Look at this example of a 3-way stopcock. You'll notice that there are three prongs (I'm not sure that's what they're actually called). Which ever direction these prongs are facing means that is the direction of flow or it is open. Notice the arrows on each prong.
So for example, you will have your patient hooked up to the bottom port in the picture. The tubing to the flush will be hooked up to the top port and the port on the right is to air or the atmosphere and is capped (this of course is where you will draw your sample).
BEFORE you do any turning of the stopcock, take your waste syringe and connect it to the "air" port (after removing the cap and preparing the port with alcohol or whatever you use.)
Then you turn the stopcock to close the flush and open the line to the patient and your syringe only. To achieve this, turn the stopcock once to the right so that no prong is facing the flush (the arrows will be facing the air port, the patient port and the third arrow will be pointing to nothing).
Draw your waste then turn the stopcock back to the original position BEFORE removing your syringe.
If you fail to do this you will have a bloody mess all over your nice clean sheets!
Repeat the process by attaching the syringe first and then drawing your specimen. Put the stopcock back into the original position BEFORE you remove your syringe
I always have a 2x2 and close the stopcock to the patient but leave it open between the flush and air (one turn to the left) and then squeeze the flush onto the 2x2 to clear the port of residual blood. Then I recap and turn it back to the origianl position and flush the line all the way to the patient.
This won't take you long to get especially if you do gases a lot.
If you just remember that where ever a prong points (the arrows) means that port is open, you'll get this in no time.
There are several different makers of stopcocks. This particular one in the picture doesn't turn 360 degrees so the air port will always be open and MUST always be capped.
Other stopcocks do turn 360 degrees and you could close off the air port. These are the types of stopcocks I work with.
Wow, I didn't expect this to be so lengthy. Let me know if I need to clarify anything.