Tidal volume is the volume of air that a person inhales and exhales in a calm state; in an adult, it is 500 ml.
Tidal volume is measured by determining the functional residual lung capacity (FRLC) and spirometry. FRLC is the amount of air left in the lungs after a quiet exhalation. Total lung capacity (TLC) is the volume of gas in the lungs after the deepest inhalation.
The FRLC is measured using gas dilution methods or plethysmography (a more accurate method in patients with signs of airflow restriction and air retention).
Gas dilution methods include:
- Leaching nitrogen;
- Helium leveling.
When nitrogen is washed out, the patient exhales to FRLC and then breathes through a spirometer containing 100% oxygen. The study ends when the nitrogen concentration in the exhaled air becomes zero. The collected volume of nitrogen is 81% of the original FRLC.
When equalizing the concentration of helium, the patient exhales to FRLC and then begins to breathe using a closed system that contains certain volumes of helium and oxygen. Helium concentration is measured until a single value is reached both on inspiration and on exhalation, which indicates its balancing with the volume of air in the lungs; subsequently, the amount of air in the lungs is estimated by a change in the concentration of helium.
Both methods underestimate FRLC because they only measure the volume of the lungs that passes through the airways. In some patients with severe airway obstruction, a significant amount of air may not enter the airways, remaining in the lungs.
Bodyplethysmography is based on Boyle’s law and allows you to measure the amount of compressible gas in the chest. It gives more accurate results than the gas dilution method. While in an airtight enclosure, the patient tries to inhale through a closed tip with FRLC. As the chest wall expands, the pressure in the sealed enclosure rises. Knowing the volume and pressure inside the body before and after inhalation, you can calculate the change in body volume, which should be equal to the change in lung volume.
Knowing FRLC, pulmonary volumes can be divided into sub-volumes that can be measured spirometrically or counted. The normal FOEL is about 40% of the TLC.