To the Editor:-We read with great interest the article by Kalfon et al., which demonstrated the efficacy of tracheal gas insufflation (TGI) during expiration (expiratory washout) in reducing PaCO2without increasing the prescribed tidal volume. The authors presented an effective method of enhancing mechanical ventilation in patients with severe respiratory failure. The use of distal airway pressure monitoring, as used by the authors, is of clinical importance during TGI administration because it provides accurate airway pressure measurement.
There are, however, certain comments we would like to make. Although gas insufflation was synchronized with expiration, it resulted in a significant increase in peak, plateau, and mean airway pressures, signifying an increase in end-expiratory lung volume and possibly in minute ventilation as well. The measurement of respiratory volumes and auto-PEEP during TGI is problematic and, at present, not feasible at bedside. The assessment of tidal volume during TGI is problematic because it is difficult to say whether the gas entering the trachea and the smaller airways adds to the tidal ventilation or participates only in airway flushing during expiration. The measurement of auto-PEEP is problematic because it requires accurate timing of the closure of the inspiratory and the expiratory valves and the cessation of TGI flow. For these reasons, it is difficult to draw conclusions about the efficacy of the ventilatory mode when improvement in ventilation is associated with a significant increase in airway pressure.
To resolve similar problems encountered when trying to compare TGI with two different gases, helium and oxygen, we devised a “coefficient of efficiency” to assess the efficacy of TGI at different flow rates and with different gases : The change in PaCO2divided by the change in peak airway pressure. This coefficient, with its easily measured variables (PaCO2and peak airway pressure), helped us to compare different strategies of ventilation according to their ability to clear CO2with minimal lung distention.
In addition, we would like to suggest that the term TGI be used in the future to describe all modes of ventilation in which CO2clearance is enhanced by intratracheal gas flow during part or all of the respiratory cycle.
Arieh Oppenheim, M.D.
Reuven Pizov, M.D.
Department of Anesthesia and Critical Care Medicine; Hadassah Hebrew University Medical Center; P.O. Box 12000; Jerusalem, 91120; Israel
(Accepted for publication November 18, 1997.)