section of septum is abnormally weak. Continued pressure on the weakened section leads to the formation of a hole in the septum. Congestive heart failure, heart murmurs, and endocarditis are some of the possible complications of a ventricular septal defect.

PTCA See angioplasty.

PULMONARY ARTERY The pulmonary artery carries oxygen-poor blood from the right ventricle to the lungs. After it leaves the right ventricle, the pulmonary artery splits into two branches, one for each lung. The pulmonary artery is the only artery in the body that carries oxygen-poor blood.

PULMONARY CIRCULATION The pulmonary circulation refers to all the blood vessels connecting the heart and the lungs. See pulmonary veins and pulmonary artery.

PULMONARY EDEMA Pulmonary edema occurs when too much fluid accumulates in the lungs. The fluid blocks the transport of oxygen from the lungs into the blood. Emergency treatment with medicines and mechanical ventilation may be necessary.

PULMONARY HYPERTENSION Pulmonary hypertension occurs when the blood pressure in the blood vessels supplying the lungs is too high. This increased pressure causes the right ventricle of the heart to become enlarged, and may result in fainting, chest pain, and heart failure. Other possible complications include, embolisms, septal defects, mitral valve diseases, and chronic lung diseases.

PULMONARY VEINS The pulmonary veins carry oxygenated blood from the lung to the left atrium. They are the only veins in the body that carry oxygen-rich blood.

PULMONIC VALVE The pulmonic valve separates the right ventricle from the pulmonary artery. The pulmonic valve is comprised of three leaflets that, when open, permit blood to flow in only one direction - from the right ventricle into the pulmonary artery.