Frequency, Tidal Volume, and Mean Airway Pressure Combinations that Provide Adequate Gas Exchange and Low Alveolar Pressure during High Frequency Oscillatory Ventilation in Rabbits

ABSTRACT: We studied healthy and saline lavaged rabbits during high frequency oscillatory ventilation to determine what combination of frequency (f), tidal volume (V_t), and mean airway pressure (P_aw) produced the lowest peak-to-peak alveolar pressure amplitude (P_alv) and physiologic blood gas tensions. Sinusoidal volume changes were delivered through a tracheostomy by a piston pump driven by a linear motor. Tracheal pressure amplitude (P_tr) was measured through a tracheal catheter and alveolar pressure amplitude was measured in a capsule glued to the right lower lobe. Pao₂, Paco₂, P_tr, and P_alv were measured at the following settings: FiO₂ = 0.5, frequency 2-28 Hz, V, 1-3 mL/kg (50 150% dead space) and P_aw 5-15 cm H₂O. Many combinations of frequency and V_t resulted in the same Pao₂ and Paco₂. P_aw had a large effect on P_alv and minimal effect on blood gas tensions. In lavaged rabbits, the composite variable f × V_t² described the trends in P_alv and blood gas tensions. As the product of f × V_t² increased, Pao₂ initially increased and then decreased, whereas Paco₂ decreased and P_alv increased. No single combination of frequency, V_t and P_aw simultaneously provided the lowest P_alv and physiologic blood gas tensions. Adequate blood gas tensions and low P_alvwere obtained at frequencies less than 12 Hz, a V, of 2 mL/kg and a P_aw of 10 cm H2O. In healthy and lavaged rabbits Pao₂ increased and Paco₂ decreased as frequency increased at lower V_t · Pao₂ decreased as frequency increased at higher V_t in lavaged rabbits only. P_alvtended to be greater in lavaged rabbits.