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Cardiac emergencies (Proceedings)

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Aug 01, 2011

Emergencies related to cardiac disease are common, and can be difficult to differentiate from non cardiac diseases with similar clinical signs. The emergency clinician must be able to differentiate between cardiac and non-cardiac diseases using subtle clues obtained from a brief physical examination, and an understanding of common cardiac conditions.

Congestive heart failure

Congestive heart failure (CHF) is the most common heart-related emergency in both dogs and cats. Congestive heart failure implies impaired cardiac function leading to increased venous pressures, with resulting congestion and edema. In response to a diseased heart (most commonly chronic valvular disease leading to mitral regurgitation), a fall in cardiac output is sensed by baroreceptors in the aorta and carotid sinus. The baroreceptors then signal the brain to increase heart rate and to constrict arterioles and veins. Reduced renal perfusion also stimulates the release of renin from the kidney, ultimately causing the following major changes:
     1. Vascular smooth muscle constriction
     2. Vasopressin release (water retention and vasoconstriction)
     3. Increased thirst
     4. Production of aldosterone (salt retention, water retention)

Thus, congestive heart failure is a state of volume overload resulting from neuroendocrine responses to reductions in cardiac output. The most common cause of congestive heart failure in dogs is chronic valvular disease (mitral regurgitation), with other causes including dilated cardiomyopathy, severe arrhythmias, and congenital heart diseases such as patent ductus arteriosus (PDA).

Dogs and cats with CHF commonly have a history of acute onset of shortness of breath. Dogs sometimes have a history of a dry cough especially at night, due to compression of the trachea by the enlarged left atrium. Coughing in cats is generally associated with asthma. Exercise intolerance and polyuria/polydipsia can also be reported in dogs with CHF. Physical examination findings consist mostly of shortness of breath. The presence of a cardiac murmur (or gallop in cats) certainly supports a diagnosis of CHF. Pulmonary crackles on auscultation suggest the presence of pulmonary edema, though causes of non-cardiogenic pulmonary edema including seizures, strangulation, contusion, and electrocution must be considered. In dogs, breed (i.e. Doberman) and auscultation (cardiac murmur) are often sufficient to create an index of suspicion for CHF. Cats with CHF are much less tolerant of handling, and the stress of the physical examination may be life-threatening. In our experience, the physical examination in cats that present with dyspnea consists of brief auscultation (i.e. murmur/gallop Yes or No) and rectal temperature. Cats with CHF are typically hypothermic from reduced perfusion, which other diseases such as asthma and chronic lung disease are more commonly associated with normothermia or even hyperthermia.

Diagnostic imaging consisting of thoracic radiography is commonly used to identify dogs and cats with congestive heart failure. Cardiomegaly, pulmonary vessel distension, and an alveolar infiltrate are commonly observed in dogs with underlying cardiac failure. In dogs, perihilar edema along with pronounced left atrial enlargement are commonly observed. In cats, distribution of pulmonary infiltrates are more variable. While radiography is used to identify whether congestive heart failure is likely, echocardiography is used to determine the nature of the underlying cardiac disease.

Treatment of dogs and cats with CHF consists first and foremost of an oxygen rich environment such as an oxygen cage (which produces up to 40-50% FIO2). There should be minimal handling, as these animals have a reduced tolerance of stress. Diuretics (Furosemide 2mg/kg) are the mainstay of therapy and can be administered intravenously or intramuscularly to reduce blood volume. Subcutaneous administration will not be effective due to reduced perfusion. As multiple injections are often necessary, a bowl of water should always be available, to prevent the development of pre-renal azotemia. Doses of furosemide can be administered every 1-2 hours until signs of dyspnea less, after which it can be administered at 2-4mg/kg q8-12hours in dogs, 1-2mg/kg q 8-12hrs in cats. Excessive dosing can lead to dehydration, electrolyte changes, and renal failure. Vasodilators can also be useful to reduce congestion. Nitroglycerine paste is a venodilator that is sometime applied to the inner ear or the groin area. Since perfusion to those areas is questionable in CHF, the efficacy of this therapy is questionable. In cases of severe CHF that does not respond to furosemide, sodium nitroprusside (0.5-1ug/kg/min to start) can be administered as a continuous infusion and titrated to effect. Sodium nitroprusside causes vascular smooth dilation of veins and arteries, thus reducing venous return to the heart as well as afterload (promoting forward flow of blood). The use of nitroprusside requires close monitoring, and should be used cautiously. In cases of DCM, dobutamine may be used to increase cardiac contractility (5-15ug./kg.min), and to control signs of CHF. Once signs of CHF are controlled, maintenance therapy is selected based on the underlying disease detected by echocardiogram. Some advocate the use of analgesics to reduce the anxiety associated with dyspnea. In general the use of any medication that cause hypotension and reduced cardiac output should be avoided. While I do not do this in practice, if you choose to do so, it is important to select drugs that are reversible (opioids), or that have minimal cardiovascular effects (diazepam). The use of Pimobendan is rapidly gaining popularity in the United States. Pimobendan increases myocardial contractility, and causes both peripheral and coronary vasodilation. As it increases cardiac output without increased energy demand, Pimobendan is thought to have beneficial effects on the treatment of heart failure and has potential to prolong the life span in animals with cardiac disease.