Species differences in upper respiratory tract deposition of acetone and ethanol vapors

Regional deposition patterns determine the dose of inhaled gaseous toxicant received by various areas of the respiratory tract. To study potential species differences in upper respiratory tract (URT) deposition of acetone and ethanol vapors, and to determine if deposition of these vapors could be described by a ventilation-perfusion (V-P) model, URT deposition efficiencies of these vapors were measured over a 12- to 18-min period at selected flow rates in the surgically isolated URT of urethane-anesthetized Fischer rats and Hartley guinea pigs. For both gases in both species, deposition efficiencies were significantly dependent upon inspiratory flow rate (p less than 0.0005). The V-P model predicts a linear relationship will exist between the ratio of the deposited to the nondeposited fraction and the inverse of the inspiratory flow rate. Such relationships were observed for deposition of acetone (r = 0.93, p less than 0.0005) and ethanol vapors (r = 0.95, p less than 0.0005) in the guinea pig and for deposition of acetone vapor (r = 0.95, p less than 0.0005) in the rat. In contrast, a linear relationship was not observed for ethanol in the rat suggesting that deposition mechanism(s) differ for this vapor in this species. Despite the fact that the surface area of the URT of the guinea pig is greater than that of the rat, URT deposition of these vapors was as much as twice as efficient in the rat as in the guinea pig (p less than 0.0005). This effect was not due to different blood:air partition coefficients as they were identical in both species. In toto, these results suggest there may be important anatomic and/or physiologic differences in the URT of the Hartley guinea pig and Fischer rat. Such differences may have to be considered when comparing the response(s) of these species to toxic gases or when extrapolating data obtained from these species to the human.