Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




We've highlighted the left side of the heart
We've highlighted the left side of the heart
(the part that pumps oxygen-rich blood to the body)
so you can see the left atrium, left ventricle and the mitral
valve in between them. The mitral valve is open when
blood pumps from the atrium to the ventricle and closes
when the ventricle pumps blood to the body. The mitral
valve is said to look like the Pope's mitre upside down.
(Photocredit: Wikimedia Creative Commons by Patrick J. Lynch)

Leaky mitral valves are extremely common in small dogs. Most of the time (>90% of dogs) they do not progress to heart failure but how will you know if your dog is one of the unlucky ones? Will you know what to look for?


The heart is divided into a left side and a right side: the left side pumping oxygenated blood to the body and the right side pumping deoxygenated blood (“used blood,” if you will) to the lung to pick up more oxygen. Blood circulates from the lung to the body and back to the lung with every beat of the heart.

Each side of the heart is further divided into a receiving chamber (an “atrium”) and a pumping chamber (a “ventricle.”) The blood collects in the atria and when the atria are full, they contract and blood goes into the pumping chambers which then send blood outward (to the lung from the right ventricle or to the body from the left ventricle).

The valves between the atria and the ventricles are the tricuspid valve (on the right) and the mitral valve (on the left). These valves are long sheets of fibrous tissue attached like sails to muscles called papillary muscles by strands called chordae tendinae. (The mitral valve is named because it looks like the pope’s mitre.) When blood rushes through the “sails,” they suck closed forming a seal. Mitre Drawing(Photocredit: Public Domain Image via Wikimedia Commons)



Normal canine heartNormal canine heart
(original graphic by
Canine with atrial enlargementCanine with atrial enlargement
(original graphic by

Older small breed dogs commonly develop “vegetations” (which can be thought of as “icky stuff” or gunk) on the leaflets of their mitral valves. When the leaflets have gunk on them, they cannot form a tight seal when they close and the valve leaks. When the full strength of the left ventricle pumps and the valve at the top of the chamber is not sealed, a lot of blood squirts through the leak back into the atrium. This back-squirting is called “mitral regurgitation.”

That squirt is audible as a murmur and if your veterinarian reports
hearing a murmur, this is most likely what he or she is hearing.

The atrium must stretch out in order to accommodate its normal volume of blood plus the regurgitated volume of blood. This enlargement is visible on a radiograph.

When the enlarged atrium pumps, the ventricle must now receive the normal amount of blood plus the regurgitated amount. The ventricle must also gets larger to accommodate all this extra blood. It also has to work harder to move all that extra blood.

Over time, the leaflets develop even more gunk plus the enlargement of the atrium itself opens the valve even more. All this makes the leak worse. When the ventricle pumps, most of the blood regurgitates backward but as long as the heart is strong enough to handle the extra work and as long as enough blood moves forward, there is no heart failure.

Murmurs are graded by loudness: I, II, III, IV, V or VI out of VI.

• Detection of Grade I murmurs require special equipment
• Grade II murmurs are very soft.
• Grade III murmurs are easily heard with a stethoscope.
• Grade IV murmurs are loud through a stethoscope.
• Grade V murmurs are accompanied by a chest vibration called a “thrill.”
• Grace VI murmurs are so loud a stethoscope is not needed to hear them.


Most dogs with mitral murmurs never go into heart failure but some dogs simply cannot keep up with pumping all this extra blood. The heart simply cannot move it all forward. The blood backs up into the pulmonary veins in the lung. Fluid leaks out into the tissues of the lung and pulmonary edema results. The patient is essentially drowning without ever having set foot in any water.

The heart is not only enlarged but has actually failed.
The heart is not only enlarged but has actually failed. It cannot keep up with all the blood it must pump and
fluid has leaked into the lung causing pulmonary edema. This is visible as the white material on top of the heart.

(original graphic by

The patient is coughing and has an elevated heart rate as the heart struggles to process all the extra blood. Respiratory rate and effort rises. If the patient can be recognized in this stage, prognosis is better but these changes can be subtle and easily missed.

Eventually, breathing requires so much focus that the patient does little else besides coughing and struggling to breathe. Activity falls off and appetite as well. If there is too much fluid in the lungs for proper oxygenation, the patient may even have a bluish/lavender color to the gums. A crisis is in progress and at this point emergency life-saving treatment is needed.


Pug Dog(Photo Credit:

Let’s back up. In 2009, the American College of Veterinary Internal Medicine published guidelines on the evaluation and treatment of this condition and revised them in 2017 in light of new research. Patients are classified into one of four stages with the idea that each stage corresponds to a general treatment plan (with room for individual nuances, of course). By determining what stage a patient is in, it becomes more clear what management should be.

