Measuring tracheal pressure during jet ventilation.

Abstract

We were very interested to read Patel and Diba's paper on measuring tracheal airway pressures during transtracheal jet ventilation (Anaesthesia 2004; 54: 248–51). They stated that our paper was the only ‘report of airway pressures measured in vivo in man’[1]. However, one of us (SSD) has reported measurement of airway pressure during jet ventilation on three other occasions [2–4]. In these reports, the ventilating catheters were not passed through the cricothyroid membrane but were placed through the glottic opening for jet ventilation. Tracheal pressure and the partial pressure of carbon dioxide were monitored in the same way as during the transtracheal technique referred to. The authors need to clarify why ‘the fixed interval between the luminal openings of the catheter…limits the usefulness of the technique’ and to clarify the ideal location for measurement. The purpose of pressure monitoring in this semi-closed system is the detection of unacceptably high pressure in the airway. In our experience, monitoring via the middle lumen works well. Although the authors are rightly concerned about the ‘whipping’ of a soft catheter inside the trachea, this can be minimised by several means. We used a stiffer catheter, low driving pressure, high frequency of ventilation, and the largest channel for ventilation. We also minimised the intratracheal length of the catheter, and fixed it at the point of puncture at the cricothyroid membrane. This model can easily be reproduced in vitro to verify how small the movement of the tip is. The authors used 4 bar of driving pressure with a 2 s inspiratory time through a 13G catheter at a rate of 10.min−1 or lower. This is, effectively, volume ventilation delivered through a small cannula and is quite different from high frequency jet ventilation. The pressure fluctuations inside the airway will be at a different range. Barotrauma does not result from overpressure alone but can also occur from impingement of the catheter tip on to the tracheal wall during manual jet ventilation at a high driving pressure. Measurement of tracheal airway pressure may not detect this problem and continued manual ventilation at 4 bar could be catastrophic. Most mechanical jet ventilators, in addition to having the adjustable airway pressure alarms and ‘cut off’ pressures, are also equipped to detect an ‘obstructed’ jet, and can then stop further delivery of jet ventilation. Finally, obstruction-to-exhalation cannot be prevented by a relaxed glottis alone. The soft palate and tongue can also obstruct the expiratory pathway. To prevent barotrauma, constant provision of a clear upper airway is required. Using a fibreoptic scope through the upper airway and glottis can separate the epiglottis and the tongue from the posterior pharyngeal wall. By creating a relatively patent upper airway, the authors may not have detected the subglottic negative airway pressure they expected.