The most common causes are defective heart muscle, valvular disease, cardiac tamponade, electrical malfunction, heartworms
or administration of drugs with profound cardiac effects. In any case, when heart failure results in circulatory collapse
and inadequate tissue oxygenation, the patient is said to be in cardiogenic shock. The term "cardiogenic shock" describes
a shock syndrome where cardiac output is reduced as a direct result of heart failure. Cardiac output is a function of heart
rate and stroke volume. Stroke volume is determined by preload (circulating blood volume), afterload (the forces that impede
ventricular ejection such as arterial vascular resistance and blood viscosity) and contractility. Therefore, abnormalities
in any of these factors can result in diminished cardiac output. Understanding the specific processes of the common causes
of heart failure is essential to successful management of ensuing cardiogenic shock.
Heart failure
Heart failure can result from a number of diseases/disorders. However, there are only two types of heart failure. Low output
(forward) failure and left or right sided congestive (backward) heart failure (CHF). Low output failure results from an inability
to pump blood forward from the left ventricle to the systemic circulation. This results in weak pulses, low blood pressure,
depressed mentation, cold periphery, prolonged CRT, oliguria, muscle weakness, syncope and inadequate perfusion. Left sided
CHF results in pulmonary edema, crackles, rales, hypoxemia, cough, dyspnea while right sided CHF results in hepatomegaly,
jugular distention, ascites, body cavity effusions. Each of the following disorders causes one or more type of heart failure:
Cardiomyopathy
Dilated cardiomyopathy (DCM)
DCM is associated with generalized heart enlargement (cardiomegaly) and dilation of all cardiac chambers. Poor left ventricular
contractility leads to reduced ejection of blood and subsequent low output failure. Atrial fibrillation or ventricular tachycardia
with DCM is a common finding
DCM is common in large breed dogs especially Doberman pinchers and boxers and most commonly in young to middle aged dogs.
DCM also occurs in cats but with much less frequently due to improved, taurine regulated diets. Any process that disturbs
myocardial function and structure can result in DCM. Infection, toxins, metabolic and nutritional abnormalities, genetic
and immunological mechanisms have all been implicated as possible causes.
Clinical signs include, pale mucous membranes, tachypnea, dyspnea, syncope, weakness and collapse. ECG shows atrial fibrillation
or ventricular premature contractions. Pulse deficits are often present. Generalized cardiomegaly is apparent on radiographs.
Echocardiography is useful in determining the extent of impaired contractility.
Therapy is aimed at improving cardiac contractility and stroke volume. It may include diuretics, positive inotropic drugs
(dobutamine) or vasodilators (nitroprusside). Antiarrhythmic drugs may be indicated in some animals.
Hypertrophic cardiomyopathy (HCM)
HCM is associated with ventricular thickening and stiffening. Diastolic ventricular filling is impaired. Poor filling capacity
results in CHF and forward failure. The left atrium becomes dilated in response and stagnation of blood occurs in the chamber.
HCM is most commonly found in cats. Hyperthyroidism is a common underlying disease. Cats with HCM often present with aortic
saddle thrombus as a result of persistent atrial torture and the unique hyperactivity of feline platelets. Embolization of
a left atrial thrombus results in occlusion at the aortic bifurcation causing ischemic injury to distal tissues.
Clinical signs include tachypnea, dyspnea, weakness, syncope, anorexia, depression and emesis.
Auscultation reveals a gallop rhythm, systolic murmur, pronounced heartbeat and tachycardia. Weak arterial pulses are evident
on palpation.
Treatment involves administration of positive inotropes (Diltiazem in cats, dobutamine in dogs) to improve cardiac contractility.
Diuretics are administered to reduce pulmonary edema. Cats with saddle thrombus may also be treated with anticoagulants (heparin).
Valvular disease
Valvular disease is the most common cause of heart failure in dogs. It is rare in cats. Mitral disease is more common than
tricuspid disease. Middle aged, small breed, male dogs show predisposition.
1. Mitral insufficiency and/or chordae tendineae rupture results in left sided congestive heart failure.
2. Triscupid insufficiency results in right-sided congestive heart failure.
