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

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).