3. Dilated cardiomyopathy
Dilated cardiomyopathy (DCM) usually occurs in young to middle age dogs (range 6
months to 14 years, mean 4-6 years) of giant breeds, however in the boxer the mean
age at presentation is reported to be 8 years (Fox 1988) and so the condition is
included in this chapter. In boxers more males are affected than females and there
is greater prevalence in some breeding lines.
DCM affects mainly young to middle aged cats, and is often associated with taurine
deficiency. Taurine deficiency has been identified as a cause of DCM in cats fed
commercial petfoods that failed to maintain satisfacplasma taurine concentrations.
It is not considered further here.
Severe dilatation of all chambers of the heart is characteristic of the condition
in most breeds, but this is not true in the boxer. There is thinning of the ventricular
walls (unless compensatory hypertrophy is present) and atrophy of papillary muscles
and trabeculae. Focal endocardial fibrosis is present.
In boxers there is usually thickening of the atrioventricular valves (mitral, but
sometimes the tricuspid or aortic valve).
The aetiology is unknown in most cases. Metabolic defects have been demonstrated
in some species and carnitine-related problems in the myocardium have been reported
in some dogs, including boxers with DCM.
Selenium deficiency has been implicated but not proven in some cases as have toxins,
infective agents (viruses) and immunological factors.
Impaired ventricular contractility leads to reduced ejection volume, though compensatory
mechanisms such as increased heart rate may maintain cardiac output for a short
period. Reduced renal blood flow stimulates the renin-angiotensin-aldosterone-antidiuretic
hormone (ADH) pathway causing sodium and water retention increasing preload and
afterload. Increased sympathetic tone also increases preload, and results in peripheral
vasoconstruction further reducing cardiac output. Congestive heart failure eventually
congestive heart failure
pale membranes, prolonged capillary refill times
arrhythmias (atrial fibrillation often - but not in boxers)
cardiomegaly on radiographs
About one-half of the boxers with DCM have no significant radiographic abnormalities
present, and about one-third of boxers are asymptomatic.
ECG changes most typical for the boxer are ventricular premature complexes and
paroxysmal ventricular tachycardia.
Endomyocardial biopsy has confirmed carnitine-deficiency in some boxers with DCM,
and dietary supplementation may be beneficial.
- inotropic agents
- antiarrhythmic drugs
- dietary modification
- restricted exercise.