To the Editor:
While the ability of high-flow nasal cannula to achieve apneic oxygenation is well established, the ability to remove carbon dioxide is less clear. The recent investigation by Riva and colleagues on the issue, in which various flow rates were examined during apneic periods, interested us. The study could not identify a meaningful ventilatory contribution attributable to high-flow nasal cannula. While we applaud the authors’ methodology, we feel that the working methodology of high-flow nasal cannula needs to be considered in context with the conclusion and interpretation.
Variations in lung mechanics need to be considered when the interplay of high-flow nasal cannula oxygenation and carbon dioxide exchange are examined. Respiratory mechanics during high-flow nasal cannula oxygenation is likely to differ substantially between the paralyzed and unparalyzed states as in spontaneous and assisted or controlled breathing. The high-flow nasal cannula is believed to generate a level of pharyngeal pressure. Thus, the air being expired from the lung will be opposed by the fresh gas flow from the high-flow nasal cannula, which will produce a positive end-expiratory pressure–like effect, changing the lung volume and influencing ventilation. With paralysis, the resistance against the expiratory flow will be lost.
Furthermore, the continuous positive airway pressure–like effect of the high-flow nasal cannula with the patient’s mouth open due to jaw thrust maneuvers, laryngoscopy, or the presence of an oropharyngeal airway in the paralyzed patient is difficult to envisage and probably negligible. Nevertheless, these factors are likely to alter oropharyngeal gas washout, the mechanism whereby a high-flow nasal cannula provides carbon dioxide removal. Therefore, in our opinion, it will be premature to entirely dismiss the ventilatory effect of high-flow nasal cannula, especially in nonparalyzed patients.