‘The episodes only happen when he gets really excited, like when I get home from work and he rushes to the door to greet me. I am to the point I am afraid he is not going to wake up next time,” explained Coco’s mom regarding why he had been referred to see me (Dr. Perry Jameson).

Coco is a 17-year-old, male Dachshund who had started fainting when overly excited, like greeting Mom at the door after a day away at work. That event should be a happy, stress-relieving event, not an anxiety-filled stressful episode as it had become for her.

As she described his episodes, my mind raced through possible causes such as seizures, low blood sugar or calcium, or heart disease. My physical examination quickly revealed the cause, as he had a loud murmur on the left side of his chest consistent with mitral valve regurgitation.

The heart has four one-way valves (mitral, tricuspid, aortic and pulmonic), which allow blood to leave the heart but not return. The mitral valve is located between the left atrium and left ventricle. Once blood has been oxygenated in the lungs, it flows into the left atrium.

When full, the left atrium pumps blood through the mitral valve and into the left ventricle. When the ventricle is full, it then pumps blood out the aorta and to the body. When this happens, the mitral valve should close, preventing blood from flowing back into the left atrium.

My suspicion for Coco was that his mitral valve was leaking, allowing blood to flow backward instead of forward to his body. An echocardiogram confirmed this to be his problem and that it was from a condition called mitral valve endocardiosis.

Mitral valve endocardiosis is due to a degenerative change in the valve. The valve leaflets become thickened and no longer can close completely. Clinical evidence in dogs is commonly noted at 6-9 years old but can occur earlier in certain breeds, especially Cavalier King Charles spaniels, or much later in life as seen in Coco.

Mitral valve endocardiosis usually is first suspected based on a murmur noted over the mitral valve during a physical examination. In some dogs, the disease never progresses; however in others, serious life-threatening symptoms may develop.

Most clinical symptoms are related to the lungs since this is the source of blood flow into the left atrium. The left atrium gradually will enlarge as more and more blood flows backward through the mitral valve instead of leaving and flowing to the body.

The first symptom of left atrial enlargement is often a cough. As the heart increases in size, it may compress the trachea, which tickles the throat, resulting in coughing.

Over time, the left atrium can no longer enlarge further. This compromises the ability of blood to flow out of the lungs and into the heart. The vessels become engorged, and when they reach a certain point, fluid will begin to leak out and into the space between the airway and blood vessels, a condition called pulmonary edema.

This accumulation of fluid is called congestive heart failure and prevents oxygen from reaching the red blood cells. Coughing and shortness of breath occur as a result. If not treated, this will be fatal.

In humans, valve replacement would be considered, but unfortunately in dogs, this is only an option at research facilities. We instead treat with medications to decrease fluid retention and to improve the flow of blood out the aorta instead of back through the valve.

Aggressive therapy and monitoring (thoracic radiographs, echocardiogram, ECG, blood pressure and blood work) usually will help control symptoms for a while. Unfortunately, since we are not reversing the underlying problem, the disease continues to progress and eventually will be refractory to medications.

Some dogs may develop a condition called pulmonary hypertension, which is essentially hypertension in the lungs but not the rest of the body. The heart does not respond well to having to pump blood into this high-pressure system. This often leads to exercise intolerance, shortness of breath, fainting and right-sided heart failure.

Treatment involves taking care of the underlying heart problem as well as medications to decrease pressure in the pulmonary arteries. These arteries do not typically respond to systemic blood-pressure-lowering drugs (anti-hypertensive drugs). There are a few drugs that have the potential to lower pulmonary arterial pressure, with the most famous being Viagra.

Coco was started on medications for his heart and Viagra to lower his pulmonary arterial pressure. Over the past two weeks, he has had only one collapsing episode, making his mom happy to come home again.

Until a technique is developed to replace the mitral valve in dogs, we know his heart disease will progress. But medically, we can provide Coco and his mom a little more quality time together.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.