Originally Posted by Sister S
I'm trying to come to grips with understanding the difference between ALI and ARDS. All of my resources state that the difference is:
ALI has a specific definition: a PaO2/FiO2 ratio of less than 300 (i.e. if the patient is on 30% O2, the PaO2 is less than 90mmHg)
ARDS – acute respiratory distress syndrome, has the same definition as ALI except that the PaO2/FiO2 ratio is less than 200 (i.e. on 60% O2, the patient’s PaO2 is less than 120mmHg).
I just can't understand the ratio formular. I know I must be dim, but can anyone highlight the obvious for me?
Eternally grateful - Sister S x 
They are essentially the same thing. ALI is a less severe form of ARDS. ARDS us usually caused from ALI and is basically a diffuse inflammation of the tissue of the lungs. ARDS and ALI can be defined by the P/F ratios that you decribed, however there are usually other factors the define it. In ARDS we often see and significant increase in inflation or peak airway pressures and often have to switch ventilation strategies to one of higher respiratory rates and lower tidal volumes to achieve the same minute volume previous to the ARDS. Lung tissue become stiff and less compliant and is more difficult to inflate at safe pressures.
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