Proximal, Tracheal, and Alveolar Pressures during High-Frequency Oscillatory Ventilation in a Normal Rabbit Model

Abstract

To study the effect of different high-frequency oscillatory ventilation parameters on airway pressure we measured oscillatory pressure amplitude (| P_aw |) and mean airway pressure (P_aw)at three sites in open-chested normal rabbits: proximal, trachea, and alveolus. Five animals were studied to test a new pleural capsule design, which was then used in two groups of animals to measure right upper (n = 4)or middle (n = 5) lobe alveolar pressures. Animals were randomly sequenced through combinations of frequency (10, 15, and 20 Hz) and fractional inspiratory time (T) (0.3 and 0.5) while normoxic and encapnic. During capsule testing, we noted that alveolar pressures increased (p < 0.05) with increasing capsule mass. suggesting that compressive forces from the capsule may alter the capsule measurement. We thus used a lowmass (430 mg) transducer system in the rabbit high-frequency oscillatory ventilation experiments. Using multifactorial analysis of variance, we found significant main effects of T_i on P_aw, and of measurement site on both |P_aw| and P_aw (p < 0.009). Frequency did not influence variations in either |P_aw| P_aw. For both T_i settings, alveolar upper lobe P_aw was lower compared with that of the middle lobe (p < 0.0005). Lengthening T_i (03 to 0.5) increased tracheal P_aw in each capsule group (p < 0.0005). At T_i = 0.5, tracheal P_aw exceeded P_aw measured proximally (p < 0.05). Our data support in vivo alveolar P_aw inhomogeneity and demonstrate significant changes in pressures within the lung related to T_i during high-frequency oscillatory ventilation.