Air entrainment during high-frequency jet ventilation

Editor—We read with great interest the article on ‘Air entrainment during high-frequency jet ventilation (HJFV) in a model of upper tracheal stenosis’.¹ The authors concluded that ventilation delivered below the stenosis (BSV) is the safest option from the point of view of low tracheal pressure and consistent oxygen concentrations of injected gas. We have several concerns about the suggestions made by the authors. From our experience, the results of in vitro studies cannot always be transferred to patients. Transtracheal jet ventilation is an invasive technique and in obese patients, small infants, and neonates, the transcutaneous puncture of the cricothyroid membrane may be difficult. Although the airway pressure might be lower in this technique, displacement of the cannula, cervical emphysema, pneumomediastinum, and pneumothorax is frequently described in literature.^2–4 Subglottic jet ventilation via transglottal cannulas is associated with the movement of the cannula; air trapping and barotrauma can occur if the air outflow is not ensured. Further clinical investigations should underscore the results of the study presented. Contrary to the author's results and conclusion, we prefer in our department supraglottic superimposed high/low-frequency jet ventilation (SHFJV) via jet laryngoscopes during endo-laryngotracheal surgery, because it is a non-invasive ventilation technique.^{5 6} In more than 10 yr of experience with this technique and in literature studies, no barotrauma has been detected in supraglottic SHFJV.⁷ The jet laryngoscope is a modification of a Kleinsasser laryngoscope with two integrated nozzles applying simultaneously HFJV and low-frequency jet ventilation. The nozzles are integrated in the wall of the jet laryngoscope; therefore, optimal jet propulsion is ensured and the distance between nozzle orifice and the stenosis remains constant.⁸ The results of the method may be based on a precise body weight depending basic setting of the high and low driving pressure of the jet ventilator, which possibly influence air entrainment and airway pressure. When using HFJV in endo-laryngotracheal surgery, the least invasive method for the patient should be chosen whenever possible.

*E-mail: [email protected]

Editor—We would like to thank Drs Mausser and Schwarz for their interest in our article.¹ We would agree that as with all laboratory investigations, extrapolation of findings to clinical practice must to be carefully guarded and is certainly not appropriate to small infants, neonates, or obese patients. However, our aim was to add to knowledge of the basic physical processes involved during HFJV and not to promote a preferred way of administration of HFJV. The particular advantage of our model is its simplicity which allows us to study different configurations under controllable and reproducible steady-state conditions. We recognize the limitations of the model and problems of direct extrapolation of the results, but would note that high pressures or hyperventilation have been previously reported in animal and human studies using HFJV. These points are clearly stated in the discussion.

Our main conclusion was that air entrainment is likely to be responsible for the higher airway pressures observed during ASV and a lack of entrainment during BSV results in lower airway pressures. The degree of stenosis also has a significant effect on entrainment, delivered volumes, and pressures distal to the stenosis. We would congratulate Drs Mausser and Schwarz on their clinical series using SHFJV but feel it is impossible to directly compare SHFJV with HFJV: SHFJV steady state is difficult if not impossible to define, the jet is placed laterally in relation to the airway inlet, and the distance from the jet orifice to the airway inlet is longer. Furthermore, the jet laryngoscope used during SHFJV described by Rezaie-Majd and colleagues⁷ encroaches on the supralaryngeal area; air entrainment (or obstruction to outflow of gases) cannot be assumed to be similar to ASV using a cannula. Although these authors stated that no barotrauma has been detected in supraglottic SHFJV, we note that recordings of airway pressure were presented for only 13 patients (out of 1515) and would suggest that it is impossible to draw any conclusions regarding airway pressures on such low numbers. In addition, the results of that paper and our own are not directly comparable because of differences in ventilator settings, degree of stenoses (which are not stated), and other clinical variables. In contrast to Rezaie-Majd's assumptions that: ‘The pressure below the stenosis cannot be higher than the pressure above the stenosis with any supraglottic technology. Stenosis will reduce the inflow of jet gas, and the resulting distal airway pressure behind the stenosis will be reduced as well’,⁷ our data show that the converse is true using ASV. We agree that further work is required before our results could be safely applied to clinical practice. However, when choosing a method of ventilation in cases of upper airway stenosis, we would strongly recommend caution and attention to the details of configuration of the interface between the ventilator and the respiratory spaces of the patient's lung.

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