Heart Conditions in the Boxer

Boxer ARVC - Arrhythmogenic Right Ventricular Cardiomyopathy
One of the most common causes of sudden/unexpected death in boxers, both young and old, is a condition thought to be inherited and characterized by abnormal heart rhythms involving the ventricles (the main blood pumping chambers) of the heart. This condition can cause varying degrees of disability and occasionally results in congestive heart failure. Current research has determined that the disease is the result of an electrical conduction disturbance which causes the heart to contract too early, thus producing an extra, ineffective beat, the so called Premature Ventricular Contraction (PVC). While many dog breeds may have a few of these PVC's, the boxer seems to be prone to having more of these premature beats. When large numbers of these PVC's occur together, the heart muscle cannot produce a normal, effective contraction, which results in a lack of blood flow to vital organs including the brain and the heart itself. This can cause the animal to experience a  fainting episode (syncope). A prolonged sequence of PVC's can lead to complete cardiac arrest unless the heart resumes a normal pattern of contractions. Less commonly, the lack of adequate contractions may cause the heart to fail and the dog's heart may dilate, causing congestive heart failure with symptoms such as shortness of breath, exercise intolerance, abdominal swelling, coughing and symptoms similar to those occurring in humans with heart failure. This is known as dilated cardiomyopathy -- a less frequent condition in the boxer than the more commonly seen ARVC, electrical conduction defect.

DNA research is currently underway at Washington State University, but research of this type can take a considerable amount of time and funding. Since affected dogs are often asymptomatic until sudden death occurs, current attempts in screening for the disease are directed at the healthy appearing animal. The most sensitive tool has been determined to be a Holter Monitor examination (24-hour EKG), which records the dog's electrocardiographic activity over at least 24 hours. The monitor is strapped to the dog and electrodes are placed on the chest while the dog goes about its normal activities. The total number of beats, including the abnormal PVC's, are recorded. The electrodes must be correctly placed and the results must be interpreted by trained personnel. While current research at Washington State University by Dr. Kate Meurs, funded by the American Boxer Charitable Foundation and the AKC Canine Health Foundation, has suggested certain thresholds of PVC's or other abnormalities for diagnosis of the condition, she has also discovered that an individual dog's numbers may vary from day to day or week to week. Her ongoing research, and that of others, will hopefully determine with greater certainty the value of this screening tool in the future until a definitive genetic test is available. While it is true that a symptomatic dog usually has thousands of beats in a 24 hour period and in severe cases may be diagnosed by simple auscultation of the heart or by a brief EKG, the Holter monitor is still probably the best screening tool for early detection, due to the often intermittent occurrence of the PVCs in the asymptomatic dog.

Of recent interest to boxer cardiology researchers is a somewhat similar condition found to occur in human families, the so-called arrhythmogenic right ventricular dysplasia (ARVD), in which young and old persons, previously in good health, experience sudden death. To our knowledge, attempts to find a precise DNA marker have not been successful so far, but if this occurs, it may help veterinary cardiologists find the canine genetic defect sooner. BCM appears to be unique to boxers, so far, in the dog world, and is dissimilar in many respects to cardiomyopathy occurring in other breeds, such as the Doberman and Great Dane. Continuing research and follow-up of dogs already under study will hopefully provide better insight and understanding of this unique condition in the boxer, as well as more widespread availability of screening.

Holter - a 24 hour holter monitor test should be performed every year to 2 years on breeding dogs. This is to test for Boxer ARVC, Arrhythmogenic Right Ventricular Cardiomyopathy and it can affect all boxers and all boxers that are bred should be tested. A short 10 minute test is not long enough to determine if there are numerous VPC or runs and couplets.  A dog with many runs or couplets should not be used for breeding and one that has over 100 VPCs should be tested again before breeding and the results should be much lower.  Dogs used for breeding should be under 50 VPCs in a 24 hour period. 

Aortic / Subaortic Valvular Stenosis: AS/SAS
One of the most common heart defects occurring in dogs, boxers in particular, is aortic or subaortic stenosis. In most cases the stenosis, or narrowing, is produced by a fibrous ring of tissue below the aortic valve, hence the term "subaortic." The disease is inherited but its mode of transmission is not known at this time.

Oxygen-rich blood flows from the left ventricle of the heart, through the aortic valve and into the aorta, which transports the oxygenated blood to all organs and tissues in the body except the lungs. Narrowing of the aortic valve requires the left ventricle to work harder to pump the necessary amount of blood. This increased workload can result in hypertrophy (thickening) of the left heart muscle. Since the blood is being forced through a smaller-than-normal opening, there is also increased pressure generated by the pumping action of the heart. This increase in pressure can cause dilation (ballooning) of the aorta. Reduced flow can produce symptoms of fainting (syncope) and even sudden death, although abnormal heart rhythms (arrhythmias) may also contribute to these symptoms.

The stenosis creates a change in the flow of blood through the valve causing turbulence which results in swishing sound called a heart murmur. Often the stenosis can be seen on echocardiography. Murmurs are graded from one to six, but a weak murmur may not always be detectable, even by a trained cardiologist. Exercising the dog during the cardiac exam may increase detection of murmurs in some cases. Not all murmurs are the result of aortic stenosis/subaortic stenosis, but may be so-called "innocent," or physiologic murmurs, particularly when they occur in young animals.

The diagnosis of AS/SAS is best made by a veterinary cardiologist, or one with equivalent experience and training. When a murmur is identified and not presumed to be physiologic, further investigation is warranted. The least invasive and most available testing consists of Echo Color Flow Doppler. This testing is best performed when the animal is full grown or at least one year of age, unless the dog is experiencing symptoms of heart disease, in which case testing should be pursued promptly.

As in many instances in medicine, these tests have limitations and are not perfect. False positive and false negative diagnoses may occur. In some cases this is simply because the abnormality is too subtle to be diagnosed with currently available knowledge and/or technology. Echo Color Flow Doppler flow rates can vary considerably in the same animal depending on the proficiency of the operator and the amount of stress to which the animal is subjected. These limitations may be minimized in part by examiners with advanced training using the appropriate ultrasound equipment, techniques, and standards established by the American College of Veterinary Internal Medicine, Specialty of Cardiology.