Abstract
Objective: Identify variables independently associated with delivered tidal volume (Vt) and measured mean airway pressure during high-frequency oscillatory ventilation across the range of pediatric endotracheal tube sizes.
Design: In vitro study.
Setting: Research laboratory.
Interventions: An in vitro bench model of the intubated pediatric respiratory system during high-frequency oscillatory ventilation was used to obtain delivered Vt and mean airway pressure (in the distal lung) for various endotracheal tube sizes. Measurements were taken at different combinations of ventilator set mean airway pressure (Paw), amplitude (Delta P), frequency, and test lung compliance. Multiple regression analysis was used to construct multivariable models predicting delivered Vt and mean airway pressure.
Measurements and Main Results: Variables independently associated with higher delivered Vt for all endotracheal tube sizes include higher Delta P (p < 0.001), lower frequency (p < 0.001), and higher test lung compliance (p < 0.001). A multiplicative interaction between frequency and Delta P magnifies the delivered Vt when Delta P is high and frequency is low (p < 0.001). Delivered mean airway pressure becomes lower than set Paw as Delta P increases (p < 0.001) and frequency increases (p < 0.05). Ventilator set Paw is the largest determinant of delivered mean airway pressure; however, increasing Delta P resulted in a lower delivered mean airway pressure. For example, in a 4.0 mm ID endotracheal tube, increasing Delta P by 10 cm H2O resulted in an average decrease of delivered mean airway pressure by 4.5%.
Conclusions: This is the first study to quantify the interaction between Delta P and frequency in delivered Vt and the effect of Delta P and frequency on delivered mean airway pressure. These results demonstrate the need to measure or estimate Vt and delivered pressures during high-frequency oscillatory ventilation and may be useful in determining optimal strategies for lung protective ventilation during high-frequency oscillatory ventilation.