BACKGROUND: Tracheal Gas Insufflation (TGI) is one of the newer ancillary measures in mechanical ventilation employed to enhance carbon dioxide elimination.7G1 exerts its effect through reduction of deadspace ventilation, but the factors determining its effect are not well studied yet. METHOD: The subjects were seven mechanically-ventilated patients (58.8+/-10.6 yrs) who showed increased Physiologic deadspace greater than 60%. After 30 min of stabilization with 100% oxygen on pressure control ventilation, continuous flow TGI was administered via the insufflation lumen of Hi-Lo Jet Tracheal Tube (Mallincrodt, USA) for 15 min at 3 L/min and 5 L/min each. RESULTS: PaCO2 was decreased (51.4+/-17.6 at baseline,49.1+/-18.9 at TGI 3 L/min, 45.0+/- 14.9 mmHg at TGI 5 L/min, p=0.050), and pH was increased (7.37+/-0.12, 7.38+/-0.13, 7.39+/-0.12, respectively, p=0.037) while mixed expired CO2 (PECO2) was not changed significantly from baseline (p=0.336) by TGI. Physiologic deadspace(Vdphy) was decreased (73.0+/-7.9% at baseline, 69.8+/-10.0% at TGI 3 L/min, and 67.1+/-10.1% at TGI 5 L/mi% p=0.015). AaDO2(p=0.147), Vt(p=0.2140), Pmean(p=0.7788) and mean arterial pressure(p=0.4169) were not changed. The correlation between % maximal decrease of Vdphy were r=0.790 with the ratio of baseline Vdana/vdphy(p=0.035) and r=-0.754 with baseline Vdalv(p=0.050). CONCLUSION: TGI was effective in reducing PaCO2 and deadspace, and the deadspace-reducing effect was best correlated with baseline anatomic/physiologic deadspace ratio.