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
Sympathetic nervous system activation (tachycardia, pallor from vasoconstriction)
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
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)
Often present in conjunction with left heart failure (see above)
Are there signs of heart disease?
If so, what are the likely causes?
Is the animal compensating for the heart disease? (Or are there signs of sympathetic drive- sign of decompensation in dogs)
Loss of sinus arrhythmia (dogs)
Observe for abnormal respiration
With coughing dogs, are there signs of left atrial enlargement on radiographs?
If not, the coughing is probably not due to cardiac disease
With pleural effusions in cats (modified transudates or chylothorax), other signs of cardiac disease?
ECG often abnormal with cardiac disease
Cardiomegaly generally present on radiographs
Echo diagnostic for cardiac disease