My eighteen-pound dog was recently diagnosed with congestive heart failure. The veterinarian prescribed furosemide and digoxin. For the past few days the dog has refused to eat and has been vomiting. She is losing weight and has diarrhea. Could any of these problems be caused by either medication?

Yes, there are several side effects associated with toxic doses.

You must take your dog back to your veterinarian promptly. Although the signs you mentioned may have many causes, these signs are often seen in digoxin toxicity. The dosage of one or both drugs may need to be reduced.

Digoxin and furosemide are frequently prescribed together in the treatment of congestive heart failure (CHF) in dogs. In cats furosemide alone is usually sufficient. In CHF, the heart is not able to effectively accommodate the volume of fluid in the body. As a result, fluid backs up into the lungs (pulmonary edema) in affected dogs and cats. Dogs may also develop fluid back up into the abdominal cavity (ascites) and the legs (peripheral edema). Cats, unlike dogs, rarely display ascites or peripheral edema associated with CHF. Digoxin (LanoxinG ) improves the ability of the heart to pump blood and influences sodium and water balance in the body. Furosemide (LasixG ), a diuretic ("water pill"), promotes excretion of excess fluid by the kidneys.

When too much digoxin builds up in the blood, it can cause loss of appetite, vomiting, and diarrhea. Heart arrhythmias also may occur, although these seem to occur after the previously mentioned gastrointestinal signs begin. Determining the proper dose of digoxin is often difficult; adjustments are often necessary until a maintenance dose is found. Therapeutic drug monitoring allows veterinarians to determine the drug level in the blood stream at a specific time and to target specific levels.

Toxic levels of digoxin may be due to the dose being too high initially, or other problems may be present that increase the activity of the drug. Low blood levels of calcium, potassium, and sodium, as well as hypothyroidism and renal failure may all intensify digoxin toxicity. Certain drugs, such as the antibiotic tetracycline and quinidine, a heart medication, may increase digoxin concentrations also. Additionally, some dogs appear to be extremely susceptible to the effects of digoxin even when blood levels are within the normal therapeutic range. Dobermans are especially likely to experience these effects.

Furosemide has a number of side effects that can cause many of the same signs as digoxin toxicity and can also contribute to digoxin toxicity itself. Furosemide can lead to dehydration, and excess loss of potassium, sodium and chloride. It can cause hearing problems and low blood pressure.

Treatment involves reducing (or temporarily discontinuing) the doses of digoxin and furosemide, and re-establishing normal fluid and electrolyte (sodium, calcium, potassium, and chloride ions) balance. The goal of treatment eventually is to find the appropriate maintenance doses of these drugs so that the congestive heart failure can be treated as optimally as possible without drug side effects.

05/23/02