We thank Dr. Zeng for the thoughtful comments regarding our article. We totally agree that estimating effective lung volume (ELV) by the capnodynamic method may offer an early detector for loss in lung volume. In the lavage-depleted lung model, we mimic a heterogeneous lung, a condition that is often encountered in pediatric clinical practice. Although we agree that neonates have a continuum state between normal and surfactant-depleted lungs, we still face a heterogeneous condition with regional differences in ventilation distribution. This condition was well demonstrated recently by our research group1 and is confirmed by the significant increase in lung clearance index observed in the current study.2
Regarding the percentage change in ELV after lavage, the trend at different positive end-expiratory pressure levels with ELV was more uniform than end-expiratory lung volume, which may reflect that the former is a functional volume parameter and not a measure of anatomical volume. Furthermore, the ELV value is the only lung volume assessment available at bedside for the clinician, and any recruitment maneuver will be based on the changes in this index. Accordingly, the aim of the clinician will be to re-establish its former value by recruitment maneuver, which eliminates the risk of lung overdistension. It is noteworthy that the lavage-induced decreases in ELV follow a more uniform pattern than end-expiratory lung volume.
There is no doubt that measurement of different lung functional parameters is necessary to reinforce the detection of airway closure and/or reopening. Accordingly, it is very hard to promote one single measure in clinical practice to guide ventilation strategy. Measuring ELV may contribute to better understand the changes observed in respiratory mechanics, but it cannot be considered as a single parameter to detect loss in lung volume. Particularly, changes in pulmonary blood volume may interfere with its absolute value as discussed in the article, which is more obvious at low positive end-expiratory pressure levels. Nevertheless, this method is under improvement and continuous development by our research team. We are currently investigating the relation between ELV and structural global and regional changes in the lungs to improve the algorithm.
Dr. Habre received a research grant from Maquet, Solna, Sweden, for acting as consultant in the respiratory developments. The laboratory where the experiments were performed received a fund from Maquet, Solna, Sweden, to partially cover the material costs.
Drs. Wallin and Hallbäck work at the Research Development Department of Maquet, Solna, Sweden, and they established the theoretical bases of the measurement technique tested in the corresponding article.