Pulmonary Hypertension in Dogs: Causes, Diagnosis and Treatment

Pulmonary hypertension (PH) is a haemodynamic condition defined by abnormally elevated blood pressure within the pulmonary arterial system — the network of blood vessels that carries deoxygenated blood from the right side of the heart through the lungs for gas exchange. While systemic hypertension (elevated blood pressure in the general circulation) is well recognised in veterinary medicine, pulmonary hypertension has gained increasing attention over the past two decades as advances in echocardiography have made non-invasive diagnosis more accessible, revealing that PH is far more common in dogs than previously appreciated.

"Pulmonary hypertension is not a primary disease — it is almost always the consequence of another underlying condition. Our most important job as clinicians is to identify and, where possible, treat that underlying cause. When we can successfully address the primary problem — whether it is heartworm disease, chronic respiratory disease, or left-sided heart failure — the pulmonary pressures often improve significantly. Targeted pulmonary vasodilator therapy is a valuable adjunct, but it works best when combined with treatment of the root cause." — Dr. Sarah Chen, DVM

In a normal dog, the pulmonary circulation is a low-pressure, low-resistance system. Systolic pulmonary arterial pressure is typically 15 to 30 mmHg — roughly one-quarter to one-fifth of systemic arterial pressure. This low pressure is essential because the pulmonary capillaries must be thin-walled enough to allow efficient gas exchange (oxygen in, carbon dioxide out), and high pressure would damage these delicate structures and cause fluid to leak into the lung tissue (pulmonary oedema). Pulmonary hypertension is generally defined as systolic pulmonary arterial pressure exceeding 30 to 35 mmHg, with moderate PH at 50 to 75 mmHg and severe PH above 75 mmHg. When this pressure rises, the right ventricle — which is designed to work against low resistance — must generate increasingly greater force to push blood through the lungs, leading to progressive right-sided cardiac dysfunction.

The consequences of sustained pulmonary hypertension are progressive and serious. The right ventricle initially compensates by hypertrophying (thickening its muscular wall) to generate the increased pressure needed to maintain blood flow through the lungs. However, unlike the left ventricle, the right ventricle is not anatomically designed for sustained pressure work, and over time it dilates and fails — a condition called cor pulmonale. This right-sided heart failure leads to systemic venous congestion, manifesting as jugular distension, hepatic congestion, ascites (abdominal fluid), and pleural effusion. Understanding PH as both a consequence of underlying disease and a cause of progressive right heart failure is essential for effective management.

Pulmonary hypertension in dogs is classified into groups based on the underlying mechanism, following a system adapted from the human World Health Organization classification. This classification is clinically important because the treatment approach and prognosis differ significantly depending on the underlying cause.

Group 1 — Pulmonary arterial hypertension (PAH): This category includes conditions that primarily affect the pulmonary arteries themselves, causing narrowing and increased resistance within the pulmonary vasculature. In dogs, this includes idiopathic PAH (where no underlying cause can be identified), heartworm disease (in which adult worms and the associated inflammatory response directly damage and obstruct the pulmonary arteries), and congenital heart defects with left-to-right shunting (such as patent ductus arteriosus or ventricular septal defect) that expose the pulmonary vasculature to excessive blood flow and pressure, eventually causing irreversible vascular remodelling (Eisenmenger syndrome). Heartworm disease is one of the most common causes of PH in endemic regions, and resolving the heartworm infection is often the most effective way to reduce pulmonary pressures in these patients.

Group 2 — PH secondary to left-sided heart disease: This is the most common cause of PH in dogs. When the left side of the heart fails — due to myxomatous mitral valve disease (MMVD), dilated cardiomyopathy, or other conditions that elevate left atrial pressure — the increased pressure is transmitted backwards through the pulmonary veins into the pulmonary capillary bed and eventually into the pulmonary arteries. Initially this 'passive' or 'post-capillary' PH is proportional to the degree of left-sided heart disease and is partially reversible if left heart function improves. However, prolonged elevation of pulmonary venous pressure can trigger reactive pulmonary arterial vasoconstriction and structural remodelling of the pulmonary vessels, creating a 'reactive' or 'combined pre- and post-capillary' component that persists even if left heart pressures normalise. Advanced mitral valve disease is the most common scenario, particularly in older small-breed dogs with severe valvular heart disease.

