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Pulmonary hypertension is increased pressure in the pulmonary circulation. It has many secondary causes; some cases are idiopathic. In pulmonary hypertension, pulmonary vessels become constricted. Severe pulmonary hypertension leads to right ventricular overload and failure. Symptoms are fatigue, exertional dyspnea, and, occasionally, chest discomfort and syncope. Diagnosis is made by finding elevated pulmonary artery pressure (estimated by echocardiography and confirmed by right heart catheterization). Treatment is with pulmonary vasodilators and diuretics. In some advanced cases, lung transplantation is an option. Prognosis is poor overall if a treatable secondary cause is not found.
Pulmonary hypertension is defined as a mean pulmonary arterial pressure ≥ 25 mm Hg at rest and a normal (≤ 15 mm Hg) pulmonary artery occlusion pressure (pulmonary capillary wedge pressure) as measured by right heart catheterization.
Many conditions and drugs cause pulmonary hypertension. The most common overall causes of pulmonary hypertension are
Pulmonary hypertension is currently classified into 5 groups (see Table: Classification of Pulmonary Hypertension) based on a number of pathologic, physiologic, and clinical factors. In the first group (pulmonary arterial hypertension), the primary disorder affects the small pulmonary arterioles.
A small number of cases of pulmonary arterial hypertension occur sporadically, unrelated to any identifiable disorder; these cases are termed idiopathic pulmonary arterial hypertension. Hereditary forms of pulmonary arterial hypertension (autosomal dominant with incomplete penetrance) have been identified; 75% of cases are caused by mutations in bone morphogenetic protein receptor type 2 ( BMPR2 ). Other identified mutations include activin-like kinase type 1 receptor ( ALK-1 ), caveolin 1 ( CAV1 ), endoglin ( ENG ), potassium channel subfamily K member 3 ( KCNK3 ), and mothers against decapentaplegic homologue 9 ( SMAD9 ) but are much less common, occurring in ~1% of cases. In about 20% of cases of hereditary pulmonary arterial hypertension, the causative mutations are unidentified.
Patients with hereditary causes of hemolytic anemia, such as sickle cell disease, are at high risk of developing pulmonary hypertension (10% of cases based on right heart catheterization criteria). The mechanism is related to intravascular hemolysis and release of cell-free Hb into the plasma, which scavenges nitric oxide, generates reactive oxygen species, and activates the hemostatic system. Other risk factors for pulmonary hypertension in sickle cell disease include iron overload, liver dysfunction, thrombotic disorders, and chronic kidney disease.
Classification of Pulmonary Hypertension
Pathophysiologic mechanisms that cause pulmonary hypertension include
Increased pulmonary vascular resistance is caused by obliteration of the pulmonary vascular bed and/or by pathologic vasoconstriction. Pulmonary hypertension is characterized by variable and sometimes pathologic vasoconstriction and by endothelial and smooth muscle proliferation, hypertrophy, and chronic inflammation, resulting in vascular wall remodeling. Vasoconstriction is thought to be due in part to enhanced activity of thromboxane and endothelin-1 (both vasoconstrictors) and reduced activity of prostacyclin and nitric oxide (both vasodilators). The increased pulmonary vascular pressure that results from vascular obstruction further injures the endothelium. Injury activates coagulation at the intimal surface, which may worsen the hypertension. Thrombotic coagulopathy due to platelet dysfunction, increased activity of plasminogen activator inhibitor type 1 and fibrinopeptide A, and decreased tissue plasminogen activator activity may also contribute. Platelets, when stimulated, may also play a key role by secreting substances that increase proliferation of fibroblasts and smooth muscle cells such as platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and transforming growth factor-β (TGF-β). Focal coagulation at the endothelial surface should not be confused with chronic thromboembolic pulmonary hypertension, in which pulmonary hypertension is caused by organized pulmonary emboli.
Increased pulmonary venous pressure is typically caused by disorders that affect the left side of the heart and raise left chamber pressures, which ultimately lead to elevated pressure in the pulmonary veins. Elevated pulmonary venous pressures can cause acute damage to the alveolar-capillary wall and subsequent edema. Persistently high pressures may eventually lead to irreversible thickening of the walls of the alveolar-capillary membrane, decreasing lung diffusion capacity. The most common setting for pulmonary venous hypertension is in left heart failure with preserved ejection fraction (HF-PEF), typically in older women who have hypertension and the metabolic syndrome. When the transpulmonary gradient (mean pulmonary artery pressure to pulmonary artery occlusion pressure gradient) is > 12 mm Hg or the pulmonary artery diastolic pressure to pulmonary artery occlusion pressure gradient is > 6 mm Hg, prognosis is poor.
In most patients, pulmonary hypertension eventually leads to right ventricular hypertrophy followed by dilation and right ventricular failure. Right ventricular failure limits cardiac output during exertion.
Progressive exertional dyspnea and easy fatigability occur in almost all patients. Atypical chest discomfort and exertional light-headedness or presyncope may accompany dyspnea and indicate more severe disease. These symptoms are due primarily to insufficient cardiac output caused by right heart failure. Raynaud syndrome occurs in about 10% of patients with idiopathic pulmonary arterial hypertension; the majority are women. Hemoptysis is rare but may be fatal. Hoarseness due to recurrent laryngeal nerve compression by an enlarged pulmonary artery (ie, Ortner syndrome) also occurs rarely.
