Heart disease is common in dogs; heart failure less so. In many instances, the presence of heart disease alone does not warrant treatment, whereas heart failure always requires active therapeutic intervention. Accurate identification of when heart failure is responsible for clinical signs can be perplexing. An understanding of the pathophysiology of heart failure can be helpful in pinpointing the telltale signs of congestive failure.

Definition of heart failure

 When the heart cannot pump enough blood forward to meet the body's requirements, and/or

 It can only supply enough blood if atrial pressures are raised

Forward failure is another term sometimes used for the situation where there is inadequate blood supply to the tissues. Clinical signs of forward failure include weakness, syncope and peripheral vasoconstriction (pallor, cold extremities). Syncope often occurs at exercise in animals with cardiac disease, as the heart cannot suddenly increase cardiac output to meet demand (even though the animal may be well-compensated at rest).

Backwards failure can be said to occur when filling pressures (or atrial pressures) are raised. Backwards failure is also termed congestive failure, because it is usually associated with abnormal fluid accumulation. Clinical signs of backwards failure include dyspnea (pulmonary edema, pleural effusion), coughing (left atrial enlargement in dogs), ascites.

Acute heart failure vs chronic heart failure

The effects of cardiac disease may differ according to the time available for compensatory mechanisms to develop (such as the time required for sodium and water retention).

Acute heart failure

 Rapid reduction in cardiac output

 No time for many of the neuroendocrine mechanisms to become activated

 Systemic blood pressure falls

 Profound weakness

 Sympathetic nervous system activation (tachycardia, pallor from vasoconstriction)

Examples

 Sudden sustained ventricular tachycardia

 Ruptured chordae tendineae (would also involve sudden rise in filling pressures)

Chronic heart failure

In chronic heart failure, many mechanisms contribute to the maintenance of normal systemic arterial pressure, which is the main homeostatic priority. This is primarily achieved by:

 Stimulation of the sympathetic nervous system

 Increase in preload (sodium and water retention)

CLINICAL RECOGNITION OF CONGESTIVE HEART FAILURE

Left heart failure vs right heart failure

The signs of congestive failure are different according to whether the left or right heart is predominantly affected.

Left heart failure

 Interstitial pulmonary edema

 Alveolar pulmonary edema

Any increase in left atrial pressures will be transmitted to the pulmonary veins, and to the pulmonary capillaries. The increase in hydrostatic pressures in the pulmonary capillaries results in a net increase in fluid movement out of the capillaries into the pulmonary interstitium. Although the lymphatics can increase the rate of drainage of fluid from the pulmonary interstitium, this may be insufficient in moderate to severe left heart failure. As pulmonary capillary pressures increase still further, fluid may move form the interstitium into the alveoli, causing alveolar edema, and even spillage of edema into the airways in very severe cases.

Key clinical features of LEFT HEART FAILURE

  Respiratory rate (>40/min)

 Lung sounds may be normal if only interstitial edema present

 Quiet inspiratory crackles if alveolar edema present (i.e., only with severe pulmonary edema)

 Heart rate moderately elevated, and constant rhythm-no slowing as the dog relaxes

 Pulses may or may not be weak

 Capillary refill time may or may not be

 Generally will be concurrent signs of heart disease (murmur, arrhythmia, gallop sounds)

Clinical features associated with PRIMARY RESPIRATORY DISEASE

 May have marked expiratory component in respiration with small airway disease

  Respiratory noise/ stridor is usually associated with airway obstruction, not CHF

 Inspiratory crackles often loud when associated with airway disease/ pulmonary fibrosis

 Heart rate often normal, with sinus arrhythmia

Radiographic features of left heart failure

 Left atrial enlargement nearly always present with pulmonary edema (nb: may not be with endocarditis or ruptured chordae tendineae)-see below

 Pulmonary vessels may be wider than normal

 Generalized cardiac 'enlargement' in absence of left atrial enlargement unlikely to be associated with significant left heart failure (same exceptions as above-endocarditis, ruptured chordae)

 Pulmonary edema may be evident as hazy increase in radiodensity in hilar area

 Air bronchograms may not be obvious even with severe alveolar edema

 Cats may have patchy infiltrates, with left atrial enlargement more obvious on ventrodorsal view

Right heart failure

 Ascites

 Pleural effusion

 Jugular distension

With right heart failure the right atrial pressures are raised. This leads to an increase in central and peripheral venous pressures. The increase in hepatic venous pressure causes hepatomegaly and ascites. Pleural effusions arise due to increase in fluid production (increase in hydrostatic pressures in systemic capillaries) and a decrease in drainage (the thoracic duct drains into the cranial vena cava, where the pressures are also increased).

Key clinical features of RIGHT HEART FAILURE

 Ascites & jugular distension

 Pleural effusion in cats (nb. cats may have distended jugulars with noncardiac causes of pleural effusions)

 Hepatomegaly

 Often present in conjunction with left heart failure (see above)

Summary