Definitions of Heart Failure
Within the body, the cardiovascular system is responsible for maintaining normal arterial blood pressure, normal blood flow
to the tissues, and normal venous and capillary pressures. Heart failure results when heart disease is so severe that the
cardiovascular system can no longer maintain one or more of these functions. Heart failure is defined in simple terms as the
inability of the heart to pump blood at an appropriate rate to sustain the tissues of the body. This produces a clinical syndrome
divided into the categories of low cardiac output (forward) heart failure, and left or right sided congestive (backward) heart
failure. Commonly, we refer to these as the two types of heart failure, forward or backward. In addition, cardiogenic shock
is defined as drastically reduced forward blood flow with concurrent low blood pressure which results in inadequate oxygen
delivery to the tissues of the body.
Causes of Heart Failure
The most common causes of heart failure are myocardial systolic failure, valvular regurgitation, and diastolic myocardial
dysfunction. Examples of these include respectively; dilated cardiomyopathy (DCM), mitral or tricuspid regurgitation, and
constrictive or hypertrophic cardiomyopathy (HCM).
Signs of Heart Failure
Forward heart failure (systolic failure or low cardiac output) typically results in poor tissue perfusion. Clinical presentation
may include weakness and fatigue (exercise intolerance), cold extremities, slow capillary refill time, poor mucous membrane
color, and if output is severely decreased, oliguria, azotemia, and lactic acidosis. Usually systemic arterial blood pressure
is maintained by means of one or more neurohumeral responses (vasoconstriction, sodium and water retention, increased heart
rate, increased contractility, or possibly an increase in myocardial mass). With a slow, progressive systolic failure (DCM),
the clinical signs specific to forward failure may not be obvious until generalized heart failure (left and right backward)
has occurred and the patient cannot compensate any longer. Owners may actually notice signs of right sided backward heart
failure (ascites as one example) first due to visual perception of abdomen size compared to noting a change in exercise tolerance.
Left or right backward heart failure is commonly referred to as congestive heart failure (CHF) due to the signs of congestion
and edema it produces. Left sided CHF typically results in tachypnea, dyspnea, coughing, crackles on auscultation, and hypoxemia
secondary to pulmonary edema. Right sided CHF typically presents as ascites, hepatic enlargement, pleural effusion, and sometimes
jugular vein distension. Exceptions may occur in the cat where pleural effusion often develops with left sided disease.
Examples of Heart Failure
Dilated cardiomyopathy is an example of decreased systolic myocardial function resulting in a forward, and possibly concurrent
backward, heart failure. The dilated ventricles of the heart cannot produce adequate cardiac output. Usually this is a chronic
development, and may go unrecognized until heart failure is severe and generalized. Presenting signs are consistent with forward
and backward failure (exercise intolerance, ascites, dyspnea, pale mucous membranes). Ventricular tachyarrhythmias and atrial
fibrillation are common with DCM. Treatment must address both forward and backward failure and frequently includes positive
inotropic drugs to increase contractility and improve cardiac output, diuretics to reduce pulmonary edema, and venodilators
to reduce systemic congestion. Treatment protocols would be tailored to each patient and would become more aggressive as needed
according to lack of response to therapy. Early management of the disease might include diuretics (furosemide, spironolactone,
hydrochlorothiazide), an ACE (angiotensin converting enzyme) inhibitor (enalapril, benazepril), and a positive inotrope (Digoxin).
Intermediate treatments may include a B-blocker (metoprolol) or Pimobendan (a calcium sensitizer and phosphodiesterase inhibitor)
which both require careful monitoring. Pimobendan has both positive inotropic and vasodilator properties. During myocardial
failure and congestion treatment would be ramped up and may include diuretics, mixed veno and arteriodilator (sodium nitroprusside),
and positive inotropes (dobutamine, amrinone, milrinone). Antiarrythmic drugs are usually indicated. Treatment would vary
dependant on diagnosis, but they are typically tachyarrhythmias of supraventricular or ventricular origin. Large breed, male,
middle age dogs are predisposed (particularly Boxers and Doberman Pinschers).