Re-inspiration of CO2 from ventilator circuit: effects of circuit flushing and aspiration of dead space up to high respiratory rate

Introduction: Dead space negatively influences carbon dioxide (CO2) elimination, particularly at high respiratory rates (RR) used at low tidal volume ventilation in acute respiratory distress syndrome (ARDS). Aspiration of dead space (ASPIDS), a known method for dead space reduction, comprises two mechanisms activated during late expiration: aspiration of gas from the tip of the tracheal tube and gas injection through the inspiratory line - circuit flushing. The objective was to study the efficiency of circuit flushing alone and of ASPIDS at wide combinations of RR and tidal volume (V-T) in anaesthetized pigs. The hypothesis was tested that circuit flushing and ASPIDS are particularly efficient at high RR. Methods: In Part 1 of the study, RR and V-T were, with a computer-controlled ventilator, modified for one breath at a time without changing minute ventilation. Proximal dead space in a y-piece and ventilator tubing (VDaw, prox) was measured. In part two, changes in CO2 partial pressure (PaCO2) during prolonged periods of circuit flushing and ASPIDS were studied at RR 20, 40 and 60 minutes(-1). Results: In Part 1, VDaw, prox was 7.6 +/- 0.5% of V-T at RR 10 minutes(-1) and 16 +/- 2.5% at RR 60 minutes(-1). In Part 2, circuit flushing reduced PaCO2 by 20% at RR 40 minutes(-1) and by 26% at RR 60 minutes(-1). ASPIDS reduced PaCO2 by 33% at RR 40 minutes(-1) and by 41% at RR 60 minutes(-1). Conclusions: At high RR, re-breathing of CO2 from the y-piece and tubing becomes important. Circuit flushing and ASPIDS, which significantly reduce tubing dead space and PaCO2, merit further clinical studies.