Pulmonary Arterial Hypertension (PAH)

Transcription

1 Pulmonary Arterial Hypertension (PAH)

2 Contents Introduction 2 What is PAH? 3 Classifi cation of PAH 5 How common is PAH? 8 Why does PAH develop? 9 What are the symptoms of PAH? 12 How is PAH diagnosed? 13 How is PAH severity classifi ed? 20 How is PAH treated? 21 References 23 1

3 Introduction Pulmonary arterial hypertension (PAH) is a serious disease of the arteries connecting the heart to the lungs (the pulmonary arteries). As PAH develops, blood fl ow through the pulmonary arteries is restricted and the right side of the heart is put under increasing strain to pump blood through to the lungs. This leads to the main symptoms of PAH - breathlessness, chest tightness limited exercise capacity and fatigue. Untreated, PAH is a disease with a very poor prognosis. The early symptoms of PAH (such as breathlessness, chest tightness and fatigue) can be mild and are common to many other conditions. Reaching the diagnosis can be delayed and as a consequence patients with PAH may fi rst present when the disease has already progressed. This brochure explains how PAH develops, describes the symptoms associated with this disease and outlines how it can be diagnosed and treated. Treatment is aimed at improving symptoms, exercise tolerance, long-term outcomes and quality of life. Until the mid 1980s there were limited treatment options for patients and PAH was associated with poor prognosis. Today, treatment options have improved the prognosis for patients with this condition. 2

4 What is PAH? Pulmonary Arterial Hypertension (PAH) is a syndrome characterised by a progressive increase in pulmonary vascular resistance leading to right ventricular overload and eventually to right ventricular failure and premature death. 1 The increase in pulmonary vascular resistance is related to a number of progressive changes in the pulmonary arterioles, including: vasoconstriction obstructive remodelling of the pulmonary vessel wall through proliferation in the various layers of the blood vessel wall (smooth muscle cell and endothelial cell proliferation) infl ammation in-situ thrombosis. The main histological features include medial hypertrophy, intimal thickening, adventitial thickening, plexiform lesions and in-situ thrombosis (Figure 1). The plexifom lesion represents a focal proliferation of endothelial and smooth muscle cells and is pathognomonic of PAH. 3

5 Figure 1 PAH is defi ned as a sustained elevation of mean pulmonary arterial pressure to more than 25 mmhg at rest or to more than 30 mmhg while exercising, with a normal pulmonary wedge pressure (< 15 mmhg). 1 In most cases the earliest symptom is dyspnoea on physical exertion. Other symptoms include syncope or near syncope, fatigue and peripheral oedema. 1 Chest tightness and pain similar to angina may occur, particularly on physical exertion. 4

6 Classification of PAH Pulmonary Arterial Hypertension (PAH) represents Group 1 within the Pulmonary Hypertension WHO clinical classifi cation system (Venice 2003 revision) and is one of fi ve such groups. The groups are divided based on aetiology. 2 Group I. Group II. Group III. Group IV. Group V. Pulmonary arterial hypertension (PAH) Idiopathic (IPAH) Familial (FPAH) Associated with (APAH): - Connective tissue disease - Congenital systemic-to-pulmonary shunts - Portal hypertension - HIV infection - Drugs and toxins - Other (thyroid disorders, glycogen storage disease, Gaucher s disease, hereditary haemorragic telangiectasia, haemoglobinopathies, myeloproliferative disorders, splenectomy) Associated with signifi cant venous or capillary involvement - Pulmonary veno-occlusive disease (PVOD) - Pulmonary capillary haemangiomatosis (PCH) Persistent pulmonary hypertension of the newborn (PPHN) Pulmonary hypertension associated with left heart diseases Pulmonary hypertension associated with respiratory diseases and/or hypoxemia (including chronic obstructive pulmonary disease) Pulmonary hypertension due to chronic thrombotic and/or embolic disease Miscellaneous group eg. sarcoidosis, histiocytosis X and lymphangiomatosis 5

7 Idiopathic PAH (IPAH), which by defi nition has no identifi able underlying cause, is one of the more common types of PAH. Familial PAH (FPAH) accounts for at least 6% of cases of IPAH and mutations in the bone morphogenetic protein receptor 2 (BMPR2) have been identifi ed in the majority of cases of FPAH. 3,4 PAH can also be associated with a number of conditions (Associated Pulmonary Arterial Hypertension - APAH), which together account for most other cases of PAH. These conditions include; 1. Connective Tissue Diseases including systemic sclerosis (scleroderma) and systemic lupus erythematosus (SLE) 2. Congenital Heart Disease including Eisenmenger s syndrome 3. Human immunodeficiency virus (HIV) infection 4. Sickle Cell Disease 6

