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What is Pulmonary Hypertension?

Pulmonary (lung) hypertension (PHT) is high blood pressure in the arteries 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. Dogs with severe pulmonary hypertension 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 a cardiac ultrasound (also called Doppler echocardiography). It 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, you cannot diagnose pulmonary hypertension by measuring your blood pressure.

PHT Classification

Veterinarians classify pulmonary hypertension based on the underlying cause.  There are 6 broad “types” of pulmonary hypertension.

Group 1: Pulmonary arterial hypertension

1. Primary (idiopathic, essential) pulmonary hypertension
2. Heritable
3. Associated with left-to-right shunting congenital cardiac disease (e.g. Patent Ductus arteriosus, Ventricular Septal Defect, Atrial Septal Defect).

Group 2: Left-sided heart disease

1. Left ventricular dysfunction (Dilated Cardiomyopathy)
2. Mitral valve disease (Mitral regurgitation), acquired and congenital
3. Mitral stenosis or left atrial obstruction

Group 3: Respiratory disease/hypoxia

1. Chronic obstructive pulmonary disease (COPD)
2. Severe tracheal or mainstem bronchial collapse
3. Pneumonia (severe)
4. Sleep apnea (severe)
5. Chronic exposure to high altitudes
6. Bronchiectasis, emphysema
7. Pulmonary fibrosis
8. Pulmonary idiopathic eosinophilia

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 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 lodge, partially or completely blocking a pulmonary artery. At other times, the clot can form directly within the pulmonary artery.  The effect is the same – 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 pressure by causing inflammation.  They also physically block the vessels.  Finally, they can induce 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 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.

When pulmonary hypertension appears suddenly and seriously (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 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 6 months was 84% and 73% at 1 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.