Figure 3. Volume versus pressure drop across the endotracheal tube (ETT) during a given respiratory cycle. Curves were obtained for a healthy participant using setup B described in Figure 1, either by measuring the pressure drop (broken line) or by computing the pressure drop using the acoustic-Blasius method (solid line). Curves were plotted for ETTs with and without artificial obstruction (upper = fully patent ETTs; lower = obstructed ETTs) and in the presence and absence of mechanical assistance (left = no assistance; right = pressure support).