Pulmonary hypertension (PH) is the term used to describe very elevated pulmonary arterial pressure. It is a separate disease entity than systemic hypertension, which describes high blood pressure in the rest of the body.
In order for you to understand how this disease may affect your dog, it is important to understand how blood travels through the heart. The venous blood from the body drains into the right atrium, through the tricuspid valve, and into the right ventricle. The right ventricle then pumps this un-oxygenated blood to the lungs through the pulmonary artery. Moving through capillaries, the blood picks up oxygen from the lungs. This blood then drains through large pulmonary veins into the left atrium, through the mitral valve, and into the left ventricle where it is pumped through the aorta and back to the body.
When dogs develop significant pulmonary hypertension, the right side of the heart enlarges (sometimes dramatically) in order to pump blood into the elevated pulmonary arterial circulation. If the pressure on the right side rises enough, the enlarged right side can hinder normal filling of the left side. Subsequently, the amount of blood pumped into the systemic circulation to the body is reduced, causing weakness, exercise intolerance, and sometimes fainting. Additionally, the elevation in right-sided pressure can back up into the venous return from the body causing fluid to ooze out of the capillaries and into the abdominal cavity. This is called right-sided congestive heart failure.
The causes of pulmonary hypertension are numerous. In some cases, it is secondary to chronic pulmonary thromboembolism (PTE). Diseases that make dogs likely to have PTE include immune diseases, severe liver disease, kidney disease, endocrine disease (such as Cushing’s disease), and cancer. Other causes of pulmonary hypertension are chronic lung disease; chronic, severe left heart disease; heartworm disease; and idiopathic diseases. The term idiopathic is used when the underlying cause cannot be identified. And regarding pulmonary hypertension, for most dogs, the underlying cause cannot be identified. However, an attempt should be made to identify the cause, because, in some cases, it can be treated or the disease process slowed.
The single most important test used to identify and diagnose pulmonary hypertension and assess its severity is the echocardiogram. Dogs with mild PH do well and rarely experience clinical signs. Dogs with moderate PH are variably affected; most do not experience obvious clinical signs, although they may be somewhat exercise intolerant or faint/collapse with exertion. Dogs with severe PH always experience clinical signs, which include trouble breathing, fainting with exertion, and/or right-sided congestive heart failure. The tests used to identify the cause of pulmonary hypertension are labwork and abdominal ultrasound if cancer or other abdominal disease is suspected.
If the underlying cause can be identified, every attempt should be made to treat it. Many of the diseases that cause PH are unfortunately not amenable to therapy. In most dogs, treatment is directed at lowering the blood pressure in the lungs. This is done medically, and the most effective medication for PH treatment is Sildenafil (Viagra).
Approximately 65% of dogs respond to Viagra and although their pulmonary arterial pressure may not normalize, it is lowered enough to cause marked improvement in clinical signs. Viagra is moderately expensive in small- to medium-sized dogs and can be prohibitively expensive in large dog. There are other medications that can be beneficial and may be prescribed earlier.
Dogs with pulmonary hypertension should be monitored for a change in breathing pattern (increased abdominal effort with each breath; rapid, shallow breathing; etc.); fainting or collapse; exercise intolerance; and abdominal swelling, which could indicate the onset of right-sided congestive heart failure.
All this being said, it is important to remember that many afflicted dogs never develop problems with pulmonary hypertension. Additionally, it is important to monitor their echocardiograms so if changes occur, appropriate therapy is prescribed.