Current practice employs low tidal volumes (Vt) in the ventilation of patients with ARDS. Studies supporting this approach also suggest that plateau alveolar pressures (Pplat) associated with such tidal volumes (25 cmH2O) are much lower than Pplat values (<35 cmH2O) previously demonstrated to be safe. These studies assume that Pplat during mechanical ventilation remains constant. We have observed fluctuating airway pressures during positive pressure ventilation of patients with ARDS. The “snapshot” measurements of mechanics in the above studies could thus misrepresent the actual Pplat, explaining the studies’ discrepancies. We have measured serial pulmonary mechanics and pressures during short periods of ventilation of patients with ARDS in order to evaluate this possibility.
In each of 20 patients using mechanical ventilation for ARDS, clinical and ventilator data were extracted from the chart. Subsequently compliance, airway resistance (Raw), peak airway pressure (Ppeak) and Pplat were measured every 30 minutes for 6 hours. These measurements were performed in the absence of any spontaneous breathing effort or change in ventilator settings. Mean values and the range of values of each parameter were measured in each patient.
7 males and 13 females, aged 72+/-17 years were studied. All utilized volume ventilation with a Vt of 9+/-2.6ml/kg and inspired oxygen concentration of 48+/-21%. Compliance was 32+/-10ml/cm with an intraindividual variation of 12+/-6ml/cm. Raw was 18+/-6.6l/s/cm with a variation of 12+/-6.6l/s/cm. Pplat was 23.8+/-8.9cmH2O with ranges of values from 5 to 18 cmH2O. Ppeak was 32 +/-11cmH2O with ranges from 7 to 17.
There is substantial short term variation in airway and alveolar pressures during mechanical ventilation of lungs with ARDS. This may expose such lungs to barotrauma if initial ventilator settings cause alveolar pressures at the high end of the safe pressure range, since these pressures may be exceeded during continued mechanical ventilation.
When optimizing ventilation and lung expansion of patients with ARDS, Pplat should be monitored serially to prevent exposure of the lung to traumatic pressures.
William Marino, None.