In advanced disease, signs of right heart failure may include right ventricular heave, widely split 2nd heart sound (S2), an accentuated pulmonic component (P2) of S2, a pulmonary ejection click, a right ventricular 3rd heart sound (S3), tricuspid regurgitation murmur, and jugular vein distention. Liver congestion and peripheral edema are common late manifestations. Pulmonary auscultation is usually normal. Patients also may have manifestations of causative or associated disorders.
Initial confirmation: Chest x-ray, spirometry, ECG, echocardiography, and CBC
Identification of underlying disorder: Ventilation/perfusion scan or CT angiography, high-resolution CT (HRCT) of the chest, pulmonary function testing, polysomnography, HIV testing, liver function testing, and autoantibody testing
Determination of severity: 6-min walk distance, plasma levels of N-terminal brain natriuretic peptide (BNP) or pro-BNP and right heart catheterization
Pulmonary hypertension is suspected in patients with significant exertional dyspnea who are otherwise relatively healthy and have no history or signs of other disorders known to cause pulmonary symptoms.
Patients initially undergo chest x-ray, spirometry, and ECG to identify more common causes of dyspnea, followed by transthoracic Doppler echocardiography to assess right ventricular function and pulmonary artery systolic pressures as well as to detect structural left heart disease that might be causing pulmonary hypertension. CBC is obtained to document the presence or absence of erythrocytosis, anemia, and thrombocytopenia.
The most common x-ray finding in pulmonary hypertension is enlarged hilar vessels that rapidly prune into the periphery and a right ventricle that fills the anterior airspace on lateral view. Spirometry and lung volumes may be normal or detect mild restriction, and diffusing capacity for carbon monoxide (DLco) is usually reduced. Common ECG findings include right axis deviation, R > S in V1, S1Q3T3, (suggesting right ventricular hypertrophy) and peaked P waves (suggesting right atrial dilation).
Additional tests are obtained as indicated to diagnose secondary causes that are not apparent clinically. These tests can include
Ventilation/perfusion scanning or CT angiography to detect thromboembolic disease
HRCT for detailed information about lung parenchymal disorders
Pulmonary function tests to identify obstructive or restrictive lung disease
Serum autoantibody tests (eg, antinuclear antibodies [ANA], rheumatoid factor [RF], Scl-70 [topoisomerase I], anti-Ro (anti-SSA), antiribonucleoprotein [anti-RNP], and anticentromere antibodies) to gather evidence for or against associated autoimmune disorders
Chronic thromboembolic pulmonary hypertension is suggested by CT or lung scan findings and is confirmed by arteriography. CT angiography is useful to evaluate proximal clot and fibrotic encroachment of the vascular lumen. Other tests, such as HIV testing, liver function tests, and polysomnography, are done in the appropriate clinical context.
When the initial evaluation suggests a diagnosis of pulmonary hypertension, pulmonary artery catheterization (see Monitoring and Testing the Critical Care Patient : Pulmonary Artery Catheter Monitoring) is necessary to measure right atrial, right ventricular, pulmonary artery, and pulmonary artery occlusion pressures; cardiac output; and left ventricular diastolic pressure. Right-sided O2 saturation should be measured to exclude atrial septal defect. Although finding a mean pulmonary arterial pressure of > 25 mm Hg and a pulmonary artery occlusion pressure ≤ 15 mm Hg in the absence of an underlying disorder identifies pulmonary arterial hypertension, most patients with pulmonary arterial hypertension present with substantially higher pressure (eg, mean of 60 mm Hg). Vasodilating drugs, such as inhaled nitric oxide, IV epoprostenol, or adenosine, are often given during catheterization. Decreasing right-sided pressures in response to these drugs may help in the choice of drugs for treatment. Lung biopsy, once widely done, is neither needed nor recommended because of its associated high morbidity and mortality. Echocardiography findings of right heart systolic dysfunction (eg, tricuspid annular plane systolic excursion) and certain right heart catheterization results (eg, low cardiac output, high mean pulmonary artery pressures and high right atrial pressures) indicate that pulmonary hypertension is severe. Other indicators of severity in pulmonary hypertension are assessed to evaluate prognosis and to help monitor responses to therapy. They include a low 6-min walk distance and high plasma levels of N-terminal pro-brain natriuretic peptide (NT-pro-BNP) or brain natriuretic peptide (BNP).
Once pulmonary hypertension is diagnosed, the patient's family history should be reviewed to detect possible genetic transmission (eg, premature deaths in otherwise healthy members of the extended family). In familial pulmonary arterial hypertension, genetic counseling is needed to advise mutation carriers of the risk of disease (about 20%) and to advocate serial screening with echocardiography. Testing for mutations in the BMPR2 gene in idiopathic pulmonary arterial hypertension can help identify family members at risk.