Group 3 — PH secondary to respiratory disease or hypoxia: Chronic lung disease that impairs gas exchange leads to chronic hypoxia (low blood oxygen levels), which triggers pulmonary arterial vasoconstriction — the lungs' attempt to redirect blood flow away from poorly ventilated areas toward better-oxygenated regions. While this hypoxic vasoconstriction is an appropriate short-term compensatory mechanism, chronic activation leads to sustained PH. Conditions in this category include chronic bronchitis, tracheal collapse, brachycephalic airway syndrome, interstitial lung disease (including pulmonary fibrosis, particularly common in West Highland White Terriers), chronic aspiration pneumonia, and severe laryngeal paralysis. Brachycephalic breeds (Bulldogs, Pugs, French Bulldogs) are particularly predisposed due to their chronic upper airway obstruction.

Group 4 — PH due to pulmonary thromboembolism: Blood clots lodging in the pulmonary arteries increase resistance to blood flow and elevate pulmonary pressure. Conditions that predispose to pulmonary thromboembolism include protein-losing nephropathy (which depletes antithrombin), hyperadrenocorticism (Cushing's disease), immune-mediated haemolytic anaemia, neoplasia, and heartworm disease. Group 5 — PH due to multifactorial or unclear mechanisms encompasses cases where multiple factors contribute or where the precise mechanism is not fully elucidated.

The clinical signs of pulmonary hypertension are often insidious in onset and may be mistakenly attributed to the underlying primary disease, ageing, or simple deconditioning. Recognising the signs that specifically suggest PH helps prompt appropriate diagnostic evaluation, even when an underlying condition has already been diagnosed.

Exercise intolerance and lethargy are typically the earliest signs, often noticed by attentive owners as a gradual decline in the dog's willingness or ability to walk, play, or engage in activities they previously enjoyed. The dog may lag behind on walks, stop to rest more frequently, or pant excessively after minimal exertion. These signs occur because the compromised right heart cannot increase cardiac output adequately during exercise — the pulmonary vasculature is too constricted or obstructed to accommodate the increased blood flow demands of physical activity. Unfortunately, many owners attribute these changes to normal ageing, particularly in older dogs already diagnosed with heart or respiratory disease, and may not report them until they become severe.

Syncope (fainting) is one of the most alarming and diagnostically significant signs of PH. Syncopal episodes in dogs with PH typically occur during or immediately after exertion or excitement, and result from a transient drop in cardiac output and cerebral blood flow. The dog may collapse suddenly, become limp and unresponsive for several seconds to a minute, then recover spontaneously and appear relatively normal shortly afterward. Some dogs show pre-syncopal signs — staggering, glazed expression, or brief disorientation — before full collapse. Syncope may also be triggered by coughing fits, which transiently elevate intrathoracic pressure and further compromise an already marginal cardiac output. Any dog experiencing syncopal episodes should receive urgent cardiac evaluation, as syncope indicates haemodynamically significant disease. If your dog faints or shows sudden weakness, this is one of the key signs of distress that warrants immediate veterinary attention.

Coughing and respiratory distress may be present, although these signs are often attributable to the underlying respiratory or cardiac disease rather than to PH specifically. A dry, non-productive cough may result from pulmonary arterial distension compressing adjacent airways, or from concurrent left-sided heart disease causing pulmonary oedema. Cyanosis — bluish discolouration of the gums, tongue, or mucous membranes — indicates severe hypoxaemia (dangerously low blood oxygen levels) and is a sign of advanced disease requiring urgent intervention. In dogs with right-sided heart failure secondary to PH, abdominal distension from ascites (fluid accumulation in the abdominal cavity) may be the presenting complaint, along with jugular vein distension visible along the neck when the dog is in a standing position.

