Evidence for variations in NO concentrations within the inspiratory limb related to the bolus effect during continuous administration in a lung model. Nitric oxide is administered into a lung model in a continuous mode after the ventilator. The inspiratory limb of the ventilator consists of a 475 cm-long tube with a provision for sampling the gas at points corresponding to 0.5 (site 1), 1.0 (site 2), 1.5 (site 3) and 2.0 (site 4) tidal volumes. (a) Nitric oxide is administered from a 22.5 ppm cylinder. Concentrations at sampling sites corresponding to 1 and 2 tidal volumes are higher than those from sites corresponding to 0.5 and 1.5 tidal volumes suggesting the existence of a bolus of NO moving in front of the tidal volume. (b) Nitric oxide is administered from a 900 ppm cylinder. The bolus effect is less pronounced than with a 22.5 ppm cylinder. There is no detectable bolus at the site corresponding to 2 tidal volumes, suggesting an early homogenization of the inspired gas. Published with permission .