Five-year survival for treated patients is about 50%. However, recent findings in some patient registries suggest lower mortality (eg, 20 to 30% at 3 to 5 yr in the French registry and 10 to 30% at 1 to 3 yr in the REVEAL registry), presumably because currently available treatments are superior. Indicators of a poorer prognosis include lack of response to vasodilators, hypoxemia, reduced overall physical functioning, low 6-min walk distance, high plasma levels of NT-pro-BNP or BNP, echocardiographic indicators of right heart systolic dysfunction (eg, tricuspid annular plane systolic excursion) and right heart catheterization showing low cardiac output, high mean pulmonary artery pressures, and high right atrial pressures. Patients with systemic sclerosis, sickle cell anemia, or HIV infection with pulmonary arterial hypertension have a worse prognosis than those without pulmonary arterial hypertension. For example, patients with sickle cell disease and pulmonary hypertension have a 40% 4-yr mortality rate.
Avoidance of activities that may exacerbate the condition (eg, cigarette smoking,high altitude, pregnancy, use of sympathomimetics)
Idiopathic and familial pulmonary arterial hypertension: IV epoprostenol; inhaled, oral, or sc prostacyclin analogs; oral endothelin-receptor antagonists; oral phosphodiesterase 5 inhibitors, and/or soluble guanylate cyclase stimulators
Secondary pulmonary arterial hypertension: Treatment of the underlying disorder
Adjunctive therapy: Supplemental O2, diuretics, and/or anticoagulants
Treatment is rapidly evolving.
IV epoprostenol, a prostacyclin analog, improves function and lengthens survival even in patients who are unresponsive to a vasodilator during catheterization. Epoprostenol is currently the most effective therapy for pulmonary arterial hypertension. Disadvantages are the need for continuous central catheter infusion and frequent, troubling adverse effects, including flushing, diarrhea, and bacteremia associated with the indwelling central catheter. Prostacyclin analogs that are inhaled (iloprost and treprostinil) or given sc or IV (treprostinil) are available.
Three oral endothelin-receptor antagonists, bosentan, ambrisentan, and macitentan, are now available. Sildenafil and tadalafil, oral phosphodiesterase 5 inhibitors, can also be used. Riociguat is the first available soluble guanylate cyclase stimulator. Drugs improve exercise capacity and reduce composite endpoints of clinical worsening, often defined by hospitalization for right heart failure. No studies have compared oral drugs to each other. Most patients prefer to begin treatment with an oral drug, adding a 2nd oral drug if necessary based on clinical response. Exercise capacity is improved if the second drug is from a different class (endothelin-receptor antagonist or phosphodiesterase 5 inhibitor). However, phosphodiesterase 5 inhibitors cannot be combined with riociguat because both drug classes increase cyclic guanosine monophosphate (cGMP) levels and the combination can lead to dangerous hypotension. Patients with severe right heart failure who are at high risk of sudden death may benefit from early therapy with an intravenous or subcutaneous prostacyclin analog.
Lung transplantation offers the only hope of cure but has high morbidity because of rejection (bronchiolitis obliterans syndrome) and infection. The 5-yr survival rate is 50%. Lung transplantation is reserved for patients with New York Heart Association class IV disease (defined as dyspnea associated with minimal activity, leading to bed to chair limitations) or complex congenital heart disease in whom all therapies have failed and who meet other health criteria to be a transplant candidate.
Many patients require adjunctive therapies to treat heart failure, including diuretics, and most should receive warfarin unless there is a contraindication.
Primary treatment involves management of the underlying disorder. Patients with left-sided heart disease may need surgery for valvular disease. Patients with lung disorders and hypoxia benefit from supplemental O2 as well as treatment of the primary disorder. Treatments for patients with severe pulmonary hypertension secondary to chronic thromboembolic disease include riociguat and surgical pulmonary thromboendarterectomy. During cardiopulmonary bypass, an organized endothelialized thrombus is dissected along the pulmonary vasculature in a procedure more complex than acute surgical embolectomy. This procedure cures pulmonary hypertension in a substantial percentage of patients and restores cardiopulmonary function; operative mortality is < 10% in patients treated in centers that have extensive experience.
Patients with sickle cell disease who have pulmonary hypertension are aggressively treated using hydroxyurea, iron chelation, and supplemental O2 as indicated. In patients with pulmonary arterial hypertension and elevated pulmonary vascular resistance confirmed by right heart catheterization, selective pulmonary vasodilator therapy (with epoprostenol or an endothelin-receptor antagonist) can be considered. Sildenafil increases incidence of painful crises in patients with sickle cell disease and so should be used only if patients have limited vaso-occlusive crises and are being treated with hydroxyurea or transfusion therapy.
Pulmonary hypertension is classified into 5 groups.
Suspect pulmonary hypertension if patients have dyspnea unexplained by another clinically evident cardiac or pulmonary disorder.
Begin diagnostic testing with chest x-ray, spirometry, ECG, and transthoracic Doppler echocardiography.
Confirm the diagnosis by right heart catheterization.
Treat group 1 by giving pulmonary vasodilators and, if these are ineffective, considering lung transplantation.
Treat groups 2 to 5 by managing the underlying disorder, treating symptoms, and sometimes other measures.
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