What is degenerative mitral valve disease?
Blood is propelled through the heart due to the actions of 4 valves which allow blood to flow in one direction but close to stop flowing backwards (regurgitating). Although all four valves can be affected by degeneration, it most commonly affects the mitral and tricuspid valves which are on the left and right sides respectively. The valve becomes thickened and the edges roll up, allowing blood to regurgitate. This high speed jet of blood causes an audible murmur that your veterinarian may detect when listening to your pet’s heart. The chordae tendineae (“heart strings”) stop the valve in the closed position. These can also be affected by this degenerative disease and as a result may rupture causing valve prolapse or the leaflet to flail back into the left atrium when the ventricle contracts during the heartbeat. This can cause a sudden deterioration in the disease.
Although all breeds can be affected, small breeds tend to be more common such as cavalier King Charles spaniels, Maltese, Chihuahuas, etc. It is also more common in older dogs but in some dogs 50% will have murmurs by 5 – 6 years. Progression of the disease is variable. In some dogs, a murmur can remain for the rest of their lives with no significant deterioration but in others it can be more rapidly progressive causing premature death. It is the most common heart disease seen in dogs, making up 75% of all cardiac cases. It is estimated that 5-7 million dogs in the USA are affected by this disease.
Initially, the volume of regurgitated blood is small and so no changes are seen. In many, but not all cases, the disease progresses and the leak gets worse, causing enlargement of the left atrium and left ventricle which can be detected on radiographs or with echocardiography. If the heart is enlarged, treatment with pimobendan has been shown to slow the progression of the disease. Eventually, pressure in the left atrium rises enough to allow fluid to spill into the lungs as pulmonary edema. At that point, increased respiratory rate and effort are seen as congestive heart failure develops. Drugs can be used to control the heart failure including diuretics (e.g. furosemide), ACE inhibitors (e.g. benazepril and enalapril) and spironolocatone.
Concurrent Diseases (diseases often seen at the same time as MVD)
- Tricuspid Valve Disease: About 1/3 of dogs with degenerative mitral valve disease also have tricuspid valve disease and dogs can develop right sided congestive heart failure with fluid gathering in the abdomen (ascites). Surgery to the mitral valve usually benefits the right side by decreasing pressures in the lungs (pulmonary hypertension).
- Pulmonary hypertension: Degenerative mitral valve disease causes blood pressure to rise in the lungs causing pulmonary hypertension. The signs of this can often be vague but fainting is commonly seen. Drugs such as sildenafil can help decrease the pressure in the pulmonary artery.
- Tracheal collapse and bronchomalacia: The trachea and bronchi are held open by incomplete rings of cartilage. In some breeds these are weak, allowing the airway to collapse during breathing, resulting in a cough. With the enlarged heart secondary to degenerative mitral valve disease, the pressure on the airway in increased resulting in severe coughing. Cough suppressants such as hydrocodone may help control signs.
Once dogs have heart enlargement, the average time to heart failure is 3 years, 4 months. Once heart failure develops the average survival is 9 months with medical treatment. However, mitral valve repair surgery offers a cure for this disease.
Mitral Valve Repair Surgery
The surgery is not recommended until dogs develop heart enlargement. About 50% of the dogs who have the surgery have had congestive heart failure and are being treated with medication. The aim of the surgery is to reconstruct the valve, reducing the leaking and stopping progression of the disease.
The dogs are anesthetized and tubes are placed in the main vessels to connect the circulation to the bypass machine which acts as the heart and lungs while the heart is stopped, allowing the blood to continue circulating and providing oxygen. Once this is functioning, the heart is stopped and the left atrium is opened. There are two parts to the surgery:
- Chordal replacement: The chordae tendineae that are stretched or broken are replaced by artificial PTFE sutures.
- Mitral annuloplasty: The mitral valve is made smaller by a circling suture. This helps restore normal valve function and prevent further dilation.
Once these are completed, the heart is closed, filled with blood and restarted.
- Preoperative lung disease: Dogs with significant lung disease have a higher risk of death during the anesthesia and may require mechanical ventilation. The University of Florida commonly uses mechanical ventilation for a variety of conditions.
- Postoperative congestive heart failure: The weakened heart may be unable to contract adequately. The tricuspid regurgitation may worsen causing right sided heart failure and ascites.
- Postoperative lung disease: Causes include bleeding in the lungs, pain, reduced contractility of the heart, reduced lung function due to collapsed lung lobes or blood clots, paralysis of the diaphragm and airway obstruction.
- Intracardiac hematoma: A bleed in heart muscle may occur during or after surgery. It may gradually reduce in size but may require drainage or may result in death. The cause is unknown.
- Hemorrhage: The patient is given anticoagulants to prevent clotting during surgery. This effect can persist postoperatively, allowing significant blood loss.
- Anemia or low platelet count: Anemia is common postoperatively due to dilution of the blood by the fluids administered. The platelet count may also be low, predisposing to hemorrhage. Both of these usually improve over the first week but a blood transfusion may be needed.
- Transfusion reaction: Although blood is typed according to DEA 1.1, other factors may result in transfusion reactions.
- Blood clots (thromboembolism): Blood clots can form in the heart during or after surgery. They can move to lodge in anywhere in the body causing a variety of signs:
- Neurological signs: Coma, seizure or paralysis
- Acute myocardial infarct (clot in the muscle of the heart): Congestive heart failure
- Pulmonary embolism (clot in the lungs): Respiratory distress
- Pancreatitis: Poor appetite, vomiting, diarrhea, abdominal pain
- Peripheral arterial embolism (clot in the arteries throughout the body): Paresis (weakness), necrosis (death of affected tissue)
- Adverse effects on organ function: Organs such as the liver, kidney and pancreas may not receive adequate blood during bypass and this can affect their function post-operatively. Pancreatitis is one of the more common problems.
- Arrhythmia: An irregular heart beat may occur during or after surgery. This usually disappears after 48 hours but drugs to control the arrhythmia or a pacemaker may be needed.
- Diaphragmatic paralysis: This can result in rapid shallow breathing and shortness of breath as the diaphragm cannot contribute to breathing. It is usually temporary and spontaneous resolution is common.
- Infection: This may occur due to a weakened immune system but infection of the implanted material is uncommon