Inclusion Criteria:  Patient ≥ 18 years of age. The patient or patient's surrogate decision maker must understand and sign the informed consent form (ICF). Hospitalized (Emergency Room (ER) or inpatient) with: Imaging (computed tomography pulmonary angiography (CTPA) or ventilation/perfusion (VQ) lung scan) proven acute pulmonary embolism (PE) PE meets the following intermediate risk PE criteria (or massive, see below): Troponin > .1 AND Imaging (computed tomography (CT) or transthoracic echocardiogram (TTE)) signs of RV compromise (at least 1 of the following): RV:LV>1 on TTE or CTPA OR RV dilation (TTE or CTPA OR RV dysfunction on TTE. Massive PE Intensive care unit (ICU) level of care (Patient moving to ICU, ICU level of care in ER, or currently in ICU) Ability to comply with study protocol in investigator's judgement  Exclusion Criteria:  Pregnancy or breastfeeding Inability to administer iNO through current mode of O2 delivery (i.e. BiPAP) Active hemoptysis Known allergy to iNO. Any serious medical condition of lab abnormality that, in the investigator's judgement, precludes the patient's safe participation in the study. Methemoglobin reductase deficiency Unable to obtain consent or patient or patient surrogate decision maker declines Patients already on iNO prior to study enrollment