INTRODUCTION AND EPIDEMIOLOGY
The pulmonary vascular system is normally a high-flow, low-resistance circuit, with a mean pulmonary arterial pressure that constitutes approximately 15% to 20% of the systemic circulation.1 Normal pulmonary arterial systolic pressures range from 15 to 30 mm Hg, and diastolic pulmonary arterial pressures range from 4 to 12 mm Hg.1 Pulmonary hypertension exists when a mean pulmonary arterial pressure is >25 mm Hg at rest or >30 mm Hg during exertion.1,2
Pulmonary hypertension classification uses measurements of pulmonary arterial pressure, pulmonary vascular resistance, and pulmonary capillary wedge pressure (Table 58-1).1,3 Although echocardiography estimates pulmonary arterial pressure in a patient with suspected pulmonary hypertension, definitive diagnosis requires right heart catheterization.
TABLE 58-1World Health Organization Classification of Pulmonary Hypertension ||Download (.pdf) TABLE 58-1 World Health Organization Classification of Pulmonary Hypertension
Group 1: Pulmonary Arterial Hypertension
Drug or toxin induced
Associated with known risk factors for pulmonary arterial hypertension (human immunodeficiency virus, portal hypertension, pulmonary veno-occlusive disease, collagen vascular disorders)
Group 2: Pulmonary Venous Hypertension (left heart disease)
Group 3: Chronic Hypoxemic Lung Disease
Obstructive lung disorders (chronic obstructive pulmonary disease)
Interstitial lung disease
Sleep-disordered breathing (obstructive sleep apnea)
Chronic exposure to high altitude
Group 4: Chronic Thromboembolic Pulmonary Hypertension
Group 5: Miscellaneous
Hematologic disorders: myeloproliferative disorders
Systemic disorders: sarcoidosis, neurofibromatosis
Metabolic disorders: glycogen storage disease, thyroid disorders
Patients with group 1 pulmonary arterial hypertension have a mean pulmonary arterial pressure >25 mm Hg, a pulmonary vascular resistance >240 dynes/s/cm5, and a pulmonary capillary wedge pressure <15 mm Hg.2 Group 2 disease is caused by left heart disease and is the most common etiology.2 Group 3 occurs with chronic hypoxemic lung disease. Chronic thromboembolic pulmonary hypertension, group 4, develops in up to 4% of patients with thromboembolic disease.4-6 Regardless of the cause, pulmonary hypertension is associated with high rates of morbidity and mortality,4,7 with a 5-year death rate for patients with idiopathic pulmonary arterial hypertension exceeding 30%.8
Pulmonary arterial hypertension consists of vascular remodeling in all layers of small and mid-sized pulmonary arterioles, plus inflammation and in situ thrombosis formation.9 Additional pathology includes alterations in microvascular permeability, hypoxic vasoconstriction, and plexiform lesion formation in arteriolar walls. The cumulative effect of these changes is sustained elevations of pulmonary vascular resistance and impaired pulmonary blood flow.
With persistent elevations in pulmonary vascular resistance, the right ventricle (RV) dilates. RV dilation leads to increased ventricular wall tension during systole, increased oxygen consumption, and eventually decreased contractility. With progressive RV dilation, the intraventricular septum displaces toward the left ventricle, which inhibits left ventricular filling and ultimately ...