Congestive heart failure

Owners usually first notice faster breathing at rest or during sleep, which is the best early warning of decompensation. Dogs then show restlessness, cough and laboured breathing, and advanced cases develop ascites. Cats present with rapid or laboured breathing, sometimes with low body temperature, a gallop sound, arrhythmia, or muffled lung sounds from pleural effusion. A cat may also arrive with sudden hind-limb paresis or paralysis from arterial thromboembolism. Radiographs show cardiac enlargement and pulmonary oedema; echocardiography shows an enlarged left atrium.

Heart failure is the clinical syndrome caused by heart dysfunction; it is congestive (backward) when raised venous pressure pushes fluid out of vessels. In dogs the commonest cause is myxomatous mitral valve disease: degenerating leaflets stop sealing, blood regurgitates into the left atrium, and the resulting volume overload remodels and enlarges the atrium and ventricle until the ventricle fails. Rising left atrial pressure is transmitted back to the pulmonary veins, and fluid floods the lungs as pulmonary oedema. The renin-angiotensin-aldosterone system and sympathetic nervous system activate to support output, retaining sodium and water; helpful early, this becomes harmful over time. In cats, cardiomyopathies such as hypertrophic cardiomyopathy enlarge the left atrium and cause pulmonary oedema or pleural effusion.

Myxomatous mitral valve disease accounts for about 75% of canine heart disease cases seen in primary care practice, and is about 1.5 times more common in males. Cavalier King Charles Spaniels develop it unusually early, while large-breed dogs progress faster with more obvious muscle dysfunction. Dogs are staged from A (at risk) through B1 and B2 (structural disease without signs) to C (current or past heart failure) and D (refractory failure). In cats, most with hypertrophic cardiomyopathy have subclinical disease, and only a proportion progress to congestive failure or arterial thromboembolism.

Primary respiratory disease is the main confusion. In coughing dogs with a mitral murmur, chronic tracheobronchial airway disease must be excluded. In cats, asthma and other respiratory causes of distress are separated from heart failure by a point-of-care NT-proBNP test (a negative result favours respiratory disease), by thoracic ultrasound showing effusions or B-lines with a severely enlarged left atrium, and by findings such as hypothermia and gallop sounds. Radiographic evidence of cardiogenic oedema confirms congestion.

Heart (mitral valve, left atrium, left ventricle, myocardium), lungs and pulmonary veins, pleural space, peritoneal cavity (ascites), kidneys and RAAS, systemic arteries (thromboembolism)

No true prevention. Stage predisposed dogs and cats early by echocardiography and radiographs; owners track sleeping respiratory rate; mitral valve repair at experienced centres; dietary sodium restriction; thromboprophylaxis in at-risk cats.

Progressive. Dogs in stage C respond to treatment for a time before becoming refractory (stage D); large breeds have a more guarded outlook. Cats survive much less long once heart failure or thromboembolism develops than with subclinical disease.