Equipment review: Tracheal gas insufflation

Abstract

Tracheal gas insufflation (TGI) is an adjunctive ventilatory technique that delivers fresh gas into the trachea either continuosly or only during a specific segment of the respiratory cycle (phasic flow) [1,2,3,4]. Two mechanisms are responsible for improving the efficacy of conventional tidal breaths during TGI [5,6,7]. First, fresh gas introduced by the catheter during expiration flushes the series (anatomic) deadspace compartment proximal to its tip free of CO2. Consequently, during the subsequent inspiration, less CO2 is recycled back to the alveoli thus improving CO2 elimination. Second, at high catheter flow rates, turbulence generated at the tip of the catheter can enhance gas mixing in regions distal to the catheter tip, thereby contributing to CO2 removal. Carbon dioxide elimination during TGI depends on catheter flow rate because fresh gas flushes a greater portion of the proximal deadspace at higher flow rates. Moreover, at higher flow rates, turbulence generated at the catheter tip may further enhance distal gas mixing. The volume of fresh gas introduced into the trachea during TGI depends on expiratory time (TE) and catheter flow rate (Vc). At a certain TE × Vc, fresh gas completely sweeps the proximal anatomic deadspace during expiration. At that point, increasing Vc most likely does not dilute the CO2 residing in the series deadspace any further. This operational charactertistic of TGI, and the fact that the decrease in the partial pressure of CO2 (PaCO2) caused by a reduction in total physiologic deadspace fraction (VD/VT) is much less at lower VD/VT, limits the decrement in PaCO2 afforded by TGI at high Vc [8]. Nevertheless, at high Vc (< 10–15 1/min) PaCO2 continues to decrease with increasing Vc, but at a slower rate [8,9,10]. Once the series deadspace is flushed completely by the fresh gas during expiration, the flow dependence of PaCO2 is thought to be secondary to enhanced turbulent mixing in the airways distal to the catheter tip [5,11,12]. TGI is unlikely to be very effective when the alveolar as opposed to the series compartment dominates the total physiologic deadspace; yet, at small tidal volumes (whenever series deadspace is especially high) or when alveolar ventilation is very low. TGI should be a helpful adjunct to conventional mechanical ventilation (CMV) [8].