Page 382 - Small Animal Internal Medicine, 6th Edition

354 PART II Respiratory System Disorders

dogs or cats. However, an estimation of pulmonary artery clinical signs. The drug most commonly used in dogs is
pressure can be made by Doppler echocardiography in sildenafil citrate (Viagra, Pfizer), a phosphodiesterase V
VetBooks.ir patients with pulmonary or tricuspid valvular insufficiency inhibitor that causes vasodilation through a nitric oxide
pathway. A dosage of 1 mg/kg orally every 8 hours was
(see Chapter 6). The widespread availability of this technol-
ogy has increased awareness of the existence of pulmonary
tension to decrease systolic pulmonary arterial pressure and
hypertension in veterinary medicine. Causes of pulmonary shown in a cross-over study of dogs with pulmonary hyper-
hypertension include obstruction to venous drainage as can result in increased exercise capacity and quality of life com-
occur with left-sided heart disease (see Chapter 6), increased pared with placebo (Brown et al., 2010). However, a retro-
pulmonary blood flow caused by congenital heart lesions spective study documented improvement in quality of life
(see Chapter 5), and increased pulmonary vascular resis- but not pressure measurements (Kellum and Stepien, 2007).
tance. When no underlying disease can be identified to A range of dosages has been reported from 0.5 to 3.0 mg/kg
explain the hypertension, a clinical diagnosis of primary every 8 to 12 hours. Monitoring for both beneficial and
(idiopathic) pulmonary hypertension is made. adverse effects is important, with adjustments to dosage as
Pulmonary vascular resistance can be increased as a result indicated by response.
of pulmonary thromboembolism (PTE; see later) or heart- Pimobendan, a phosphodiesterase III inhibitor, results in
worm disease (see Chapter 10). Vascular resistance can also decreased pulmonary artery pressure in dogs with pulmo-
be increased as a complication of chronic pulmonary paren- nary hypertension associated with chronic valvular heart
chymal disease, such as canine chronic bronchitis (see disease (Atkinson et al., 2009). In a retrospective study,
Chapter 21) and idiopathic pulmonary fibrosis (see prior pimobendan was not shown to improve survival times in
discussion). An overly simplistic explanation for increased dogs with severe pulmonary hypertension due to lung
vascular resistance as a complication of pulmonary disease disease when combined with sildenafil, compared with treat-
is the adaptive response of the lung to improve the matching ment with sildenafil alone (Murphy et al., 2017). Pimoben-
̇ ̇
of ventilation and perfusion (V/Q) through hypoxic vaso- dan is discussed further in Chapter 3.
constriction. However, other factors are thought to contrib- Long-term anticoagulation with warfarin or heparin is
ute significantly to the development of hypertension often prescribed for people with primary pulmonary hyper-
associated with pulmonary disease, including endothelial tension to prevent small thrombi formation. Its potential
dysfunction, vascular remodeling, and thrombosis in situ. benefits for veterinary patients are not known.

Clinical Features and Diagnosis Prognosis
Pulmonary hypertension is diagnosed more commonly in The prognosis for pulmonary hypertension is presumably
dogs than in cats. Clinical signs include those of progressive influenced by the severity of hypertension, the presence of
hypoxemia and can be difficult to distinguish from any clinical signs, and any underlying disease.
underlying cardiac or pulmonary disease. Signs include exer-
cise intolerance, weakness, syncope, and respiratory distress.
Physical examination may reveal a loud split second heart PULMONARY THROMBOEMBOLISM
sound (see Chapter 6). Radiographic evidence of pulmonary (PTE)
hypertension may be present in severely affected patients
and includes pulmonary artery enlargement and right-sided The extensive low-pressure vascular system of the lungs is a
cardiomegaly. Radiographs are evaluated closely for underly- common site for emboli to lodge. It is the first vascular bed
ing cardiopulmonary disease. The diagnosis of pulmonary through which thrombi from the systemic venous network
hypertension is most often made through Doppler echocar- or the right ventricle pass. Respiratory signs can be profound
diography. Use of this modality to estimate pulmonary artery and even fatal in dogs and cats. Hemorrhage, edema, and
pressure requires the presence of pulmonary or tricuspid bronchoconstriction, in addition to decreased blood flow,
regurgitation and a skilled echocardiographer. can contribute to the respiratory compromise. The attendant
increased vascular resistance secondary to physical obstruc-
Treatment tion by emboli and vasoconstriction results in pulmonary
Pulmonary hypertension is best treated by identifying and hypertension, which can ultimately lead to the development
aggressively managing any underlying disease process. In of right-sided heart failure.
people, pulmonary hypertension associated with chronic Microthrombi are thought to play a role in pulmonary
bronchitis is usually mild and is not directly treated. Long- hypertension, as discussed in the previous section. However,
term oxygen therapy is often provided, but this treatment is most patients who present primarily with signs of thrombo-
rarely practical for veterinary patients. embolism have a predisposing disease in organs other than
Treatment targeted specifically to address the pulmonary the lungs, and a search for the underlying cause of clot for-
hypertension itself is indicated for patients showing clinical mation is therefore essential. Abnormalities predisposing to
signs of pulmonary hypertension when no underlying clot formation include venous stasis, turbulent blood flow,
disease is identified, or when management of underlying endothelial damage, and hypercoagulation. In addition to
disease fails to improve pulmonary arterial pressures or emboli originating from thrombi, emboli can consist of