Hypertrophic Cardiomyopathy: Acute Therapy

By | 2013-06-07

Just as with dilated cardiomyopathy, cats that have respiratory distress suspected of having heart failure secondary to hypertrophic cardiomyopathy may need to be placed in an oxygen-enriched environment as soon as possible. If possible the cat should be initially evaluated by doing a cursory physical examination, taking care not to stress the patient during this or any other procedure, because stress exacerbates dyspnea and arrhythmias and often leads to death. Most, but not all, cats with severe hypertrophic cardiomyopathy that are in heart failure will have a heart murmur and gallop rhythm. A butterfly catheter should be used to perform thoracentesis on both sides of the chest to look for pleural effusion as soon as possible. Generally this should be done with the cat in a sternal position so that it does not become stressed during the procedure. Clipping of the hair is not needed. If fluid is identified, it should be removed. Most cats that arc dyspneic due to pleural effusion have 150 to 250 mL of fluid in their pleural space. If none is identified, a lateral thoracic radiograph to identify pulmonary edema may be taken with the veterinarian present to ensure that the cat is not stressed (e.g., the clinician should make sure no one stretches the cat out or in any way interferes with its ability to breathe). If the patient struggles or appears to be stressed or fractious during or before radiographic examination, the procedure should be canceled and the patient placed into an oxygen-enriched environment. A preferable alternative to blind tapping is to perform a superficial ultrasonographic examination to identify and locate fluid accumulation.

Furosemide

Furosemide should initially be administered intravenously or intramuscularly to the cat in severe respiratory distress. The route of administration depends on the stress level of the patient. Furosemide should bo administered intramuscularly to cats that are very distressed and cannot tolerate restraint for an intravenous injection. Cats that can tolerate an intravenous injection may benefit from the more rapid onset of action (within 5 minutes of an intravenous injection versus 30 minutes for an intramuscular injection).The initial rurosemide dose to a cat in distress should generally be in the 1 to 2 mg/lb range, intramuscularly or intravenously. This dose may be repeated within 1 hour to 2 hours ().

High-dose parenteral furosemide therapy commonly produces electrolyte disturbances and dehydration in cats. Cats with severe heart failure that require intensive therapy are often precarious. They may be presented dehydrated and electrolyte-depleted because of anorexia. They may remain anorexic and consequently dehydrated and depleted of electrolytes once the edema, the effusion, or both are lessened. Judicious intravenous or subcutaneous fluid administration may be required to improve these cats clinically. Overzealous fluid administration will result in the return of congestive heart failure. If fluid administration is required, the furosemide administration must be discontinued for that time.

Nitroprusside

Anecdotally, nitroprusside may be beneficial in cats with severe pulmonary edema due to hypertrophic cardiomyopathy. As with dilated cardiomyopathy, it may be administered empirically at 2 ug/lb/min or titrated, using blood pressure measurement to document efficacy, starting at a dose of 1 to 2 pg/lb/min. Nitroprusside has a very short half-life. Consequently, if clinically significant systemic-hypotension is produced (e.g., weakness, collapse, poor capillary refill time) cessation of the infusion will result in the systemic blood pressure returning to normal within several minutes.

Nitroglycerin

Nitroglycerin cream may be beneficial in cats with severe edema formation secondary to feline cardiomy-opathy. However, no studies have examined effects or efficacy. Nitroglycerin is safe and some benefit may occur with its administration in some cats. Consequently, one-eighth inch to one-fourth inch of a 2% cream may be administered to the inside of an ear every 4 to 6 hours for the first 24 hours as long as furosemide is being administered concomitandy. Nitroglycerin is not a primary drug. Tolerance develops rapidly in other species, and prolonged administration is probably of even lesser benefit.

Once drug administration is complete, the cat should be left to rest quietly in an oxygen-enriched environment. Care should be taken not to distress the cat. A baseline measurement of the respiratory rate and assessment of respiratory character should be taken when the cat is resting. This should be followed at 30-minute intervals and furosemide administration continued until the respiratory rate starts to decrease (a consistent decrease of the respiratory rate from 70 to 90 breaths per minute into the 50 to 60 breaths per minute range is a general guide), the character of the cat’s respiratory effort improves, or both occur. When this happens, the furosemide dose and dose frequency should be curtailed sharply.

Sedation or Anesthesia

In some cats, sedation with acepromazine (0.02 to 0.5 mg/lb intramuscularly or intravenously) may help by producing anxiolysis. Oxymorphone (0.02 to 0.04 mg/lb every 6 hours intramuscularly, intravenously, or sub-cutaneously) or butorphanol tartrate (0.04 mg/lb intravenously or 0.18 mg/lb every 4 hours subcutaneously) may also be used but are secondary choices because they can produce respiratory depression. Oxymorphone may produce excitement in some cats.

In some cats with fulminant heart failure, anesthesia, intubation, and ventilation are required to control the respiratory failure. Although this method is not preferred for most severely dyspneic cats, it can be life saving in some. This procedure has the advantage of being able to administer 100% oxygen and to be able to drain or suction fluid from the large airways in a controlled environment. The disadvantage is the administration of anesthetic agents to a cat that has cardiovascular compromise.