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Heart Failure - Pathophysiology (view source)
Revision as of 16:13, 22 March 2011
, 16:13, 22 March 2011no edit summary
==Introduction==
== Introduction ==
Heart failure occurs when the heart is unable to maintain an adequate perfusion of the body's tissues at normal filling pressures (Nb, shock is an inability to maintain adequate circulation at low filling pressures).
The heart pumps unoxygenated blood from the venous circulation into the lungs where it is oxygenated. Newly oxygenated blood then travels to the left atria and ventricle where it is pumped into the arterial circulation to meet the oxygen demands of the body. Heart Failure is a condition that begins when the heart is unable to pump enough blood (cardiac output) at normal filling pressures to meet the body’s requirements. Heart Failure is the end result of heart disease (heart abnormality). Under normal circumstances the heart can adapt to moderate increases of pressure or volume using its functional reserve capacity.
The cardiovascular system has a large reserve capacity so overt clinical signs are only seen with severe disease when the heart cannot compensate for the decreased function.
The cardiovascular system has a large reserve capacity so overt clinical signs are only seen with severe disease when the heart cannot compensate for the decreased function.
Factors affecting cardiac output:
Factors affecting cardiac output include:
'''Preload.''' Preload is the end-diastolic volume and when this increases so does the systolic function of the myocardium based on the '''Frank-Starling Law.'''
'''Afterload:''' Refers to the resistance the left ventricle encounters as it ejects blood into the peripheral circulation. Depends upon many variables; for example ventricular volume, arterial tone etc. In the failing heart, in order to maintain blood pressure with reduced cardiac output the peripheral resistnace and so the afterload has to increase contractility, heart rate, distensibility and synergy of contraction.
Heart failure can affect the right ventricle, the left ventricle, or both ventricles (biventricular). Clinical signs are characteristic and can help determine which ventricle is affected.<br>
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===Volume overload===
== Mechanisms of failure ==
'''Myocardial failure '''e.g Dilated cardiomyopathy. Causes a failure in contractility.
Initial response based on the '''Frank-Starling Law''' and stroke volume increases as preload increases and enables the heart to expel the extra blood. Ultimately the heart decompensates and the chamber enlarges. Fluid will build up in the compartment preceding it. For example, failure of the left atrium in this manner due to mitral regurgitation leads to pulmonary oedema.
'''Volume overload '''e.g. Valve regurgitation and shunts (PDA). Initial response based on the '''Frank-Starling Law''' and stroke volume increases as preload increases and enables the heart to expel the extra blood. Ultimately the heart decompensates and the chamber enlarges. Fluid will build up in the compartment preceding it. For example, failure of the left atrium in this manner due to mitral regurgitation leads to pulmonary oedema.
'''Pressure overload '''e.g. aortic stenosis or systemic overload. Increase afterload so heart muscle hypertrophies. The abnormally thick myocardium may predispose to diastolic failure and arrhythmias.
'''Compliance failure '''e.g. Cardiac tamponade. Prevents adequate relaxation of the ventricles and don't allow sufficient filling. Results in diastolic failure.
'''Abnormal rate/rhythm '''e.g sustained bradycardia leads to a low output failure.
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== Clinical Signs ==
==== Forward-Low output failure ====
Decreased blood supply to the lungs and other organs. Left failure results in decreased blood returning to the right and so both sides fail simultaneously and vice versa. There will be low systemic blood pressure, exercise intolerance, pallor, tachycardia, weak femoral pulses and pre-renal failre and azotaemia.
==== Backward-Congestive failure ====
Clinical signs are different for each side. In left sided failure signs include dyspnoea and tachypnoea. There may also be lung crackling on ausculatation due to pulmonary oedema and a cough due to left cardiomegaly comressing the left main stem bronchus. In right sided failure there may be jugular distension, hepatomegaly and splenomegaly, ascites, positive [[Hepato-jugular reflux|Hepato-jugular reflux ]] and pleural effusion.
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== Compensatory Mechanisms ==
==== Renin-angiotensin-aldosterone system; ====
Causes sodium and water retention by the kidney as well as vasoconstriction.
==== Sympathetic nervous system ====
Decreased blood pressure stimulates release of noradrenaline. Increase heart rate and contractility by the efect on beta-receptors. Peripheral vasoconstriction is activated by its action on alpha-receptors. Sinus arrhythmias are abolished. Increased heart rate and rhythm imposed by the sympathetic nervous system increases the hearts oxygen consumption. As diastole is shortened the time available for blood to enter the coronary circulation is also shortened, decreasing blood flow to the myocardium. Resulting myocardial hypoxia may cause arrhythmias.
====Right sided failure====
=== [[Cardiac Hypertrophy|Myocardial hypertrophy]] ===
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===Renin-angiotensin-aldosterone system===
== Classification ==
=== New York Heart Association Classification ===
Classification of congestive heart failure used in human medicine.
*Class 1: No clinical signs but evidence of heart disease.
*Class 2: Exercise intolerance or dyspnoea.
*Class 3: Marked exercise intolerance.
*Class 1: No clinical signs but evidence of heart disease.
*Class 2: Exercise intolerance or dyspnoea.
*Class 3: Marked exercise intolerance.
*Class 4: Cannot exercise, dyspnoea at rest.
*Class 4: Cannot exercise, dyspnoea at rest.
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==Introduction==
== References<br> ==
Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition) W.B. Saunders Company<br>Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2)W.B. Saunders Company<br>Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition) Mosby Elsevier<br>Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial<br>Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition) Mosby Elsevier. <br>
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[[Category:Heart Failure]][[Category:To_Do_-_Cardiovascular]]
[[Category:Heart_Failure]] [[Category:To_Do_-_Review]]