Flow-independent nitric oxide exchange parameters in healthy adults

Currently accepted techniques utilize the plateau concentration of nitric oxide (NO) at a constant exhalation flow rate to characterize NO exchange, which cannot sufficiently distinguish airway and alveolar sources. Using nonlinear least squares regression and a two-compartment model, we recently described a new technique (Tsoukias et al. J Appl Physiol 91: 477-487, 2001), which utilizes a preexpiratory breath hold followed by a decreasing flow rate maneuver, to estimate three flow-independent NO parameters: maximum flux of NO from the airways (J_NO,max, pl/s), diffusing capacity of NO in the airways (D_NO,air, pl·s^-1·ppb^-1), and steady-state alveolar concentration (C_alv,ss, ppb). In healthy adults (n = 10), the optimal breath-hold time was 20 s, and the mean (95% intramaneuver, intrasubject, and intrapopulation confidence interval) J_NO,max, D_NO,air, and C_alv,ss are 640 (26, 20, and 15%) pl/s, 4.2 (168, 87, and 37%) pl·s^-1·ppb^-1, and 2.5 (81, 59, and 21%) ppb, respectively. J_NO,max can be estimated with the greatest certainty, and the variability of all the parameters within the population of healthy adults is significant. There is no correlation between the flow-independent NO parameters and forced vital capacity or the ratio of forced expiratory volume in 1 s to forced vital capacity. With the use of these parameters, the two-compartment model can accurately predict experimentally measured plateau NO concentrations at a constant flow rate. We conclude that this new technique is simple to perform and can simultaneously characterize airway and alveolar NO exchange in healthy adults with the use of a single breathing maneuver.