Overview of Common Cardiac Medications

There are a large number of medications used in people and animals to treat heart disease. The following is a list of some of the more common drugs in use, along with a description of what they do, how they do it, and what problems (“complications”) may arise with use.

Diuretics: Furosemide (Lasix, Salix) and others (Torsemide, Spironolactone, HCTZ)

• Mechanism of action: diuretics, or “water pills”, increase loss of salt and water through the kidneys via increased urination (termed “diuresis”).
• Congestive heart failure (CHF) is characterized by increased salt and water retention and increased fluid in or around the lungs and/or abdomen. Diuretics result in alleviation of this congestion by causing increased urination, which results in salt and water loss. All blood salts, also called “electrolytes”, are lost to varying degrees.
• Torsemide and Lasix are similar drugs; torsemide is absorbed more reliably, lasts longer, but is more expensive. It also may have some use in treatment of high pulmonary blood pressure.
• Spironolactone is a different kind of diuretic; it is not very effective at relieving congestion, but does help the body retain potassium. It may help slow down the course of heart failure, though this is still unclear.
• Hydrochlorthiazide is another kind of diuretic. It is also weaker than Lasix, and results in a high degree of potassium loss. It is typically reserved for advanced cases of CHF, and is used in addition to the above diuretics.
• Indications: Lasix is the cornerstone of therapy of congestive heart failure in dogs and cats. Torsemide and/or hydrochlorthiazide are indicated for pets that are resistant to Lasix therapy. Spironolactone is frequently added to therapy for CHF, especially if potassium is low.
• Adverse effects: Primarily adversely affects the kidneys; may even result in kidney dysfunction or failure at high doses. At home, the increased amount of urine can become a problem, especially in dogs without free access to the outdoors. Animals on diuretics will also drink a lot of water, so it is important to have water available at all times. The primary sign of adverse effects is a reduction in or loss of appetite. If this is identified, stop and call a veterinarian immediately. Electrolyte loss through the urine is common, and may require supplementation (especially potassium). These side effects are why we recommend checking kidney values and electrolytes on a blood test about a week after starting diuretic therapy, and periodically during the course of therapy.

Pimobendan (Vetmedin)

• Mechanism of Action: Strengthens the heart muscle and dilates the blood vessels. This results in improved forward blood flow, improving heart function and alleviating congestion.
• Has been shown to increase quality of life and lifespan in dogs with heart failure due to mitral valve disease or dilated cardiomyopathy. It is now considered the second most important drug in the treatment of these conditions, following Lasix.
• Often, dogs will feel better, have more energy, eat better, and cough less while on Pimobendan.
• Indications: The second drug of choice (with Lasix) in the treatment of CHF in the dog due to mitral valve disease or dilated cardiomyopathy. No clear use in cats at this time, but may be effective in some situations.
• Adverse effects: Usually very minimal. Can be difficult to give due to the large pill size (it is a chewable tablet, but not all pets will eat it). May cause excitation, especially in cats. May actually increase the severity of mitral valve damage, though this is controversial.

Angiotensin Converting Enzyme (ACE) Inhibitors (Enalapril, Benazepril, others)

• Mechanism of action: inhibit ACE, which is a hormone that is over-expressed in congestive heart failure (CHF)
• Elevations in ACE can result in salt and water retention, leading to congestion. It can also result in elevated blood pressure. Another adverse effect of ACE is “scarring” of the heart muscle and blood vessels.
• ACE inhibitors have been shown to result in a longer survival time in dogs with congestive heart failure, presumably by counteracting the above effects. They also can be helpful in treating hypertension and kidney disease.
• Indications: Primarily used for the treatment of CHF in dogs and cats; second-line medication behind Lasix.
• Some controversy exists about treating dogs with ACE inhibitors prior to the onset of heart failure; this decision is usually made on a case-by-case basis.
• Adverse effects: May cause kidney dysfunction. The initial sign of kidney dysfunction in these patients is often a reduction in or loss of appetite. If this is identified, stop and call a veterinarian immediately. May cause reduced blood pressure (uncommon).

