Heart murmurs come in all sizes, sounds and locations. Because mammalian hearts have four chambers and four
valves, it is possible to have a defect and thus a murmur in any of those four valves.
In addition, heart murmurs occuring as congenital (present at birth) abnormalities such as patent ductus arteriosus (PDA) and Tetralogy of Fallot (ToF) can be associated with murmurs, even though the murmur sound does not originate from a valve.
Murmurs occur in the very young, the very old and at every age in between.
The National Institutes of Health (NIH) defines a heart murmur as “blowing, whooshing or rasping sounds heard during a heartbeat. The sound is caused by turbulent blood flow through the heart valves or near the heart.”
Today we will study what is by far the most commonly-occurring murmur in dogs, the mitral valve murmur, which occurs with damage or defect in the valve between the left atrium and left ventricle. Thus the valve is also called “Left AV valve.” Its name is derived from the valve’s similarity in appearance to a bishop’s headgear or mitre.
Mitral valve disease is caused by endocardiosis. According to Carl D. Sammarco, BvSc, MRCVS, Diplomate ACVIM, it is an “age-related thickening of the mitral valve due to fibroblast infiltration and an increase in collagen and elastic fibers.” In other words, there is scarring, thickening and stiffness of the valve’s tissues, making them less pliable and ill-fitting, leading to blood leaking through the valve when it tries to close.
A heart valve’s main job is to control the flow of blood and keep it going in the right direction. When the mitral valve works properly blood in the left ventricle goes out of the heart, into the aorta and is distributed to the body through arteries. A properly-functioning valve allows no leakage, forcing all blood to travel in the correct direction. A leaky valve, one causing a murmur, allows blood to “regurgitate,” or go in the wrong direction, back into the left atrium. Eventually, sufficient leakage may occur that increased blood pressure occurs in the lung circulation, resulting in fluid collection in the lungs. This condition is called congestive heart failure (CHF) and the fluid in the lungs where air is supposed to be is called pulmonary edema. Since gas exchange cannot occur where fluid is in the lungs’ air sacs (alveoli), waste gasses are not eliminated from the body and fresh oxygen is not transferred to the bloodstream. The patient then finds it hard to breathe, especially with exertion.
Murmurs are graded with a Roman numeral system. Grade I is the softest (least loud) murmur and Grade VI is the loudest and harshest. It can be heard over a wide area of the chest when your pet’s doctor uses his stethoscope to listen to your dog’s heart. A Grade 1 murmur on a 0-6 scale would be expressed as I/VI.
Treatment for a mitral heart valve murmur can begin as early Grade I. While these patients are rarely symptomatic, many veterinary cardiologists believe that early medical intervention can extend life expectancy.
One drug used for this condition is enalapril, an angiotensin converting enzyme inhibitor (ACE inhibitor). Enalapril (generic for Vasotec®) is used to allow relaxation of the blood vessels all over the body, termed vasodilation. Doing so lowers blood pressure and reduces back-pressure on the blood leaving the heart. The net effect is less reverse flow through the mitral valve, back into the atrium and back into the lungs. Think of a garden hose connected to a water pump. Put your finger over the hose and the pump has to work harder to make water flow. Remove your finger and the pump works less hard and more water comes out. That is essentially what enalapril is doing for your dog’s weakened heart.
Other classes of drugs that can be useful in patients with mitral valve disease include beta blockers (beta-adrenergic antagonists) and calcium channel blockers.
Beta blockers inhibit the action of the hormones epinephrine and norepinephrine, both of which can cause arteries to constrict. Commonly-used forms include propanolol and atenolol.
Calcium channel blockers (CCBs), also called calcium antagonists, work by interfering with cells’ action. In order for muscle cells to contract, calcium must enter the cells. CCBs decrease the flow of calcium into muscle cells. Examples are diltiazem (Cardizem® and Tiazac®), amlodipine (Norvasc®).
Pimobendan (Vetmedin®) is a calcium channel blocker that not only allows arteries to relax and dilate, but has the same effect on veins, allowing blood to flow back to the heart more easily.
And, it has yet another important effect: it is a positive inotrope. The word comes from the Greek prefix inos, meaning “fiber,” or, more broadly, “muscle fiber,” and the Greek root trope, which means “turning toward or strengthening.” Therefore, a positive cardiac muscle inotrope is a medication that strengthens cardiac muscle contraction. Importantly, pimobendan does this without increasing oxygen consumption by muscle tissue.
In some patients these drugs are used singly and in others they are used in combination.
Medications to remove fluid from wet alveoli are primarily in the diuretic family and include furosemide (Lasix®) and hydrochlorthiazide (Microzide® and Esidrix®). Low-sodium diets, such as Hill’s Pet Nutrition’s h/d can also help by preventing fluid retention in the first place.
Digoxin, an older drug that is still sometimes used, has the ability to lower heart rate as well as its positive inotrope capabilities.
It is important to note that many dogs with mitral valve murmurs never progress to congestive heart failure. Not only is this crucial for pet owners, but, someday, dogs will be receiving heart valve replacements, just as people do now. Identifying which dogs will develop heart failure and which ones won’t is an important aspect of ongoing research.
Dental disease is considered to be a factor that can possibly instigate, accelerate and exacerbate heart valve disease. Click here to read more.
See you next week, Dr. Randolph.
MMMITRAL
