Animal and lung model studies of tracheal gas insufflation
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Tracheal gas insufflation (TGI) is the continuous or phasic insufflation of fresh gas into the central airways for the purpose of improving the efficiency of alveolar ventilation and/or minimizing the ventilatory pressure requirements. Fresh gas is insufflated near the main carina, usually at flow rates of 2-15 L/min. During expiration, TGI clears the anatomic and apparatus dead space proximal to the catheter tip, thus improving carbon dioxide (CO2) clearance. Moreover, at high catheter flow rates turbulence generated at the tip of the catheter may enhance distal gas mixing. CO2 elimination during TGI depends on catheter flow rate, as at higher flow rates a greater portion of the proximal dead space is flushed clear of CO2. Consequently, as TGI flow is increased, arterial carbon dioxide tension (PaCO2) decreases. Eventually, with increasing catheter flow rate, fresh gas completely flushes the available dead space during expiration and the PaCO2 reaches a plateau. At that point, increasing catheter flow rate decreases PaCO2 much less, probably because of turbulent mixing in the airways distal to the catheter tip. In clinical practice, TGI can be applied either to decrease PaCO2 while maintaining tidal volume constant or to decrease tidal volume while keeping PaCO2 constant. In the former strategy, TGI is used to protect pH, whereas in the latter it is used to minimize the stretch forces acting on the lung parenchyma, to minimize ventilator-associated lung injury.