The objective of this study is to assess the safety, tolerability and pharmacokinetics of intravenous sildenafil (10 mg) administered as a bolus injection to patients with Pulmonary Arterial Hypertension already receiving and stable on oral Revatio 20 mg TID.

Inclusion Criteria:

• Subjects (≥18 years old) receiving oral Revatio 20 mg TID for PAH within the approved indication (EU Summary of Product Characteristics) for a minimum of 1-month and who, in the investigator's opinion, have demonstrated good tolerability. Subjects with PAH who are nil by mouth for any reason (eg, elective surgery or medical procedure, TPN feeding, gastrointestinal disturbance etc) may also be included in the study provided they are haemodynamically stable (see Inclusion Criterion 4) and the 6 hour period of blood pressure and pulse monitoring in the study does not interfere with any planned medical or surgical intervention.
• Evidence of a personally signed and dated informed consent document indicating that the subject has been informed of all pertinent aspects of the study.
• Subjects who are willing and able to comply with scheduled visits, treatment plan and study tests and procedures.

Exclusion Criteria:

• Any subject receiving Revatio outside ('off-label') of the current Summary of Product Characteristics (SmPC).
• Other severe acute or chronic medical or psychiatric condition that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the subject inappropriate for entry into this study.
• Introduction of potent CYP 3A4 inhibitors (eg, ketoconazole, itraconazole and ritonavir) within the previous 1 month of screening.