Page 209 - Clinical Small Animal Internal Medicine

18 Pathophysiology of Heart Failure 177

Contractility is the inherent ability of the cardiac or when the heart can do so only with an elevated
VetBooks.ir myocytes to shorten and generate force independent of filling pressure. The heart’s inability to pump a suf-
ficient amount of blood to meet the needs of the
preload, afterload, or heart rate. Increased contractility
represents either increased calcium movement or sensi-
cardiac filling and/or impaired contraction and
tization of the contractile proteins at a given level of body tissues may be due to insufficient or defective
cytosolic calcium. Although criticisms of this concept emptying. Compensatory mechanisms increase
include the lack of noninvasive measures of isolated con- blood volume and raise cardiac filling pressures,
tractility and the impossibility of separating the molecu- heart rate, and cardiac muscle mass to maintain
lar mechanisms of enhanced function from preload and the heart’s pumping function and cause redistribu-
afterload from contractility, this concept is still useful as tion of blood flow. Eventually, however, despite
we discuss the pathophysiology of heart failure. these compensatory mechanisms, the ability of the
The Bowditch staircase effect (also known as the heart to contract and relax declines progressively,
Treppe effect or the positive force‐frequency relation- and the heart failure worsens.
ship) defines that increased heart rate not only increases
cardiac output via more frequent contractions, it also Although common long‐standing cardiac conditions
enhances the force of ventricular contraction. It is typically contribute to volume overload and develop-
hypothesized that an increased rate of depolarization and ment of pulmonary (left‐sided) or systemic (right‐sided)
the subsequent increased sodium and calcium current congestion, there are conditions (e.g., cardiac tampon-
across the sarcolemma produce overload of the sodium– ade, acute pulmonary thromboembolism, acute severe
potassium pump. The activity of the sodium–calcium valvular insufficiency) that may severely impair the abil-
exchanger is therefore augmented in the “reverse” mode, ity to provide vital organs with their necessary cardiac
extruding sodium from the cell in exchange for increased output without fluid accumulation. Therefore, the term
calcium into the cell. Cross‐bridge cycling is supported heart failure is often preferred over congestive heart
by the higher cytosolic calcium level. failure.
A final determinant of ventricular systolic function is
the coordinated activation (and hence contraction) of the Classification of Heart Failure
ventricular walls, termed ventricular synchrony. An
altered activation sequence and the lack of coordinated The American College of Veterinary Internal Medicine
mechanical contraction, as witnessed with ventricular (ACVIM), for animals, and the ACCF/AHA, for humans,
arrhythmias, right ventricular apical pacing, or in humans recognize a functional classification scheme, and a sepa-
with markedly prolonged QRS durations, can contribute rate, complementary classification system that empha-
to functional and clinical deterioration. Cardiac (pace- sizes the risk, development, and progression of heart
maker) resynchronization therapy has been shown to off- disease. The modified New York Heart Association
set some of these negative consequences in people. (NYHA) functional classification is defined as follows.
Class I describes patients with asymptomatic heart
●
disease (e.g., heart disease is present but there are no
The Failing Heart clinical signs even with exercise).
Class II describes patients with heart disease that
●
Definition of Heart Failure causes clinical signs only during strenuous exercise.
Class III describes patients with heart disease that
The 2013 American College of Cardiology Foundation and ● causes clinical signs with routine daily activities or
American Heart Association (ACCF/AHA) guideline for mild exercise.
the management of heart failure defines heart failure broadly Class IV describes patients with heart disease that
as “a complex clinical syndrome that results from structural ● causes severe clinical signs even at rest.
or functional impairment of ventricular filling or ejection of
blood.” The biochemical mechanisms behind this complex A newer veterinary system outlined in the ACVIM
syndrome continue to be elucidated but the macroscopic Consensus Statement detailing canine valvular heart
view and description of heart failure has not varied signifi- disease highlights four basic stages of heart disease and
cantly from the conclusion of a panel of the National Heart, failure. Because this system was devised for valvular
Lung and Blood Institute in 1994. They stated: heart disease, it is not always easily incorporated into
discussion of other forms of congenital and acquired
Heart failure occurs when an abnormality of heart disease in dogs and cats, but nonetheless it is still
cardiac function causes the heart to fail to pump useful for discussion of heart failure. The stages of this
blood at a rate required by the metabolizing tissues classification scheme are defined as follows.