Intratidal Analysis of Intraoperative Respiratory System Mechanics
First, if inspiratory flow is sufficiently constant, inspiratory volume increases linearly and thus, the inspiratory pressure–time and the pressure–volume curves proceed in similar fashion. Consequently, the presented approach is quite close to the calculation of the “stress index.”3 This method has been applied to patients suffering from acute respiratory distress syndrome and animal models to identify intratidal recruitment/derecruitment or overdistension depending on the inspiratory pressure–time curve deflecting convexly or concavely, respectively. To calculate the stress index, an exponential function and, in the present study, a parabolic function is fit to the pressure–time/pressure–volume curve and then the slopes of the resulting functions are evaluated. Consequently, both approaches underlie the same limitations, which means that the appearance of recruitment/derecruitment and overdistension within 1 breath cannot be reasonably addressed as the underlying functions are limited to either concave or convex curvature.3
Second, the presented methodological approach presumes a strictly constant resistance. Changes of inspiratory flow are supposedly small during volume-controlled ventilation but depend on the ventilator’s performance. Due to the nonlinearity of the endotracheal tube’s resistance, even slight changes of inspiratory flow may skew the slope of the pressure–volume curve to a relevant extent. The analyses did not consider the pressure gradient across the endotracheal tube as they were based on airway pressure and not on tracheal pressure. Hence, the impact of the endotracheal tube’s resistance on the calculated slopes of the pressure–volume curves is uncertain.
The gliding-SLICE method4 provides an approach that does not underlie these limitations. It considers the nonlinearity of resistance and is not restricted to volume-controlled ventilation. The resulting compliance and resistance curves are not preassigned on a specific curvature. Thus, recruitment/derecruitment and overdistension are detectable within a single breath, if compliance increases at early inspiration and decreases at late inspiration. For these reasons, we feel that the gliding-SLICE method would be preferable for analyses of intratidal respiratory system mechanics during controlled mechanical ventilation.