Definition

Any structural or functional abnormality of the ventricles of the heart that results in enlargement or dysfunction.

Clinical Signs

Dilated Cardiomyopathy

Clinical signs will look like those of CHF:

• The rat tires quickly.
• A dry cough, bronchial wheeze, or moist lung sounds making breathing labored.
• An enlarged abdomen, although this is not a common sign in rats.
• Swelling in limb(s) and feet from fluid retention.
• Little to no appetite. May see the rat willing to eat but will only manage a few bites or leave the rest of food uneaten.
• A gradual weight loss.
• Signs of dehydration may be present (skin slow to return to normal position when gently pinched into tent position at back of shoulders).

Hypertrophic Cardiomyopathy

Clinical signs in the rat may be so subtle as to appear asymptomatic in the beginning with signs progressing over time to include any of the following:

• Single events of same type that may occur several weeks apart, example:
  1. Falls not associated with roughhousing or jumping. May appear to owner as if rat has fallen asleep and suddenly falls from a cage shelf (syncopal episode otherwise known as “passing out” which may be due to a heart arrhythmia).
  2. Unexplained swelling of the feet in an uninjured rat.
• Tires very quickly, reluctant to move and seems to want to sleep or keep eyes closed all the time.
• Labored breathing (appears as if using abdominal muscles to help breathe). Hangs head over cage shelf when laying down, or props head, in an attempt to make breathing easier.
• Hind limb paresis due to thromboemboli (passage of small blood clots).
• Evidence/showing signs of weight loss.

Although weight loss may, in fact, be gradual for both hypertrophic and dilated cardiomyopathy the owner may not detect this unless weighing the rat regularly. As such it may be interpreted as a sudden dramatic weight loss.

Death may appear to occur suddenly.

It is important to note that respiratory infections (a common cause of disease in rats) may be caused by more than one organism and may, if not effectively treated, lead to heart failure. It is therefore recommended to treat respiratory infections persistently and aggressively while determining, or treating for, Cardiomyopathy or CHF. In some cases, a more intensive treatment regimen for respiratory infections may actually rule out a diagnosis of heart disease.

For more information regarding aggressive respiratory treatment modalities refer to the articles on Pneumonia and Mycoplasma.

Etiology

Cardiomyopathy can result from a variety of disease processes and factors such as bacterial, viral, parasitic and fungal infections. It can also be brought about by neoplasms, degenerative inflammatory diseases, toxins, and metabolic disorders related to thyroid diseases.

This condition tends be more severe, and seen more often, in male rats compared to female rats.

Although there are three major classifications of cardiomyopathy the two most pertinent to rats are: dilated (congestive) cardiomyopathy, and hypertrophic cardiomyopathy.

In dilated cardiomyopathy there is damage to the muscle fibers of the heart. The heart progressively loses its ability to contract and pump blood out of the heart and into the circulatory system. As the blood volume remaining in the ventricles increase the pressure increases in the chambers causing the ventricle (or both ventricles) to dilate (enlarge) leading to congestion (see CHF) and clot formation.

The goal of therapy in dilated cardiomyopathy is to improve the ability of the heart to contract and reduce the congestion.

In hypertrophic cardiomyopathy (thickening of the heart muscle, often genetic in origin) the intraventricular septum thickens, and the ventricular (usually the left) filling time is shortened as it becomes stiff resisting the filling of blood into the chamber. This in turn increases pressure in the atrium of the heart causing dilation leading to an increase in pulmonary venous pressure, and causing pulmonary edema. Hypertrophic cardiomyopathy eventually leads to heart failure. The goal of therapy here is to decrease contractility allowing time for the ventricle to fill, and to relieve pulmonary edema and congestion.

The complications of cardiomyopathy are severe heart failure (See CHF), pulmonary effusion, thromboembolisms (blood clots), pulmonary embolus, DIC (disseminated intravascular coagulation) and sudden death.

Diagnostics

Obtain history.

The owner may state the rat has been losing weight, having difficulty breathing, and has not responded to previous treatment for what was believed to be a respiratory illness.

Check for weight loss or gain.

An x-ray (or CT, or MRI if equipment available), can be taken to determine size, shape of heart, or presence of congestion.

Early signs tend to reflect no change in the cardiac silhouette. Late sign may reflect increased cardiothoracic ratio.

Thoracic radiographs may or may not show cardiomegaly (enlarged heart). Often difficult to see in the presence of pulmonary effusion or pulmonary edema. A deviated trachea may be indicative.

Lab draw for electrolytes can reflect hyponatremia (low sodium) due to water retention.

Blood Chemistry may show an elevated BUN (blood, urea nitrogen), creatnine, and creatine kinase.

Treatment

Information on the use of the following drugs can be found in Rat Medication Guide Index.

Dilated Cardiomyopathy:

• Lasix (a diuretic)
• Digoxin/Cardoxin (a positive inotropic cardiac glycoside)
• Pimobendan (in the same therapeutic class as digoxin, but is both a positive inotrope and vasodilator)
• Enalapril (or similar ACE inhibitor)
• Atenolol (or similar beta blocker)
• Aspirin (prevent thrombus formation).
• Careful consideration of fluids by SQ or IV (via tail vein) for dehydration should be determined by vet, once lung congestion is under control.
• Administration of aminophylline , or theophylline may be of benefit to aid breathing where pulmonary congestion is secondary to heart failure.

Hypertrophic Cardiomyopathy:

• Atenolol (or similar beta blocker)
• Aspirin
• Lasix (use when symptomatic heart failure and congestion present)
• Enalapril (use ACE inhibitor only when symptomatic heart failure present)

Oxygen as needed to relieve difficulty in breathing and improve oxygen saturation to heart and tissues.

Nursing Care

• Provide one level cages to prevent difficulty to breathing from the exhaustion of climbing different levels.
• Ensure food and water are within easy reach.
• Provide for healthy adequate nutrition with reduced salt content. Including such supplements in the diet as, omega-3 oils in flax oil (25 mg/day), co-enzyme Q10 (10-30 mg/day), L-carnitine (25 mg/day), and taurine (50 mg/day) may be beneficial. ³
• Monitor for weight gain, which indicates fluid retention.
• Monitor for dehydration using pinch test by gently pinching skin behind shoulders up into a tent position and letting it fall back to normal position. In dehydrated animals skin will be very slow to return to the normal position.
• Monitor effectiveness of medications to see if there is improvement in condition. Medications may need to be adjusted or increased as disease progresses. If you see your rat’s condition worsening return to the Vet for proper adjustment of medication.

Outcome

• Relief of heart failure symptoms
• Enhance heart’s performance
• Improve quality of life
• Emotional support for those having to consider euthanasia for their rat

Prevention

• Maintain a healthy cage environment that includes room to exercise.
• Ensure a healthy diet low in fat content, in order to prevent obesity.
• Ensure adequate treatment is provided for any illness.
• Check with breeder, if possible, for any known history of cardiac disease in the line.

References

1. 44th Annual Pathology of Laboratory Animals. (1999). Retrieved June 28, 2003, from http://www.afip.org/vetpath/POLA/99/NOLDPOLA99.htm.