FUNCTIONAL RESIDUAL CAPACITY MEASUREMENT DURING TRACHEAL GAS INSUFFLATION

Objective. Tracheal gas insufflation (TGI) is considered an adjunctive method to enhance carbon dioxide elimination during permissive hyperc apnia in patients with acute respiratory distress syndrome. Due to inc reasing tidal volume and/or expiratory resistance, TGI may cause intri nsic PEEP (PEEPi), and may lessen the advantages oi permissive hyperca pnia. There is no reliable method to measure PEEPi during TGI. Using a n argon washout method to evaluate dynamic hyperinflation, we develope d a method to measure FRC with TGI flow. Methods. We measured FRC duri ng TGI by washing out both the ventilator and TGI circuit with 100% ox ygen (O-2) previously equilibrated with 10% argon and 90% O-2. To test the accuracy of our system, we measured the volume in a model lung co mposed oi two flasks. The FRC oi the model lung was changed by varying its volume oi water, to active 500, 1000, and 1500 mL. The change oi FRC (Delta FRC) of the model lung was measured at a flow of 0, 4, 8, a nd 12 L/min. Then the FRC oi a bellows-type model lung was measured at the same TGI flow. PEEPi oi the model lung was also recorded as the p ressure inside the bellows at end-expiration. Results. Our FRC measure ments were accurate within 10% except for that oi 500 mt without TGI ( 12.7% +/- 1.1%). As inspiratory time (Ti) and/or TGI flow increased, t he FRC of the bellows-type model lung increased. PEEPi and Delta FRC s howed a positive correlation (r = 0.843, p < 0.001). The higher the TG I flow, the greater was the Delta FRC with both continuous and expirat ory-phase TGI. FRC during continuous TGI was higher than during expira tory-phase TGI especially during long TI and high TGI flow. Conclusion s. The system developed in this study can be used as a method to detec t air-trapping during TGI.