Pulmonary Arterial Hypertension (PAH) is a serious, progressive condition characterized by high blood pressure in the arteries of the lungs, which ultimately restricts blood flow and can lead to right-sided heart failure. Because PAH symptoms often relate to a patient’s ability to perform physical tasks, doctors need a consistent way to measure the disease’s impact on daily life. The World Health Organization Functional Classification (WHO FC) system serves as the globally accepted standard tool for assessing a patient’s level of functional impairment and monitoring the disease’s progression over time. This classification provides a simple, four-tiered scale to quantify the severity of symptoms experienced by individuals living with PAH.
The Purpose of Functional Classification
The WHO Functional Classification system standardizes the assessment of PAH severity based on the patient’s reported symptoms and physical limitations during routine activities. This classification is subjective, relying on the patient’s description of their comfort level and tolerance for physical exertion. It is distinct from objective measurements, such as the 6-Minute Walk Test, but both tools are used together to provide a complete clinical picture.
Assigning a functional class gives physicians a common language to communicate a patient’s status consistently across different healthcare settings and countries. This shared metric provides a baseline for tracking changes, whether the disease is worsening or improving. The classification helps establish a clear picture of how the condition affects daily life, which is a major goal of PAH treatment.
Detailed Breakdown of the Four Classes
The WHO FC system is a four-point index, adapted from the New York Heart Association (NYHA) classification used for heart failure. A higher class indicates more severe disease and greater functional impairment, focusing on the patient’s experience of shortness of breath, fatigue, chest pain, or near fainting in relation to physical activity.
WHO Functional Class I represents the mildest form, where patients have PAH but experience no resulting limitation of physical activity. Ordinary physical activity does not cause undue symptoms. Diagnosis in this class is rare. Class II patients are comfortable at rest but experience a slight limitation of physical activity, where ordinary activities, such as climbing stairs, cause symptoms like shortness of breath or fatigue.
In WHO Functional Class III, patients are comfortable at rest but have a marked limitation of physical activity. Less than ordinary activity results in undue symptoms, meaning they may struggle with normal chores and require breaks. Class IV represents the most severe functional impairment, where patients cannot carry out any physical activity without symptoms. These patients often experience dyspnea or fatigue while at rest and may manifest signs of right heart failure.
Classification and Treatment Decisions
The WHO Functional Classification is a primary factor in determining the appropriate treatment strategy for a patient with PAH. Patients in lower classes (I or II) have a significantly better prognosis and survival rate compared to those in higher classes (III or IV). This classification influences the choice and intensity of medication, such as whether to initiate oral therapy or more aggressive intravenous or subcutaneous prostacyclin treatment.
Patients diagnosed in Class III or IV are considered high risk and may be candidates for initial combination therapy with multiple drugs. A change in functional class over time is a key metric used to assess the efficacy of the current treatment regimen. If a patient deteriorates and moves to a higher class, doctors will often escalate or change the medication to improve their functional status.
The goal of treatment is typically to move a patient into a lower-risk status, ideally Class I or II. The functional class is a component in multiparameter risk calculators used for prognosis. Monitoring the WHO FC, along with parameters like the 6-Minute Walk Distance and blood biomarkers, allows the medical team to make informed, timely decisions to stabilize the disease. The WHO FC must be assessed frequently, typically every three to six months during stable disease, or whenever there is a change in treatment.