Introduction

• The pulmonary pressure results from the interaction between:
  • The pulmonary blood flow; this is the right ventricular cardiac output.
  • The arterial vascular resistance (pre-capillary pressure).
  • The venous vascular resistance (post-capillary pressure or left atrial pressure).
• Pulmonary hypertension (PHT) is defined as pulmonary arterial systolic pressure > 30 mmHg, pulmonary arterial diastolic pressure > 15 mmHg, or pulmonary arterial mean pressure > 20 mmHg.
• PHT is not a diagnosis but rather the result of several disease processes, often associated with poor prognosis.
• Cause: PHT results from:
  • Increased pulmonary vascular resistance.
  • Increased pulmonary blood flow.
  • Increased left atrial pressure.
  • Or a combination of the three.
• Signs: severe PHT can cause right-sided heart failure and tachypnea, with associated poor left-sided filling. As consequence this may lead to poor left-sided cardiac output causing exercise intolerance and syncope.
• PHT is usually secondary to other disease conditions such as pulmonary thromboembolism, heartworm infestation, chronic lung parenchymal disease, or left-sided heart failure.
• Diagnosis: thoracic radiographs, cardiac ultrasound, heartworm antigen tests.
• Treatment: treatment of PHT involves addressing the primary cause, supplemental oxygen, and vasodilator therapy.
• Treatment of severe disease is often unrewarding.
• Prognosis: very poor if severe with right heart failure; if mild or moderate may be successfully managed depending on the underlying cause.

Pathogenesis

Etiology

• PHT can be caused by several different abnormalities.
• The most common cause for PHT in the cat is related with an increased pulmonary blood flow. When this flow exceeds the capacity of the pulmonary system, increased pulmonary artery pressures develop.
• Patients with increased pulmonary vascular resistance may have experienced obstruction of the pulmonary vessels (ie pulmonary thromboembolism), have abnormalities in circulating vasoactive substances (ie endothelin), have hypoxia induced vasoconstriction (ie chronic lung disease), or have increased pulmonary pressures due to elevated downstream left atrial pressures (ie mitral valve regurgitation of mitral valve stenosis Atrioventricular valve dysplasia).
• All these different causes for PHT result in vasoconstriction, vascular smooth muscle proliferation and thrombosis affecting the pulmonary circulation vessels. Therefore, if left untreated, may become progressively more severe, irreversible and refractory to treatment.
• In cases where no underlying cause can be identified, a diagnosis of primary or idiopathic PHT is occasionally made.

Most common causes of PHT in practice

• It is relatively rare for cats to develop clinically significant PHT.
• The most common cause of PHT is reversed patent ductus arteriosus (PDA), but any other abnormality that causes pulmonary overcirculation from birth may fall in this category.
• Many cases of PHT due to reversed PDA in the cat may be reversible and therefore, surgical treatment to ligate the abnormally patent vessel may still be an option in a selection of cats.
• Acquired causes of PHT in the cat are rare. Possibly the most common cause is pulmonary thromboembolism secondary to immune mediated hemolytic anemia, sepsis, protein losing nephropathy, protein losing enteropathy, neoplasia or others.
• Depending on the geographical area, lung worm (Aelurostrongylus abstrusus) or even heart worm (Dirofilaria sp) may be prevalent.
• Pulmonary parenchymal disease is relatively rare in the cat, but it may cause PHT.

Pathophysiology

• PHT causes pressure overload to the right ventricle, which suffers remodeling in the form of concentric hypertrophy (thickening of the ventricular walls) and dilation.
• Dilation of the tricuspid valve annulus can cause secondary tricuspid regurgitation Heart: tricuspid valve dysplasia and further promote the development of right heart failure.
• Right heart failure secondary to PHT is commonly referred to as cor pulmonale Cor pulmonale.
• Due to the increased pressure, and in virtue of the Bernoulli equation, if systolic tricuspid regurgitation and diastolic pulmonic insufficiency are present, these will have faster velocities than normal and proportional to the severity of the PHT.
• In severe cases of PHT, the right heart cannot generate enough force to overcome the high pulmonary pressures → right-sided cardiac output falls → left-sided cardiac output falls as consequence → weakness and fainting during exertion can occur.
• Once present, pulmonary hypertension can worsen due to abnormal blood flow, endothelial disruption, platelet aggregation, release of vasoactive substances such as endothelin, thromboxane A2, and serotonin, and further obstruction of vessels.

Timecourse

• The time course of disease is often dependent on the underlying disease, being most commonly a silent chronic manifestation in the cat.
• Many episodes of pulmonary thromboembolism are sudden and the patients present with acute signs while cases of chronic lung disease may slowly develop PHT over long periods of time.

Diagnosis

Treatment

Outcomes