THEORETICAL INTERACTIONS BETWEEN VENTILATOR SETTINGS AND PROXIMAL DEADSPACE VENTILATION DURING TRACHEAL GAS INSUFFLATION

Objective: To investigate the theoretical interactions between ventila tor settings, tracheal gas insufflation (TGI), and alveolar ventilatio n. Design: We derived differential equations governing compartmental v olume changes in a one-compartment model of TGI-assisted ventilation a nd 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 predicti ons for proximal ventilation were then generated over wide ranges of f requency, duly cycle, and tidal volume. Results: Significant interacti ons were identified between ventilator settings and proximal ventilati on. The persistence of end-expiratory flow from the lung decreased pro ximal dilution by fresh gas and thereby reduced TGI-aided proximal ven tilation. Changes in end-expiratory lung flow resulting from alteratio ns in ventilator settings were correlated inversely with proximal vent ilation, Conclusions: During TGT with constant catheter flow, ventilat or settings that promote end-expiratory flow of gas from the lung dimi nish proximal ventilation. When frequency increases, the decrease in d ilution efficiency of the individual breath is partially offset by the increase in cycle number, an effect which is magnified by any concomi tant decrease in inspired tidal volume. Prolongation of the duty cycle tends to decrease proximal ventilation. Increases in expiratory resis tance, including those arising from the external ventilator circuit or the endotracheal tube, also impair proximal ventilation.