A model for the uptake of inhaled vapors in the nose of the dog during cyclic breathing.

A model was developed to simulate the uptake of inhaled vapors in the nasal airway of the Beagle dog during cyclic breathing. Input data to the model were morphological and physiological data for the dog, and physiocochemical data for the vapors. The model simulates the nasal airway as a slit-like duct, where air passes between the two airway walls in an ideal plug flow. The thickness of the walls corresponds to the distance between the air interface and the average position where vapor molecules are removed into the capillary blood. All resistance to radial mass transfer is assumed to arise on the liquid side in the diffusion of vapors through the air/blood tissue barrier and in transport by the blood. The model results agreed reasonably well with experimental data. The nasal absorption of vapors on inhalation increased from 1% for a compound with a blood/air partition coefficient of 1 to around 95% uptake for a compound with a partition coefficient of 2000. Desorption from the nasal tissues on exhalation increased from approximately 1% to approximately 30% over the same range of partition coefficients. The nasal uptake over one complete breathing cycle, defined as absorption on inhalation minus desorption on exhalation, was almost zero for low partition coefficient compounds and plateaued at around 65% for high partition coefficient compounds. The model indicates that diffusional resistance and inertia of the nasal tissues result in temporary storage of absorbed vapors upon inhalation, followed by desorption of vapors back into the airstream upon exhalation. An important consequence of this phenomenon is a shift in exposure to inhaled vapors from the lungs to the nasal airway during cyclic flow compared with predictions from experiments and models based on unidirectional flow.