Muscle Relaxation and Lower Airway Characteristics May Affect Tidal Volume during Pressure-controlled Ventilation

With great interest, we read the article by Sato et al.,¹  which reported the effect of sleep-disordered breathing on tidal volume with mask ventilation during anesthetic induction.

Additional details might better clarify this study and affect the interpretation of the results.

The exact location of the intravenous catheter (upper or lower extremities), which would affect the onset of rocuronium after injection, should be mentioned. Calibration of TOF-Watch is recommended before neuromuscular function monitoring during clinical research² ; however, it was not adopted in this study. Although statistically insignificant, the uncalibrated train-of-four ratio was higher in the sleep-disordered breathing group. More information about the muscle paralysis in each group should have been provided.

The investigation used a ventilator with pressure-controlled ventilation (peak inspiratory pressure, 15 cm H₂O). The resistance and compliance were not reported in this article, which could be important, especially in obese subjects. Both the airway and lungs participate in the process of ventilation, not only the pharyngeal airways. The details of lower respiratory airway and lungs are important to interpret the results. Pressure-controlled rather than volume-controlled ventilation was used. The pressure needed for each subject might vary, and could not be preset individually, especially for obese subjects. If volume-controlled ventilation was chosen, however, the tidal volume could be preset conveniently according to ideal body weight. The inspired volume was the specific aim of mask ventilation. Perhaps the peak inspiratory pressure during volume-controlled ventilation could be a better parameter to describe difficult mask ventilation. And the amount of increased peak pressure to achieve the preset tidal volume among patients with sleep-disordered breathing could be explained by a narrowed pharyngeal airway.

To understand better the dynamic changes of ventilation during mask ventilation, and difficult one-hand mask ventilation in patients with obesity and severe sleep-disordered breathing, particularly when expiratory flow limitation occurs during mask ventilation, additional details would be useful.

Supported by National Natural Science Foundation of China (grant No. 81371251; to Dr. Wang).

The authors declare no competing interests.