To evaluate the effectiveness of high assistance proportional assist ventilation (PAV+) (objective 80% gain) as main ventilatory support in early stage of critically ill patients in comparison with standard volume-assist control ventilation (ACV).

The goal of this proposal is to apply PAV+ as routine ventilatory mode in the early stage of critically ill patients, taking advantages of spontaneous breathing and better patient-ventilator interaction.

The standard treatment in patients with acute respiratory failure is mechanical ventilation in control-mode for the first days of acute illness. This procedure is usually associated with patient-ventilator dyssynchrony, higher needs of sedation and/or relaxation, muscle atrophy, etc. PAV + is a new ventilatory mode that applies pressure in proportion to spontaneous patient inspiratory effort allowing better adaptation to changes in internal homeostasis.

Up to now, several reports compare PAV with assisted modes as a feasible alternative only in the weaning phase. However, PAV is able to unload patient effort in different levels, suggesting that high-assistance PAV (about 80%) could be comparable with assist-control modes in terms of respiratory muscles unload.

Whether PAV is as effective as traditional ACV in terms of ventilation muscle unload in the acute phase of illness has not been established and we aim to address this question.

We plan to prospectively enroll patients on mechanical ventilation early at ICU admission, and to ventilate them randomly under ACV (volume-assist control ventilation) or PAV+ (beginning with 80% if possible). We will evaluate length of mechanical ventilation, sedation requirements and respiratory-hemodynamic variables from the very beginning and until attending clinicians decide that patients are ready to be weaned.

Inclusion Criteria:

• Patients 18 years of age or older
• Anticipated MV > 24 hours
• Availability of informed consent from patient or next of kin
• Ventilation parameters measured under PAV+ 80% gain:

PaO2/FiO2 >100 RPAV <10 cm H2O/l/s CPAV > 30 ml/cm H2O WOBTOT <1.5 J/l VE <18 l/min

Exclusion Criteria:

• Patients on moribund state or with life-sustaining therapy withholding decision.
• Patients with unstable respiratory/hemodynamic state, PaO2/FiO2 <100, Dopamine >15 microg/Kg/min or epinephrine >0.1 microg/kg/min.
• Pregnancy.
• Air leak.
• Patients needing deep sedation or muscle paralysis
• Patients needing hyperventilation (brain trauma).
• Patients with severe muscle weakness.
• Recruitment maneuvers or prone position.