Frequency adjustment does not effect hemodynamics during high frequency oscillatory ventilation in an animal model

Background: Controversy exists concerning the ideal frequency settings to use during high frequency oscillatory ventialtion (HFOV). The HFOV frequency setting affects the ventilator rate and the delivered tidal volume. We hypothesized that a lower frequency would improve gas exchange but at the expense of cardiopulmonary function. Method: 10 swine with a mean weight of 6.13 ±0.97 kg were placed on HFOV (Sensormedics 3100B, Yorba Linda, CA). Each animal was instrumented with an arterial line, pulmonary artery flow probe (Cardiac Output, CO), and pulmonary artery pressure catheter (PAP). Pulmonary vascular input resistance (PVR) was calculated as the ratio of PA to CO. HFOV settings included amplitude 40 cmH₂O, mean airway pressure 10 cmH₂O, FiO₂ 0.40, and inspiratory time 33%. Animals were ventilated at 5 Hz, 5 Hz + 11pm inhaled CO₂, or 12 Hz for periods of 15 minutes in a random order. At the end of each 15 minute period, data were collected. Up to ten data sets were collected on each animal, a total of 98 data sets were analyzed. Effects of the frequency setting were tested using ANOVA with repeated measures. Conclusions: We have demonstrated that a lower HFOV frequency (5 Hz) results in a predictable decrease in PaCO₂ by increasing tidal volume. There are no hemodynamic differences seen between 5 Hz + CO₂ and 12 Hz. This data suggests that increasing tidal volumes in HFOV does not adversely effect hemodynamics. Results: Frequency (Hz) PaCO ₂ (torr) PaO₂ (torr) PAP (mmHg) CO (ml/min) PVR (dyne-sec/cm⁵) 5 26±7 206±19 16±6 610±140 2411 ±1426 5+CO2 46±9*191 ±33 20±8 *575±183 3581±3105 12 49±10 *166±24*20±8*584±159 3315±2483 (Mean ± SD,*p<0.005 vs 5Hz).