Number of subjects with changes in Epoprostenol dose from baseline maintained continuously for 6 months. Increased = Epoprostenol dose continuously more than 20% greater than core study Baseline for at least 6 months. Decrease = Epoprostenol dose continuously more than 20% less than core study Baseline for at least 6 months. No change = Epoprostenol dose change met neither Increase or Decrease criteria. Stopped = Epoprostenol dose stopped for at least 6 months.

Safety population: all subjects who took at least one dose of study medication in core study. N=number of subjects with evaluable data at core study Baseline. Includes subjects who were only treated in the core study and those who continued into the extension study.

Yearly survival status: number of subjects who survived, discontinued, and died. Analysis includes post-treatment visit data from subjects who discontinued study treatment. Time to death was taken relative to the first dose of study treatment in A1481141, and was censored on the last day the subject was known to be alive in A1481141 or A1481153.

Full analysis set: all randomized and treated subjects recruited into core study. N=number of subjects with evaluable data at core study Baseline.

Number of subjects with categorized change in 6-minute walking distance. Distance that a subject could walk in 6-minutes at a comfortable pace with as many breaks as needed. Performed as close to trough levels of sildenafil as possible (just before dosing; at least 4 hours after the previous dose of study drug). Scores for categorized changes for missing visits were imputed as the worse score of its non-missing neighbors. If a visit was missing and there was no subsequent score, the score was coded to missing.

FAS. m = meters. N=number of subjects with evaluable data at core study Baseline. Treatment groups shown by core study randomization; includes subjects who were only treated in the core study and those who continued into the extension study.

Pulmonary hypertension (PH) criteria: Class I: PH without limitation of physical activity (PA) (no undue dyspnea, fatigue, chest pain, near syncope); Class II: PH with slight limitation in PA, comfortable at rest, ordinary PA causes undue dyspnea, fatigue, chest pain, near syncope; Class III: PH with marked limitation in PA, comfortable at rest, less than ordinary activity causes undue dyspnea, fatigue, chest pain or syncope; Class IV: PH with inability to carry out PA without symptoms, signs of right heart failure, dyspnea or fatigue may be present at rest, discomfort increased by any PA.

FAS; N=number of subjects with evaluable data at core study Baseline. Treatment groups shown by core study randomization; includes subjects in core study and in extension study. Scores for categorized changes for missing visits imputed as worse score of its non-missing neighbors; missing visits with no subsequent score: score coded to missing.

Subject-rated measure of health status comprised of 36 items: 8 subscale scores (physical functioning, role limitations due to physical health problems, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems, and mental health), 2 summary scores (physical component and mental component), and a self-evaluated change in health status. Subscale and summary scores range: 0-100. Higher subscale and summary scores = better health status. Change from baseline = score at observation minus score at baseline.

FAS. N=number of subjects with evaluable data at core study Baseline; n=number of subjects with SF-36 score at observation, Placebo, Sildenafil, respectively. Treatment groups shown by core study randomization; includes subjects who who were only treated in the core study and those who continued into the extension study. Phys = physical.

EQ-5D: subject rated questionnaire to assess health-related quality of life in terms of a single utility score. Health State Profile component assesses level of current health for 5 domains: mobility, self-care, usual activities, pain and discomfort, and anxiety and depression; 1 indicates better health state (no problems); 3 indicates worst health state (confined to bed). Scoring formula developed by EuroQol Group assigns a utility value for each domain in the profile. Score is transformed and results in a total score range -0.594 to 1.000; higher score indicates a better health state.

FAS. N=number of subjects with evaluable data at core study Baseline; n=number of subjects with evaluable data at observation, Placebo, Sildenafil, respectively. Treatment groups shown by core study randomization; includes subjects who who were only treated in the core study and those who continued into the extension study.

EQ-5D: subject rated questionnaire to assess health-related quality of life in terms of a single index value. The VAS component rates current health state on a scale from 0 (worst imaginable health state) to 100 (best imaginable health state); higher scores indicate a better health state.

FAS. N=number of subjects with evaluable data at core study Baseline; n=number of subjects with evaluable data at observation, Placebo, Sildenafil, respectively. Treatment groups shown by core study randomization; includes subjects who who were only treated in the core study and those continued into the extension study.

Subject-rated measure of health status (36 items): 8 subscale scores (physical functioning, role limitations due to physical health problems, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems, mental health), 2 summary scores (physical component, mental component), and a self-evaluated change in health status. Change from Baseline in SF-36 Health Transition score at each visit. I=much better than 1 year ago; II=somewhat better than 1 year ago; III=about the same as 1 year ago; IV=somewhat worse than 1 year ago; V=much worse than 1 year ago

FAS. N=number of subjects with evaluable data at core study Baseline. Treatment groups shown by core study randomization; includes subjects who who were only treated in the core study and those continued into the extension study. BL=Baseline.

BORG Dyspnea score: change from core study Baseline. Subject rating of maximum degree of dyspnea experienced at any time during the 6-Minute Walk Test. Range: 0 (no breathlessness at all) to 10 (maximum breathlessness).

FAS. N=number of subjects with evaluable data at core study Baseline; n=number of subjects with evaluable data at observation, Placebo, Sildenafil, respectively. Treatment groups shown by core study randomization; includes subjects who were only treated in the core study and those who continued into the extension study.