OBJECTIVE: Tracheal gas insufflation (TGI) reduces PaCO(2) by flushing the tracheal and mechanical deadspace, and may have its maximum benefit when TGI gas is unopposed by significant expiratory gas flow. Thus, limiting TGI to the late expiratory period may diminish tracheal exposure to TGI gas while preserving the efficacy of TGI. This study examined the gas exchange consequences of such late-expiratory TGI. DESIGN AND SETTING: Randomized controlled trial, animal study. MATERIALS: Eleven pigs. INTERVENTIONS: After stable lung injury was established using oleic acid 11 pigs were ventilated using a standardized lung protective strategy. Phasic expiratory TGI was applied for 30 min stages during the last 20%, 40%, 60%, and 100% of expiration in random sequence. PaCO(2) was continuously measured via an indwelling blood gas analysis system. MEASUREMENTS AND RESULTS: PaCO(2) at baseline was 86.1+/-4.7 mmHg, and decreased progressively with increasing TGI duration of 20%, 40%, and 60%, but not 100%, of expiration (PaCO(2)=75.7+/-5.2, 68.8+/-3.6, 65.1+/-5.3 and 65.2+/-5.2 mmHg, respectively). For all stages the reduction in PaCO(2) relative to baseline was significant. Trends of increasing PaO(2) and airway pressure with increasing TGI duration were noted and most likely associated with a TGI-induced increase in lung volume. CONCLUSIONS: Under these conditions confining TGI to the final 60% of expiration achieved effective PaCO(2) reduction, not significantly different from panexpiratory TGI, while limiting exposure of the trachea to TGI gas, and reducing the potential for TGI-induced hyperinflation. These findings suggest that TGI is most effectively applied in a phasic manner in late expiration, with its duration titrated to effect.