Background and Rationale :

Mechanical ventilation is an essential component of the care of patients with respiratory failure.Biphasic positive Airway Pressure (BiPAP) and Airway Pressure release ventilation (APRV) are relatively new modes of mechanical ventilation which can be used in treatment of patients with impaired oxygenation.The effect of using BiPAP and APRV modes on reducing the physiological dead space had not been previously investigated. The investigators hypothesize that using APRV mode will decrease physiological dead space more than BiPAP mode in the mechanically ventilated critically ill patients.

Objectives :

To assess the physiological dead space with each mode. To assess lung mechanics during the use of the two modes. To assess the effectiveness of ventilation during the use of the two modes.

Study population & Sample size :

Sixty adult patients more than 18 years old who are mechanically ventilated patients with P/F ratio less than 300. This sample size was calculated based on the assumption that APRV will decrease dead space by 20% with alpha error 0.05 and power 80%. The mean and Standard deviation of the volume of the dead space assessed in a previous study using BIPAP was 40

Study Design :

A randomized controlled non-blinded study with cross-over design. In the Trauma and surgical ICU at 185-Hospital (Kasr Alainy Hospitals).

Methods :

All mechanically ventilated patients in Trauma and surgical ICU at 185-Hospital (Kasr Alainy Hospitals) will start on pressure controlled ventilation mode (PCV) with inspiratory pressure achieving tidal volume 6-8 ml/kg for 2 hours then they will be randomized into one of the two study groups the BIPAP group or the APRV group .

Possible Risk (s) to study population :

By adjusting the ventilator parameters properly and continuous monitoring of the patients in the study, there will be no risk facing the patients.

Outcome parameter (s):

Primary outcome: Physiological dead space will be measured in the two groups after 30 minutes.

Secondary outcomes

• Physiological dead space after 3 hours.
• PO2/FiO2 ratio.
• Peak airway and Mean airway pressures.
• PCO2 and PH.
• Dynamic compliance.

All mechanically ventilated patients in Trauma and surgical ICU at 185-Hospital (Kasr Alainy Hospitals) will start on pressure controlled ventilation mode (PCV) with inspiratory pressure achieving tidal volume 6-8 ml/kg for 2 hours then they will be randomized into : A-group (APRV group) & B-group (BiPAP group). which are described later in the 2 arms of the study.

Measurement tools :

The following data will be recorded :

• Demographic data ( Age , Sex , Weight and Height ).
• P/F ratio before inclusion.
• All patients then will be connected to Volumetric capnography which is included in the metabolic module on General Electric ventilator (Engstrom Carestation, GE Health care, USA). and physiological dead space will be recorded after 30 minutes and at the end of the 3 hours on each APRV & BiPAP modes .
• Peak air way pressure.
• Mean airway Pressure. They will be also recorded after
• Dynamic Compliance. 30 Minutes and at the end of the
• P/F ratio , PH and pCO2. 3 hours On APRV & BiPAP modes.
• Minute ventilation.
• Blood pressure , Heart rate and central venous pressure will be recorded before inclusion . then , they will be also recorded every 2 hours .

Inclusion Criteria:

• Adults above 18 years old.
• Patients who are recently mechanically ventilated ( within 48 hours ).
• Patients with P/F ratio less than 300.

Exclusion Criteria:

• patients with COPD or pneumothorax.
• Patients with acute lung injury.
• patients with Emphysema and Emphysematous bullae .
• patients with broncho-pleural fistula.
• patients with severe hemodynamic instability (on IV Noradrenaline > 0.8 mic/kg/min).
• patients with cardiomyopathy , and those with stenotic valvular diseases.
• Patients with increased intracerebral pressure.