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A Guide to Heart Diseases in Dogs: Valves and Holes - Simplified by Dr. Jean Dodds of Hemopet.  For more information please visit her website, see link below  

A Guide to Heart Diseases in Dogs: Valves and Holes | Hemopet April 21, 2023 / General Health / By Hemopet 

The dilated cardiomyopathy (DCM) controversy has settled down somewhat now that the Food & Drug Administration (FDA) will not be providing public updates until more meaningful scientific research is available to share. As well, the agency has apparently reversed its position on grain-free kibble diets because the information to date does “not supply sufficient data to establish a causal relationship”.  

With all of the focus on DCM, we wonder if other heart conditions could have been ignored or overlooked or even if well-meaning pet companion caregivers just assumed it was DCM instead of another condition – heart or not.  

We thought a review of common heart conditions in dogs is needed because it has been estimated that up to 75% of senior dogs may have some form of heart disease that goes undetected. We hope this will help pet companion parents understand what might be happening to their companion pet’s heart when speaking with their veterinarians.  

The Heart’s Basic Anatomy The heart is one of the most complicated organs in the body. We are simplifying this review of cardiac anatomy and physiology to include the basics.  

The pericardium is a membrane – or sac – that surrounds the heart. The muscles of the heart, termed the myocardium, make up the middle and thickest layer of the heart wall. “Myo” refers to muscle.  

The heart has four main arteries: Left Coronary, Right Coronary, Aorta and Pulmonary. Think of them as the highways of the body.  

The heart has four chambers: Left Atrium, Left Ventricle, Right Atrium and Right Ventricle.  

Valves are located between each of the chambers. They are flaps that prevent blood from flowing in the wrong direction or backwards. When the valve is faulty, backward flow can occur. Think of a valve as a dam, a gate or a guard. The primary valves are: Mitral, Aortic, Tricuspid, and Pulmonary.  

The Heart’s Basic Physiology 

Left Side 
#1. Oxygenated blood from the lungs goes to the Left Atrium via the Left Coronary Artery.  
#2. The blood passes through the Mitral Valve to the Left Ventricle. 
#3. From the Left Ventricle, the blood passes through the Aortic Valve to the Aorta. 
#4. The Aorta sends the blood to the rest of the body. 

Right Side 
#1. Deoxygenated blood from the body goes to the Right Atrium via the Right Coronary Artery. 
#2. The blood passes through the Tricuspid Valve to the Right Ventricle. 
#3. From the Right Ventricle, blood passes through the Pulmonary Valve to the Pulmonary Artery.  
#4. The Pulmonary Artery sends the blood back to the lungs. 

Signs That Your Dog May Have a Heart Condition 

These are the signs to look for at home and to relay to your veterinarian: persistent cough, difficulty breathing, fainting or collapse, fluid accumulation around the belly, behavior changes, and inability to exercise. Yes; these signs may signal other diseases as well. 

Diagnosing Heart Disease 

Probably the most important part of an annual veterinary examination, your companion dog’s pulse, respiratory system, and abdominal distension will be checked. Additionally, the veterinarian will listen to the heart for sounds, murmurs, and arrhythmias/flutters. Based on this initial exam, further testing may be suggested such as bloodwork, an echocardiogram and/or electrocardiogram for confirmation.  

A murmur creates a whooshing sound in the heart. An arrhythmia refers to an abnormally high heart rate (tachycardia), low heart rate (bradycardia), or fluttering (“skipping a beat”). To keep it simple, some veterinarians may tell you your dog has a murmur and/or an arrhythmia. In vet school, veterinarians learn the location of these in the heart and the difference in heart murmur sounds.  

An electrocardiogram (ECG/EKG) measures the pattern of electric pulses generated by the heart’s rhythm. The echocardiogram uses sound waves to check the heart’s structure and how well the heart is pumping blood. Sometimes, an x-ray might be needed too. 

Our Review of Heart Diseases Most reviews review heart diseases based upon their anatomy. We are taking a different approach here. We will start with the significant heart valve conditions in dogs by the route blood flows through the heart, and then discuss holes in the septums separating the heart chambers or in the valves. In another blog post, we will focus on pericardial disease and cardiomyopathies. We will not be addressing heart diseases that are identified as originating from ticks, protozoa, mosquitoes, bacteria, parasites, medications, or viruses at this point in time. Although they also are very important!  

