Rapid Shallow Breathing Index

An anesthesiologist recently explained to me the RSBI or Rapid Shallow Breathing Index.  This index is used to predict whether a patient is ready to be extubated.  It is most commonly used in the ICU, but can be used in the operating room as well.  The rapid shallow breathing index is the ratio of respiratory frequency to tidal volumes in liters.

RSBI = (f/VT)

For example, if a patient is breathing 25 breaths/min with a tidal volume of 100mL, there RSBI = (25)/(0.1) = 250 breaths/min/L

So what does this number mean?

An RSBI >100 means they are NOT ready for extubation
An RSBI <100 means they are ready for extubation