Canis ISSN: 2398-2942

Pulmonary arterial hypertension (PHT)

Contributor(s): Mark Oyama, Jordi Lopez-Alvarez

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 prssure >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 any combination of the three.
  • Severe PHT can cause right heart failure and tachypnea, with associated poor left-sided filling. As a 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 respiratory disease, or left-sided heart failure.
  • Treatment: involves addressing the primary cause, supplemental oxygen, and vasodilator therapy.
  • Treatment of severe disease is often unrewarding.

Pathogenesis

Etiology

  • PHT can be caused by several different abnormalities.
  • Patients with left-to-right cardiac shunts have increased pulmonary blood flow, and if this flow exceeds the capacity of the pulmonary system, increased pulmonary artery pressures develop, and reversed, right-to-left shunts can occur.
  • Patients with increased pulmonary vascular resistance may have experienced obstruction of the pulmonary vessels (ie pulmonary thromboembolism Lung: 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 Heart: mitral valve degenerative disease).
  • 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

  • Congenital abnormalities Congenital heart disease: overview: reversed patent ductus arteriosus is the most common congenital abnormality associated with PHT, but any other abnormality that causes pulmonary overcirulation from birth may fall in this category.
  • Acquired:
    • Degenerative mitral valve disease is the most common acquired cardiac disease in the dog and, as consequence, this is also the most common cause of PHT.
    • The second most common is pulmonary parenchymal disease (pulmonary fibrosis, particularly common in West Highland White terriers) and other causes of chronic hypoxia (chronic bronchopneumopathy, upper airway disease, high altitude, etc).
    • Depending on the geographical area, lung worm (Angiostrongylus spp Angiostrongylosis) or heart worm (Dirofilaria spp) may also be common. 
    • Pulmonary thromboembolism is a frequent complication of several systemic diseases, including immune-mediated hemolytic anemia Anemia: immune-mediated hemolytic, neoplasia, protein-losing nephropathy Protein-losing nephropathy, protein-losing enteropathy Protein-losing enteropathy, pancreatitis Pancreatitis: acute Pancreatitis: chronic and sepsis Shock: septic.

Pathophysiology

  • PHT causes pressure overload to the right ventricle, which suffers remodelling in the form of concentric hypertrophy (thickening of the ventricular walls) and dilation.
  • Dilation of the tricuspid valve annulus can cause secondary tricuspid regurgitation 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.
  • 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.
  • Reactive PHT or secondary to an increased left atrial pressure may be subtle and respond to treatment of congestive heart failure, at least initially. This chronic form is common in dogs with advanced degenerative mitral valve disease.

Diagnosis

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Treatment

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Kellihan H B & Stepien R L (2010) Pulmonary hypertension in dogs: diagnosis and therapy. Vet Clin North Am Small Anim Pract 40 (4), 623-641 PubMed.
  • Stepien R L (2009) Pulmonary arterial hypertension secondary to chronic left-sided cardiac dysfunction in dogs. J Small Anim Pract 50 (Suppl 1), 34-43 PubMed.
  • Mikhail G W & Prasad S K et al (2004) Clinical and hemodynamic effects of sildenafil in pulmonary hypertension: acute and mid-term effects. Eur Heart J 25 (5), 431-436 PubMed.
  • Glaus T M, Hauser K & Hässig M et al (2003) Non-invasive measurement of the cardiovascular effects of chronic hypoxemia on dogs living at moderately high altitude. Vet Rec 152 (26), 800-803 PubMed.
  • Rich S & McLaughlin V V (2003) Endothelin receptor blockers in cardiovascular diseaseCirc 108 (18), 2184-2190 PubMed.
  • Johnson L, Boon J & Orton E C (1999) Clinical characteristics of 53 dogs with Doppler-derived evidence of pulmonary hypertension: 1992-1996. J Vet Intern Med 13 (5), 440-447 PubMed.
  • Gavaghan B J, Lapointe J M & Thomas W P (1998) Acute onset of pulmonary necrotizing arteritis in a dogs with a left to right patent ductus arteriosusAust Vet J 76 (12), 786-791 PubMed.
  • Rawlings C A (1978) Pulmonary vascular response of dogs with heartworm disease. Can J Comp Med 42 (4), 452-459 PubMed.

Other sources of information

  • Fox P R, Sisson D D, Moise N S (eds) (1999) Textbook of Canine and Feline Cardiology, 2nd edn. WB Saunders, Philadelphia.
  • Kittleson M D & Kienle R D (1998) Small Animal Cardiovascular Medicine. Mosby, St. Louis.
  • Johnson L R, Hamlin R L (1995) Recognition and treatment of pulmonary hypertension. In:Current Veterinary Therapy XII: Small Animal Practice. WB Saunders, Philadephia. pp 887-892.


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