Non-invasive determination of respiratory system mechanics in pressure support ventilation using the expiratory time constant?

Abstract

In the previous issue of Critical Care, we read with interest the article by Al-Rawas and colleagues [1], who proposed a method for the determination of respiratory system mechanics (RSM) in diff erent ventilatory modes. We have some concerns regarding the applicability of the described method during pressure support ventilation (PSV). In particular, all equations proposed in the article contain a term called ‘inhaled fl ow rate’. Since there is no constant inspiratory fl ow during PSV, it is unclear how the authors determined its exact value. Furthermore, all of these equations are transformations of the equation of motion of the respiratory system and require the knowledge of the inspiratory driving pressure, for which the authors apparently used the inspiratory airway pressure. However, in PSV, the patients’ spontaneous breathing activity contributes to inspiratory driving pressure to a variable degree [2]. It would be interesting to know how the authors took the patients’ muscular eff ort (P mus ) into account. Finally, we would like to know why the numbers of patients in the PSV and in the volume-controlled continuous mandatory ventilation groups diff er from the numbers of measurements plotted in the corresponding fi gures. For example, 23 patients were in the PSV group (according to Table 1), but 25 measure ments are shown in Figure 4a,b and only 14 are shown in Figure 4c. A non-invasive determination of RSM during PSV is certainly helpful in a clinical setting, and we would appreciate if the authors provided a detailed description of how they handled the non-constant qualities of fl ow and P mus during PSV.