Time and Concentration Relationships of Gentamicin in Serum and Bronchial Lavage Fluid of Horses Administered Gentamicin Intravenously and by Aerosol

This study was performed to compare the delivery of the antimicrobial gentamicin to the respiratory tract of adult horses following aerosol and intravenous administration. Nine adult horses were used in a crossover design. Aerosol administration of gentamicin was performed using a close fitting facemask and an ultrasonic nebulizer. Intravenous gentamicin was administered via a jugular venous catheter. Samples of pulmonary epithelial lining fluid were collected by bronchial lavage performed at 0.5, 4, 8 and 24 hours after gentamicin administration. All samples were analyzed for gentamicin concentration, and cytologic examination was performed on aliquots of bronchial lavage fluid from times 0.5, 8 and 24 hours. Comparisons were made using the Wilcoxon signed-rank test. The bronchial lavage fluid gentamicin concentration after aerosol administration was significantly greater (p<0.05) than after intravenous administration at 0.5, 4, and 8 hours. The bronchial lavage fluid total nucleated cell count increased significantly (p<0.05) from 0.5 to 24 hours following both routes of gentamicin administration, with the increase observed following aerosol administration being significantly greater (p<0.05) than that observed following intravenous administration. A significant increase in neutrophil count was detected between bronchial lavage fluid samples taken at 0.5 hours and 24 hours, regardless of route of gentamicin administration. We conclude that aerosol administration of gentamicin to the equine respiratory tract achieves bronchial lavage fluid gentamicin levels that are significantly higher than levels obtained following intravenous administration for at least the first 8 hours after administration, while inciting a mild inflammatory response.