The Science Journal of the American Association for Respiratory Care
Background: High frequency oscillatory ventilation (HFOV) is a technology increasingly utilized to support intensive care patients with acute lung injury. Controversy exists concerning the ideal frequency settings to use. The HFOV frequency setting not only affects the ventilator rate but also alters the delivered tidal volume (frequency is inversely proportional to tidal volume). The purpose of this study was to evaluate the effect of different HFOV frequencies on gas exchange and cardiopulmonary hemodynamics. We hypothesized that a lower frequency (i.e. increased tidal volume) would improve gas exchange but at the expense of cardiopulmonary function.
Method: Five infant swine with a mean weight of 6.06 ± 1.4 kg were placed on HFOV. Each animal was instrumented with an arterial line, pulmonary artery flow probe, right ventricular pressure catheter, and pulmonary artery catheter. HFOV settings included amplitude 40 cmH20, mean airway pressure 10 cmH20, FiO2 0.40, inspiratory time 33%, and bias flow 20 lpm. Animals were ventilated at 5 Hz or 12 Hz for periods of 20 minutes in a random order. At the end of each 20 minute period, data were collected. Up to six data sets were collected on each animal, a total of twenty nine data sets were analyzed. Effects of the frequency setting were tested using ANOVA with repeated measures.
Results: Mean PaCO2 levels were lower when the ventilator was set at 5 Hz. No statistically significant differences were found for PaO2, cardiac output (CO), pulmonary artery pressure (PA), or pulmonary vascular resistance (PVR).
|12||42.7 ± 6.8*||159.2 ± 25.4||18.8 ± 10.0||639.5 ± 197.7||3175 ± 3437|
|5||23.1 ± 3.7||191.7 ± 11.5||15.7 ± 6.9||653.6 ± 159.9||2297 ± 1971|
|(mean ± SD, *p< 0.001)|
Conclusions: We have demonstrated that a lower HFOV frequency results in a predictable decrease in PaCO2 by increasing tidal volume. However, this increase in tidal volume did not have the expected detrimental effect on hemodynamics.