Page 170 - Small Animal Internal Medicine, 6th Edition

142 PART I Cardiovascular System Disorders

resistance, and volume retention (see Chapter 3). Chronic of screening echocardiography and Holter monitoring, more
neurohormonal activation is thought to contribute to pro- commonly performed in breeding or show dogs. Some giant-
VetBooks.ir gressive myocardial damage, as well as to CHF. Coronary breed dogs with mild to moderate LV dysfunction are rela-
tively asymptomatic, even in the presence of AF.
perfusion can be compromised by poor forward blood flow
Clinical signs of DCM may seem to develop rapidly, espe-
and increased ventricular diastolic pressure; myocardial
ischemia further impairs myocardial function and predis- cially in sedentary dogs in which early signs may not be
poses to development of arrhythmias. Signs of low-output noticed. Sudden death before CHF signs develop is relatively
heart failure and left-sided, right-sided, or biventricular CHF common. Presenting complaints include any or all of the
(see Chapter 3) are common in dogs with DCM. following: weakness, lethargy, tachypnea or dyspnea, exer-
Atrial fibrillation (AF) often develops in dogs with DCM, cise intolerance, cough (sometimes described as “gagging”),
particularly giant-breed dogs and dogs with severe left atrial anorexia, abdominal distention (ascites), and syncope. Loss
(LA) enlargement. Approximately 30% of Doberman Pin- of muscle mass (cardiac cachexia), accentuated along the
schers and more than 80% of giant breed dogs with DCM dorsal midline, may be severe in advanced cases.
have concurrent AF. In Irish Wolfhounds, AF may precede Physical examination findings vary with the degree of
echocardiographic changes. Atrial contraction contributes cardiac decompensation. Some dogs with occult disease have
importantly to ventricular filling, especially at faster heart normal physical examination findings. Others have a soft
rates. The loss of the “atrial kick” associated with AF reduces murmur of mitral or tricuspid regurgitation or an arrhyth-
cardiac output and can cause acute clinical decompensation. mia. Dogs with advanced disease and poor cardiac output
Persistent tachycardia associated with AF probably also have increased sympathetic tone and peripheral vasocon-
accelerates disease progression. Ventricular tachyarrhyth- striction, with pale mucous membranes and slowed capillary
mias are common as well and can cause syncope and sudden refill time. The femoral arterial pulse and precordial impulse
death. In Doberman Pinschers, serial Holter recordings have often are weak and rapid. Uncontrolled AF and frequent
documented the appearance of ventricular premature com- VPCs cause an irregular and usually rapid heart rhythm,
plexes (VPCs) months to more than a year before early echo- with frequent pulse deficits and variable pulse strength (see
cardiographic abnormalities of DCM were identified. Once Fig. 4.1). Signs of left- and/or right-sided CHF include tachy-
left ventricular (LV) function begins to deteriorate, the fre- pnea, increased breath sounds, pulmonary crackles, jugular
quency of tachyarrhythmias increases. Excitement-induced venous distention or pulsations, pleural effusion or ascites,
bradyarrhythmias have also been associated with low-output and/or hepatosplenomegaly. Heart sounds may be muffled
signs in Doberman Pinschers. because of pleural effusion or poor cardiac contractility. An
Dilation of all cardiac chambers is typical in dogs with audible third heart sound (S 3 gallop) is a classic finding,
DCM, although LA and LV enlargement usually predomi- although it may be obscured by an irregular heart rhythm.
nate. The ventricular wall thickness may appear decreased Soft to moderate-intensity systolic murmurs of mitral and/
compared with the lumen size. Flattened, atrophic papillary or tricuspid regurgitation are common.
muscles and endocardial thickening also occur. Concurrent
degenerative changes of the AV valves are generally only RADIOGRAPHY
mild to moderate, if present at all. Histopathologic findings
include scattered areas of myocardial necrosis, degeneration, Diagnosis
and fibrosis, especially in the left ventricle (LV). Narrowed The stage of disease, chest conformation, and hydration
(attenuated) myocardial cells with a wavy appearance may status influence the radiographic findings. Dogs with early
be a common finding. Inflammatory cell infiltrates, myocar- occult disease are likely to be radiographically normal. Gen-
dial hypertrophy, and fatty infiltration (mainly in Doberman eralized cardiomegaly (predominately left heart enlarge-
Pinschers and Boxers with ARVC) are inconsistent features. ment) is evident in those with advanced DCM (Fig. 7.1). In
Doberman Pinschers and other deep-chested breeds, the
Clinical Findings heart might appear minimally enlarged, except for the left
The prevalence of DCM increases with age, although most atrium (LA). In other dogs, generalized cardiomegaly can be
dogs with CHF are 4 to 10 years old. Among Doberman severe and can mimic the globoid cardiac silhouette typical
Pinschers, prevalence of DCM approaches 50% in dogs of large pericardial effusions. Distended pulmonary veins
greater than 8 years of age. Male Doberman Pinschers gener- and pulmonary interstitial or alveolar opacities accompany
ally show signs at an earlier age than females and are more left-sided CHF with pulmonary edema. The distribution of
likely to experience CHF. pulmonary edema infiltrates in DCM often is diffuse (see
DCM appears to develop slowly, with a prolonged preclin- Fig. 7.1). Pleural effusion, caudal vena cava distention, hepa-
ical (occult) stage that may evolve over several years before tomegaly, and ascites usually accompany right-sided CHF.
clinical signs become evident. Further cardiac evaluation is Biventricular CHF is common.
indicated for dogs with a history of reduced exercise toler-
ance, weakness, or syncope or in those in which an arrhyth- ELECTROCARDIOGRAPHY
mia, murmur, or gallop sound is detected on routine physical The electrocardiogram (ECG) findings in dogs with DCM
examination. Occult DCM is often recognized through use also are variable. Sinus rhythm usually is the underlying