Risks and precautions None. Avoid eating heavy meals for three hours before the test. Wear comfortable clothing that doesn't restrict breathing. Loose-fitting dentures may be removed. A clip is placed over your nose.
Some people feel claustrophobic, and some are uncomfortable breathing against a closed shutter.
The door of the plethysmograph is closed and tightly sealed. You breathe quietly through the mouthpiece, then are instructed to pant lightly. While you are panting, place your palms flat on your cheeks to make sure the air you exhale is leaving your mouth and not filling your cheeks instead. As you perform various breathing exercises, the pressure in the breathing tube and in the box are recorded. You will be talked through the test by a technician who is usually sitting right next to the plethysmograph.