Comparison of volume control and pressure control ventilation: Is flow waveform the differences

Objective: To examine the hypothesis that a decelerating inspiratory flow waveform is responsible for improvements in gas exchange during pressure control ventilation for acute lung injury. Design: Prospective, controlled, crossover study. Measurements and Main Results: Twenty-five patients with acute lung injury requiring mechanical ventilation with a positive-end expiratory pressure greater than or equal to 10 cm H2O, ventilator frequency of greater than or equal to 8 bpm, inspired oxygen concentration of greater than or equal to 0.50, peak inspiratory pressure greater than or equal to 40 cm H2O, and requiring sedation and paralysis were studied, Patients were ventilated at a tidal volume of 10 mL/kg, respiratory frequency was set to maintain a pH > 7.30 and Paco(2) < 50 mm Hg, and positive end-expiratory pressure (PEEP) set to maintain Pao(2) > 70 mm Hg or Sao(2) > 93% with an Fio(2) less than or equal to 0.50. In random sequence, ventilator mode was changed from volume control with a square flow waveform, pressure control ventilation with a decelerating non waveform, or volume control ventilation with a decelerating flow waveform. Tidal volume, minute ventilation and airway pressures were continuously measured at the proximal airway. After 2 hours of ventilation in each mode, arterial and mixed venous blood gases were drawn and cardiac output determined by thermodilution, Dead space to tidal volume ratio was determined from mixed expired gas concentrations and Paco(2). During volume control ventilation with a square flow waveform, Pao(2) was decreased (75 +/- 11 mm Hg vs. 85 +/- 9 mm Hg and 89 +/- 12 mm Hg), p < 0.05, and peak inspiratory, pressure was increased (50 +/- 9 cm H2O vs. 42 +/- 7 cm H2O and 39 +/- 9 cm H2O) p < 0.05 compared to volume control with a decelerating flow waveform and pressure control ventilation. Mean airway pressure was also lower with volume control with a square flow waveform (17 +/- 4 cm H2O vs. 20 +/- 4 cm H2O and 21 +/- 3 cm H2O) compared to volume control with a decelerating flow waveform and pressure control ventilation. There were no differences in hemodynamic parameters. Conclusions: Both pressure control ventilation and volume control ventilation with a decelerating flow waveform provided better oxygenation at a lower peak inspiratory pressure and higher mean airway pressure compared to volume control ventilation with a square flow waveform. The results of our study suggest that the reported advantages of pressure control ventilation over volume control ventilation with a square flow waveform can be accomplished with volume control ventilation with a decelerating now waveform.