Volume-controlled ventilation and pressure-controlled inverse ratio ventilation: a comparison of their effects in ARDS patients.

Volume-controlled ventilation with positive end-expiratory pressure (PEEP) (CPPV) is the conventional ventilatory approach in adult respiratory distress syndrome (ARDS) patients, but some reports suggest that pressure-controlled ventilation with an inverse inspiratory to expiratory ratio (PCIRV) may improve gas exchange in these patients. We analysed the acute effects on gas exchange, lung mechanics and haemodynamics induced by CPPV and PCIRV in ARDS patients. CPPV and PCIRV were applied randomly in ARDS patients. During CPPV, external PEEP was titrated according to the initial inflection point in the static pressure-volume (P-V) curve of the respiratory system, or it was 10 cmH2O when there was no inflection. During PCIRV, external PEEP was not applied, and inspiratory to expiratory (I/E) ratio was inversed until total PEEP was equal to the inflection point in the P-V curve, or it was 10 cmH2O. Respiratory rate, fractional inspiratory oxygen (FIO2), and tidal volume (VT) were kept constant in both modes. Eight ARDS patients were studied prospectively and admitted to a general Intensive Care Unit (ICU) of a University Hospital. Haemodynamic measurements, airflow (V), airway pressure (Paw) and VT were obtained using standard methods. We did not observe any significant change between CPPV and PCIRV with respect to: arterial oxygen tension (PaO2) 117 +/- 12 vs 107 +/- 15 mmHg (16 +/- 2 vs 14 +/- 2 kPa), arterial carbon dioxide tension (PaCO2) 40 +/- 2 vs 39 +/- 2 mmHg (6 +/- 0.3 vs 5 +/- 0.3 kPa), intrapulmonary shunt function (QS/QT) 36 +/- 3 vs 38 +/- 4%, cardiac output (CO) 7.1 +/- 0.7 vs 7 +/- 0.8 l.min-1, and total PEEP 9.7 +/- 0.6 vs 9 +/- 0.3 cmH2O. Oxygen transport and total respiratory system compliance remained unchanged in both modes. Mean Paw was slightly lower during CPPV (17 +/- 1 cmH2O) than during PCIRV (19 +/- 1 cmH2O). PCIRV does not appear to have clinical advantages over CPPV in terms of gas exchange, haemodynamics, or static lung mechanics when using the same total PEEP and minute ventilation.