Definitive diagnosis of pulmonary hypertension requires estimation or measurement of pulmonary arterial pressure, and echocardiography (cardiac ultrasound) has become the primary diagnostic tool for non-invasive assessment of PH in dogs. While right heart catheterisation remains the gold standard for direct measurement of pulmonary pressures in human medicine, it is invasive, requires specialised equipment and expertise, and is performed in only a small number of veterinary referral centres. Fortunately, Doppler echocardiography provides reliable, non-invasive estimates of pulmonary pressure that are sufficient for clinical diagnosis and management in the vast majority of cases.

The most commonly used echocardiographic method for estimating pulmonary pressure relies on measuring the peak velocity of tricuspid regurgitation (TR) using continuous-wave Doppler. The tricuspid valve separates the right atrium from the right ventricle, and in many dogs with PH, elevated right ventricular pressure causes the tricuspid valve to leak (regurgitate). The velocity of this regurgitant jet is directly proportional to the pressure difference between the right ventricle and the right atrium during systole. Using the modified Bernoulli equation (pressure gradient = 4 times the square of the TR velocity), the cardiologist can estimate right ventricular systolic pressure, which in the absence of pulmonic valve stenosis, closely approximates systolic pulmonary arterial pressure. A TR velocity greater than 2.8 to 3.0 metres per second (corresponding to a pressure gradient of approximately 31 to 36 mmHg) is generally considered consistent with PH. Velocities exceeding 3.5 metres per second suggest moderate PH, and those above 4.5 metres per second indicate severe PH.

Echocardiography also reveals important structural changes associated with PH. Right ventricular hypertrophy (wall thickening) and dilation, right atrial enlargement, flattening or paradoxical bowing of the interventricular septum toward the left ventricle (indicating right ventricular pressure approaching or exceeding left ventricular pressure), and dilation of the main pulmonary artery are all echocardiographic findings that support the diagnosis and indicate the severity and chronicity of PH. Additionally, the echocardiographic examination evaluates left heart structure and function, which is essential for determining whether PH is secondary to left-sided heart disease (Group 2) — this distinction is critical because the treatment approach differs significantly.

Supporting diagnostic tests include thoracic radiographs (which may show right heart enlargement, enlarged pulmonary arteries, and changes in the lung parenchyma suggesting underlying respiratory disease), arterial blood gas analysis (to assess oxygen levels and identify hypoxaemia), complete blood work (to screen for conditions predisposing to thromboembolism), urinalysis with urine protein-to-creatinine ratio (to rule out protein-losing nephropathy), heartworm testing, and coagulation profiles when thromboembolism is suspected. Advanced imaging with computed tomography (CT) angiography may be performed at referral centres to evaluate for pulmonary thromboembolism, interstitial lung disease, or other structural lung pathology not visible on radiographs.

Management of pulmonary hypertension requires a dual approach: treating the underlying cause whenever possible, and targeted therapy to reduce pulmonary arterial pressure and improve right heart function. The relative importance of each component depends on the PH classification and the reversibility of the underlying condition.

Treating the underlying cause is the most impactful intervention in many cases. For Group 2 PH (secondary to left heart disease), optimising management of the primary cardiac condition with diuretics (furosemide), pimobendan, and ACE inhibitors can reduce left atrial pressure and consequently lower pulmonary venous and arterial pressures. For Group 3 PH (secondary to respiratory disease), managing the underlying airway or lung disease — whether through bronchodilators, anti-inflammatory medications, weight loss for obese pets, or surgical correction of airway obstruction in brachycephalic dogs — addresses the hypoxia driving pulmonary vasoconstriction. For heartworm-associated PH, successful adulticide therapy to eliminate the worm burden can dramatically improve pulmonary pressures over time, though vascular remodelling from chronic infection may cause some degree of persistent PH.

