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What is Pulmonary Hypertension?

Pulmonary (lung) hypertension (PHT) is high blood pressure in the arteries and pulmonary veins leading in and out of your pet's lungs. If the high blood pressure becomes too severe, it can cause disease and failure of the right side of the heart.

Generally speaking, pulmonary hypertension results from other diseases the pet already has, so it's referred to as a secondary disease rather than a primary one. 

Diseases that can cause pulmonary hypertension include heartworm disease and pulmonary thromboembolism, both of which block the arteries in the lung; diseases of the lungs, such as pulmonary fibrosis or chronic obstructive pulmonary disease; or diseases affecting the left side of the heart, such as degenerative mitral valve disease or dilated cardiomyopathy. However, in some instances, it can be the primary, or only, disease. If a disease has no known cause, it’s called idiopathic, and primary pulmonary hypertension has no known cause.

Pulmonary hypertension is seen far more commonly in dogs than in cats.

Signs

Clinical signs associated with pulmonary hypertension depend on the severity of the hypertension and the underlying primary disease process.  Dogs with pulmonary hypertension can have no clinical signs (if the hypertension is mild or moderate), and even dogs with severe pulmonary hypertension can appear relatively normal at rest. However, clinical signs often appear when the dog exercises or exerts itself. 

Such signs include exertional dyspnea (shortness of breath), syncope (fainting), or ataxia (staggering and wobbliness) when exercising. With severe pulmonary hypertension, dogs can be short of breath even at rest and have enough trouble breathing that their gums can become bluish from lack of oxygen. Some develop heart murmurs (which your veterinarian should be able to detect with a stethoscope), and some develop a fluid-filled and distended abdomen (right-sided congestive heart failure).  Dogs with severe pulmonary hypertension can die suddenly.

Dogs with pulmonary hypertension will also often show signs associated with the underlying primary disease process. For example, dogs with severe lung disease might cough (note: pulmonary hypertension does not cause coughing, but diseases that also cause coughing can cause pulmonary hypertension). Your veterinarian might hear crackles and wheezes when they listen to the chest with a stethoscope. Dogs with left-sided heart disease might have a heart murmur. 

Diagnosis

The diagnostic test of choice is cardiac ultrasound (Doppler echocardiography). This provides a noninvasive and readily available method of diagnosis and is generally performed by a veterinary cardiologist. It does not require the pet to be anesthetized or sedated.

Pulmonary hypertension is unrelated to systemic hypertension (the type of hypertension that your doctor measures with a blood pressure cuff or an “at-home” blood pressure monitor). Therefore, pulmonary hypertension can't be diagnosed by measuring blood pressure.

PHT Classification

Veterinarians classify pulmonary hypertension based on the underlying cause. There are six broad “types” of pulmonary hypertension. Below, you will see the medical term followed by explanations in parentheses.

Group 1: Pulmonary Arterial hypertension

1. Primary (idiopathic, essential) pulmonary hypertension
2. Heritable (It can be passed down from parent to offspring.)
3. Associated with left-to-right shunting congenital cardiac disease (e.g. Patent Ductus arteriosus, Ventricular Septal Defect, Atrial Septal Defect). These are heart defects commonly referred to as “holes in the heart”.

Group 2: Left-sided heart disease

1. Left ventricular dysfunction, Dilated Cardiomyopathy (The left ventricle, or main pumping heart chamber, is failing to contract properly)
2. Mitral valve disease, Mitral regurgitation (The mitral valve, or valve that lies between the left heart chambers, leaks)
3. Mitral stenosis or left atrial obstruction (The valve between left heart chambers fails to open properly.)

Group 3: Respiratory disease/hypoxia

1. Chronic obstructive pulmonary disease (COPD - Decreased airflow, irritation, and swelling.)
2. Severe tracheal or mainstem bronchial collapse (The main tube to the lungs becomes narrow/closed.)
3. Pneumonia, severe (An infection in the lung’s air sacs)
4. Sleep apnea, severe (Periods of absent breathing during sleep)
5. Chronic exposure to high altitudes
6. Bronchiectasis, emphysema (Bronchial tubes are dilated, air sac walls are damaged.)
7. Pulmonary fibrosis (Lung scarring makes it hard for lungs to expand.)
8. Pulmonary idiopathic eosinophilia (An immune reaction occurs where large numbers of white blood cells called eosinophils infiltrate the lungs.)

Group 4: Embolism (blood vessel blockage) or Thromboembolism (blood clot)

Pulmonary embolism is rare in dogs. Causes of embolism include fat (associated with long-bone fractures), or implanted cardiac devices (e.g. coils used to occlude a Patent Ductus Arteriosus). Blood clots that form elsewhere in the body can occasionally float up to the heart and into the pulmonary arteries, where they may partially or completely block a pulmonary artery. At other times, a clot can form directly within the pulmonary artery.  The effect is the same – the blockage of the vessels.

Group 5: Parasitic diseases

Heartworm (Dirofilaria immitis) and the lungworm (Angiostrongylus vasorum) are parasitic worms that live within pulmonary arteries. They cause damage to the walls of the arteries, causing them to thicken and become less elastic, increasing blood pressure by causing inflammation. They also physically block the vessels. Finally, they can cause blood clots to form within the pulmonary arteries.

Group 6: Multifactorial disorders or unclear mechanisms

1. Cases where a dog has multiple causes that span multiple groups (e.g. mitral valve disease (Group 2) and heartworm disease (Group 5). 
2. Tumors that physically compress a pulmonary artery or obstruct blood flow from inside the artery

How is PHT Treated?

Pulmonary hypertension can be difficult and frustrating to treat. Your veterinarian will attempt to identify its cause and treat the underlying disease. Medications can be used to improve the patient’s quality of life.

Sudden Onset

When pulmonary hypertension appears suddenly and severely (those cases are called acute), treatment focuses on giving oxygen because most patients improve with oxygen; significant exercise restriction; and alleviating congestive heart failure and any underlying diseases. At-home oxygen cages can be used to help provide relief for dogs with severe pulmonary hypertension. 

Longer-Term Management

Longer-term management generally relies on the medication sildenafil citrate (Viagra® – yes, the Viagra® you’ve heard about, but it also works well to relieve signs of pulmonary hypertension as it was originally developed to treat pulmonary hypertension in humans) or tadalafil (Cialis®). Additionally, your veterinarian might prescribe pimobendan, a heart medication. Newer human drugs that decrease pulmonary pressure (e.g., iloprost and bosentan) may be cost-prohibitive for most clients.

Mild and moderate pulmonary hypertension generally does not require treatment. Rarely, moderate pulmonary hypertension can cause clinical signs that require retreatment.

Prognosis

The prognosis for a dog with pulmonary hypertension depends on the underlying disease and how advanced it is. One study shows that in dogs who survive the first week of therapy, the probability of survival at six months was 84% and 73% at one year. 

Pulmonary hypertension is a truly debilitating disease, but on occasion, it is cured or at least managed well with drugs: patients’ clinical signs improve when they take sildenafil citrate. However, many patients die suddenly because of pulmonary hypertension or have a quality of life that the owners consider intolerable.