Myxomatous Mitral Valve Disease (MMVD) 

MMVD is a degenerative valve disease (DVD) specifically affecting the mitral valve. It is a progressive and chronic disease. On average, initial signs – particularly heart murmurs – start around the age of six and even younger in some breeds.  

Basically, the mitral valve is degenerating and lesions form that can affect the functionality of the valve. The leaflets of the valve become thickened and they go in the wrong direction – into the left atrium instead of the left ventricle. Thus, a backward flow of blood (mitral regurgitation) could occur with possible enlargement of the left atrium. As the disease progresses, dogs may develop high blood pressure in the capillaries of the lungs that could result in fluid in the lungs.  

Approximately 75% of heart disease diagnoses are attributed to degenerative valve disease (DVD). 60% of all DVDs are MMVD. Are those percentages inflated compared to other heart disease diagnoses in dogs? Possibly. Veterinarians do know to look for MMVD in small to midsized breeds – particularly Dachshunds and Cavalier King Charles Spaniels. We know this is an inherited trait in both of these breeds.  

Medical treatment might include diuretics to decrease fluid retention that will help lower blood pressure. Additional treatment options may include pimobendan and an angiotensin converting enzyme inhibitor (ACEI), such as benazepril or enalapril. 

Mitral Valve Dysplasia (MVD) MVD is different than MMVD. It is a developmental, congenital (present at birth) malformation of the mitral valve. Valves are complex; the simplest way to think of MVD is as a box top that does not fit perfectly on the box because it has one or more imperfections. So, the severity of the malformation can vary between affected dogs.  Signs are usually present within the first few years of life. Companion pet parents would notice the signs of rapid breathing and coughing. The coughing is occurring due to a build up of fluid in the lungs.  Veterinarians would detect a heart murmur and possibly arrhythmias if the left atrium is dilated. Additionally, the left ventricle could become enlarged. Sadly, the prognosis is poor.  Some veterinarians will call MVD a heritable trait, but others will not. We know that Bull Terriers, German Shepherds, Newfoundlands, Dalmatians, Mastiffs, and Great Danes are susceptible to MVD. 

Aortic Stenosis Aortic stenosis is the narrowing of the aortic valve that usually affects large breeds such as Newfoundlands, German Shepherds, Rottweilers, Boxers and Golden Retrievers. It is a congenital and progressive disease that varies by severity.  Remember, valves are complex. So, the narrowing could occur in three different areas of the aortic valve location: below the valve in the left ventricle (subvalvular or subaortic), aortic valve (valvular), or above the aortic valve in the aorta (supravalvular). Most dogs have subaortic stenosis.  Signs companion parents may see are lethargy, exercise intolerance, and fainting in severe cases. Veterinarians will detect a murmur on the left side of the heart. Fainting usually points to an arrhythmia and dogs should be fitted with a Holter monitor. In general, we manage aortic stenosis with a beta-blocker, which helps decrease the heart’s activity, thereby reducing the work of the left ventricle to push blood through and risk becoming enlarged. Surgical intervention is possible, but is expensive, not widely available, and risky. 

Myxomatous Tricuspid Valve Disease (MTVD) The same degeneration and creation of lesions occurs with MTVD, like MMVD. However, only 10% of DVDs are MTVD, and 30% are both MMVD and MTVD.  Above, we talked about the importance of location and that the tricuspid valve regulates the deoxygenated blood coming from the body, whereas the mitral valve regulates oxygenated blood from the lungs. That means the signs and symptoms are slightly different with MTVD as compared to MMVD. When the tricuspid valve is affected, elevated blood pressure in the body’s veins occurs, and fluid accumulation in the abdomen may develop due to the backflow of blood.  

Tricuspid Valve Dysplasia (TVD) Similar to MVD, TVD is a congenital malformation just of the tricuspid valve. Again, the box top and box do not quite fit together.  Again, location, location, location is important – just like with DVDs. A heart murmur may be heard on the right side of the chest, the jugular veins in the neck may be enlarged or have a pulse, the right atrium might become enlarged, and fluid may accumulate in the abdomen or in the soft tissues of the legs. Most affected breeds are Labrador Retrievers, German Shepherds, Weimaraners, Irish Setters, Dogue de Bordeaux, Great Pyrenees, and Old English Sheepdogs. 