Sildenafil (brand name Viagra in human medicine, Revatio for pulmonary hypertension) is the most widely used targeted pulmonary vasodilator in veterinary medicine. Sildenafil is a phosphodiesterase-5 (PDE5) inhibitor that works by enhancing the vasodilatory effects of nitric oxide in the pulmonary vasculature, selectively reducing pulmonary arterial pressure without significantly affecting systemic blood pressure. It is typically administered orally two to three times daily, and multiple studies have demonstrated its efficacy in reducing echocardiographic estimates of pulmonary pressure, improving exercise tolerance, reducing syncopal episodes, and improving quality of life in dogs with PH of various causes. Side effects are generally mild and may include mild systemic hypotension (low blood pressure), gastrointestinal upset, and flushing. Tadalafil (Cialis), another PDE5 inhibitor with a longer duration of action allowing once-daily dosing, is increasingly used as an alternative to sildenafil, though published veterinary data are more limited.

Pimobendan, an inodilator (combining positive inotropic and vasodilatory effects) widely used in canine cardiology, also has pulmonary vasodilatory properties and may be beneficial in dogs with PH, particularly those with concurrent right ventricular dysfunction. Its combination of strengthening right ventricular contraction while reducing pulmonary vascular resistance makes it a logical addition to the treatment protocol for many PH patients. Supplemental oxygen therapy provides immediate relief of hypoxaemia and reduces hypoxic pulmonary vasoconstriction, and may be used during acute decompensation or as palliative home therapy in advanced cases. For dogs with PH secondary to thromboembolism, anticoagulant therapy (typically clopidogrel or low-dose aspirin, with rivaroxaban gaining use) is indicated to prevent recurrent clot formation.

Regular monitoring through serial echocardiography (typically every two to four months initially, then every four to six months once stable) allows assessment of treatment response and disease progression. Consistent veterinary follow-up is essential, as medication doses may need adjustment based on clinical response, and the underlying disease process may require ongoing management modifications.

The prognosis for dogs with pulmonary hypertension varies considerably depending on the underlying cause, the severity of PH at the time of diagnosis, and the response to treatment. While PH is a serious condition that can be life-limiting, many dogs respond well to appropriate therapy and enjoy meaningful improvements in quality of life and survival time.

Dogs with PH secondary to treatable underlying conditions generally have the most favourable prognosis. When heartworm disease is successfully treated and the worm burden eliminated, pulmonary pressures often improve substantially — though some degree of persistent PH from chronic vascular remodelling is possible. Dogs with PH secondary to left heart disease that responds well to cardiac medications may experience significant reduction in pulmonary pressures as left atrial pressure normalises. Dogs with PH caused by correctable upper airway obstruction (for example, brachycephalic dogs that undergo surgical airway correction) may show dramatic improvement in pulmonary pressures post-operatively.

Dogs with moderate-to-severe PH at diagnosis, particularly those presenting with syncope, right-sided heart failure, or severe hypoxaemia, carry a more guarded prognosis. Published survival data suggest median survival times of approximately 3 to 14 months for dogs with moderate-to-severe PH, though individual variation is substantial. Some dogs stabilise on medical therapy and live for two years or more, while others progress despite aggressive treatment. Dogs with idiopathic pulmonary arterial hypertension (no identifiable underlying cause) tend to have a poorer prognosis than those with treatable secondary causes, as the underlying vascular pathology is progressive and irreversible.

Quality of life should be the primary focus of management for dogs with PH. Practical measures that support comfort and wellbeing include maintaining gentle, low-intensity exercise (short, leash-controlled walks at the dog's own pace), avoiding heat and humidity (which increase respiratory demand), preventing excitement and stress (which can trigger syncope), maintaining a healthy body weight to reduce cardiorespiratory workload, and ensuring a comfortable, well-ventilated living environment. Owners should be counselled about realistic expectations — the goal of treatment is to improve quality of life and slow disease progression, not necessarily to achieve a cure. Monitoring for signs of disease progression, including increasing exercise intolerance, more frequent syncopal episodes, progressive abdominal distension, or persistent cyanosis, helps guide decisions about treatment intensification and, ultimately, compassionate end-of-life care when quality of life can no longer be maintained. For comprehensive support, consider our guide on caring for an ageing pet.