Figure 6. (Upper) Endotracheal tube (ETT) area (patient 3) versus distance in the presence of a catheter (described in the text). The area obtained using an acoustic reflection method was compared with the theoretical area. The theoretical area accounts for the presence of the catheter. The segment between 10 and 40 cm corresponds to the ETT, and the segment between 5 and 10 cm to the connecter used to introduce the catheter into the ETT. The end of the wave tube is located at 5 cm. The vertical lines represent the standard deviation associated with the measurement. (Lower) Comparison between measured and estimated pressure drop values through the ETT of patient 3 during an inspiratory cycle. Measurements were taken using the catheter method. Estimations were obtained using the acoustic-Blasius method, the Blasius formula, and the theoretical ETT diameter given by the manufacturer.