Breath-by-breath quantification of inhaled nitric oxide absorption and modeling of methemoglobin kinetics

Recent studies uncovered extrapulmonary effects of inhaled nitric oxide (NO) therapy. However, no method exists to quantify NO absorption. This study developed a breath-by-breath system for inhaled NO absorption measurement, which consists of a rapid chemiluminescent NO analyzer and a pneumotachograph. The accuracy of the system was validated in vitro and in an animal model, achieving excellent agreement with the reference gas-collection method (linear regression R² 0.98 and 0.97; mean bias 1.5% ± 15.4% and -3.6% ± 14.4%, respectively). We established a methemoglobin kinetic model that followed single-compartment first-order elimination kinetics, and included NO absorption rate, methemoglobin conversion factor, time constant, treatment duration, hemoglobin, and blood volume. The model was trained and validated in 11 pigs receiving 81 treatments at 20 - 300 parts per million (ppm) and 14 human subjects, including 10 healthy subjects and 4 patients who received 64 treatments at 20 - 300 ppm. The approach yielded stable parameters of elimination constant 0.012 min^-1 (coefficient of variation [CV] 4.1%) and conversion factor 0.015 g/μmol (CV 2.3%). When comparing methemoglobin-estimated NO absorption and measured values, linear regression R² reached 0.89, and the mean bias was 1.0% ± 26% in human subjects. Over 84% of estimates fell within ±30% of measured values, indicating strong estimation capability of the model. This study demonstrated methemoglobin as a bedside marker for NO absorption and provides a framework for optimizing NO dosing strategies and advancing dose-response studies in inhaled NO therapy.