Biomedical Engineering 403
Cardiovascular Diseases
Cardiovascular Diseases
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14 Cardiac Disease Schedule change HIV placental transmission Valsalva maneuver--heart rate change Training Effects Ischemic Heart Disease Hypoxia versus Ischemia The leading single cause of mortality in the western world Usually affects males in their forties or older Caused by arteriosclerosis (in particular atherosclerosis) Generally no symptoms until arteriosclerosis has progressed sufficiently to produce at least a 70% narrowing on one major coronary artery. Angina Pectoris--chest pain lasting up to several minutes resulting from transient reversible myocardial ischemia. Result of a temporary disparity between myocardial oxygen demand and supply (usually caused by exertion or emotional stress and relieved by rest). Myocardial Infarction--sudden irreversible ischemic injury to myocardium Critical factors in prognosis-- Size of infarct Limit of its spread--noncontracting tissue segment will be stretched by contraction, and may spread General condition of heart Ability of uninvolved myocardium to compensate Treat IHD by increasing coronary blood flow or reducing myocardial oxygen need. Nitroglycerin and other nitrites reduce venous tone and peripheral resistance, reducing preload, afterload, and arterial pressure. Less important, nitrites also dilate coronary arteries. Drop in BP causes baroreflex-mediated tachycardia, but reduction in myocardial effort overrides this disadvantage. Beta-adrenergic blockade (e.g., propranolol) minimizes increase in heart rate and myocardial contractility that may occur during stress. Contraindicated in heart failure, since it would interfere with compensatory sympathetic mechanisms. Direct myocardial revascularization (surgical construction of a bypass vessel) Cardiac rehabilitation--program of exercise training, diet and stress-reduction counseling to improve overall condition. Congestive Heart Failure--not a single disease but a symptom with many causes Etiology--Basic cause may be cardiac or extracardiac Cardiac a. myocardial contractile failure (including that due to ischemic injury) b. a disorder that prevents proper filling or emptying (valvular stenosis, pericardial disease, etc.) Extracardial pressure overloads, such as hypertension, renal failure, etc. In any case, cardiac output is reduced, so kidneys retain water and sodium to raise BP--vicious cycle leading to progressive increases in end-diastolic volume. Compensatory Changes Starling Effect--as long as preload stays within the ascending range of the Starling curve, the increased pressure helps the heart to beat with greater force. Sympathetic Inotropic Effect--increased sympathetic activity and circulating catecholamines has both myocardial and peripheral circulatory effects. The increase in force of contraction helps. Ventricular Hypertrophy--increases the force-generating capacity of the ventricle. Occurs in athletes as well as CHF patients. Peripheral Circulation-- Blood flow is redistributed in CHF. Reduced CO reduces renal and cutaneous blood flow to ensure adequate perfusion of coronary and cerebral beds. (Vasoconstriction is sympathetic probably related to baroreflex.) This helpful, but reduced renal flow results in increases in blood volume, interstitial fluid, and sodium levels. Treatment-- Reduce cardiac work load--rest, minimize stress, nitrites if necessary Enhance Cardiac Contractility --digitalis and other glycosides Control Fluid Retention--diuretics Pericardial disorders Pericarditis Pericardial Tamponade Pericardial pressure or inelasticity restricts heart filling Valve Disorders Stenosis--thickening of valve that narrows opening Incompetence--distortion of valve that prevents full closure Mitral Stenosis--Mitral valve narrows. Left atrium must use increased pressure to force blood into ventricle. Increased pressure is transmitted back to the pulmonary circulation. The right ventricle must also pump extra hard to force blood through pulmonary circulation. Results in right heart failure. Mitral Incompetence--Blood leaks back into left atrium from left ventricle with each beat. Heart must work harder to pump extra load. May result in left heart failure. Both may result from Rheumatic fever, etc. Aortic Stenosis--Aortic valve narrows. Left ventricle works harder. Leads to left sided heart failure. Aortic Incompetence--Blood leaks back into left ventricle. Ventricle works harder. Left or total heart failure. Tricuspid Stenosis and Incompetence--Rare. Occur only in conjunction with other valvular disease. Lead to heart failure. Pulmonary Stenosis & Incompetence--Very rare.
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