8 PAH is also a rare side effect of certain anorexigenic agents, such as fenfl uramine and dexfenfl uramine. However, the incidence of drug-induced PAH is decreasing as these agents are no longer available. 1. PAH associated with connective tissue disease PAH is a well-recognised complication of connective tissue diseases such as systemic sclerosis and SLE and in affected patients may also occur in association with interstitial lung disease. The prevalence of PAH in patients with connective tissue disease has been reported to be up to 38% 5 and in systemic sclerosis patients, pulmonary complications, such as interstitial lung disease and PAH, are now the leading causes of death. Patients with PAH associated with systemic sclerosis have a particularly poor prognosis compared to those with systemic sclerosis without PAH PAH associated with congenital heart disease Congenital heart disease is relatively common, affecting around 1% of the population. Within this population 10% will go on to develop PAH. 7 As determined by the level of pulmonary vascular resistance, the most severe form of PAH is Eisenmenger s syndrome, which is associated with the reversal of an initial left to right shunt causing cyanosis and limited exercise capacity PAH associated with HIV infection PAH is a rare (estimated prevalence in patients with HIV: 0.5% 9 ) but relatively well-documented complication of HIV infection. With the advent of highly active anti-retroviral therapy (HAART) and markedly improved survival, PAH and other non-infectious manifestations of HIV infection are increasingly responsible for HIV-associated morbidity and poor prognosis. In patients with HIV, the HIV-1 envelope glycoprotein GP120 may stimulate the production of endothelin by macrophages. 10 HIV-associated PAH shows a similar clinical picture to IPAH and seems to be independent of the degree of immunosuppression. 7

9 4. PAH associated sickle cell disease Sickle cell disease (SCD) is a genetic disorder of the haemoglobin which results in a chronic haemolytic anaemia. Over time and due to the chronic haemolysis the vasculature becomes damaged and pulmonary hypertension can develop. The prevalence of PAH in sickle cell patients is 20-40%. 11 The two year mortality of pulmonary hypertension related to SCD is reported up to 50% and is one of the leading causes of death in SCD. 12 How common is PAH? Although PAH is a rare disease, with an estimated prevalence of cases per million, 13 the prevalence of PAH in certain at-risk groups is substantially higher. For example, in HIV-infected patients the prevalence is 0.5% 9, in patients with sickle cell disease the prevalence is 20-40% 11 and in patients with systemic sclerosis the prevalence has been reported to be up to 38%. 5 Idiopathic PAH is more common in young women with a mean age of diagnosis of 36 years. However it can occur at any age. 1 Idiopathic PAH is twice as common in women as in men. 1,3 Although the true relative prevalence of IPAH, FPAH and APAH are unknown, it is likely that IPAH accounts for at least 40% of cases, and APAH for most of the remaining cases. 14,15 Due to the non-specifi c nature of the symptoms, PAH is most frequently diagnosed when patients have reached an advanced stage of disease (WHO Functional Class III and IV), 15 suggesting that the true prevalence may be higher than documented in the literature. 8

10 Why does PAH develop? The exact cause behind the development of PAH remains unknown. However, research has led to a better understanding of the underlying mechanisms. PAH is recognised as a complex, multi-factorial condition involving numerous biochemical pathways and different cell types. Endothelial dysfunction is believed to occur early on in disease pathogenesis, leading to endothelial and smooth muscle cell proliferation and structural changes or remodelling of the pulmonary vascular bed resulting in an increase in pulmonary vascular resistance. Vascular remodelling itself involves all layers of the vessel wall and is characterised by proliferative and obstructive changes involving many cell types, including endothelial, smooth muscle and fi broblasts. Infl ammatory cells and platelets may also play a signifi cant role in PAH. Endothelial cell dysfunction results in reduced production of vasodilators, such as nitric oxide (NO) and prostacyclin, and over production of vasoconstrictors, such as thromboxane A 2 and endothelin-1 (ET-1). ET-1, NO and prostacyclin have been the principal focus of research into new treatment options for patients with PAH. 9

11 Endothelin Endothelin is produced by the endothelial cells and is essential for maintenance of normal vascular tone and function. However, high levels of endothelin are seen in patients with PAH due to various aetiologies and correlate with disease severity, 19 resulting in a number of detrimental effects, primarily in the vasculature 1 : Fibrosis Hypertrophy and proliferation of cells, which can lead to thickening, narrowing and occlusion of blood vessels. Inflammation Vasoconstriction Endothelin binds to 2 receptors, ETA and ETB. Both receptors are implicated in PAH and mediate the deleterious effects of endothelin 20. Endothelin receptor antagonism can either mitigate the effects of only one (single ETA antagonist) or both (dual ETA and ETB receptor antagonist) receptor types. 10

