Evaluation of respiratory system resistance in mechanically ventilated patients: the role of the endotracheal tube

Objective: To investigate the role played by the endotracheal tube (ETT) in the correct evaluation of respiratory system mechanics with the end inflation occlusion method during constant flow controlled mechanical ventilation. Setting: General ICU, university of Rome 'La Sapienza'. Patients: 12 consecutive patients undergoing controlled mechanical ventilation. Methods: We compared the values of minimal resistance of the respiratory system (i.e. airway resistance) (RRS min) obtained: i) subtracting the theoretical value of ETT resistance from the difference between P max and P1, measured on airway pressure tracings obtained from the distal end of the ETT; ii) directly measuring airway pressure 2 cm below the ETT, thus automatically excluding ETT resistance from the data. Results: The values of RRS min obtained by measuring airway pressure below the ETT were significantly lower than those obtained by measuring airway pressure at the distal end of the ETT and subtracting the theoretical ETT resistance (4.5 ± 2.8 versus 2.5 ± 1.6 cm H 2O/l/s, p < 0.01). Conclusion: When precise measurements of ohmic resistances are required in mechanically ventilated patients, the measurements must be obtained from airways pressure data obtained at tracheal level. The 'in vivo' positioning of ETT significantly increases the airflow resistance of the ETT