Objectives: We compared biologically variable ventilation (BVV) (as previously described) (1) with conventional control mode ventilation (CV) in a model of acute respiratory distress syndrome (ARDS) both at 10 cm H2O positive end-expiratory pressure.
Design: Randomized, controlled, prospective study.
Setting: University research laboratory.
Subjects: Farm-raised 3- to 4-month-old swine.
Interventions: Oleic acid (OA) was infused at 0.2 mL/kg/hr with Fio2 = 0.5 and 5 cm H2O positive end-expiratory pressure until Pao2 was ≤60 mm Hg; then all animals were placed on an additional 5 cm H2O positive end-expiratory pressure for the next 4 hrs. Animals were assigned randomly to continue CV (n = 9) or to have CV computer controlled to deliver BVV (variable respiratory rate and tidal volume; n = 8). Hemodynamic, expired gas, airway pressure, and volume data were obtained at baseline (before OA), immediately after OA, and then at 60-min intervals for 4 hrs.
Measurements and Main Results: At 4 hrs after OA injury, significantly higher Pao2 (213 ± 17 vs. 123 ± 47 mm Hg; mean ± sd), lower shunt fraction (6% ± 1% vs. 18% ± 14%), and lower Paco2 (50 ± 8 vs. 65 ± 11 mm Hg) were seen with BVV than with CV. Respiratory system compliance was greater by experiment completion with BVV (0.37 ± 0.05 vs. 0.31 ± 0.08 mL/cm H2O/kg). The improvements in oxygenation, CO2 elimination, and respiratory mechanics occurred without a significant increase in either mean airway pressure (14.3 ± 0.9 vs. 14.9 ± 1.1 cm H2O) or mean peak airway pressure (39.3 ± 3.5 vs. 44.5 ± 7.2 cm H2O) with BVV. The oxygen index increased five-fold with OA injury and decreased to significantly lower levels over time with BVV.
Conclusions: In this model of ARDS, BVV with 10 cm H2O positive end-expiratory pressure improved arterial oxygenation over and above that seen with CV with positive end-expiratory pressure alone. Proposed mechanisms for BVV efficacy are discussed.
The use of positive end-expiratory pressure is the conventional means to improve oxygenation in patients with clinically significant acute respiratory distress syndrome (ARDS) (2, 3). Positive end-expiratory pressure can be associated with barotrauma and hemodynamic dysfunction in such patients. We have previously demonstrated improved arterial oxygenation without a concomitant increase in airway pressure by using a computer-controlled mechanical ventilator programmed for biological variability in a porcine model of ARDS at zero end-expiratory pressure (1). This ventilator mimics the normal spectrum of breathing by incorporating breath-to-breath variability in respiratory rate and tidal volume while in control mode.
Because positive end-expiratory pressure increases mean airway pressure (Paw), it was uncertain if the benefits seen with biologically variable ventilation (BVV) also would be seen in the presence of positive end-expiratory pressure. Therefore, we have studied whether BVV would have salutary effects if used with positive end-expiratory pressure. We used the same porcine ARDS model as in our previous study and compared BVV with conventional control mode ventilation (CV), both at 10 cm H2O positive end-expiratory pressure.