12 Prostacyclin Prostacyclin is a potent vasodilator as well as an inhibitor of platelet activation. It is believed that patients with PAH have low levels of prostacyclin, which could result in vasoconstriction in the pulmonary vasculature and a tendency for smooth muscle cell proliferation and platelet activation, encouraging the formation of thrombi in both the micro-circulation and the pulmonary arteries Therapy with synthetic forms of prostacyclin can help to correct this defi ciency, although administering this form of treatment is complex Nitric oxide Nitric oxide is an endothelial-derived substance that, like prostacyclin, is a potent vasodilator and also possesses anti-proliferative properties. PAH patients appear to produce insuffi cient NO and this may contribute to the development of PAH. 1 The vasodilatory effect of NO is mediated by cgmp, which is rapidly degraded by phosphodiesterases. The inhibition of the degradation of cgmp with phosphodiesterase 5 inhibitors promotes the accumulation of intracellular cgmp, resulting in vasodilatation

13 What are the symptoms of PAH? The changes to the pulmonary vasculature described above lead to the typical symptoms of PAH. The symptoms are caused by the high resistance to blood fl ow through the lungs and result in increased stress on the heart. This can severely impact the ability to exercise or carry out normal daily activities. Common early symptoms include: Breathlessness (dyspnoea), particularly on physical activity Fatigue Dizziness Syncope, also on physical activity (rare) Peripheral oedema Chest pain, again particularly during physical activity. The symptoms may not be obvious at fi rst and are often attributed to more common conditions such as asthma, general fatigue, or lack of physical fi tness. Over time, however, they can become more severe and begin to limit normal activities. As the disease progresses, some patients may experience constant dyspnoea and fatigue so that even simple tasks such as getting dressed and walking short distances become diffi cult. 12

14 How is PAH diagnosed? The early symptoms of PAH (such as dyspnoea, dizziness and fatigue) are often mild and are common to many other conditions. At rest there are often no symptoms and no apparent signs of illness. As a result, diagnosis can be delayed for months or even years meaning that PAH is frequently not recognised until the disease is relatively advanced. 1 PAH is often diagnosed only once other conditions have been investigated and ruled out. The non-specifi c nature of symptoms associated with PAH means that the diagnosis cannot be made on symptoms alone. A series of investigations is required to make an initial diagnosis, to refi ne that diagnosis in terms of clinical class of pulmonary hypertension and to evaluate the degree of functional and haemodynamic impairment (Figure 2). Consequently, it can be useful to adopt a four stage approach: 13

15 1. Clinical suspicion of pulmonary hypertension Breathlessness (dyspnoea) without overt signs of specifi c heart or lung disease Screening of patients with associated conditions (Connective Tissue Disease, Congenital Heart Disease, HIV, Sickle Cell Disease) Incidental fi ndings on examination for other clinical reasons 2. Detection of pulmonary hypertension ECG (echocardiogram) Doppler echocardiogram (Figure 2) Chest radiograph, may show evidence of cardiomegaly and enlarged pulmonary arteries (Figure 3) 3. Identify other causes of pulmonary hypertension Pulmonary function tests (PFTs) and arterial blood gas samples Ventilation and perfusion lung scan High resolution computed tomography (HRCT) Pulmonary angiography 4. PAH evaluation and classification (type, functional capacity, haemodynamics) Blood tests and immunology, HIV test, abdominal ultrasound scan 6 minute walk test (6-MWT) and peak VO 2 Right heart catheterisation and vasoreactivity testing 14

16 Figure 2 Figure 3 15

17 Echocardiography - value as a screening tool Transthoracic Doppler-echocardiography (TTE) is a non-invasive screening test for pulmonary hypertension. TTE is able to estimate pulmonary arterial systolic pressure, which is equivalent to right ventricular systolic pressure in the absence of pulmonary outfl ow obstruction. It can provide additional information about the cause and consequences of PH, including right and left ventricular dimensions and function, heart valve abnormalities, right ventricular ejection and left ventricular fi lling characteristics and presence of a pericardial effusion. In the initial investigation of patients with PAH it is important to obtain adequate images of the right heart. Pulmonary arterial pressure can be estimated from the tricuspid regurgitant (TR) jet (Figure 4). Figure 4 16

