By Dr. Gail Mason,
DVM, MA, DACVIM
Chief Medical Officer, Portland Veterinary Emergency & Specialty Care
Pulmonary (lung) hypertension refers to elevated blood pressure within the arteries and veins contained within your dog’s lungs. This differs from systemic hypertension, which involves elevated pressures in the peripheral circulation. They are not directly related issues. While considered uncommon and may have some genetic basis, the disease usually is a sequela to a pre-existing medical illness. It occurs most frequently in older, small-breed canines.
The lungs contain a complex network of blood vessels structured much like tree branches. The larger arteries continue to branch, carrying unoxygenated blood to the smallest branches called capillaries. These are where fluids and gases are exchanged between blood vessels and lung tissue. The capillaries connect to veins and start the oxygenated blood’s return trip to the heart. Pulmonary hypertension refers to elevated pressures within the lung vasculature. As the condition progresses, the right side of the heart enlarges from pushing against the high pressures and may fail. The lining of the capillaries thickens as well, reducing oxygen exchange.
Known causes of pulmonary hypertension (PH) include heartworm disease, lung cancer, formation of a clot (referred to as a pulmonary thromboembolism, PTE), and congenital heart defects. Chronic lung diseases such as pneumonia, bronchitis, pulmonary fibrosis, and chronic pulmonary obstructive disease (COPD) are also known causes. COPD (higher incidence in West Highland White Terriers) is the result of excess scar tissue in the lungs which narrows the blood vessels and elevates pressures.
Pulmonary hypertension can be insidious and may not produce symptoms in the dog until it is advanced. The most noted symptom is exercise intolerance and heavy breathing after even mild exertion. Other signs include shortness of breath, rapid breaths, syncope (fainting), or ataxia (wobbly gait after exercise). Pulmonary hypertension does not cause coughing, but several of the pre-disposing illnesses do. Your veterinarian may notice “crackles and wheezes” which are abnormal lung sounds as heard through a stethoscope and/or the presence of a new heart murmur. Sudden death can result from advanced lung changes.
Diagnosis
Diagnostics utilized on patients suspected of having chronic pulmonary disease and/or PH often start with thoracic radiographs (chest X-rays) for an overview of lung health. A heartworm test plus routine blood work is often recommended. A CAT scan is a sensitive modality to visualize intricate lung structures and potential abnormal processes. These tests help determine if there are underlying medical conditions that can be treated or managed. Pulmonary pressures cannot be assessed using a pressure cuff that is used for systemic hypertension. The most sensitive tool involves a noninvasive echocardiogram performed by a board-certified cardiologist. Rarely, a right-heart catheterization is required by inserting a special wire electrode into the jugular vein.
Classification of PH patients
• Functional Class 1: This class includes dogs who are functionally quite normal. They can play, run, and jump without any obvious consequences. They do not demonstrate changes in respirations.
• Functional Class 2: These dogs are normal at rest, but develop symptoms such as fatigue, fainting, and increased respiration rate and effort.
• Functional Class 3: Class 3 patients cannot sustain exercise or even mild activity without developing fatigue, increased respiratory difficulty, apparent chest pain or collapse.
• Functional Class 4: Dogs in this category are significantly symptomatic even at rest.
Treatment
Pulmonary hypertension can be difficult to treat. If the symptoms are severe and sudden in onset, emergency assistance is essential. Oxygen therapy will often relieve the severity of the symptoms and make the dog more comfortable. If heart failure is present, specific treatment for that condition would improve the dog’s ability to respire and oxygenate more normally. Identification and treatment of any other underlying conditions can also improve the quality of life of the patient by reducing symptoms of PH.
Unfortunately, by the time PH is recognized, the underlying conditions and lung damage are often advanced. However, there are several medications that can be beneficial for improving the life of the patient specific to PH. Dogs with chronic lung disease may improve with bronchodilators such as theophylline and/or anti-inflammatory drugs such as corticosteroids. Drugs that specifically dilate pulmonary arteries (reducing the hypertension) include sildenafil (yes, Viagra), and tadalafil (Cialis). Home oxygen therapy can certainly benefit patients with PH, but it requires a very motivated owner and an oxygen cage.
Prognosis
The prognosis for PH patients depends on the functional class at the time of diagnosis and degree of underlying disease(s). Dogs with Class 4 disease may survive only a few days to weeks. However, some dogs with less severe PH may be successfully managed for months to years. If your dog seems to have chronic or relapsing episodes of a respiratory problem, taking early action can result in a good outcome!