To the Editor:
In a recent issue of Anesthesiology, we read with great interest the article by Dres et al.,1 who prospectively studied the prevalence and risk factors of pleural effusion in patients in the intensive care unit. They showed that the prevalence of pleural effusion had no significant impact on weaning failure, the duration of mechanical ventilation, or the intensive care unit length of stay. We appreciate this research for providing insight into the presence of pleural effusion at the time of liberation from mechanical ventilation among patients in the intensive care unit.
However, several factors that could potentially affect the study results should be discussed. First, the procedure for quantification of pleural effusion is still controversial. The authors adopted the procedure recommended by Balik et al.,2 who quantified the pleural effusion volume using the following formula: pleural effusion volume (ml) = 20 × Sep (mm), where Sep was defined by Balik et al. as the maximal end-expiratory distance between the parietal and visceral pleura on ultrasound. However, Balik et al.2 suggested several potential limitations associated with this procedure. They excluded patients with a small volume of pleural effusion (Sep less than 10 mm), Sep and pleural effusion were not linearly correlated in patients with a Sep of less than 17 mm (i.e., pleural effusion of less than 340 ml), and the Sep value was affected by patient height (size of the thoracic cavity). However, Dres et al.1 included patients with a small volume of pleural effusion, and information regarding the patients’ height is lacking. An additional analysis with consideration of these factors would be helpful. Furthermore, whether the pleural effusions were detected unilaterally or bilaterally and whether the total volumes were calculated as a sum remains unclear. Because the effect of pleural effusion on the respiratory condition and gas exchange might differ, unilateral and bilateral effusions should be analyzed separately.
Second, information regarding interventional and supportive therapy after extubation is lacking. Noninvasive ventilation and high-flow nasal cannula deliver positive pressure to the lungs without intubation, thus improving the lung volume and unloading the respiratory muscles. Previous studies demonstrated that the prophylactic use of noninvasive ventilation and high-flow nasal cannula reduced the risks of postextubation respiratory failure and reintubation.3,4 Considering the effects of these supportive therapies is important to ensuring accurate evaluation of the effect of pleural effusion.
Third, a failed spontaneous breathing trial and an extubation requiring reintubation should be analyzed separately. Extubation failure is commonly defined as the inability to sustain spontaneous breathing after removal of the tracheal tube. Although the most common cause of extubation failure is respiratory failure, which can be evaluated by a spontaneous breathing trial, other frequent causes include airway edema, excessive secretions, inadequate muscle strength, and glottic incompetence.5 The presence of pleural effusion does not appear to affect these causes equally. Provision of the etiologies of extubation failure, and separate analysis of a failed spontaneous breathing trial and extubation requiring reintubation would be helpful to ensure a better understanding of the impact of pleural effusion.
The authors thank Angela Morben, D.V.M., E.L.S., from Edanz Group (Fukuoka, Japan; http://www.edanzediting.com/ac), for editing a draft of this manuscript.
This work was supported by KAKENHI Grants from the Japan Society for the Promotion of Science (JSPS, Tokyo, Japan; Nos. JP 16K09541 and 17K11573), as well as by the Strategic Information and Communications Research and Development Promotion Program (SCOPE, Tokyo, Japan), and Japan Agency for Medical Research and Development (AMED, Tokyo, Japan).
The authors declare no competing interests.