Introduction
Heart disease in dogs and cats can vary from mild disease that will not affect the lifespan of the patient to disease that can cause premature death, through heart failure or sometimes acutely with sudden cardiac death.
Apart from dilated cardiomyopathy and degenerative valve disease, there is little evidence that therapy in the asymptomatic stages of heart disease delays the onset of heart failure or prolongs life. Heart failure can be described as a syndrome whereby the heart is unable to maintain blood pressure or can only do so with elevated filling pressures. Once dogs and cats develop heart failure, their clinical signs can often be managed medically in the long term. Unfortunately, many diseases are progressive and so eventually the signs return and may not be amenable to control. In considering heart disease in small animals, it helps to recall the classification scheme:Class A: Animals with no evidence of heart disease but a predilection to develop it, for example, a 6-year-old cavalier King Charles spaniel with no murmur, or a Maine coon cat with the myosin binding protein abnormality and no structural changes.
Class B: This class can be subdivided:
• Class B1: Evidence of heart disease but no structural changes or clinical signs, for example, a cavalier King Charles spaniel with a heart murmur suggestive of degenerative mitral valve disease but no chamber enlargement.
• Class B2: Evidence of heart disease and structural change but no clinical signs, for example, a cavalier King Charles spaniel with a heart murmur and left atrial enlargement or a Doberman with dilated cardiomyopathy with evidence of left ventricular dilation and systolic dysfunction on echocardiography.
Class C: Evidence of congestive heart failure or on treatment for congestive heart failure (compensated).
Class D: Severe congestive heart failure requiring hospitalization and intensive care that is refractory to conventional medications.
Patients progress from one class to the next but cannot move in the reverse direction, in other words, once a patient develops heart failure, they stay in class C even when compensated. Not every patient will develop heart failure but for those that do, treatment has two phases and goals. Initial treatment of acute congestive heart failure is aimed at ensuring that the patient does not die. In the case of acute pulmonary edema, this effectively involves stopping the patient from drowning and treatments are aimed to achieve this even if their effects may be detrimental in the long run. However, once this is achieved, the goal moves to controlling
Chronic Disease Managementfor Small Animals, First Edition. Edited by W. Dunbar Gram, Rowan J. Milner and Remo Lobetti.
© 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.
clinical signs and extending longevity. In doing this, it should be remembered that heart failure is a neurohormonal disease that involves the activation of a number of hormones that may be beneficial in the setting of acute blood loss but are detrimental in patients with heart failure and stimulation of the adrenergic system. Activation of the renin-angiotensin-aldo sterone system
results in increases in levels of angiotensin II and aldosterone both of which have harmful effects chronically. Angiotensin II causes vasoconstriction, increased aldosterone levels, sodium retention, increased thirst and myocardial remodeling. Increased aldosterone levels result in increased sodium and water retention with potassium loss, as well as sympathetic potentiation. Aldosterone is also responsible for myocardial fibrosis and cell death. Furthermore, the sympathetic system itself is stimulated and again, chronic stimulation is harmful causing increased heart rate and contractility, vasoconstriction. Baroreceptor reflexes become blunted and there is down-regulation of beta receptors.
While the benefits of angiotensin converting enzyme (ACE) inhibitors and aldosterone antagonists have been demonstrated in dogs and to a lesser extent in cats, the benefits of beta blockage have yet to be convincingly demonstrated. In humans, they are commonly used to treat congestive heart failure and disease due to ischemic myopathy. However in veterinary patients who suffer from different diseases, it may be that survival times are too short to show advantage.