What is pulmonary arterial hypertension?
Pulmonary arterial hypertension (PAH) is a sustained increase in systolic and/or diastolic pulmonary artery pressure. Severe pulmonary hypertension is often associated with high morbidity and mortality in humans and animals, and the diagnosis often carries a grave prognosis. These patients typically have a guarded prognosis once they have been diagnosed with severe PAH. Pulmonary hypertension is not commonly seen in feline patients. Normal pulmonary artery pressures (PAP) are 15-25mmHg and in those patients with severe pulmonary hypertension, the PA pressures will often be similar to or exceed systemic systolic pressures (often >100mmHg).
What causes pulmonary arterial hypertension?
Pulmonary hypertension is a multifactorial disorder in dogs and people. In veterinary patients, the most common causes are secondary to other diseases. These include increased pulmonary blood flow due to left-to-right shunts (e.g. patent ductus arteriosus, ventricular septal defect), pulmonary venous hypertension (secondary to left-sided heart failure usually due to chronic valvular disease), increased pulmonary vascular resistance, chronic parenchymal disease (e.g. pulmonary fibrosis, pneumonia, and neoplasia), pulmonary thromboembolism, and heartworm disease.
Clinical signs & physical exam findings
Most patients diagnosed with PAH are middle-aged to older, small breed dogs. Clinical signs of PAH are variable and typically depend on the severity of the disease. Patients may be asymptomatic in the earlier stages of disease or severely dyspneic with weakness or syncope during the latter stages of the disease. The most common presenting complaints may include the following: lethargy, coughing, respiratory distress, weakness, exercise intolerance or syncope. Physical exam findings may include crackles in the absence of left-heart failure, tachypnea, dyspnea, respiratory distress, heart murmur, and abdominal or pleural effusion. Many patients will often have more than one presenting complaint or abnormal physical exam finding. Screening of small breed patients with clinical signs and physical exam findings of cardiorespiratory disease may be beneficial especially if syncope is reported.
Diagnosis of PAH in canine patients can be difficult and usually requires an echocardiogram. Cardiac catheterization and direct measurement of the PAP are the gold standards for diagnosis in humans. Unfortunately, this is often impractical in our patient population since this would require sedation or general anesthesia. A diagnosis of PAH can be made by estimating pulmonary artery pressures during the echocardiographic exam. Measuring the velocity of tricuspid and/or pulmonic regurgitation can provide an estimate of systolic PAP and diastolic PAP, respectively. In the absence of tricuspid or pulmonic regurgitation, other echocardiographic findings can be supportive of the diagnosis of PAH. These findings include right ventricular dilation and/or hypertrophy, right atrial enlargement, dilated pulmonary artery, interventricular septal flattening, mass/clot within pulmonary artery and right-sided heart failure. Radiographs can also aide in the diagnosis of PAH. These findings include right-sided heart failure (pleural or abdominal effusion), right heart enlargement, dilated pulmonary arteries, and parenchymal disease.
Treatment in veterinary patients is aimed at ameliorating the clinical signs associated with the disease and improving the overall quality of life as there is no cure for PAH. Addressing the primary disease process is important (i.e. treatment for heartworms or left-sided heart failure). In most cases, treatment for PAH is not initiated until clinical signs have developed or the disease is in the moderate-severe range. In human patients with PAH, there has been no treatment that has been shown to improve survival and many treatment options are cost-prohibitive in our patients. Unfortunately, treatment options for PAH are limited in our patients and there are no approved medications. Oxygen therapy is the most effective pulmonary vasodilator. It is used in acutely dyspneic patients but is impractical for long-term therapy in dogs. Currently, the most commonly used medication for the treatment of PAH is sildenafil (Viagra®). Sildenafil is a phosphodiesterase-5 inhibitor that leads to dilation of the pulmonary artery. This medication was recently approved by the FDA for the treatment of PAH in humans and has been relabeled as Revetio®. Sildenafil has been reported to improve the quality of life in dogs. This medication is now available as a generic and is no longer as cost-prohibitive for some patients especially large breed dogs.
The prognosis for pulmonary hypertension is variable and depends on the underlying cause of the hypertension and response to therapy. It is generally guarded once severe pulmonary hypertension has developed. The goals of therapy are to decrease the clinical signs related to PAH, ideally, to decrease pulmonary artery pressure and most importantly, improve the quality of life in these patients.