Effects of tracheal gas insufflation and tracheal gas exsufflation on intrinsic positive end-expiratory pressure and carbon dioxide elimination

BACKGROUND: Tracheal gas insufflation (TGI) reduces arterial carbon dioxide tension (P(aCO₂)) when used in conjunction with mechanical ventilation, but antegrade TGI flow increases intrinsic positive end- expiratory pressure (auto-PEEP) or end-expiratory lung volume (EELV). Tracheal gas exsufflation (TGE) might reduce TGI-elevated EELV while retaining the CO₂ elimination effects of TGI. METHODS: An apparatus was designed to control TGI and/or TGE during selected portions of expiration. The first phase of the study examined the general effects of expiratory TGI and TGE on EELV and CO₂ elimination in a mechanical test lung. In the second phase, effects on normal physiology were tested in uninjured dogs. Changes in EELV were measured by respiratory inductance plethysmography, and P(aCO₂) was continuously monitored. TGI and TGE were applied separately and in combination during all or half of the expiratory phase. TGI/TGE combinations found to be most effective at CO₂ elimination in the test lung model were applied in the animal tests. RESULTS: TGI raised EELV and TGE lowered EELV in both models, and exerted their greatest effects on EELV in the latter half of expiration. In the animal model, the TGI/TGE interventions tested reduced P(aCO₂) by 16-32% (p < 0.01) compared to baseline P(aCO₂), similar to or greater than reported clinical studies. CONCLUSIONS: Combining TGI and TGE allowed precise control of EELV in both models. Second-half TGI provided the most effective CO₂ elimination.