Beta-Blockers: Atenolol, Carvedilol, Sotalol, Propranolol, others

• Mechanism of Action: block the effects of adrenaline (also called epinephrine) on the heart. Each beta-blocker we use has slightly different characteristics that make them better or worse for specific conditions.
• Adrenaline causes the heart to beat faster and harder, which uses more oxygen and energy. This can result in worsening of pre-existing heart disease, especially if there is obstruction to blood flow through the heart or the heart is thicker than normal (such as with hypertrophic cardiomyopathy in cats or subaortic stenosis in dogs).
• Additionally, adrenaline can result in arrhythmias (irregular heart beats). Beta blockers can stop these arrhythmias, while also slowing down the heart muscle. This allows the heart to “save energy”
• In humans, beta blockers extend life in a number of heart diseases, by reducing the damage to the heart over time. This has not been demonstrated in pets, although long-term studies have not been performed.
• Indications: Primarily used for hypertrophic cardiomyopathy (HCM) in cats, and for the treatment of some kinds of arrhythmias in dogs.
• Other uses: may help slow the onset or progression of heart failure in some dogs and cats with heart disease (unproven). Can use to help treat high blood pressure.
• Adverse effects: Some dogs and cats do not tolerate beta-blockers, as the reduction in heart strength may cause congestive heart failure or low blood pressure. For this reason, we frequently prescribe a very low dose, and increase the dose over time. If instituted carefully, beta-blockers are well tolerated by most animals. Beta-blockers may also worsen pre-existing lung disease, such as asthma in cats.

Calcium-channel blockers (Diltiazem, Amlodipine)

• Mechanism of action: Calcium is an important component of blood vessel and heart muscle contraction, as well as the heart rate. Calcium channel blockers prevent calcium from entering the cell, which results in blood pressure reduction, slowed heart rate, and some degree of weakening of the heart muscle.
• Indications:
• Diltiazem – primarily used for slowing the heart rate (antiarrhythmic), especially for atrial fibrillation. May be used for treatment of hypertrophic cardiomyopathy in cats.
• Amlodipine – first-line drug for the therapy of hypertension (high blood pressure) in dogs and cats. May be used for therapy of advanced congestive heart failure in dogs or cats.
• Adverse effects: Vomiting, weakness, lethargy, or collapse due to excessive blood pressure reduction. May cause excessive slowing of the heart rate, resulting in similar signs. These signs are uncommon with judicious use.

Antithrombotic (“clot preventing”) drugs: Aspirin, Plavix (Clopidogrel), Heparin

• Mechanism of action: Work through a variety of different pathways to inhibit clot formation, which can be a result of heart disease (especially in cats).
• The above drugs are the most commonly prescribed in this class. There are numerous other drugs available for humans, but are either cost-prohibitive, uninvestigated, or associated with unacceptable side effects.
• Indications:
  bq. Aspirin: in low doses, used to inhibit clot formation in cats with heart disease and enlarged hearts. Sometimes used in dogs with clots in the lungs.
  Plavix (Clopidogrel): A newer drug that may be more effective (still under investigation) in preventing clot formation in cats with enlarged hearts.
  Heparin agents (unfractionated heparin, Lovenox, Fragmin): injectable drugs that help to reduce clot formation; usually used in patients in-hospital or with venous clots.
• Adverse Effects:
  bq. Aspirin – may cause stomach upset or GI ulceration/bleeding, but tolerated extremely well at the low doses we use.
  Plavix (Clopidogrel)- may cause inappetance and vomiting, especially if given without food. May cause bleeding, but rare at the prescribed dose. Can be expensive, especially in dogs.
  Heparin agents- have to give injections once to three times a day, which leads to risk of infection and administration errors. More likely to cause bleeding that Aspirin or Plavix, although the risk is still low at prescribed doses.

Digoxin

• Mechanism of action: Slows the heart rate, strengthens the heart muscle, and improves hormone balance in heart disease.
• Digoxin is a compound that is naturally found in purple foxglove plants. As such, it is the oldest drug used specifically for heart failure in western medicine (identified in 1779)
• Has complicated and multiple effects on different parts of the body, which contributes to a high degree of toxicity and variability in effects.
• Indications: Primarily used for dogs with atrial fibrillation (antiarrhythmic), especially if the pet is also in congestive heart failure. Rarely used in cats.
• Adverse effects: Many potential complications, which is why we generally start a very low dose (lower than in the past). Vomiting, diarrhea and inappetance are usually the initial signs of toxicity, so if noted it is very important to stop Digoxin and call your veterinarian or cardiologist immediately. If toxicity progresses, it can be fatal – arrhythmias (irregular heartbeats), collapse and shock may occur. This is particularly prevalent in patients with kidney disease, dehydration or electrolyte imbalances. In general, though, low doses of Digoxin have been useful and beneficial for years, and are tolerated well by most dogs.