Pulmonary hypertension (PH) is characterized by abnormally high blood pressure in the arteries of the lungs, which carry blood from the right side of the heart. This elevated pressure forces the heart to work harder to pump blood, often leading to the enlargement and eventual failure of the right ventricle. Because PH can stem from many different issues, doctors rely on specific classification and grading systems to determine the condition’s origin, monitor its severity, and guide effective treatment plans.
Diagnostic Thresholds for Pulmonary Hypertension
The initial diagnosis of PH is based on precise hemodynamic measurements obtained through right heart catheterization. This invasive test involves guiding a thin tube into the pulmonary artery to directly measure the pressure inside the vessels. The mean pulmonary arterial pressure (mPAP) represents the average pressure exerted in the pulmonary arteries.
The traditional threshold for diagnosing PH was an mPAP of 25 millimeters of mercury (mmHg) or higher at rest. However, the definition has been revised to an mPAP greater than 20 mmHg. This lower threshold aims to identify patients earlier in the disease course, as pressures above 20 mmHg are associated with increased risk.
Pulmonary vascular resistance (PVR) is often used to further characterize the disease. PVR measures the resistance to blood flow through the pulmonary circulation, and a value greater than 2 Wood units (WU) is currently used as a secondary criterion to define pre-capillary PH.
The Five Major Etiological Groups
The World Health Organization (WHO) classification system divides PH into five distinct groups based on the underlying cause, as the treatment approach differs significantly for each type. Delineating the specific group is fundamental, as it directs the diagnostic workup and determines which therapies are appropriate.
- Group 1: Pulmonary Arterial Hypertension (PAH). This is caused by structural changes in the small pulmonary arteries, making them narrowed or thickened. It includes idiopathic PAH, heritable forms, or PAH associated with connective tissue diseases and HIV.
- Group 2: PH due to left heart disease. When the left side of the heart is weak or stiff, blood backs up into the lungs, raising pressure in the pulmonary veins and arteries. This is the largest and most common category.
- Group 3: PH caused by lung diseases and/or chronic low oxygen levels. This includes conditions such as Chronic Obstructive Pulmonary Disease (COPD), interstitial lung disease, and sleep apnea.
- Group 4: Chronic Thromboembolic Pulmonary Hypertension (CTEPH). This results from organized blood clots that fail to dissolve and permanently block the pulmonary arteries, creating a physical obstruction.
- Group 5: A heterogeneous category for PH with unclear or multifactorial mechanisms, including various hematologic disorders, systemic diseases like sarcoidosis, or metabolic disorders.
Assessing Symptom Severity
Doctors assess how severely PH limits a patient’s daily life using the WHO Functional Classification system, which is adapted from the New York Heart Association (NYHA) scale. This grading system focuses exclusively on the patient’s symptoms and their ability to perform physical activities, providing a measure of disease impact across four classes.
- Class I: Patients experience no limitation of physical activity. They do not experience undue shortness of breath, fatigue, chest pain, or near fainting during ordinary physical activity.
- Class II: Patients have a slight limitation of physical activity. They remain comfortable at rest but experience symptoms during ordinary activities like climbing stairs or grocery shopping.
- Class III: Patients have a marked limitation of physical activity. They are comfortable at rest but may have difficulty completing simple chores around the house without needing to take breaks.
- Class IV: This represents the most severe stage. Patients are unable to carry out any physical activity without symptoms, and discomfort or shortness of breath is often present even at rest. They may also show signs of right heart failure.
How Grading Informs Treatment Decisions
The combination of the etiological group and the functional class is used to create a comprehensive risk stratification that guides treatment decisions. Treatment for Group 1 PAH is highly specific, often involving targeted pulmonary vasodilators that help relax and open the narrowed blood vessels. Conversely, for Group 2 or Group 3 PH, the primary strategy is to manage and treat the underlying heart failure or chronic lung disease, as the PH is a consequence of that primary condition.
The functional class directly dictates the urgency and aggressiveness of medical intervention, especially for Group 1 PAH. Patients in Classes I and II are generally considered lower risk and may be managed with less intensive therapy. Patients presenting in Classes III or IV are at a higher risk of mortality and typically require immediate, often combination, therapy to quickly improve their functional status and prognosis. The ability to accurately grade both the cause and the severity of symptoms allows doctors to select the most appropriate therapeutic pathway and ultimately improve long-term outcomes.