Concentrations of gentamicin and carbenicillin in bronchial secretions

A canine model was used to study the kinetics of antibiotic penetration into and clearance from normal respiratory tract secretions. Gentamicin and carbenicillin appeared in bronchial secretions within 5 min after iv injection and reached peak concentrations between 15 and 30 min, when levels in serum were maximal. During a 2-hr period, levels of antibiotic fell more slowly than in serum, indicating that clearance from the respiratory tract may be prolonged or that the antibiotics may accumulate in the secretions. Concentrations of gentamicin in bronchial secretions were greater than the MIC for 63% of isolated Pseudomonas; bronchial levels of carbenicillin exceeded the MIC of only 43% of pseudomonas isolates. In contrast, penicillin G was found to achieve easily the therapeutic levels in bronchial secretions for such susceptible organisms as Diplococcus pneumoniae. This study indicates a degree of unpredictability in obtaining therapeutic concentrations of gentamicin and carbenicillin in bronchial secretions in spite of adequate levels in serum.