Non-invasive ventilation (NIV) strategy was introduced into clinical practice more than two decades ago. Its early application in case of acute respiratory failure (ARF) may consistently reduce the number of patients requiring endotracheal intubation (ETI) and invasive mechanical ventilation. Particularly its efficacy has been shown in immunocompromised patients, when ETI is often correlated to higher mortality rate than non-invasive ventilation . However, if advantages of NIV are ascertained, also in patients suffering from cardiogenic pulmonary oedema and chronic obstructive pulmonary disease (COPD), some debate still exists concerning NIV in patients with de novo ARF without the two above cited conditions. Indeed, in such situations, NIV may be beneficial in order to avoid ETI, but any delay in intubation in patients who did not show any significant improvement in oxygenation within a few hours, has been related to higher mortality [3, 4]. Some authors have tried to identify all the factors associated to NIV failure, in order to recognize patients who will need ETI and do it promptly. Nonetheless, many of these factors are quite aspecific (i.e. severity score, patient discomfort, etc.) and often depend also upon the skills of the team providing NIV.
In a recent paper Carteaux et al., investigated the role of tidal volume (TV) in NIV failure in patients presenting to the hospital with ARF. The authors prospectively collected data of adult patients admitted to their intensive care unit (ICU) needing NIV, excluding those affected by cardiogenic pulmonary edema or COPD. Sixty-two patients were finally enrolled in a two-years and a half period, among which 32 experimented NIV failure. Both SAPS II and SOFA scores were higher in patients with NIV failure (41[35–51] vs. 30 [22–38] P< 0.001 and 6 [5-8] vs. 4 [3-7] P=0.01, respectively), similarly PaO2/FiO2 before NIV was lower in patients who required ETI (122 [98–191] vs.177 [133–219], P=0.02), however such differences disappeared after 1 hour of NIV treatment. Though patients who did not require ETI had more days on NIV (3 [2-4] vs. 2 [1-3] P=0.05), total time spent under NIV was similar (16.3 [7.0–27.7] vs. 15 [6.5–22.6] P=0.5), meaning that those who did not were intubated underwent shorter NIV sessions. Finally, TV was significantly higher in patients with NIV failure (10.6 [9.6–12.0] vs. 8.5 [7.6–10.2] P=0.001). In a multivariate model, mean expired TV during NIV remained significantly associated with NIV failure (1.286 [1.069-1.547], P=0.008) and a ROC curve of mean expired TV showed an AUC of 0.85 to predict NIV failure, and a volume of 9.5 ml/kg predicted failure with a sensitivity of 82% and a specificity of 87%.
The role of TV as potential predicting factor of NIV failure was already known in the literature, however this paper underlines that higher TV are not necessarily related to lower PaO2, as one could speculate. Furthermore, in the present paper PaO2/FiO2 differences disappeared in both groups after 1 hour of treatment, meaning that other factors than gas exchange may be implicated in higher TV generation and NIV failure. NIV was delivered in sessions and not continuously, with less than 30% of total time during the first 2-3 days of treatment actually spent in NIV. At the moment, however, we do not have any data to argue that interruption of NIV session may induce a recruitment/derecruitment phenomenon which can further lead to VILI in some patients. Also, PEEP levels were kept quite low during NIV session (5 cmH2O) to avoid leaks, but patients who required intubation were treated with higher PEEP (12 cmH2O) and this may have led to a low recruitment in patients possibly responding to PEEP adjustment even during NIV session. Finally, no information regarding transpulmonary pressure during NIV session were provided in the paper. Though it may be difficult to use an oesophageal catheter in awake patients during NIV, the information about transpulmonary pressure are crucial to understand the mechanisms behind NIV failure in patients who develop higher TV.
In conclusion, this study highlights the importance of mean expired TV during NIV in patients with de novo ARF, indicating that those who reach high TV (∼10 mL/kg) are likely to experiment NIV failure. However, the role of uninterrupted NIV session and the actual effect of high TV on transpulmonary pressure during NIV need further studies to be better understood.
This article review was prepared and submitted by ESICM Journal Review Club and NEXT member Antonio Dell’Anna.
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