Pulmonic Stenosis Pulmonic stenosis is a fairly common congenital disease commonly found in English Bulldogs, Boxers, Beagles, Boykin Spaniels, French Bulldogs, Keeshonds, and West Highland White Terriers.  Stenosis means narrowing. The question is the cause of the narrowing? Typically, with pulmonic stenosis, we see a fusion or a dysplasia of the pulmonary artery’s valve leaflets. Since the heart is working diligently to pump blood through to the lungs, the right ventricle’s muscle and the valves become thickened – causing an obstruction or narrowing. Again, with the right side of the heart, signs are fluid accumulation in the midsection, murmurs, and arrhythmias.  Again, location matters. Similar to aortic stenosis, we can manage the condition with beta blockers. Unlike aortic stenosis though, surgery by inserting a balloon to open the passageway is more successful and easier to perform on severe or moderate cases.  

HolesPatent Ductus Arteriosus (PDA) The majority of mammals – humans and dogs as well – develop as fetuses in a womb prior to birth. Lungs are nonfunctional then because they would otherwise choke and suffocate on the amniotic fluid. Yet, mammals still need oxygen. So, there is a duct or hole between the pulmonary artery and the aorta called the ductus arteriosus. This duct takes blood from the right ventricle and passes the blood to the aorta, which sends it on to the rest of the body. During the first few breaths, this duct is supposed to close.  If the duct does not close properly, it could lead to failure of a heart’s left side because excess oxygenated blood is now flowing from the aorta to the pulmonary artery and then the lungs.  While rare, misdirected blood flow from the pulmonary artery to the aorta can occur if the hole is really large. Right-sided heart failure is then the concern.   PDA is a common heart condition particularly in small-breed dogs, but any dog could be affected. Specific breeds thought to be susceptible to PDA are Maltese, Pomeranian, Shetland Sheepdog, English Springer Spaniel, American Cocker Spaniel, Keeshond, Bichon Frise, German Shepherd, Collie, Irish Setters, Kerry Blue Terrier, Labrador Retriever, Newfoundland, Miniature and Toy Poodle, Chihuahua, and Yorkshire Terrier. Again, veterinarians will detect a heart murmur the majority of the time and other symptoms present will help point them in the right direction.  Treatment really depends on how big the hole is, how the blood is flowing through the duct, how soon it is caught and if heart failure already has started to set in. If detected early and treated prior to heart failure in dogs with left-to-right blood shunting, surgery generally has a good outcome. Sometimes, medications will be prescribed. For right-to-left blood shunting, the prognosis is poor and management includes addressing pulmonary hypertension and withdrawing blood periodically.  

Ventricular Septal Defect (VSD) VSD is another, but less common, heart “hole” condition in dogs. Prior to birth, the wall between the left and right chambers closes before the fetus is born. If this does not occur, blood initially leaks from the left ventricle into the right ventricle. The right ventricle could then enlarge and the pulmonary artery could narrow. That creates excessive pressure. So, the blood might then reverse and go from the right side into the left side.  We typically do not treat dogs with small defects and their outlooks are fairly good. For more severe cases and if the blood is still in a left-to-right direction, surgery may be considered along with blood pressure medication. If the direction has changed, again, we look to management with periodic withdrawal of blood. Dogs with VSD should not be used for breeding. We also know it is a heritable condition in English Springer Spaniels.  

References Axelsson, Erik et al. “The genetic consequences of dog breed formation-Accumulation of deleterious genetic variation and fixation of mutations associated with myxomatous mitral valve disease in cavalier King Charles spaniels.” PLoS Genetics vol. 17,9 e1009726. 2 Sep. 2021, doi:10.1371/journal.pgen.1009726,  Besche, Beatrice et al. “Efficacy of oral torasemide in dogs with degenerative mitral valve disease and new onset congestive heart failure: The CARPODIEM study.” Journal of Veterinary Internal Medicine vol. 34,5 (2020): 1746-1758, doi:10.1111/jvim.15864,  Parker, Heidi G, and Paul Kilroy-Glynn. “Myxomatous mitral valve disease in dogs: does size matter?” Journal of Veterinary Cardiology: The Official Journal of the European Society of Veterinary Cardiology vol. 14,1 (2012): 19-29, doi:10.1016/j.jvc.2012.01.006,