Body Plethysmography (Body Box)

Body Plethysmography (Body Box) is an advanced, highly accurate pulmonary function test performed inside an airtight, telephone booth-sized transparent chamber called a plethysmograph, used to measure lung volumes and airway resistance that cannot be determined by standard spirometry alone.

During the test, the patient sits inside the sealed chamber and breathes through a mouthpiece while the door is closed. At a specific point, a shutter briefly blocks airflow and the patient is asked to pant gently against the closed shutter. The pressure changes inside the box and at the mouth are measured simultaneously by sensitive sensors, and using Boyle's Law — which states that pressure and volume of a gas are inversely related at constant temperature — the device calculates precise measurements of lung volumes and airway mechanics.

The key measurements obtained include Total Lung Capacity (TLC), which is the total volume of air the lungs can hold at maximum inhalation; Residual Volume (RV), which is the amount of air remaining in the lungs after a complete forced exhalation that cannot be expelled; Functional Residual Capacity (FRC), which is the volume of air remaining in the lungs at the end of a normal passive exhalation; and Specific Airway Resistance (sRaw) and Airway Resistance (Raw), which measure the resistance to airflow within the airways, useful for detecting early or subtle airway obstruction.

Clinically, body plethysmography is used to confirm and characterize obstructive lung diseases such as asthma and COPD by demonstrating air trapping and hyperinflation through elevated RV and TLC; to diagnose restrictive lung conditions such as pulmonary fibrosis or chest wall disorders by showing a reduced TLC; to differentiate between true restriction and pseudo-restriction that may appear on spirometry alone; to evaluate patients before lung surgery to assess remaining lung function; and to monitor disease progression and response to treatment over time.

Body plethysmography is considered the gold standard for measuring absolute lung volumes and is superior to other lung volume techniques such as gas dilution methods, particularly in patients with severe airflow obstruction or air trapping, where gas dilution tests tend to underestimate lung volumes. When combined with spirometry and FENO, it provides the most complete and detailed assessment of a patient's overall respiratory function.