Airway resistance or thoracic gas volume.

• To measure the volume of air in the lungs.
• To diagnose lung disease or assess its severity.
• To determine whether the airways are obstructed and to what extent.

The test is conducted inside a tightly sealed box where changes in air pressure and volume as you inhale and exhale can be measured and compared to normal values for someone of your age, sex, height, and weight.

• 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.

• You sit inside a plethysmograph, a glass-walled, airtight box about the size of a refrigerator, and insert the mouthpiece into your mouth with your lips sealed around it.
• 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.

You remove the nose clip and exit the plethysmograph. You replace dentures if necessary, and are free to leave.

• Lack of a tight seal over the mouthpiece.
• Bulging of the cheeks during breathing out.

Pressure of the exhaled air against the shutter and changes in lung volume are used to evaluate the ability of your lungs to fill up with air, and to assess the flow of air through the airways.

It's noninvasive.

It detects a problem but doesn't identify its cause.

Test results should help your doctor more precisely understand your pulmonary physiology.