Remodulin (Treprostenil sodium) is synthetic prostacyclin analog available as a solution for administration in the intravenous route. It is approved for treatment of class I - pulmonary hypertension functional capacity stage III-IV. A key factor for the success of pulmonary hypertension therapy is maintaining near constant blood level of a given medications along the day. A sudden decrease in the blood levels might lead to rebound phenomenon; abrupt increase in the pulmonary artery pressure which might lead to acute right ventricular failure and even to sudden death. As some PHT patients suffer are on hemo-dialysis therapy due to end-stage renal disease there is a need to document the variation of the blood level along the dialysis cycle.

After giving informed consent 10 ml of blood will be drawn from the study population at the following occasions.

• Baseline measurement immediately before starting HD therapy.
• During the hemo-dialysis therapy - every one hour.
• After the end of the hemo-dialysis therapy - every one hour (a total of four measurements) to record recovery of Treprostenil blood levels.

Inclusion Criteria:patients with PAH who receive chronic (more than 3 months) Treprostenil treatment administered intravenously as a single agent therapy who maintain chronic hemo-dialysis therapy. Patients must be in stable condition for at least one month. Patients other than class I PAH will be not included in this study.

Exclusion Criteria:

• none

The study population includes patients with PAH who receive chronic (more than 3 months) Treprostenil treatment administered intravenously as a single agent therapy who maintain chronic hemo-dialysis therapy. Patients must be in stable condition for at least one month. Patients other than class I PAH will be not included in this study.