Fig. 2
An example of the fitted balloon volume-pressure curve using the sigmoid regression equation. a An example of the fitting of balloon pressure and balloon volume in a SmartCath-G balloon at end-expiratory occlusion during simulated mechanical ventilation. Circles are individual data points. The solid line represents the fitted curve. The equation (shown on the top of the figure) has four fitting parameters: a, in units of volume (ml), representing the lower asymptote of the fitted sigmoid curve; b, also in units of volume (ml), representing the vertical distance from the lower to the upper asymptote; c (cmH2O), representing the pressure at the midpoint of the sigmoid curve where the concavity changes direction; and d (cmH2O), is proportional to the minimal balloon pressure change as balloon volume increasing. We defined the balloon volume at balloon pressure of c − d as the minimal inflating volume (V MIN, equal to [a + b/2] − 0.231b) and c + d as the maximal volume (V MAX, equal to [a + b/2] + 0.231b). Within these two points, the balloon pressure change was minimal during the inflation of the balloon. Balloon working volume (V WORK) was defined as the difference between V MIN and V MAX. b The analysis of V WORKs during end-expiratory (EEO) and end-inspiratory occlusion (EIO) in the same balloon in a. The V MIN and V MAX both satisfying the two phases were defined as the low and the high limit of the optimal balloon volume range