Table 1. World Health Organization functional classification (WHO FC) definitions.

Class	Definition
WHO FC I	Patients with PH but without resulting limitation of physical activityOrdinary physical activity does not cause undue dyspnoea or fatigue, chest pain or near syncope
WHO FC II	Patients with PH resulting in slight limitation of physical activityThey are comfortable at restOrdinary physical activity causes undue dyspnoea or fatigue, chest pain or near syncope
WHO FC III	Patients with PH resulting in marked limitation of physical activityThey are comfortable at restLess than ordinary activity causes undue dyspnoea or fatigue, chest pain or near syncope
WHO FC IV	Patients with PH with inability to carry out any physical activity without symptomsThese patients manifest signs of right-heart failureDyspnoea and/or fatigue may even be present at restDiscomfort is increased by any physical activity

PH: pulmonary hypertension.