The primary objective is to evaluate the efficacy of the treatment with supplementary oxygen added to conventional anticoagulant treatment in patients with Intermediate-risk pulmonary embolism.

The secondary objective is to evaluate the safety of the treatment with supplementary oxygen added to conventional anticoagulant treatment in patients with Intermediate-risk pulmonary embolism.

Inclusion Criteria:

• Confirmed PE by multidetector computed tomographic pulmonary angiography (MDCT).
• Disfunction of right ventricle (RV to left ventricle [LV] diameter ratio >1.0 in the apical 4-chamber view) in the echocardiogram performed in the first 12 hours after PE diagnosis.
• Signed and dated informed consent of the subject.

Exclusion Criteria:

• <18 years old.
• Allergy to iodinated contrast.
• Renal failure defined as a creatinine clearance less than 30 mL/min, according to the Cockcroft-Gault formula.
• Use of chronic oxygen therapy.
• Hypercapnia (pCO2 >50 mmHg at the time of diagnosis).
• Technically inadequate basal echocardiography.
• Contraindication to anticoagulant therapy.
• Symptoms duration >10 days.
• Haemodynamic instability.
• Participation in other clinical trial for PE treatment during the present clinical trial.
• Inability to use mask or nasal prongs.
• Life expectancy less than 90 days.