We thank Dr. Haghshenas for his interest in our recent paper.1We appreciate his comments and suggestions and will address his concerns in this letter.
First, it is important to define the primary purpose of this study; to determine if nasal ventilation was more effective than oral-nasal ventilation in the patient with difficult mask ventilation. We created difficult mask ventilation by maintaining the patients’ head in the neutral position since patients under anesthesia in head neutral position more likely have airway obstruction than in head extension position.2Our goal was not to identify the level of airway obstruction. We assumed that the level of obstruction would be constant if the head was kept in the neutral position regardless of the approach to mask ventilation. Thus our primary end point was the effectiveness of ventilation. We chose to determine effectiveness by the amount (volume) of carbon dioxide removed per breath using either ventilation approach. Other variables also contribute to the assessment of effective ventilation and indeed we also assessed tidal volume, peak inspiratory pressure and volume of carbon dioxide removed per breath/cm H2O pressure applied during the two approaches.
Dr. Haghshenas suggests that we should have used the pressure control mode instead of volume control mode. We believe that either approach to ventilatory assistance could have been used to assess the effectiveness of ventilation. However, each has its limitations. In pressure mode ventilation, pressure is limited and in volume mode ventilation, volume is limited. We believe we accounted for these limitations by determining the volume of gas delivered per cm H2O peak airway pressure. Peak inspiratory pressure was limited to about 25 cm H2O to avoid putting the studied patients at risk. Thus, we do not consider this a limitation of the study since avoidance of situations increasing the likelihood of gastric insufflations and subsequent vomiting and aspiration is always emphasized during mask ventilation. However, we concede that, if airway pressure is not a consideration, almost any patient can be effectively ventilated if airway pressure applied is high enough.
Since our data were determined by analyzing the expired volume from both the nose and mouth, we were able to determine the total volume of carbon dioxide removed. In addition, we calculated carbon dioxide removal regardless of the size of the tidal volume. Indeed even with an exhaled tidal volume smaller than anatomic dead space, carbon dioxide was still exhaled sometimes because inhalation and exhalation did not occur through the same orifice, nose or mouth.1,3We are aware of the three articles Dr. Haghshenas suggested and agree that they illustrate excellent methods to study upper airway obstruction. However, as indicated above the purpose of this study was not to determine the degree of airway obstruction but to determine the effectiveness of ventilation when the head was in the neutral position using two different approaches.
We agree with Dr. Haghshenas that the patient’s head in the neutral position should not be a common scenario in routine anesthesia. However, again our purpose was to compare to effectiveness of nasal versus oral-nasal mask ventilation in a nonoptimized situation. We also agree that nasal ventilation only needs to be assessed in other settings; however we disagree based on our data with Dr. Haghshenas statement that the “combined mode is probably a better way of ventilation than nasal.” In fact, others have shown that during ventilation through a mouth-piece with head elevated 20 degree no flow is provided if the nose is closed.4
Dr. Haghshenas indicates that auto-positive end-expiratory pressure may have contributed to our negative results during full face mask ventilation. Although we did not measure auto- positive end-expiratory pressure, we believe that auto-positive end-expiratory pressure is an unlikely cause of the differences. None of the patients studied had chronic lung disease, the respiratory rate was maintained at 10/min and tidal volumes were normal or decreased. As a result in spite of the upper airway obstruction sufficient time was provided for exhalation of these relatively small tidal volumes. In addition, previous data from Safar et al. demonstrated that expiratory upper airway obstruction occurs if the anesthetized person was ventilated through the nose and the mouth was kept closed during exhalation.5In our study, we kept the mouth open during both combined and nasal only ventilation. Finally even if auto-positive end-expiratory pressure did develop during full facemask ventilation, it can be considered part of the problem with the difficult airway created by the neutral position and in this setting nasal ventilation produced better results than full face mask ventilation.
Finally, as Dr. Haghshenas’s questions imply, this is simply the beginning of work exploring the use of nasal ventilation during anesthesia and considerably more work must be done before this approach can be recommended for routine practice.
*Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts. email@example.com