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Clinical evidence highlights the importance of limiting airway pressure during mechanical ventilation. In addition, experimental results suggest the importance of avoiding lung overdistension and cyclic end-expiratory airspace collapse and reexpansion, indicating that both phenomena promote mechanical damage and release of inflammatory mediators.1–3 Unfortunately, interventions that can attenuate the structural insult caused by mechanical ventilation, such as the use of low tidal volume, high positive end-expiratory pressure, and reduced respiratory rate, can limit total minute ventilation.4,5 In this context, transtracheal oxygen therapy and tracheal gas insufflation (TGI) could have a role as adjuncts to mechanical ventilation.6–10
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TGI attempts to minimize dead space by delivering fresh gas through an intratracheal catheter to flush the anatomic dead space free of CO2. During TGI, low-to-moderate flows of fresh gas introduced near the carina, either continuously or in phases, dilute the CO2 in the anatomic dead space proximal to the catheter tip. Because CO2 is washed out during expiration, less CO2 is recycled back into the alveoli during the subsequent inspiration. Any catheter flow during inspiration contributes to the inspired tidal volume (VT) but bypasses the anatomic dead space proximal (mouthward) to the catheter tip. At higher catheter flow rates, turbulence generated at the tip of the catheter by the jet stream can enhance gas mixing in regions distal to the catheter tip, thereby contributing to CO2 removal.11–14 The fresh gas stream exiting the catheter tip rapidly establishes an expiratory front beyond the catheter tip between CO2-rich alveolar gas and CO2-free fresh catheter gas.13 This front is practically abolished by inverting the catheter tip and directing the catheter jet mouthward, thus eliminating the distal effect of TGI.12 These observations indicate that the primary mechanism of CO2 elimination during TGI is expiratory washout, and the forward-directed TGI penetrates a substantial distance into the central airways, extending the compartment susceptible to CO2 washout with a smaller contribution of turbulence beyond the straight catheter tip. Consequently, partial pressure of arterial carbon dioxide (PaCO2) during TGI falls as a nonlinear function of catheter flow rate. Initially, modest flow rates achieve large decrements in PaCO2, but once the anatomic dead space is flushed free of CO2, the effect on PaCO2 diminishes as catheter flow rate increases.6,11
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During TGI, fresh gas can be delivered continuously, or delivery can be timed to occur in phases during a specific portion of the respiratory cycle by gating a solenoid valve that either directs the flow to the catheter or diverts it to the atmosphere.15,16 During continuous TGI, closure of the expiratory valve during inspiration causes catheter flow to deliver variable portions of the inspired V...