STAGE A: These are patients considered “at risk” for heart disease but who don’t actually have any evidence of heart disease, not even a murmur. Basically, every Cavalier King Charles Spaniel with no murmur goes in this category because this breed has a strong genetic predisposition to mitral insufficiency. No treatment is recommended but if a murmur develops before ages 8, these individuals should probably not be bred.

STAGE B: These are patients with audible heart murmurs. This category is divided into two subcategories: STAGE B1 AND STAGE B2. Experts recommend that any dog with a murmur get a blood pressure measurement, chest radiographs, and ideally an echocardiogram. A pro-BNP test (a blood test measuring heart muscle stretch) is useful. A urinalysis, red blood cell count, and a creatinine (a kidney parameter) test is also recommended for all patients as a minimum baseline. This represents a change from prior recommendations because in the past it did not appear that there was medication that could delay or prevent heart failure in these patients. Now there is and we need to identify the patients who may benefit.

Cavalier King Charles SpanielCavalier King Charles Spaniel
(Photo Credit:

STAGE B1: These are patients with murmurs but with normal radiographs (no evidence of heart enlargement). For these dogs, no therapy is recommended.

STAGE B2: These are patients with murmurs AND heart enlargement on radiographs. It turns out that heart failure can be prevented or delayed if patients in this group meet certain criteria: a murmur of Grade III/VI or more and a heart enlargement of a certain size (as determined by the aforementioned diagnostics). These patients should begin a salt-restricted diet as well as a regimen of pimobendan, a medication to help the heart contract with greater strength.

This is the stage where it is important to start watching resting respiratory rate. To do this, the patient must genuinely be at rest, even asleep if possible. No sniffing or attending to something as that will alter respiratory rate. A normal resting respiratory rate (less than 30 breaths per minute consistently) means no heart failure is present. If resting respiratory rate jumps up to 50-60 then we have probably entered Stage C and more aggressive medication is needed. If the resting respiratory rate jumps to 80 or 90 then you have an emergency on your hands and need to get to a veterinary office as soon as possible. The resting respiratory rate is an extremely important test in heart disease monitoring and it turns out that this very crucial test costs nothing and is something you can do yourself.

Not sure how to calculate resting respiratory rate? Here is a video:

(Video credit: segolladay via

STAGE C: These patients are either in heart failure or have been in heart failure. Stage C patients are divided into Acute/Hospitalized patients and Chronic therapy patients.

The hospitalized patients will need injectable diuretics (usually furosemide) to relieve the extra fluid in the lung and extra burden on the heart, pimobendan as mentioned for a stronger heart contraction, oxygen, and perhaps a vasodilator to redistribute the extra fluid. After stability has been reached, patients can go home and be treated as Chronic therapy patients.

The chronic therapy patients may have recovered adequately to have been discharged from the hospital or they may never have been critical enough to have needed hospitalization. For these patients, “quadruple therapy” is now considered standard. This would consist of four medications:

Pimobendan – for improved heart contractility.
Furosemide – short-acting diuretic (removes fluid from the body by inducing urine production)
An ACE inhibitor (usually enalapril) – a vasodilator to redistribute the extra blood volume so the heart is not overwhelmed.
Spironolactone – a long acting diuretic (removes fluid from the body by inducing urine production).

Salt restricted diet would also be important for these patients.

Resting respiratory rate is an even more important parameter in this group as it indicates when medication has failed to control the disease.

Any changes in medication doses will most likely need some kind of kidney parameter check
(usually a creatinine level) 5-7 days after the regimen alteration.

An extra dose of furosemide (frequently referred to by its human product brand name: lasix®)
can be life-saving in a patient whose resting respiratory rate takes an unexpected rise.

STAGE D: These patients are in heart failure that the above standard therapy cannot control. Similarly, patients can be in an Acute/Hospital based group or in a Chronic/Home treatment group. For patients such as these, consultation with a cardiology specialist is in order as management will be highly individual-based and advanced. Hopefully, your pet will never enter this realm.


Once heart failure has been confirmed and medication has been started, a fair goal is a one year life expectancy. Escalating doses of furosemide will most likely be needed. About 50% of heart failure patients will have died within 6 months of diagnosis. About 80% will have died by 1.5-2 years.

Of course, dogs with murmurs and no heart failure commonly live normal life spans.

When your dog has mitral insufficiency, heart failure is a possibility, either in the present or in the future. Knowing what to watch for and what to expect is important in caring for a patient with mitral insufficiency with or without heart failure. If you have further questions, please use the “contact us” function or ask your regular veterinarian.


Page last updated: 6/4/2022