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Heart Failure - Pathophysiology (view source)
Revision as of 10:51, 30 June 2016
, 10:51, 30 June 2016→References
* Blood Pressure (BP) = Cardiac Output (CO) x Total Peripheral Resistance (TPR)
* Blood Pressure (BP) = Cardiac Output (CO) x Total Peripheral Resistance (TPR)
* Cardiac Output (CO) = Venous Return (VR)
* Cardiac Output (CO) = Venous Return (VR)
The primary determinants of cardiac performance are:
* '''Preload''': The volume of blood or hydrostatic pressure within the ventricles at the end of diastole.
* '''Afterload''': The force that opposes ejection of blood into the peripheral arterial system, of which arterial blood pressure is the primary factor
* '''Contractility''': The ability of the myocardium to function as a pump and eject blood
The '''Frank-Starling''' mechanism states that stroke volume increases in response to increased end-diastolic volume (preload) when all other factors remain constant. Therefore, if a larger volume of blood flows to the ventricle, there is greater wall stretch, causing greater expansion during diastole, which in turn increases the force of contraction and therefore stroke volume (quantity of blood that is pumped into the aorta during systole).
== Mechanisms of failure ==
== Mechanisms of failure ==
=== [[Cardiac Hypertrophy|Myocardial hypertrophy]] ===
=== [[Cardiac Hypertrophy|Myocardial hypertrophy]] ===
Chronic increase in cardiac work results in a geometric alteration of the chambers involved. Remodelling of the ventricular myocardium occurs in two forms: '''concentric''' and '''eccentric''' hypertrophy. Factors implicated in the development of hypertrophy include adrenergic stimulation, angiotensin II and increased intracellular calcium.
Chronic increase in cardiac work results in a geometric alteration of the chambers involved. Remodelling of the ventricular myocardium occurs in two forms: concentric and eccentric hypertrophy. Factors implicated in the development of hypertrophy include adrenergic stimulation, angiotensin II and increased intracellular calcium.
'''Concentric hypertrophy''' develops in response to pressure overload (increased afterload). Increased afterload causes replication of sarcomeres in parallel, resulting in an increase in wall thickness and a decrease in internal diameter with no overall change in the external diameter of the chamber. This is better understood by considering the '''Laplace''' law, which states that ventricular wall stress is elevated by increased pressure and increased chamber diameter; whereas wall stress decreases as the ventricular wall thickens. Therefore concentric hypertrophy occurs as a compensatory mechanism to normalise ventricular wall stress in the face of pressure overload.
'''Concentric hypertrophy''' develops in response to pressure overload (increased afterload). Increased afterload causes replication of sarcomeres in parallel, resulting in an increase in wall thickness and a decrease in internal diameter with no overall change in the external diameter of the chamber. This is better understood by considering the '''Laplace''' law, which states that ventricular wall stress is elevated by increased pressure and increased chamber diameter; whereas wall stress decreases as the ventricular wall thickens. Therefore concentric hypertrophy occurs as a compensatory mechanism to normalise ventricular wall stress in the face of pressure overload.
== Classification ==
== Classification ==
=== New York Heart Association Classification ===
=== Modified New York Heart Association Classification ===
Classification of congestive heart failure modified from human medicine. This is problematic, as cardiac debilitation is not the only factor governing exercise tolerance. This is particularly difficult to apply to cats, which tend to lead a sedentary lifestyle. Furthermore, a normal level of activity is clearly defined for humans (e.g. ability to walk a certain distance), but in veterinary medicine this may be influenced by the breed and lifestyle of the dog.
*Class I: Heart disease with no clinical signs
*Class II: Exercise intolerance
*Class III: Marked exercise intolerance and dyspnoea
*Class IV: Cannot exercise, dyspnoea at rest
===American Heart Association (AHA and American College of Cardiology (ACC)===
* Stage A: Predisposition for developing cardiac disease e.g. Cavalier King Charles Spaniel, Doberman
* Stage B: Structural heart disease, no clinical signs
* Stage C: Structural heart disease, current or prior clinical signs
* Stage D: Refractory heart failure
===International Small Animal Cardiac Health Council (ISACHC)===
The only veterinary-specific clinical classification.
*Class 1: No clinical signs but evidence of heart disease
* Class Ia: Structural heart disease, no radiographic or echocardiographic evidence of cardiac enlargement
*Class 2: Exercise intolerance or dyspnoea
* Class Ib: Structural heart disease,radiographic or echocardiographic evidence of cardiac enlargement
*Class 3: Marked exercise intolerance
* Class II: Mild clinical signs
*Class 4: Cannot exercise, dyspnoea at rest
* Class IIIa: Overt clinical signs, death or severe debilitation likely without immediate therapy but homecare possible
* Class IIIb: Overt clinical signs, death or severe debilitation likely and hospitalisation required
== References ==
== References ==
* Luis Fuentes, V, Johnson, L.R, Dennis, S. (2010) '''BSAVA Manual of Canine and Feline Cardiorespiratory Medicine (Second Edition)'''