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 (ET T) in the correct evaluation of respiratory system mechanics with the end inflation occlusion method during constant flow controlled mechani cal ventilation. Setting: General ICU, university of Rome ''La Sapienz a''. Patients: 12 consecutive patients undergoing controlled mechanica l ventilation. Methods: We compared the values of minimal resistance o f the respiratory system (i.e. airway resistance) (RRS min) obtained: i) subtracting the theoretical value of ETT resistance from the differ ence between P max and P1, measured on airway pressure tracings obtain ed from the distal end of the ETT, ii) directly measuring airway press ure 2 cm below the ETT, thus automatically excluding ETT resistance fr om the data. Results. The values of RRS min obtained by measuring airw ay pressure below the ETT were significantly lower than those obtained by measuring airway pressure at the distal end of the ETT and subtrac ting the theoretical ETT resistance (4.5+/-2.8 versus 2.5+/-1.6 cm H2O /1/s, p < 0.01). Conclusion: When precise measurements of ohmic resist ances are required in mechanically ventilated patients, the measuremen ts must be obtained from airways pressure data obtained at tracheal le vel. The ''in vivo'' positioning of ETT significantly increases the ai rflow resistance of the ETT.