EFFECT OF CATHETER FLOW DIRECTION ON CO2 REMOVAL DURING TRACHEAL GAS INSUFFLATION IN DOGS

Tracheal gas insufflation (TGI) improves the efficiency of CO2 elimina tion by replacing CO2 in the anatomic dead space proximal to the cathe ter tip with fresh gas during expiration. Turbulence generated by gas exiting the catheter tip may also contribute to alveolar ventilation. To separate distal (turbulence-related) and proximal (washout of dead space) effects of TGI, we compared the efficacy of a straight and an i nverted catheter during continuous and expiratory TGI in six mechanica lly ventilated dogs. We reasoned that the inverted catheter cannot imp rove CO2 elimination from more distal conducting airways. During conti nuous TGI with the straight catheter, arterial P(CO2) (Pa(CO2)) decrea sed significantly from baseline (without TGI) of 56 +/- 10 Torr to 38 +/- 8, 36 +/- 8, and 35 +/- 8 Torr at catheter flow rates (Vcath) of 5 , 10, and 15 l/min, respectively. For the same conditions, Pa(CO2) was always higher (P < 0.001) with the inverted catheter (42 +/- 10, 41 /- 10, and 41 +/- 10 Torr). Pa(CO2) was lower with the straight (40 +/ - 9 Torr) than with the inverted catheter (44 +/- 10 Torr, P < 0.001) during TGI delivered only during expiration at a Vcath of 10 l/min. En d-expiratory lung volume relative to baseline increased during continu ous, but not during expiratory, TGI and was significantly greater with the straight than with the inverted catheter (P < 0.0001). Our data c onfirm that the primary mechanism of TGI is expiratory washout of the proximal anatomic dead space but also suggest a minor contribution of turbulence beyond the tip of the straight catheter.