Figure 4. Comparison of values for inspiratory endotracheal tube (ETT)-related work per cycle obtained using direct pressure measurement or the acoustic-Blasius method. As recommended by Bland and Altman, [16]differences between the two values were plotted against the mean of the two values. The curves were obtained in healthy participants using setup A with three levels of pressure support (PS) (top: 0 cm H2O, mean = 0.008 joules, SD = 0.017; middle: 4 cm H2O, mean = 0.031 joules, SD = 0.039; bottom: 7 cm H2O, mean = 0.080 joules, SD = 0.067). Each graph shows the data obtained for each of the four participants using ETTs with three levels of obstruction. The extra work necessitated by the ETT increased with the level of pressure support. This can be ascribed to the increase in flow associated with the increase in pressure support.