Non-interventional monitoring of expiratory flow limitation during experimental mechanical ventilation

BACKGROUND: Expiratory flow limitation (EFL) is common among patients in the intensive care unit under mechanical ventilation (MV) and may have significant clinical consequences. In the present study, we examine the possibility of non-interventional detection of EFL during experimental MV. METHODS: Eight artificially ventilated New Zealand rabbits were included in the experiments. EFL was induced during MV by application of negative expiratory pressure (−5, −8 and −10 hPa) and detected by the negative expiratory pressure technique. Airway pressure (P(aw)) and gas flow (V′) were digitally recorded and processed off-line for the evaluation of respiratory mechanics. The method is based on the computation and monitoring of instantaneous respiratory resistance R(rs)(t). The resistive pressure (P(aw,res)(t)) is calculated by subtracting from P(aw) its elastic component and the end-expiratory pressure, as assessed by linear regression. Then, R(rs)(t) is computed as the instant ratio P(aw,res)(t)/V′(t). RESULTS: Two completely different patterns of expiratory R(rs)(t) separate the cases with EFL from those without EFL. Small and random fluctuations are noticed when EFL is absent, whereas the onset of EFL is accompanied by an abrupt and continuous rise in R(rs)(t), towards the end of expiration. Thus, EFL is not only detected but may also be quantified from the volume still to be expired at the time EFL occurs. CONCLUSION: The proposed technique is a simple, accurate and non-interventional tool for EFL monitoring during MV.