TRACHEAL GAS INSUFFLATION - CATHETER EFFECTIVENESS DETERMINED BY EXPIRATORY FLUSH VOLUME

Used adjunctively during mechanical ventilation, tracheal gas insuffla tion (TGI) improves CO2 elimination, principally by decreasing effecti ve anatomic dead space. Continuing lung deflation at end-expiration ra ises the end-expiratory CO2 concentration within the proximal airway, and could theoretically reduce the efficiency of a given catheter flow . To test this possibility, we designed a series of experiments that e xamined the influence of TGI delivery patterns on the efficiency of CO 2 elimination. Using a gating device, catheter flow was delivered sele ctively during desired portions of expiration. Paralyzed, ventilated d ogs were studied at short and extended inspiratory time fractions (TI/ TT) with inspiratory tidal volume and ventilator frequency held consta nt. The expiratory flush volume, not the pattern of gas delivery, dete rmined the observed decline in Pa-CO2, provided that the end-expirator y period was included in the catheter flush period. Despite continuing end-expiratory lung deflation (extended TI/TT), catheter effectivenes s remained the same at matched expiratory flush volumes. To determine if enhanced distal mixing at the higher catheter flows required during the extended TI/TT (to match expiratory flush volume) masked a decrea se in efficiency, we repeated the experiment with a tip-inverted cathe ter. We again found that matched catheter delivered expiratory volumes were similarly effective. With or without ongoing lung deflation, the volume of gas flushed during the expiratory period determined the eff ectiveness of TGI, provided that inspired minute ventilation remains u nchanged and end-expiration is included in the catheter flush period.