Page 278 - Clinical Small Animal Internal Medicine

246 Section 3 Cardiovascular Disease

implications. In particular, when the RAAS becomes enlarge and there is a sudden increase of left atrial pres-
VetBooks.ir activated in dogs with natural occurring MMVD has sure with development of acute pulmonary edema.
been the source of much controversy. Some studies sug-
gest that the RAAS might be activated relatively early in
the history of the disease, whereas other studies suggest Epidemiology
otherwise. Further complicating the picture is the obser- Myxomatous mitral valve disease is the most common
vation that in dogs, alternative pathways for angiotensin heart disease in dogs and the most frequent cause of con-
II production, such as the chymase pathway, may be par- gestive heart failure in this species. MMVD in dogs
ticularly active. Moreover, it is possible that release of represents 75% of all cardiovascular disease in this spe-
natriuretic hormones from the dilated left atrium cies. The prevalence of the disease varies with age and
and ventricle might balance RAAS and sympathetic whether the study is based on necropsy, echocardio-
activation. graphic or clinical findings. The prevalence of MMVD
Moderate to severe mitral valve regurgitation leads to has also been correlated with breed. In some breeds,
left ventricular volume overload, left atrial and left ven- such as the CKCS, the prevalence of the disease in ani-
tricular enlargement. Mitral valve insufficiency due to mals older than 10 years is greater than 90%. Males are
MMVD represents a unique hemodynamic condition also reported to develop the disease at a younger age
with increased preload and decreased afterload. This is than females. Although the disease is more commonly
because a significant volume of left ventricular preload is diagnosed in small‐breed dogs, it can occur in large‐
ejected into the left atrium at the beginning of systole. breed dogs.
Because the left atrium is a low‐pressure chamber, there
is no increase in cardiac work. This is likely the reason History and Clinical Signs
why many dogs with severe mitral valve regurgitation
can tolerate this situation for a long time without pre- Myxomatous mitral valve disease is a chronic disease in
senting clinical signs. which the clinical presentation is variable. Some patients
Left ventricular eccentric hypertrophy, decreased remain asymptomatic for all their lives, whereas others
afterload, and increased sympathetic activation produce develop life‐threating pulmonary edema and chronic
hyperkinetic left ventricular function. Therefore, the congestive heart failure (CHF). Most dogs with mild to
recognition of normal values for fractional shortening or moderate MMVD are generally free of clinical signs.
ejection fraction during an echocardiographic exam in The presence of clinical signs is generally related to the
dogs with left ventricular enlargement suggests the pres- severity of the disease. Dogs with pulmonary edema have
ence of a systolic dysfunction. Systolic dysfunction has tachypnea and dyspnea. It has been reported that the
been recognized to play an important role in disease pro- normal respiratory rate at rest for dogs should be less
gression and it is independent of clinical status. It has than 30 breaths per minute. Accordingly, an increased
been hypothesized that systolic dysfunction in dogs with respiratory rate at rest might represent the first clinical
MMVD is the consequence of inadequate hypertrophy. sign of worsening of the disease. Dogs with pulmonary
Wall stress imposed by pure volume overload might not edema are often anxious and have difficulties sleeping.
be a very potent stimulus for cardiac hypertrophy, and They often cannot lie in lateral recumbency and prefer a
experimental canine models of MMVD systolic dysfunc- sternal position (orthopnea). Weight loss is common in
tion were associated with inadequate left ventricular dogs with advanced stages of MMVD and CHF. Patients
hypertrophy. Systolic dysfunction is more apparent in that develop right‐sided heart failure have abdominal
large‐breed dogs affected by MMVD. distension due to ascites. Syncope is occasionally
Left atrial function also plays an important role in the reported in dogs with MMVD. Arrhythmias, pulmonary
pathophysiology of the disease. Slowly progressing mitral hypertension, vasovagal syncope, and coughing can
valve regurgitation is associated with a progressive dila- cause syncope in these patients.
tion of the left atrium. If left atrial compliance is main- Cough represents the most common complaint for
tained, filling pressure is also maintained close to normal dogs affected by MMVD, especially in small breeds. It
limits. Accordingly, patients with severe mitral regurgi- should be stressed that cough is a general clinical sign of
tation do not develop pulmonary edema. However, respiratory disease and its presence in a dog with a mur-
chronic dilation of the left atrium is associated with left mur is not an indicator for starting treatment for CHF.
atrial hypertrophy and remodeling that, with time, Old small‐breed dogs are commonly affected by tracheo-
decreases compliance and leads to development of pul- bronchial disease and by MMVD. In these patients, the
monary edema. In dogs with rupture of a major chorda cough is often the result of their primary respiratory dis-
tendinea, there is an acute worsening of mitral regurgita- ease and not heart disease. A recent study has shown
tion. In this condition, the left atrium has no time to that cough was not associated with the presence of active