Page 142 - Small Animal Internal Medicine, 6th Edition

114 PART I Cardiovascular System Disorders

not open at all (atretic). RV hypertrophy occurs in response PCV at a level where clinical signs are minimal (a general
to the pressure overload imposed by the PS and systemic goal is 62%-65%); further reduction of PCV (into the normal
VetBooks.ir arterial circulation. The volume of blood shunted from the range) can exacerbate signs of hypoxia. A β-blocker such
as atenolol or propranolol might help reduce clinical signs
RV into the aorta depends on the balance of outflow resis-
tance caused by the fixed PS compared with systemic arterial
RV (muscular) outflow obstruction, and myocardial oxygen
resistance, which varies with exercise and autonomic tone. in some dogs with T of F by decreasing sympathetic tone,
Pulmonary vascular resistance is generally normal in animals consumption. Additionally, β-blockers help limit exercise-
with T of F. A polygenic inheritance pattern for T of F has induced peripheral vasodilation that can exacerbate right-
been identified in the Keeshond. The defect also occurs in to-left shunting and cause hypercyanotic episodes. Exercise
other dog breeds, particularly terrier breeds, and in cats. restriction is also advised. Drugs with systemic vasodilator
effects should not be given. Supplemental O 2 has negligible
Clinical Features benefit in patients with T of F.
Exertional weakness, dyspnea, syncope, cyanosis, and The prognosis for animals with T of F depends on the
stunted growth are common in the history. Physical exami- severity of PS and erythrocytosis. Mildly affected animals
nation findings are variable, depending on the relative sever- and those that have had a successful palliative surgical shunt-
ity of the malformations. Cyanosis may be seen at rest in ing procedure may survive well into middle age. Neverthe-
some animals, whereas others are cyanotic only with exer- less, progressive hypoxemia, erythrocytosis, and sudden
cise. The precordial impulse is usually of equal intensity or death at an earlier age are common. Overall median survival
stronger on the right chest wall than on the left. A holosys- time following diagnosis is approximately 2 years.
tolic murmur at the right sternal border consistent with a
VSD, a systolic ejection murmur at the left base compatible PULMONARY HYPERTENSION WITH
with PS, or both may be heard on auscultation. However, SHUNT REVERSAL
some animals have no audible murmur because hyperviscos-
ity associated with erythrocytosis diminishes blood turbu- Etiology and Pathophysiology
lence and therefore murmur intensity. In dogs and cats, pulmonary overcirculation typically results
in left-sided CHF. Left-to-right shunts rarely lead to reactive
Diagnosis vasoconstriction and pulmonary arterial hypertension in
Thoracic radiographs depict variable cardiomegaly, usually small animals, because the low-resistance pulmonary vascu-
of the right heart (see Table 5.2). The main pulmonary artery lar system (with significant capacity for collateral circula-
usually appears small, in contrast to typical valvular PS. tion) normally can accept a large increase in blood flow
Reduced pulmonary vascular markings are common. The without marked rise in pulmonary arterial pressure. However,
malpositioned aorta can create a cranial bulge in the heart a small percentage of dogs and cats with shunts do develop
shadow on lateral view. RV hypertrophy causes dorsal dis- pulmonary arterial hypertension causing shunt reversal
placement of the cardiac apex on lateral views and an (right-to-left shunting). It is not clear why pulmonary hyper-
upturned cardiac apex on VD views, leading to a classically tension develops in some animals, although the defect size
described “boot-shaped” heart. The ECG typically suggests in affected animals is usually quite large. Possibly the high
RV enlargement, although a left axis deviation has been seen fetal pulmonary resistance may not regress normally in these
in some affected cats. animals, or their pulmonary vasculature may react abnor-
Echocardiography depicts the VSD, a large aortic root mally to an initially large left-to-right shunt flow. In any case,
shifted rightward and overriding the ventricular septum, PS, irreversible histologic changes are present in the pulmonary
and RV hypertrophy. Doppler studies reveal the right-to-left arteries that increase vascular resistance. These include
shunt and high-velocity stenotic pulmonary outflow jet. An intimal thickening, medial hypertrophy, and characteristic
echo-contrast study can also document the right-to-left plexiform lesions.
shunt. Typical clinicopathologic abnormalities include As pulmonary vascular resistance increases, pulmonary
increased PCV and arterial hypoxemia. artery pressure rises and the extent of left-to-right shunting
diminishes. If right heart and pulmonary pressures exceed
Treatment and Prognosis those of the systemic circulation, the shunt reverses direction
Definitive repair of T of F requires open-heart surgery. Pal- and deoxygenated blood flows into the aorta. These changes
liative surgical procedures can increase pulmonary blood typically develop in very young animals (usually by 6 months
flow by creating a left-to-right shunt. Anastomosis of a sub- of age), supporting the notion that pulmonary hypertension
clavian artery to the pulmonary artery (a modified Blalock- in these cases may represent retention of fetal pulmonary
Taussig shunt) is the most commonly used palliative vascular resistance. The term Eisenmenger physiology refers
procedure in small animals. to severe pulmonary hypertension with shunt reversal;
Severe erythrocytosis and clinical signs associated with affected animals with clinical signs sometimes are described
hyperviscosity (e.g., weakness, shortness of breath, seizures) as having Eisenmenger syndrome.
can be treated with periodic phlebotomy (see p. 116) or, alter- Right-to-left shunts that result from pulmonary hyper-
natively, hydroxyurea (see p. 116). The goal is to maintain tension cause pathophysiologic and clinical sequelae similar