The esophageal-tracheal combitube resistance and ventilatory pressures

Study Objective: To measure resistance of the Combitube, a supraglottic ventilatory device used in the management of the patients with difficult airways, and its influence on delivered ventilatory pressures. Design: Prospective study. Setting: University-affiliated hospital. Patients: A total of 20 patients with ASA status I or II who were scheduled for elective knee arthroscopy. Interventions: (Part 2 of the study) After induction of general anesthesia and insertion of the Combitube, mechanical ventilation was initiated. Airway pressures were measured using fluid-filled pressure lines at the Y-piece (P_Y-piece) of the breathing system and in the oropharynx (P _oropharynx) at a position 2 cm beyond the second proximal anterior hole of the Combitube. These pressures were simultaneously recorded and the pressure curves were compared. Measurements: (Part 1 of the study) Resistance of the esophageal and the tracheal lumen of the 37-F Combitube and standard endotracheal tubes (with internal diameters of 6, 7, and 8 mm) was compared ex vivo with a Datex AS/3 monitor. Ventilation conditions were kept constant at a tidal volume of 0.5 L, frequency of 10 breaths per minute, and ramp flow waveform and peak flow of 1 L/s. Main Results: Resistance of standard endotracheal tubes was inversely proportional to their diameters (16, 11, and 7 cm H₂O/L per second for the tubes with internal diameters of 6, 7, and 8 mm, respectively). The resistance of the Combitube's tracheal lumen was 12 cm H₂O/L per second. There was a significant difference in peak respiratory pressures between P_Y-piece and P_oropharynx (40 ± 5 and 23 ± 5 cm H₂O, respectively). Conclusions: The Combitube has significant airflow resistance that should be considered when patients are mechanically ventilated because the delivered oropharyngeal pressure is significantly lower than the pressure measured at the anesthesia breathing system.