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Pulmonary Artery Hypertension

by Risa Roland, DVM, DACVIM (Cardiology)

Pulmonary artery hypertension (PAH) refers to an increase in the pressure in the pulmonary arterial tree. Although once thought to be exceptionally rare, knowledge of its associated clinical signs as well as advances in diagnostic tests have helped reveal it to be relatively common. PAH results from reduction in the caliber of the pulmonary vessels and/or increases in pulmonary blood flow. It is of importance clinically as it contributes significantly to patient morbidity and mortality.

PAH is classified as occurring either primarily (i.e. due to an essential increase in the blood pressure in the pulmonary arteries) or secondary to other diseases (e.g. pulmonary venous hypertension, respiratory diseases, chronic pulmonary thromboembolism, and disorders of the pulmonary vasculature). In human medicine, primary pulmonary hypertension occurs as a familial trait. In veterinary medicine, PAH is most commonly secondary to other diseases. A common inciting condition is high pulmonary venous pressure which may occur as a result of high left atrial pressure. In the dog this is most commonly seen with chronic degenerative valve disease and dilated cardiomyopathy. Another secondary cause of PAH are respiratory disease including chronic tracheobronchitis, chronic pneumonia, and interstitial lung disease. Pulmonary thromboembolism can also lead to PAH and can occur secondary to any disease that leads to a hypercoagulable state. Examples of such diseases include Cushing's disease, pancreatitis, sepsis, neoplasia, proteinlosing diseases, and coagulopathies. Heartworm disease can lead to pulmonary thromboembolism as well as changes in the pulmonary arteries themselves. Both of these changes lead to PAH.

Patients with significant PAH may present in a number of ways. In addition to the clinical signs associated with the primary disease that lead to the PAH, PAH itself may result in exercise intolerance, cough, respiratory difficulty, syncope and right-sided heart failure (ascites with jugular venous distension). Physical examination findings may reveal a right-sided systolic heart murmur due to tricuspid regurgitation, a stronger precordial impulse on the right side of the chest, a split S-2 heart sound, abnormal lung sounds related to underlying respiratory disease, jugular venous distension, ascites and any other findings associated with the primary disease. Diagnostics to determine if pulmonary hypertension is a possibility include chest radiographs as well as an echocardiogram. Chest radiographs may reveal right atrial and right ventricular enlargement, main pulmonary artery enlargement or peripheral pulmonary artery enlargement, and the presence of underlying lung disease. Echocardiographic results may reveal right atrial enlargement, right ventricular enlargement, a dilated main pulmonary artery, tricuspid regurgitation, pulmonic regurgitation and evidence of underlying heart disease. A tricuspid regurgitation velocity > 2.7 m/s and/or a peak pulmonary insufficiency velocity > 2.2 m/s (with normal transpulmonic velocities) are diagnostic for PAH.

However, the absence of these changes in velocity does not rule out PAH. Once pulmonary hypertension is confirmed and evidence of significant left atrial enlargement (suggesting primary cardiac disease as a cause) is not present on the echocardiogram, a search for an underlying cause is warranted.

Unfortunately there is no cure for PAH. Thus every attempt must be made to identify and reverse underlying primary disease. If reversal is not possible, aggressive treatment of the primary underlying disease is warranted. If the animal remains clinical for PAH despite treatment of the underlying disease, or if treatment of the underlying disease is not possible, the goal of treatment of PAH is to palliate clinical signs and prolong survival. In the acute setting, oxygen is used as a potent pulmonary arterial dilator. Currently, there are novel therapeutics being used to help dilate the pulmonary arteries; the most commonly used is sildenafil, a phosphodiesterase 5 inhibitor. If right heart failure has occurred, in addition to sildenafil, diuretics, angiotensinconverting enzyme inhibitors, pimobendan, and vasodilators may also be indicated. The long-term prognosis with PAH remains guarded but if the underlying disease can be controlled the PAH may reverse.

PAH is an important clinical sequelae to many primary veterinary disorders. As PAH leads to significant morbidity and mortality, diagnosis of PAH, treatment of the underlying cause and palliation of the clinical signs associated with PAH is prudent.

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