The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

PENETRATION OF A DISTALLY DIRECTED TGI WAVE FRONT WITH OPPOSING (EXPIRATORY) FLOW

Chris Carter M.D., Alexander Adams RRT, Mary Stone RRT, John Marini M.D; Regions Hospital, St. Paul, MN

Introduction: Airflow dynamics in the upper airway during expiration are altered significantly by tracheal gas insufflation (TGI). Presumably, a cone of TGI flow emitting from the catheter tip penetrates distally to flush CO2 from anatomical, then mechanical deadspace. An artificial airway model was constructed to study distally directed TGI flow dynamics retrograde to a bias (expiratory) flow.

Methods: A #7.0 Mallinckrodt Hi-Lo endotracheal tube (ETT) was inserted into a smooth plastic tube (14 mm ID, 60 cm in length) and TGI flow consisting of 100% oxygen was emitted from the distal tip of the ETT. A bias flow of 100% nitrogen (representing expiratory flow) was delivered into the opposite end of the tube to oppose the TGI flow. An extremely sensitive oxygen analyzer (Ametek, Pittsburg, PA) sampled O2 concentrations from a matrix of sites(in a range of depths/distances from the TGI source) to profile TGI gas penetration. A range of TGI/bias flows was tested. A rigid TGI catheter positioned medially was also tested in straight, bifurcated and corrugated airway models.

Results: Under several tested conditions, TGI distal penetration was significant, extending as far as 13 cm from the TGI source. TGI gas commonly appeared at 5-10 cm from the source despite high bias flows. A non-cone front was often observed with TGI gas appearing to adhere to the simulated tracheal wall, not centrally projected as previously thought. In the bifurcated model, TGI penetration extended further along the lower bias flow limb (figure - degree of shading = O2 %).
Conclusions: In this model, retrograde directed TGI penetration (distal effect TGI) was marked and may augment CO2 washout effect. Although retrograde TGI may induce autoPEEP, the additional CO2 washout by distal effect TGI may be a favorable tradeoff. In the bifurcated model, the additional TGI penetration toward the lower bias flow limb suggests that distal effect TGI may selectively enhance CO2 washout from more slowly emptying, higher PACO2 alveolar regions. (See Original for Figure)

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