We read with great interest the study by Cristiani and colleagues 1. We acknowledge the relevance of individualized positive end-expiratory pressure (PEEP) in preventing ventilator-induced lung injury (VILI), even in the context of elective pediatric anesthesia, and we commend the authors for their thought-provoking work. However, we would like to raise some considerations regarding their findings. The authors compared lung mechanics in sedated and paralyzed children receiving mechanical ventilation during facemask assistance and after endotracheal intubation. They evaluated airway resistance, driving pressure, and static compliance (Cs) at both zero end-expiratory pressure (ZEEP) and after the application of 5 cm H₂O of PEEP in the two ventilatory conditions. They reported differences in driving pressure and Cs between facemask ventilation and postintubation ventilation at the same PEEP level, with higher driving pressure and lower compliance after intubation 1. First, we would like to remark that the authors did not describe how they assessed the adequacy of the facemask seal. Second, even in the presence of an optimal seal, part of the delivered tidal volume may be diverted to the esophagus and stomach due to the physiological laxity of the upper and lower esophageal sphincters in children 2. This phenomenon could partly explain the observed reduction in Cs, calculated as Cs = tidal volume/driving pressure, after intubation. Once intubated, air leaks are eliminated, and esophageal insufflation is prevented; as a result, the measured driving pressure (the denominator of the equation) may increase. If tidal volume is kept constant, this would inevitably lead to a reduction in calculated Cs. Third, when measuring driving pressure during facemask ventilation, a portion of the tidal volume is distributed within the upper airways (structures that are bypassed by the endotracheal tube) and therefore does not reach the lower airways. This may result in an underestimation of the true pulmonary driving pressure during facemask ventilation. This mechanism could further explain why measurements obtained at ZEEP during facemask ventilation showed lower driving pressure (and consequently higher apparent compliance) compared with those obtained after intubation. Finally, even assuming negligible gas loss within the upper airways, in the absence of PEEP, tidal recruitment in healthy lungs should theoretically be similar for the same tidal volume delivered via facemask or endotracheal tube, and therefore should not lead to differences in the recruitment of collapsed lung units between the two modalities 3. Once again, the differences observed by the authors may suggest an underestimation of driving pressure during facemask ventilation. Another critical aspect concerns the findings related to mechanical power (MP). MP inherently increases with the application of PEEP, as PEEP is included in its formula: MP = 0. 098 × respiratory rate × tidal volume × (ΔPᵢnsp + PEEP) 4. Anyhow, as stated by the authors, a decrease in MP might occur in patients who experience decreased lung inhomogeneity with the increase of PEEP. However, as they reported, this was not observed in clinical data. More recently, it was shown that only if MP is normalized to the size of the aerated lung, it decreases with increasing PEEP in patients with recruitable lungs 5, but this concept has not yet been explored in children with relatively healthy lungs shortly after induction of anesthesia. As a matter of fact, we think that it is important to note that MP is influenced both by patient-related variables (such as driving pressure, which reflects respiratory system mechanics) and by clinician-controlled ventilator settings (such as PEEP and respiratory rate). As acknowledged by the authors, MP has been conceived as a measure of the energy delivered to the respiratory system, which has been associated with the risk of VILI 6. While we agree that the optimization of ventilator settings and their impact on MP represents a potential field of investigation in pediatric critical care, we wish to emphasize that MP, being strongly influenced by clinician-controlled ventilatory settings, is not intended to be considered a marker of disease severity 7, 8. Consequently, we believe that MP should await more consistent evidence from the adult setting before application and validation in children. In conclusion, we greatly appreciate the authors' effort to translate current knowledge on respiratory mechanics into the clinical care of anesthetized children. We strongly encourage future studies to account for upper airway dead space and to further investigate the clinical relevance of MP to advance our understanding of pediatric respiratory pathophysiology. Irene Steinberg: conceptualization, writing – original draft. Zaccaria Ricci: conceptualization, writing – original draft. The author received no specific funding for this work. The authors declare no conflicts of interest. Data sharing is not applicable to this article, as no new data were created or analyzed in this study.
Steinberg et al. (Sun,) studied this question.