Effect of Morphine on Dyspnea and 6-Minute Walk Distance in Pulmonary Arterial Hypertension

Inclusion Criteria: * Aged 18 or older * Diagnosis of Group 1 pulmonary hypertension including idiopathic PAH, heritable PAH, and PAH that is drug- or toxin-induced, associated with connective tissue disease, human immunodeficiency virus (HIV) infection, congenital heart disease, or schistosomiasis23 * PAH confirmed by means of a right heart catheterization demonstrating:24 * Mean pulmonary arterial pressure of ≥ 25 mmHg * Pulmonary capillary wedge pressure ≤ 15 mmHg * Pulmonary vascular resistance of ≥ 3 Wood units * World Health Organization (WHO) Functional Class III or ambulatory Class IV * Six-minute walk test performed within the past 6 months demonstrating a distance of at least 50 metres. * Unchanged PAH medication regimen for 30 days prior to enrolment. Therapy may include endothelin-receptor antagonists, phosphodiesterase type 5 inhibitors, soluble guanylate cyclase stimulators, or oral or parenteral prostacyclin analogues. Diuretic doses may change. Exclusion Criteria: * Group 1 pulmonary hypertension due to portal hypertension * Group 1 pulmonary hypertension due to pulmonary veno-occlusive disease or pulmonary capillary hemangiomatosis * Groups 2, 3, 4, or 5 pulmonary hypertension * Severe renal impairment (estimated glomerular filtration rate \< 30 mL/minute/1.73m2 measured within 6 months) * Severe hepatic impairment (INR \> 2.0 in absence of vitamin K antagonist therapy, serum bilirubin \> 50mmol/L, cirrhosis on imaging or liver biopsy, prior hepatic encephalopathy, or Model for End-Stage Liver Disease (MELD) score \> 19, measured within 6 months, as required based on clinical suspicion) * Women who are pregnant or breastfeeding (beta-human chorionic gonadotropin (hCG) to confirm non-pregnant status in all females below age 50) * Hypersensitivity to opioid analgesic, concomitant use with Monoamine Oxidase (MAO) inhibitor or within 14 days of such treatment, concomitant use with barbiturates. Concomitant use with benzodiazepines and/or antipsychotics is permissible provided doses are stable over preceding 1 month. * Daily use of an opioid-containing medication * Unstable condition that is a contraindication to opioid use: Central Nervous System (CNS) depression, acute respiratory disease or impairment (acute hypoxia or hypercapnia), acute asthma or Chronic Obstructive Pulmonary Disease (COPD) exacerbation, untreated symptomatic obstructive sleep apnea, unstable cardiac arrhythmias, suspected hypovolemia, recent seizures (within 1 month), active drug abuse, abdominal disease and/or recent GI surgery (within 1 month), active gallbladder disease/biliary colic, untreated depression/suicidality, recent head injury (within 1 month), pre-existing intracranial lesion or increased intracranial pressure, untreated urinary tract obstruction, untreated hypothyroidism, hypopituitarism or Addison's disease. * Hypotension (resting systolic blood pressure less than or equal to 80mmHg) * Active or unstable coronary artery disease