Automatic tube compensation in patients after cardiac surgery: Effects on oxygen consumption and breathing pattern

Objective: To evaluate patients without prior pulmonary disease after cardiac surgery and to determine whether resistive unloading by automatic tube compensation, pressure support ventilation, and continuous positive airway pressure has different effects on oxygen consumption, breathing pattern, gas exchange, and hemodynamics. Design: Prospective, randomized, controlled study. Setting: Tertiary care, postoperative intensive care unit. Patients: Twenty-one patients scheduled for open heart coronary artery bypass graft surgery. Interventions: Each patient was ventilated with all three modes in random order. Measurements and Main Results: Patients were ventilated in three modes, each applied for 30 mins according to computer-generated randomization: pressure support ventilation with 5 cm H 2 O, continuous positive airway pressure, and automatic tube compensation. Oxygen consumption was calculated by means of indirect calorimetry. The hypnotic state of the patients was monitored by Bispectral Index. For hemodynamic measurements, a fiberoptic pulmonary artery catheter was inserted. The main finding of our study was that oxygen consumption and breathing pattern (tidal volume and respiratory rate) did not differ significantly during automatic tube compensation and pressure support ventilation compared with continuous positive airway pressure (oxygen consumption, 170 ± 29 vs. 170 ± 26 vs. 174 ± 29 mL.min.m 2 , respectively; tidal volume, 466 ± 132 vs. 484 ± 125 vs. 470 ± 119 mL, respectively; respiratory rate, 16 ± 4 vs. 15 ± 4 vs. 16 ± 4 breaths/min, respectively). Automatic tube compensation and pressure support ventilation had no clinical effects on gas exchange and hemodynamic variables compared with continuous positive airway pressure. None of the variables differed significantly during the three ventilatory settings. Conclusion: In postoperative tracheally intubated patients with normal ventilatory demand, automatic tube compensation and pressure support ventilation with 5 cm H 2 O lead to identical oxygen consumption, breathing patterns, gas exchange, and hemodynamics. We, therefore, suggest that this group of patients does not need any additional positive pressure support from the ventilator to overcome the additional work of breathing imposed by the endotracheal tube during the weaning phase from mechanical ventilation.