Why do we pre-oxygenate patients before intubation?

This question refers to the practice, prior to intubation, of giving the patient 100% oxygen to breathe for a period of time.

The reason for doing this 'pre-oxygenation' is that it buys us a little more time to perform our intubation. If you've done any respiratory physiology recently, you'll recall that the Functional Residual Capacity (FRC) is the volume of gas left in your lungs at the end of a passive (i.e. non-forced) expiration. In a healthy adult, this constitutes about 2.4 litres of air. You can see a nice picture of all of this here.

Now, if we give the patient 100% oxygen to breathe for a bit, this volume of air (which is largely nitrogen) is gradually replaced with ... pure oxygen. This comes in handy when we're trying to intubate -a period during which the patient isn't receiving any additional oxygen, since (s)he is paralysed or unconscious.

To see the difference, let's do a calculation. In the 2.4 litres of air that we had before, we had about 500 ml oxygen (21% of 2.4 litres). Now, we have (theoretically) 2.4 litres of oxygen - almost five times as much! This additional oxygen can diffuse across the lungs into the blood, providing us with a greater buffer of time to get the tube in before the patient develops hypoxia.