Influence of the endotracheal tube on CO2 transport during high-frequency ventilation

Low-volume, high-frequency ventilation (HFV) delivered via an endotracheal tube can maintain eucapnia in both humans and animals. Because recent animal studies have suggested that a substantial fraction of the resistance to gas transport during HFV can be attributed to the presence of the endotracheal tube, we evaluated the importance of the endotracheal tube on carbon dioxide elimination (VCO2) during HFV in humans. We compared the effectiveness of delivering the fresh gas bias flow at the proximal and the distal end of an endotracheal tube. For each bias flow position, we ventilated patients using tidal volumes of 60 ml or less and frequencies from 0.5 to 12 Hz. In each case, VCO2 was approximately 50% greater when the fresh gas was introduced at the carinal end of the endotracheal tube. Thus, the endotracheal tube contributed about one third of the resistance to HFV-induced CO2 transport in these patients. These results indicate that the position of the fresh gas source strongly influences the effectiveness of HFV.