Impaired pulmonary function as determined by PVr-P perturbation after MWCNT instillation. The PVr-P perturbation was performed in tracheotomized C57BL/6 mice instilled with vehicle (10% surfactant in saline) or increasing doses of MWCNTs. From PV loops, perturbation parameters, (A) A, (B) K, (C) Cst, (D) Est, and (E) Area were determined. The mean ± SEM of six mice per group are shown, *p < 0.05 and ** p < 0.01 compared with the naïve mice; # p < 0.05, ## p < 0.01, and ### p < 0.001 compared with the vehicle control mice; ++ p < 0.01 compared between two groups. No statistically significant differences were found between naïve and vehicle treated mice.