Expiratory phase tracheal gas insufflation and pressure control in sheep with permissive hypercapnia

Tracheal gas insufflation (TGI) has been shown to be a useful adjunct to me chanical ventilation, decreasing Pa-CO2 during permissive hypercapnia. Whil e TGI can be used either with pressure (PCV) or volume-controlled ventilati on and continuously or only during the expiratory phase (Ex-TGI), there are no controlled studies evaluating the effects of Ex-TGI with PCV in acute l ung injury when the direction of the insufflated flow or the inspiratory:ex piratory (I:E) ratio are varied. We evaluated the effect that Ex-TGI with P CV would have on CO2 removal during both direct and reverse insufflated flo w direction with varied I:E ratios when peak airway pressure, total positiv e end-expiratory pressure (PEEP), and tidal volume (VT) were kept constant. In addition we examined the effect that insufflation flow directed toward the mouth (reverse flow) would have on the generation of PEEP compared with flow directed toward the carina (direct flow). After saline lavage, nine s heep were ventilated with PCV to a baseline Pa-CO2 of 80 mm Hg. Ex-TGI (10 L/min) was then randomly applied in the reverse and direct direction with I :E set at 1:2 or 2:1. During 1:2 I:E Pao, decreased from 78 +/- 4 mm Hg to 60 +/- 7 mm Hg (23.5 +/- 8.9%) with direct flow and to 64 +/- 5 mm Hg (18.5 +/- 5.5%) with reverse flow (p < 0.05), whereas during 2:1 I:E Pa-CO2 decr eased from 80 +/- 4 mm Hg to 69 +/- 8 mm Hg (13.7 +/- 9.2%) with direct flo w and to 66 +/- 4 mm Hg (17.2 +/- 4.4%) with reverse flow (p < 0.05). Great er PEEP was developed with direct flow (2.8 cm H2O I:E 1:2 and 4.0 cm H2O I :E 2:1) than with reverse flow (-0.9 cm H2O I:E 1:2 and -0.4 cm H2O I:E 2:1 ), p < 0.05. There was no difference in the Pa-CO2 change between I:E with reverse flow, but the Pa-CO2 decrease was greater (p < 0.05) during 1:2 ver sus 2:1 I:E with direct flow. CO2 removal during PCV and Ex-TGI is more con sistent with reverse flow than with direct flow and PEEP level is less affe cted by TGI with reverse flow than with direct flow.