Ventilator Settings and Comfort

• Patients' comfort [ Time Frame: 40 minutes ] [ Designated as safety issue: No ]
  Borg score of patients' dyspnea
  
  

Device: Ventilator setting changes
Ventilator settings will be changed every 3 minutes as described above, simultaneously measuring respiratory mechanics and patients' comfort.

Methods: Demographic and physiologic data will be recorded. Investigators will begin measurements more than 60 minutes after last dose of sedative and measurements to occur when no major interventions are planned or medications are scheduled. Then patients will undergo the following ventilator settings (all of which are within ranges ordinarily employed) in random order for 3 minutes of each setting (pressures measured midway i.e. 1.5 minutes - at each interval to ensure safety; stopped if static airway pressure>30 cmH2O):

1. Assist control

   1. Tidal volumes of 6 ml/kg, 8 ml/kg and 10 ml/kg
   2. Flow regimes: constant (CF) and decelerating (dec) at 40, 60 and 80 L/min;
2. Pressure support (PS) 0, 10 and 20 cmH2O with PEEP=5 cmH2O

Patients will be randomized to receive the following in either forward or reverse order: CF60L/min in 6 then 8 then 10 ml/kg; then at 8 ml/kg CF40, Dec40, Dec60, CF60, CF80, Dec80; then PS=0, 10, 20 cmH2O.

In the last 30 second of each setting, patients will be asked to indicate their level of comfort (1-10 in increasing comfort) using a visual analog Borg Score (when they can see/write) or to indicate on their fingers when they cannot write.