Pulmonary hypertension is an elevated blood pressure of the lung vasculature (the pulmonary vein, pulmonary artery, or pulmonary capillaries). It can lead to heart failure if left untreated.
By definition, any pulmonary arterial systolic pressure over 30 mm Hg, pulmonary arterial mean pressure over 20 mm Hg, or pulmonary arterial diastolic pressure over 15 mm Hg constitutes pulmonary hypertension.

In a healthy dog with normal pulmonary blood pressure, the pulmonary arteries have distensible walls and are elastic. In dogs with the disorder, the pulmonary arteries are thickened, hypertrophic. In some severe cases, the blood vessels may develop necrosis.

Dogs with pulmonary hypertension are divided into classes depending upon the severity of the signs ranging from functional class I (the dog shows no signs of the disease) to functional class IV (the dog is unable to perform physical activity without severe signs of pulmonary hypertension, and show symptoms even at rest). Dogs with the most severe form have right-sided heart failure as well.

Pulmonary hypertension is always secondary to an underlying cause. Anything that affects pulmonary arterial vasodilation or vasoconstriction, smooth muscle cell proliferation, blood flow, or platelet activation will increase pulmonary arterial pressure. Left atrium high blood pressure can lead to pulmonary hypertension.  Underlying causes may be cancer, thrombosis, pneumonia, vascular blockage, bronchitis, adult respiratory distress syndrome, overactive adrenal glands, infection, obesity, congenital shunts, heartworm disease, inflammation of the pancreas, hypoxia, protein-losing nephropathy, high altitude disease, and paralysis.
The main cause of elevated pulmonary pressure is heart valve disease and the second main cause is heartworm infection.

Because of the wide variety of primary causes, there is no definitive way to prevent pulmonary hypertension.

There are several general signs a dog owner may notice in a dog suffering from elevated pulmonary pressure: trouble breathing, fatigue, coughing or vomiting blood, exercise intolerance, coughing, fainting, weight loss, and enlarged abdomen. It is important that any dog showing any of these symptoms be examined immediately, because untreated pulmonary hypertension leads to permanent damage and eventual death.

Dogs at Risk
Any dog may develop pulmonary hypertension as the result of an underlying disorder. Some underlying diseases may affect some breeds more than others. One of the main causes of pulmonary hypertension is degenerative mitral valve disease, which occurs most often in small breed dogs that are middle-aged and older. Cavalier King Charles Spaniels are prone to mitral valve disease, as are the Norwich Terrier, Norfolk Terrier, English Toy Spaniel, Chihuahua, and Saluki.
Dogs prone to heartworm infection are also at higher risk for pulmonary hypertension. This includes all working dogs, middle-aged male outdoor dogs, and all those dogs that live in endemic areas.

Testing and Diagnosis
A thorough physical exam and history, blood chemistry profile, heartworm antigen test, urinalysis, complete blood count, and arterial blood gas test should be performed. Any fluid in the lungs or abdomen should be drawn off for testing. The vet should listen for a split s2 sound, heart murmurs, and abnormal lung sounds.
Thoracic x-rays can show heart disease and/or heart and lung abnormalities. An electrocardiogram can test for pulmonary hypertension. An echocardiogram, currently considered the gold standard in diagnosis for this disorder, can indicate pulmonary blood clots and heart abnormalities that didn’t show on x-ray. ECG can also help to measure the pressure in the dog’s blood vessels when the heart contracts.
Right heart catheterization is another option to diagnose this disorder, but it is invasive and generally not done very often. It requires general anesthesia and is high-risk. It is also expensive.
When pulmonary hypertension is suspected, and then diagnosed, the first goal is to determine the underlying cause. Second, the degree of severity of the hypertension must be determined. The third goal of diagnosis is to assess for hemodynamic impairment, and the fourth goal is to decide upon the best treatment available.

Treatment and Prognosis
Treatment goals include decreasing clinical signs, decreasing hospitalization duration, increasing survival time, and improving quality of life.
Dogs with severe breathing problems will be hospitalized and placed on oxygen. Treatment for the underlying disease or disorder varies, and can bring about a reversal of the elevated pulmonary blood pressure.
Supportive therapy may include brochodilating medicines, chemotherapy, diuretics, corticosteroids, oxygen supplementation, heartworm therapy, and anti-inflammatory agents, depending on the primary disease. Dogs with functional class III through IV must be monitored to assess improvement or decline in function.
Prognosis depends entirely upon the underlying cause and the functional class at which the dog is diagnosed. In many cases, the underlying cause cannot be cured, only treated. This means that the goal is to make the patient more comfortable. Most dogs only live up to 91 days past diagnosis of functional class III and IV, if they receive immediate treatment. Some dogs may live up to two years past diagnosis, but this is rare.