Flow Resistance Estimation of Endotracheal Tube during High Frequency Percussive Ventilation: Preliminary Results

Abstract

Introduction. High-frequency percussive ventilation (HFPV) is an unconventional ventilatory strategy that associates the beneficial aspects of conventional mechanical ventilation (CMV) with those of high-frequency ventilation (HFV). Accurate measurement of respiratory mechanics performed on intubated patients requires to take into account the real amount of pressure dissipated by endotracheal tube (EET). The purpose of this preliminary study was to characterize the endotracheal tube pressure drop (ΔP) during HFPV at different working pressures, frequencies and mechanical loads. Materials and Methods. Two ETTs (inner diameter ID 6.5 and 8mm) in 54 different working settings were examined. Considering the inertial effects (I), distributed and concentrated pressure losses, and taking also into account asymmetry between the active inspiratory and the passive expiratory phase, a model (Δ P = KB * V̇1.75 + I * V̈) for the estimation of ETT pressure drop during HFPV, separately identified during the two phases, is proposed. Results and Discussion. For each ETT, the two estimated values of the coefficient KB are about independent from working conditions and lung loads. Moreover, the estimated inertia coefficients (I) are equal in both phases and very similar to the theoretical values, obtained from the Blasius formula applied to the concentrated pressure drops that occurs at the connector and at the proximal and distal parts of EET. The estimated inertia produces a phase delay between flow (V) and Δ P, similar to that usually present during high frequency ventilation. This pilot study offers the possibility to estimate EET pressure drop by using a simple model that could be implemented as a clinical tool for real-time ETT pressure drop monitoring during HFPV.