Management of heart failure (Proceedings)
The technician should appreciate the definition and causes of CHF, as well as methods of evaluation. The drugs used to treat CHF should be understood, as well as related side-effects. This awareness improves both patient care and client communications.
Definition of CHF
Congestive heart failure (CHF) is a common disorder of dogs and cats and is characterized by a cardiac lesion, insufficient cardiac output, and maladaptive compensations that restore blood pressure but injure the patient. For example, the sympathetic nervous system and the renin-angiotensin-aldosterone system are triggered in CHF. However, chronic activation of these systems also damage heart muscle and blood vessels. In most cases there is elevation of venous pressures behind the failing side of the heart creating "congestion" in the organs drained by that venous circulation. Left-sided CHF typically causes pulmonary edema with respiratory signs and dysfunction. Right-sided CHF is characterized in dogs by hepatomegaly and ascites. Pleural effusion is a consequence of right-sided or left-sided CHF and is typical of many cats with chronic heart muscle diseases (cardiomyopathies).Causes of heart failure
The most common causes of heart failure in DOGS include the following disorders. Valvular endocardiosis is the most common reason for CHF in older dogs. It is characterized by progressive atrioventricular valve degeneration and typical heart murmurs of mitral and tricuspid regurgitation. Atrial arrhythmias and left mainstem bronchial compression may complicate the picture. Systemic hypertension due to concurrent renal or Cushing's disease can worsen the situation. Dilated cardiomyopathy is a primary myocardial disorder wherein progressive contractility failure leads to CHF. Often associated with cardiac arrhythmias, such as atrial fibrillation and ventricular tachycardia, the condition is "idiopathic" but genetically predisposed in many breeds. Left- and right-sided CHF, as well as sudden cardiac death, are common outcomes. Infective endocarditis is a bacterial infection of the cardiac valves, typically affecting the mitral and aortic valves in dogs. This disorder is relatively uncommon and is very rare in cats, but may lead CHF. Congenital heart defects, including patent ductus arteriosus and pulmonic stenosis, are other causes of heart failure. Pericardial effusion is an under-recognized cause of CHF, but treatment is very different than for the conditions above, and does not involve cardiac drugs.
In comparison, the main causes of CHF in CATS are primary and secondary heart muscle disease or cardiomyopathies. These conditions impair the filling or pumping functions of the left ventricle. Common conditions are hypertrophic cardiomyopathy (HCM) and restrictive (fibrotic) cardiomyopathy (RCM). Chronic and severe hyperthyroidism and systemic hypertension also can lead to CHF in cats.
Diagnosis of heart failure
Diagnosis of CHF requires a careful clinical examination and no historical findings are specific for the condition. In most cases, exercise intolerance can be identified. Respiratory symptoms (tachypnea, dyspnea, or cough) will be observed in left-sided CHF. In right heart failure, the abdomen may be obviously distended with ascitic fluid (especially in dogs). Auscultation may indicate a heart murmur, arrhythmia, or gallop sound (an extra sound indicating abnormal cardiac filling). The lungs may auscult abnormally. Thoracic radiography is very helpful for diagnosis of left-sided CHF, but it may not be safe for the patient to obtain radiographs until initial patient stabilization. The key findings are 1) left-sided cardiomegaly; 2) pulmonary venous congestion or distension; and 3) lung infiltrates compatible with pulmonary edema. Pleural effusions are also readily evident from radiographs. Diagnosis of right-sided CHF is usually made from physical diagnosis, recognition of cardiomegaly, and echocardiography. The EKG may be helpful for identification of cardiac-enlargement patterns and arrhythmias; however, it is not a very sensitive test for heart disease and is often near-normal in animals with heart failure. Routine clinical laboratory tests, especially those of kidney function (serum urea nitrogen and creatinine) are important to obtain as many drugs may impair kidney function. Blood troponin is increasingly measured to identify heart muscle injury in selected cases. A pro-hormone called NT-BNP is released in higher quantities by the heart in heart failure and is commercially available as a "biomarker" for heart disease/failure. Arterial blood pressure (BP) should be measured in all patients with CHF. Echocardiography is the clinical gold standard for diagnosis of heart diseases in animals, and is very helpful in many cases of suspected CHF.