During this test, a clamp will be placed on the nose so that the patient will breathe through his/her mouth. The patient will then be instructed to breathe through a device. This device, in addition to containing oxygen, also contains two inert gases. One of these gases is soluble in blood and disappears over time at a rate proportional to pulmonary blood flow as the patient breathes. The other gas does not enter the bloodstream and allows lung volume to be measured.