Pulmonary hypertension (PH) is a term used to describe an abnormally elevated pulmonary artery pressure. In essence this means that the blood pressure within the arteries in the lungs is elevated. This is a separate disease entity from systemic hypertension which describes elevated blood pressure in the rest of the body. This elevated pulmonary artery pressure leads to decreased blood flow to the lungs and increases the workload of the right side of the heart. It also results in structural changes to the pulmonary artery and right heart chambers.
What causes pulmonary hypertension?
Causes of pulmonary hypertension are numerous but in dogs there are five major causes appreciated clinically:
- Chronic bronchial/lung disease is often seen in small breed dogs
- Pulmonary thromboembolism (PTE) is the formation of a blood clot in a blood vessel that breaks away and is carried through the blood stream where it eventually clogs another blood vessel. When the blood clot clogs a vessel in the brain the patient suffers a stroke; when it plugs a vessel in the lungs the result is a pulmonary embolism, or it may clog a blood vessel in the leg, kidney or gastrointestinal tract. PTE can occur secondary to a variety of immune-mediated, endocrine, organ dysfunction, or neoplastic (cancerous) diseases
- Heartworm disease
- Chronic left-sided heart disease
- Intracardiac or arteriovenous shunts (usually congenital)
In a significant number of cases, the underlying cause is not identified or only suspected.
- increased respiratory effort or distress
- exercise intolerance
- collapse or fainting episodes
- progressive lethargy
- pulmonary hypertension can lead to right- sided heart failure which would most likely appear as abdominal distension
Diagnosis of PH in humans usually requires cardiac catheterization and direct measurement of pulmonary artery pressures and resistance. This is rarely performed in dogs and cats because often these patients are in serious condition and it will require heavy sedation or general anesthesia. Diagnosis of PH in veterinary medicine, therefore, is most effective via an echocardiogram (cardiac ultrasound) performed by a board-certified veterinary cardiologist. The echocardiogram assesses structural changes to the heart as well as blood flow abnormalities that allow for indirect estimation of pulmonary artery pressure. Disease is classified as mild, moderate, or severe and can worsen over time.
A complete workup for the underlying cause of PH is warranted once diagnosis is confirmed, including bloodwork/urinalysis, radiographs (x- rays), and abdominal imaging.
Dogs with severe pulmonary hypertension often require immediate hospitalization and oxygen therapy while diagnostic workup and treatment are performed. Treatment is aimed at identifiable, underlying causal conditions.
- If the cause is lung disease, the dog may be started on bronchodilators.
- If the cause is a thromboembolism (or blood clot to the lungs), then we start the patient on anti-thrombotic drugs. Clopidogrel, better known by its trade name Plavix®, is an anticoagulant that is really effective in preventing blood clots in dogs and it is being used more often by veterinarians to treat their animal patients suffering from thromboembolism.
- In most cases (regardless of cause), powerful drugs that vasodilate the pulmonary arterial system are required to improve clinical condition. The drug most commonly used is Sildenafil (Viagra). Sildenafil is a vasodilator that relaxes smooth muscle cells, allowing blood to flow freely to certain tissues-especially the tissue found in the reproductive tract and in the lungs. Sildenafil relaxes the lungs, which allows blood to flow more readily, and decreases stress on the heart. Cialis (Tadalafil) is occasionally used as a substitute for Sildenafil.
Although echocardiographic parameters often do not significantly change with treatment, patients are often highly clinically responsive to the medication. Therapy for pulmonary hypertension is life-long.
The long term prognosis for PH is generally guarded to poor, but patients often do well in the short term and maintain a good quality of life with continued therapy. It may be a few days or more in the hospital before they begin to show a response to therapy, however.