BACKGROUND: Endotracheal tubes (ETTs) impose a substantial respiratory work load on mechanically ventilated patients. Automatic tube compensation (ATC) should overcome this flowdependent resistive work load; however, ETT resistance can be increased by tracheal secretions or ETT deformities. Our bench study investigated whether ATC provides effective respiratory work load relief in used ETTs. METHODS: We enrolled 20 critically ill patients requiring mechanical ventilation for longer than 48 hours. After extubation, we collected the used ETTs and measured the pressure-time products (PTPs) by using a bellows-in-a-box lung model that simulated spontaneous breathing, at a respiratory rate of 10 breaths/min, inspiratory time of 1.0 s, and tidal volumes (V T ) of 300 mL, 500 mL, and 700 mL. The ventilator was set at ATC 100% with PEEP of 5 cm H 2 O and F IO 2 of 0.21. The flow and airway pressure at the proximal (P aw ) and distal (P tr ) ends of the ETT were recorded, and the PTP integrated from P tr analyzed. RESULTS: PTP values increased with V T during ATC. Even at 100% ATC the ventilator did not completely compensate for the PTP imposed by the ETT. In used ETTs, peak flow and peak P aw were lower and PTP values were greater than in new ETTs. As V T increased, the percentage difference in the PTP values between the new and used ETTs increased. CONCLUSIONS: ATC does not necessarily compensate for an ETT-imposed respiratory work load. ETT configuration changes and tracheal secretions can increase ETT resistance and decrease the ability of ATC to compensate for the increased respiratory work load.