Congestive heart failure (CHF) is an important differential for animals presented with respiratory signs, but it is not the only differential. Accurate identification of CHF is essential for effective treatment, but can be challenging with both acute and chronic CHF. Less commonly, it may be necessary to distinguish animals with heart disease that are preclinical (i.e., still free of clinical signs) versus those that have developed early CHF.

Heart Disease vs. Heart Failure

Not all animals with heart disease will develop heart failure. Many dogs with myxomatous mitral valve disease (MMVD) and many cats with hypertrophic cardiomyopathy (HCM) will remain free of clinical signs and will have a normal life expectancy. Treatment is essential for animals with congestive heart failure, but treatment may not be necessary at all for animals with preclinical heart disease, depending on the condition. It is therefore important to be able to recognise the signs that indicate that heart disease has progressed to heart failure. Heart disease can be staged in dogs (and cats) in the same way as it is staged in humans.¹

Definition of CHF

There are many definitions of heart failure. One definition is 'the diseased heart fails when it can no longer maintain adequate delivery of oxygenated blood to the tissues at normal filling pressures.' Another set of definitions includes 'forwards' and 'backwards' heart failure. Forwards failure occurs when clinical signs develop because of inadequate cardiac output. Backwards failure occurs when atrial pressures increase sufficiently to cause excessive venous pressures, which in turn result in accumulation of fluid in the lungs or body cavities. Left-sided heart failure and right-sided heart failure result in different signs depending on the side of the heart that is affected. Usually when clinicians talk about heart failure, they are talking about congestive heart failure (CHF). This can be thought of as backwards failure, where clinical signs are associated with abnormal fluid accumulation. Respiratory distress is the most common manifestation of this, when fluid accumulates in the lungs or pleural space.

Pathophysiology of CHF

Although an acute cardiac insult can decrease cardiac output in the short term, most cardiac diseases are of gradual onset, and any drop in cardiac output is imperceptible. Blood pressure is very tightly regulated to ensure adequate perfusion pressure for the cerebral and coronary circulations. Baroreceptors will sense the change in cardiac performance, and this leads to neurohormonal activation of the sympathetic nervous system and the renin-angiotensin system. The net effect is to increase systemic vascular resistance (increasing blood pressure) and to increase sodium and water retention (increasing plasma volume and therefore preload, thereby increasing cardiac output and restoring blood pressure). This means that signs due to low cardiac output are uncommon, but signs due to excessive plasma volume expansion and increased atrial pressures are much more common.

Respiratory Distress - Is This Congestive Heart Failure?

Observation

There is very valuable information in the physical examination, but also just from observing the breathing pattern. A breathing pattern that is slow, with prolonged inspiration and associated with respiratory noise is most consistent with upper airway obstruction and is not likely to be CHF. A slow breathing pattern with prolonged expiration and marked abdominal effort is most likely to be caused by lower airway obstruction. The pattern most consistent with CHF is a rapid respiratory rate with equal effort in inspiration and expiration. Usually no abnormal sound is audible. This pattern can be associated with any pulmonary parenchymal or pleural disease and is not specific for CHF.

Auscultation

Assessment of heart rate, rhythm, and abnormal sounds is important for identifying underlying heart disease in animals presenting with respiratory distress. In dogs, heart rate is likely to be increased in dogs with CHF. This is not true in cats! Relaxed, healthy dogs often have sinus arrhythmia, and this is usually lost with the onset of CHF. Presence of arrhythmias in both dogs and cats may indicate underlying heart disease but is not a reliable sign of heart failure. Gallop sounds are often present in cats with CHF. Crackles can be heard with severe pulmonary oedema, but they may also be present with interstitial pulmonary fibrosis. Lung sounds will be quieter than normal ventrally when a pleural effusion is present.

Other Physical Exam Findings with CHF

The jugular veins are often ignored, but jugular distension can be a helpful clue with right-sided CHF. The most obvious sign with right-sided failure may be abdominal distension, with a fluid thrill present. Cats are less likely to have ascites than dogs with CHF, but even cats can have ascites with refractory heart disease. Subcutaneous oedema is uncommon in both species, but may be present with very advanced right-sided CHF. Advanced CHF is often associated with loss of lean muscle mass and cardiac cachexia.

Cardiac Imaging for CHF

Physical exam is often sufficient to raise suspicion of CHF, but it may not be enough to confirm it. Imaging can be very helpful for confirming that not only is heart disease present, but it is also severe enough to cause CHF. Imaging can be used to demonstrate the abnormal fluid accumulation or to show changes in chamber dimension. Echocardiography is often better tolerated. Presence or absence of left atrial (LA) enlargement is a vital clue - LA enlargement is expected with CHF in MMVD, HCM, and dilated cardiomyopathy (DCM). LA enlargement can be identified on radiographs or echocardiography. Identification of a dilated caudal vena cava and distended hepatic veins on ultrasound will be supportive of a diagnosis of right heart failure.

Cardiac Biomarkers

Several studies have shown that NT-proBNP can be used to help differentiate respiratory distress due to cardiac disease and respiratory disease, both in dogs^2,3 and cats⁴. A point-of-care test for identifying elevated NT-proBNP concentrations in cats has now been developed as a rapid test to rule out CHF in cats with respiratory distress (CHF is less likely in cats with NT-proBNP concentrations < 100 pmol/l).⁵ Biomarkers used in this way may change our approach to identifying CHF.

Owner Measurements of Respiratory Rate

One of the most useful tests for identifying CHF in animals with known heart disease is also the cheapest measuring respiratory rate at home. One study comparing the ability of a range of different diagnostic tests to correctly identify dogs with CHF found that the respiratory rate measured by the owner at home while the animal was resting or sleeping was the most accurate test. Although other conditions can cause an elevated respiratory rate, in an animal with severe cardiac disease that is at risk of CHF, this is probably the best test of all.

References

1.  Atkins C, Bonagura J, Ettinger S, et al. Guidelines for the diagnosis and treatment of canine chronic valvular heart disease. J Vet Intern Med. 2009;23:1142–1150.

2.  Oyama MA, Boswood A, Connolly DJ, et al. Clinical usefulness of an assay for measurement of circulating N-terminal pro-B-type natriuretic peptide concentration in dogs and cats with heart disease. J Am Vet Med Assoc. 2013;243:71–82.

3.  Oyama MA, Fox PR, Rush JE, et al. Clinical utility of serum N-terminal pro-B-type natriuretic peptide concentration for identifying cardiac disease in dogs and assessing disease severity. J Am Vet Med Assoc. 2008;232:1496–1503.

4.  Fox PR, Oyama MA, MacDonald K, et al. Comparison of NT-pro-BNP concentration in cats with acute dyspnea from cardiac or respiratory disease. J Vet Intern Med. 2008;22:65.

5.  Machen MC, Oyama MA, Gordon SG, et al. Multi-centered investigation of a point-of-care NT-proBNP ELISA assay to detect moderate to severe occult (pre-clinical) feline heart disease in cats referred for cardiac evaluation. J Vet Cardiol. 2014;16:245–255.