Due to the nature of the heart problems to which boxers are predisposed, both a 24-hour Holter EKG and an echocardiogram are necessary for an adequate evaluation. Since arrhythmias in boxer cardiomyopathy are often transiently present, a three-minute EKG is quite inadequate to rule out the likelihood of boxer cardiomyopathy. As part of the echo, a good Doppler study is necessary to determine the cause and significance of a heart murmur and to rule out the (phenotypic, not necessarily genotypic) presence of subaortic stenosis. The problems of most concern are:
- Subaortic stenosis
- Boxer cardiomyopathy
- Mitral valve disease/dysplasia
- Pulmonic stenosis
- The "innocent murmurs" in otherwise normal puppies
- Physiologic murmurs in otherwise normal puppies
- Pulmonic stenosis
- Dilated cardiomyopathy in the middle-aged or older boxer
- Heart base tumors in the middle-aged or older boxer
Screening Tests Available
- The Echocardiogram: The echocardiogram will detect the presence of all of the above except boxer cardiomyopathy. Boxers with the classic form of cardiomyopathy usually have a normal echo. However, a few will have a very abnormal echo and may have dilated cardiomyopathy (DCM). Any indication of cardiomyopathy precludes an individual from being involved in a breeding program. Subaortic stenosis and mild mitral valve disease are detected by a Doppler and Color Flow Doppler study. We have noticed that many boxers have soft heart murmurs and that oftentimes the murmur appears to be due to a small degree of mitral regurgitation. Although at this time mild mitral valve disease is not believed to be an absolute contraindication for breeding, we are monitoring for evidence that this is a heritable trait. At this time, no relationship is believed to exist between mitral valve regurgitation and cardiomyopathy. Mild subaortic stenosis also causes a soft murmur. It is most important to differentiate these two problems. A dog with SAS should never be used for breeding. Other common causes of a murmur in a puppy are the so-called "innocent murmurs" and physiologic murmurs. All of these types of murmurs commonly occur in boxers. An echo should be performed prior to a boxer being used for breeding. The echo should be repeated yearly if an abnormality is present. The young boxer puppy with a soft murmur is likely to have one of the following:
Mild mitral valve disease/dysplasia
Mild subaortic stenosis
- The 24-Hour Holter EKG: It is well known that a 3-minute EKG can severely underestimate the number of abnormal beats (i.e, miss 90% to 95% of the abnormal beats) that are occurring. Dogs of other breeds have no VPC's (ventricular premature contractions) at all or a very few VPC's on their 24-hour EKG. The exact normal number of VPC's per 24-hour period of time for canine species, and the normal number of VPC's for the boxer breed, is currently unknown. At the present time, however, it is believed that 50 to possibly 100 VPC's per 24-hour period is considered acceptable. A 24-hour Holter EKG is the only reasonably accurate test to detect early or subclinical cases of the type of arrhythmias that are seen in boxer cardiomyopathy. A Holter EKG must be performed yearly in all boxers used for breeding to detect the presence of cardiomyopathy as early as possible. These individuals should not be used for breeding.
Subaortic Stenosis (SAS)
This is a common congenital defect in the boxer and ranges in severity from undetectable (other than at autopsy) to extremely severe. It is routinely screened for during the echocardiogram by a careful Doppler study of the subaortic area (LVOT). We feel that the Doppler study must include a substernal view of the LVOT (left ventricular outflow tract) and ascending aorta, and that this reading should be less than 1.7 m/s to 1.75 m/s. "Grey zone" readings of 1.75 m/s to 1.95 m/s should be considered suspicious for SAS. Should an unexplained murmur be present along with "grey zone" LVOT readings, SAS should be considered present. No dog with an LVOT velocity of over 1.95 m/s, which in fact constitutes clear Doppler evidence that SAS is present, should ever be included in a breeding program. It is believed that although most dogs with SAS continue to worsen form birth to approximately 4 months of age, some may progress until one year of age. Therefore, the final evaluation to rule out SAS must be made no earlier than one year of age. For young puppies with a "grey zone" LVOT velocity, the entire litter may be requested for controls.
The term boxer cardiomyopathy refers to a problem that is by far the most common heart disease in the breed. The cause is unknown and it is suspected to be genetically transmitted. Boxers may be first affected at any age but are typically first diagnosed between the ages of six years and 10 years. Prior to this, the disease may be undetectable. The main problem in boxers with this disease is arrhythmias that may eventually become severe enough to cause syncope (fainting due to low blood pressure) or even sudden death. These boxers have an abnormal EKG with progressively worsening arrhythmias as the disease progresses over several years. During this time, the echocardiogram is usually near normal, especially at the onset. Eventually, life-threatening arrhythmias develop and/or myocardial failure occurs. (Chronic tachyarrhythmias are known to induce myocardial failure. It is not known whether this mechanism is present in boxer cardiomyopathy.) Treatment for these arrhythmias does decrease the frequency of syncopal events. However, it is not presently known whether such treatment extends an individual's life.
Talk presented by Dr. Rossi on Wednesday, November 3, 1999 to the Middlesex Boxer Club. The subject discussed was Heart Disease in Boxers. Click on the link to see the presentation.