To the Editor:—
I applaud the recent landmark investigation by Liang et al. 1regarding nasal mask ventilation. It is a very interesting and innovative work. The abstract was so fabulous that it led me to read article itself. The points that are not mentioned in the abstract are Bispectral Index monitoring and the number of breaths tried in each trial. Before reading the article, I was even thinking about the depth of anesthesia and its impact on metabolism and carbon dioxide production. After reading, I found out that the brevity of trial time and the use of Bispectral Index exclude the alteration of metabolism. I appreciate their work; I’d like some points to be clarified, though.
What are the criteria of the efficacy in different methods and routs of ventilation? I’d like to know why the researchers have used the term effective while they have worked on airway patency . Shall we use a pressure limiting mode of ventilation first and then use the volume of expired breathes as the end point measurement of the study to represent its efficacy? According to figure 2, the tidal volumes exhaled are much smaller in combined mode than nasal one; this is because of the high airway pressure that exceeds the limiting pressure of 25 cm H2O. When there is no inhalation, how can we expect exhalation? It is quite obvious that the volume of breaths exhaled is a main factor when we measure the volume of exhaled carbon dioxide. In some cases, the authors have used carbon dioxide content even when exhaled tidal volume is near zero; in about half cases, the same carbon dioxide content has been used when the exhaled tidal volume is less than 100 ml. These small volumes are less than anatomical dead space, and in such a condition, nobody expects the above-mentioned carbon dioxide concentration to represent alveoli.
The main aim of this article is to study the patency of airway during anesthesia. Since the above-mentioned researchers had employed tidal volume, carbon dioxide content per breath, and peak inspiratory pressure, they were in search of airway patency and severity of airway resistance. Airway patency can be measured differently. Why not to ventilate in a pressure-cycled mode and measure expiratory tidal volume? Why not to ventilate in a volume-cycled mode (without pressure limit) and measure peak pressure? Both will represent efficacy of ventilation. Higher tidal volumes in pressure preset and lower peak pressures in volume preset ventilation suggest lower airway resistance and more effective ventilation.
Why not to use other methods of investigating airway patency? Eastwood et al. 2have used pressure-flow relationships of the upper airway (simultaneous measurements of nasopharyngeal, oropharyngeal, and hypopharyngeal and esophageal pressures) to show increased airway collapsibility during anesthesia. Crawford et al. 3have employed magnetic resonance imaging to measure the dimensions of upper airway in infants during light propofol anesthesia. Nandi et al. 4have been in favor of using conventional lateral radiography to investigate the changes induced by general anesthesia in the upper airway. These are merely some instances, while other methods have also been employed which I hope to be added to the similar future studies.
These researchers have examined their patients in the neutral position that seldom happens in real practice of anesthesia. The study hypothesis is based on overcoming difficult mask ventilation and we are almost sure that when confronting difficulty, the position of head and neck should be modified. For future studies, this study could be repeated in different head and neck positions. In another position, combined mode is probably a better way of ventilation than nasal one. As a result, it seems that it is a little bit soon to decide that “Nasal Ventilation Is More Effective than Combined Oral-Nasal Ventilation.”
A question arises that weather the main locations of obstruction in the airway are the same in inhalation and exhalation or not. Is obstruction possible to happen just during exhalation; or is it less severe during inhalation? I wish the authors had used a measurement of inhaled volume as well to calculate the differences between inhaled and exhaled volumes. By not letting a complete exhalation occur, a new resistance only during exhalation may cause autopeep. Collapsibility of airway is dependent on the balance of compressing and opening forces which are different in inspiration and expiration as well as in spontaneous and mechanical ventilation. For instance, in obstructive sleep apnea, obstruction mainly happens during inspiration which is not the same in mechanical ventilation.
We all have the experience of manual face mask ventilation. At first, we use it simply; and just in the case of difficulty in ventilating, an oral or nasal airway is used; and, at the same time, head and neck position is modified. Suppose that a person is being ventilated only through face mask: the airway is mainly the nasal route and the mouth is closed. If ventilation is helpful and effective, there is no need for oral airway, one of whose characteristics is opening the nasal route by displacing the tongue. Is using nasal mask better than nasal route during face or combined mask ventilation? That is still the question.
Bushehr University of Medicine, Bushehr, Iran. smhagh@yahoo.com
