The endotracheal tube biases the estimates of pulmonary recruitment and overdistension

Abstract

To assess the impact of the endotracheal tube (ETT) and of different flow waveforms on estimates of alveolar cyclic recruitment (CR) and overdistension (AO). Numerical simulation of the respiratory system plus ETT (inertance L plus a flow-dependent resistance, K ₁ and K ₂), with the following non-linear equation of motion

$$ P_{{{\text{AW}}}} ({\text{t}}) = {\text{ }}{\left( {\left( {\left. {K_{1} + K_{2} \cdot \left| {\left. {\ifmmode\expandafter\dot\else\expandafter\.\fi{V}({\text{t}})} \right|} \right.} \right) \cdot \ifmmode\expandafter\dot\else\expandafter\.\fi{V}({\text{t}}) + L \cdot \ifmmode\expandafter\ddot\else\expandafter\"\fi{V}({\text{t}})} \right.} \right)} + {\text{Rrs}} \cdot \ifmmode\expandafter\dot\else\expandafter\.\fi{V}({\text{t}}) + {\left( {E_{1} + E_{2} \cdot V({\text{t}})} \right)} \cdot V({\text{t}}) + P_{0} $$

(P _AW pressure at the airways opening, V volume), under volume-controlled mechanical ventilation. An index %E ₂ = 100·(E ₂·V _T)/(E ₁ + E ₂·V _T) can be calculated where %E ₂ > 30% represents AO and %E ₂ < 0% represents CR. Parameters were estimated by the least-squares method, either with the complete equation or supressing L, K ₂ or both. %E ₂ is always underestimated (down to −152 percent points) with incomplete equations of motion. The estimation of %E ₂ may be strongly biased in the presence of an ETT excluded from the estimation model.