COMPARISON OF VOLUME CONTROL AND PRESSURE CONTROL VENTILATION - IS FLOW WAVE-FORM THE DIFFERENCES

Objective: To examine the hypothesis that a decelerating inspiratory f low waveform is responsible for improvements in gas exchange during pr essure control ventilation for acute lung injury. Design: Prospective, controlled, crossover study. Measurements and Main Results: Twenty-fi ve patients with acute lung injury requiring mechanical ventilation wi th a positive-end expiratory pressure greater than or equal to 10 cm H 2O, ventilator frequency of greater than or equal to 8 bpm, inspired o xygen concentration of greater than or equal to 0.50, peak inspiratory pressure greater than or equal to 40 cm H2O, and requiring sedation a nd 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 t o 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 ventilat ion with a decelerating non waveform, or volume control ventilation wi th a decelerating flow waveform. Tidal volume, minute ventilation and airway pressures were continuously measured at the proximal airway. Af ter 2 hours of ventilation in each mode, arterial and mixed venous blo od gases were drawn and cardiac output determined by thermodilution, D ead space to tidal volume ratio was determined from mixed expired gas concentrations and Paco(2). During volume control ventilation with a s quare 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 H2 O) p < 0.05 compared to volume control with a decelerating flow wavefo rm and pressure control ventilation. Mean airway pressure was also low er 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 press ure control ventilation and volume control ventilation with a decelera ting flow waveform provided better oxygenation at a lower peak inspira tory pressure and higher mean airway pressure compared to volume contr ol ventilation with a square flow waveform. The results of our study s uggest that the reported advantages of pressure control ventilation ov er volume control ventilation with a square flow waveform can be accom plished with volume control ventilation with a decelerating now wavefo rm.