Inhaled Nitric Oxide in Severe Obesity

For participants with acute hypoxemic respiratory failure Inclusion Criteria: * Acute hypoxemic respiratory failure, defined as persistent hypoxemia (PaO2/FiO2 ≤ 300 mmHg or SpO2/FiO2 ≤ 315) and on invasive mechanical ventilation for \< 72 hours * Presence of an arterial and central venous catheter (for blood gas measurement) * Admitted to a participating MGH ICU Exclusion Criteria: * Age \< 18 years * Pregnancy or known active breastfeeding * Prisoner or Incarceration * Inability or unwillingness of subject or legal surrogate/representative to give written informed consent * Use of inhaled or oral pulmonary vasodilatory therapy within the 24 hours preceding study enrollment * Contraindication to inhaled NO * Baseline Methemoglobin ≥ 3% * Known left ventricle ejection fraction \< 20% * Known history of G6PD deficiency or cytochrome issues * Prior adverse reaction to inhaled nitric oxide * Presence of pneumothorax or acute pulmonary embolism * Chronic hypoxemia requiring home supplemental non-invasive oxygen (nasal cannula or positive pressure ventilation) or home mechanical ventilation * Chronic pulmonary vascular disease on home chemical vasodilator support (e.g., sildenafil) * History of lung resection or transplant * Hemodynamic instability at the time of potential study enrollment defined as: * Persistent systolic blood pressure \< 90 mmHg or \>180 mmHg despite the use of vasopressor or vasodilators or * Requiring an increment in inotropic-vasopressors over the past two hours just before enrollment: more than 15 mcg/min for norepinephrine and dopamine, more than 10 mcg/min in epinephrine; and more than 50 mcg/ min for phenylephrine. EIT assessments of lung perfusion will only be performed in participants who are already receiving neuromuscular blocking agents (e.g., cisatracurium) at the time of study enrollment. EIT assessments of lung perfusion will not be performed in participants who have the following contraindications to EIT perfusion monitoring: * Hypernatremia (serum sodium \> 150 mEq/L) * Usage of any devices with electric current generation such as pacemaker or internal cardiac defibrillator For controls without acute hypoxemic respiratory failure Inclusion Criteria: * Receiving invasive mechanical ventilation and do not have a diagnosis of acute hypoxemic respiratory failure (PaO2/FiO2 \> 300 mmHg or SpO2/FiO2\> 315) * Presence of an arterial catheter * Admitted to a participating MGH ICU Exclusion Criteria: * Age \< 18 years * Pregnancy or known active breastfeeding * Prisoner or Incarceration * Inability or unwillingness of subject or legal surrogate/representative to give written informed consent