Methemoglobin and Nitrogen Dioxide Levels During Inhaled Nitric Oxide Therapy at 300 Parts per Million in Healthy...

High-dose inhaled nitric oxide (iNO) at 300 parts per million (iNO₃₀₀) has shown antimicrobial and anti-inflammatory effects in preclinical models. Defining its safety profile in humans is essential for therapeutic development in respiratory infections. Human data specifically evaluating methemoglobin (MetHb) and nitrogen dioxide (NO₂) kinetics during iNO are limited. This study investigated MetHb and NO₂ formation in healthy adults exposed to iNO.

In this phase 1 trial, 10 healthy adults received iNO for 30 min, 3 times daily over 5 days. To simulate oxygen therapy in pneumonia, NO was delivered at 2 inspired oxygen fractions (F_IO2): 0.21 at 50 L/min and 0.80 at 70 L/min. On day 5, subjects exercised on a stationary bike to simulate increased ventilation in critically ill adults. Continuous MetHb monitoring was performed with pulse CO-oximetry (0.5 Hz), and NO₂ was measured using an ultrafast laser-induced fluorescence analyzer (15 Hz).

One hundred forty-eight treatments were administered without major adverse effects. At rest, peak MetHb averaged 5.5 ± 1.2% (range 3.3-9.0%), increasing to 9.0 ± 1.1% (6.8-11.3%) during exercise. Six transient and asymptomatic MetHb elevations above 10% occurred in 3 out of 10 subjects, exclusively during exercise sessions, with all returning to baseline before the next dose. No MetHb accumulation was observed at day 7 follow-up. NO₂ levels in the inspiratory limb remained <5 ppm during all treatments. Occasional brief NO₂ spikes >5 ppm at the Y-piece occurred only at the inspiration-expiration transition or respiratory pause, representing 0.01-0.14% of sampling time.

Intermittent administration of iNO was safe in healthy adults under real-time monitoring of MetHb and NO₂. These findings support further clinical investigations into iNO as a potential inhaled therapy for lower respiratory tract infections.