Inhaled nitric oxide (INO) is being investigated as a therapy for pulmonary hypertension. NO2 is formed when NO reacts with O2: 2NO + O2 → 2NO2. The potential for pulmonary toxicity from NO2 formation within the ventilator circuit (VC) stimulated this study of the chemical kinetics of NO2 formation in a neonatal continuous flow (CF) VC. NO2 measurements were made using NIST's tunable diode laser absorbtion spectrometer (TDLAS) which can selectivity measure NO2 with a sensitivity of 20 ppb (nmol/mol) in a NO, O2 and water vapor matrix. NO at 800 ppm (μmol/mol) was diluted with O2 using mass flow controllers to give a mole fraction of 0.90 O2, 80 ppm NO. The flow rate of the circuit was adjusted to vary dwell times. The rate of formation of NO2 and the rate constant were calculated. Delivering 80 ppm of NO in 0.90 O2 results in the formation of 0.144 ± 0.006 ppm of NO2 per second of dwell time in a CFVC at 21° C. According to the termolecular mechanism, the rate of formation will be lower at 37° C. We obtained a rate constant, k21C= 1.27*10-11 ± 0.05*10-11 Mole2/ppm2sec. This compares favorably with published rate constant data. We conclude that: 1) NO2 formation in a neonatal CFVC is accurately described by existing chemical kinetics and rate constants, 2) INO can be delivered in neonatal CFVC using high flow rates and short dwell times with minimal NO2 formation. For example, mixing NO with O2 at 10 LPM in a 50 cm length of ventilator tubing results in a dwell time of approximately 0.3 sec and < 43 ppb NO2 formation. If greater amounts of NO2 are detected, NO source cylinder contamination, analyzer inaccuracy or NO delivery system configuration problems should be investigated.
