From: Marshall R.  A rebreathing method for measureing carbon monoxide diffusing capacity.  Amer Rev Resp Dis.  1977; 115: 587-588

“The apparatus consists of a mouthpiece, A T-piece to which bag 1 is attached by a short length of 1.2 cm bore rubber tubing that can be closed by a surgical clamp, and a 3-way tap, to the side arm of which the reservoir bag (bag 2) is attached.  Bag 2 should be able to contain about 3 liters and bag 1 500 ml when just distended.  Measurement of the capacity of bag 1 is important because the intention is just to distend the bad when collecting sample 1 so that the volume of gas removed from the re-breathing circuit is known.

“Into bag 2 is measured accurately (by gas syringe) 2 liters of the helium-CO mixture used for the single-breath method and 100 ml of oxygen.  This volume of oxygen is added to replace most of the oxygen that will be consumed during rebreathing.  The subject with noseclip attached breathes quietly through the mouthpiece.  At the end of a normal expiration the tap is turned to connect bag 2 and the subject is instructed to breath rapidly, almost emptying the bag at each breath.  At about the tenth second, most conveniently at the start of an expiration, the clamp on bag 1 is released and gentle pressure is applied to bag 2 to stop it filling so that the expired gas goes into bag 1 and just fills it.  The tube to this bag is then re-clamped.  The  subject is encouraged to breathe hard and exactly 20 sec (or similar time interval measured by a stop watch) the tap is turned to close off bag 2.  The contents of bags 1 and 2 are analyzed for CO and helium.”