Our group is currently developing an intravenous respiratory assist device that uses a centrally located pulsatile balloon within a hollow fiber bundle to enhance gas exchange rate via active mixing mechanism. We tested the hypothesis that the non-symmetric inflation and deflation of the balloon lead to both nonuniform balloon-generated secondary flow and nonuniform gas exchange rate in the fiber bundle. The respiratory catheter was placed in a 1-in. internal diameter rigid test section of an in vitro flow loop (3 L/min deionized water). Particle image velocimetry (PIV), which was used to map the velocity vector field in the lateral cross-section, showed that the balloon pulsation generated a nonuniform fluid flow surrounding the respiratory assist catheter. PIV was also used to characterize the fiber bundle movement, which was induced by the balloon pulsation. Gas permeability coefficient of the device was evaluated by using both the fluid velocity and the relative velocity between the fluid and the fiber bundle. The highest difference in the gas permeability coefficient predicted by using the relative velocity was about 17% to 23% (angular direction), which was more uniform than the 49% to 59% variation predicted by using the fluid velocity. The movement of the fiber bundle was responsible for reducing the variation in the fluid velocity passing through the bundle and for minimizing the nonuniformity of the gas permeability coefficient of the respiratory assist catheter.