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. For a similar decrement in VD during the pest-OA/VT stage, TGI was less effective in decreasing PaCO2. Our results can be explained by the inverse relationship between PaCO2 and the physiologic dead-space fraction (VD/VT), in which at high VD/VT a small decrement in VD causes a relatively large decrease in PaCO2. We conclude that application of a permissive hypercapnia strategy during ALl counterbalances the decreased CO2 removal efficacy of TGI caused by increased alveolar dead space.
|Original language||English (US)|
|Number of pages||7|
|Journal||American Journal of Respiratory and Critical Care Medicine|
|State||Published - Aug 1 1995|