Efficacy of expiratory tracheal gas insufflation in a canine model of lung injury.

Tracheal gas insufflation (TGI) improves the efficiency of CO2 elimination by reducing the CO2-laden dead space of the airways. The effect of TGI on PaCO2 diminishes in the setting of acute lung injury (ALI) because an increased alveolar component dominates the total physiologic dead space. Nevertheless, adopting a strategy of permissive hypercapnia should partially offset the decreased efficacy of TGI by increasing CO2 concentration in the proximal airways. To examine these issues we studied the CO2 removal efficacy of expiratory TGI as an adjunct to conventional mechanical ventilation (CMV) before and after oleic acid-induced lung injury (OAI). We first examined the effect of TGI before and after OAI, keeping tidal volume (VT) and frequency constant, and allowing PaCO2 to increase after OAI. We then tested TGI efficiency after matching PaCO2 after OAI to its pre-OAI level by increasing VT (post-OA/VT stage). PaCO2 was 53 +/- 3, 79 +/- 21, and 52 +/- 4 mm Hg in the pre-OAI, post-OAI, and post-OA/VT stages of CMV, respectively. The corresponding decrements in PaCO2 produced by TGI at a flow rate of 10 L/min were 16 +/- 3, 24 +/- 10, and 10 +/- 2 mm Hg, respectively. TGI decreased total physiologic dead space per breath (VD) by 56, 31, and 28 ml during the pre-OAI, post-OAI, and post-OA/VT stages, respectively. Despite a smaller reduction in VD during the post-OAI stage, the effect of TGI on PaCO2 was preserved because of the relatively high PaCO2 prior to its initiation.(ABSTRACT TRUNCATED AT 250 WORDS)