Chapter 58: Pulmonary Hypertension

INTRODUCTION AND EPIDEMIOLOGY

Normally, the pulmonary vascular system is 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, whereas diastolic pulmonary arterial pressures range from 4 to 12 mm Hg.¹ Pulmonary hypertension is defined as a mean pulmonary arterial pressure >25 mm Hg at rest or >30 mm Hg during exertion.^1,2

Pulmonary hypertension is classified based on measurements of pulmonary capillary wedge pressure (PCWP) and pulmonary vascular resistance. Patients with pulmonary arterial hypertension have a mean pulmonary arterial pressure >25 mm Hg, a pulmonary vascular resistance >240 dynes/s/cm⁵, and a PCWP <15 mm Hg.² In contrast, patients with pulmonary hypertension caused by left heart disease, the most common cause of pulmonary hypertension, have a PCWP >15 mm Hg.² Although echocardiography can estimate pulmonary arterial pressure in a patient with suspected pulmonary hypertension, definitive diagnosis requires right heart catheterization.

The World Health Organization classifies pulmonary hypertension into five categories based on cause and response to treatment (Table 58-1).^1,3 Some patients have features of multiple categories, but the majority have one predominant type of pulmonary hypertension.⁴ Accurate classification of pulmonary hypertension is key to directing treatments, which vary among the categories. Pulmonary venous hypertension is the most common cause of pulmonary hypertension, affecting almost 4 million patients in the United States.⁴ In contrast, pulmonary arterial hypertension is the least common cause of pulmonary hypertension, with an estimated prevalence of 15 patients per million.^4,5 Pulmonary hypertension develops in up to 4% of patients with chronic thromboembolic disease.^4,6,7 Regardless of the cause, patients with pulmonary hypertension have high morbidity and mortality rates,^4,8 with a 5-year mortality rate for patients with idiopathic pulmonary arterial hypertension exceeding 30%.⁵

PATHOPHYSIOLOGY

The exact pathophysiology of all forms of pulmonary hypertension remains unknown. In pulmonary arterial hypertension, the initial abnormality is endothelial dysfunction, which results in an imbalance ...