Pressure-controlled inverse ratio ventilation after cardiac surgery

Abstract

Background and objective Pressure-controlled inverse ratio ventilation was compared with controlled mechanical ventilation in patients after cardiac surgery.

Methods Ten patients were ventilated after sternal closure using a Siemens Servo 900C ventilator to a target end–tidal PCO₂ of 4.0 kPa. They were randomized to receive controlled mechanical ventilation or pressure–controlled inverse ratio ventilation. CO₂-based data were recorded on a laptop personal computer, which together with arterial PCO₂ permitted measurement of the respiratory dead space. Once measurements were complete the ventilator was switched to the other mode and new measurements taken. Results PaCO₂ and VCO₂ were virtually the same in both modes. Peak airway pressure (17.2±2.7 vs. 20.8±2.5 cmH2O, P < 0.01) and minute ventilation (4.9±1.1 vs. 5.3±1.1 cmH₂O, P < 0.01) were less during pressure–controlled inverse ratio ventilation. Physiological dead space fraction (0.39±0.06 vs. 0.51±0.05, P <0.001), airway dead space (56±15 vs. 81±15 mL, P <0.001) and alveolar dead space fraction (0.25±0.07 vs. 0.31±0.09, P<0.01) were all less during pressure–controlled inverse ratio ventilation. There were no differences in heart rate or mean arterial pressure.

Conclusions The prolonged inspiratory period and pressure–controlled flowpattern of pressure–controlled inverse ratio ventilation reduce the alveolar and airway dead spaces, and give lower peak airway pressures, compared with conventional ventilation, in cardiac surgical patients.