18 Right heart catheterisation - the diagnostic gold standard Right heart catheterisation is required for a defi nitive diagnosis of PAH (Figure 5 and 6) 1,26 to assess the severity of haemodynamic impairment and to test the vasoreactivity of the pulmonary circulation. The following parameters should always be assessed: right atrial pressure (RAP), pulmonary arterial pressure (PAP [systolic, diastolic and mean]), pulmonary capillary wedge pressure (PCWP), cardiac output / index, pulmonary (PVR) and systemic vascular resistance, blood pressure and arterial and mixed venous oxygen saturation. PAH is defi ned as a sustained elevation of mean pulmonary arterial pressure to > 25 mmhg at rest or to > 30 mmhg while exercising, with a mean pulmonary wedge pressure or left ventricular end-diastolic pressure of < 15 mmhg and pulmonary vascular resistance of 3 woods units. 1 A positive vasoreactive response is defi ned as a reduction in mean pulmonary artery pressure (mpap) 10 mmhg to reach an absolute value of mpap 40 mmhg with an increase or unchanged cardiac output. A positive response is shown in only 10-15% of patients, and sustained response is shown in even fewer (less than 7%)

19 Figure 5 Figure 6 18

20 6-minute walk test - evaluation of exercise capacity In patients with PAH, the 6-MWT to evaluate exercise capacity is refl ective of activities of daily living; 28 the distance a PAH patient can walk in 6 minutes is a critical endpoint in studies evaluating the benefi t of different therapeutic options. To allow meaningful comparisons, it is important that the 6-MWT be performed under supervision according to a standardised protocol. 29 A 30 m corridor should be available, marked at 3 m intervals The patient should rest for at least 10 minutes prior to the test and should not have performed any rigorous exercise within the previous 2 hours The patient should be asked to rate their baseline dyspnoea The patient should be instructed to walk to their maximum capacity but not to run or jog; they should be permitted to rest as necessary The supervisor should count each lap as the patient fi nishes it The patient should be asked to rate their dyspnoea at the end of the test The test should be repeated at approximately the same time of day on each occasion 19

21 How is PAH severity classified? Once diagnosed, PAH is usually classifi ed according to a functional class system initially devised by the New York Heart Association (NYHA) for Chronic Heart Failure and then adapted for PAH by the World Health Organization (WHO). This functional class measures the severity of PAH and refl ects the impact on a patient s life in terms of physical activity and symptoms. There are four classes, with class I being the least severe and class IV being the most advanced (Table 1). 26 Table 1. NYHA/WHO Classifi cation of Functional Status of Patients With Pulmonary Hypertension 26 Class Class I Class II Class III Class IV Symptomatic profile Patients with pulmonary hypertension but without resulting limitation of physical activity. Ordinary physical activity does not cause dyspnoea or fatigue, chest pain or near syncope. Patients with pulmonary hypertension resulting in slight limitation of physical activity. They are comfortable at rest. Ordinary physical activity causes undue dyspnoea or fatigue, chest pain or near syncope. Patients with pulmonary hypertension resulting in marked limitation of physical activity. They are comfortable at rest. Less than ordinary activity causes undue dyspnoea or fatigue, chest pain or near syncope. Patients with pulmonary hypertension with inability to carry out any physical activity without symptoms. These patients manifest signs of right heart failure. Dyspnoea and/or fatigue may even be present at rest. Discomfort is increased by any physical activity. 20

22 How is PAH treated? There is currently no cure for PAH but advances in understanding how the disease develops (see section: Why does PAH develop?) means that there are now treatments available which have helped to improve prognosis for patients with this disease. The main treatment options for patients with PAH are: 26 Treatments that are routinely used but with little evidence of a positive impact on the disease progression. anticoagulants, such as warfarin, to address the observed thrombotic changes and potential predisposition in the pulmonary microcirculation for in-situ thrombosis. calcium-channel blockers (CCBs). Less than 10% of IPAH patients benefi t from CCB therapy. This fi gure is even lower in other forms of PAH. If not used in appropriate candidates (patients with demonstrated vasoreactivity during right heart catheterisation), CCBs can decrease cardiac output and systemic vascular resistance without any improvement in PAP and PVR and therefore may be deleterious. 27 diuretics, for treatment of right heart failure. oxygen therapy, to maintain oxygen saturation at > 90% at all times. 21

23 Treatments that have been specifically studied in PAH endothelin receptor antagonists - endothelin is implicated in the pathogenesis of PAH through actions on the pulmonary vasculature. Endothelin is found to be elevated in patients with PAH and levels of endothelin are directly related to disease severity and prognosis. Endothelin receptor antagonists (ERAs) are oral treatments that either block the ETA receptor alone or both the ETA and ETB receptors. 30,31 phosphodiesterase 5 inhibitors - Oral agents which induce relaxation and antiproliferative effects on vascular smooth muscle cells by preventing the reduction in levels of cgmp. prostacyclin analogues - may be delivered by continuous intravenous or subcutaneous infusion or via an intermittent nebuliser. To date, oral agents have shown limited effectiveness. In very severe cases surgical options may be considered: balloon atrial septostomy heart and lung transplantation However, the use of transplantation is constrained by the limited number of donor organs. 22

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