Theoretical interactions between ventilator settings and proximal deadspace ventilation during tracheal gas insufflation

Objective To investigate the theoretical interactions between ventilator settings, tracheal gas insufflation (TGI), and alveolar ventilation.Design We derived differential equations governing compartmental volume changes in a one-compartment model of TGI-assisted ventilation and equations governing gas dilution in the airway proximal to the TGI catheter and the additional CO₂ clearing ventilation arising from this dilution. This additional ventilation was called proximal ventilation. Validation was conducted in a mechanical lung analog. Model predictions for proximal ventilation were then generated over wide ranges of frequency, duty cycle, and tidal volume.Results Significant interactions were identified between ventilator settings and proximal ventilation. The persistence of end-expiratory flow from the lung decreased proximal dilution by fresh gas and thereby reduced TGI-aided proximal ventilation. Changes in end-expiratory lung flow resulting from alterations in ventilator settings were correlated inversely with proximal ventilation.Conclusions During TGI with constant catheter flow, ventilator settings that promote end-expiratory flow of gas from the lung diminish proximal ventilation. When frequency increases, the decrease in dilution efficiency of the individual breath is partially offset by the increase in cycle number, an effect which is magnified by any concomitant decrease in inspired tidal volume. Prolongation of the duty cycle tends to decrease proximal ventilation. Increases in expiratory resistance, including those arising from the external ventilator circuit or the endotracheal tube, also impair proximal ventilation.