What is the method for calculating Functional Residual Capacity (FRC) using helium dilution technique?

From the Guidelines

To calculate Functional Residual Capacity (FRC) using the helium dilution method, the formula FRC = V1 × (He1 - He2)/He2 is used, where V1 is the initial volume of the system, He1 is the initial helium concentration, and He2 is the final helium concentration after equilibrium, which is considered complete when the change in helium concentration is less than 0.02% for 30 seconds, as stated in the American Thoracic Society technical statement 1. The helium dilution technique is based on the principle of conservation of mass, where the amount of helium remains constant while its concentration decreases as it distributes throughout the lungs.

• The test involves having the patient breathe from a closed system containing a known concentration of helium gas.
• The patient breathes normally on room air, then is connected to the spirometer containing a known volume and concentration of helium.
• As the patient breathes, the helium distributes throughout the lungs and the system until equilibrium is reached.
• The FRC calculation is then performed using the formula, providing an estimate of the lung volume. This method is widely used in clinical practice, but it may underestimate FRC in patients with severe airflow obstruction because poorly ventilated areas may not equilibrate within the testing period, as noted in the technical statement 1.
• The test typically takes 2-3 minutes to complete and requires the patient to maintain a good seal on the mouthpiece.
• The results are displayed with other volume parameters in a physiologically rational order, allowing for easy interpretation of lung function.

From the Research

Calculating FRC from Helium Dilution

To calculate Functional Residual Capacity (FRC) from helium dilution, the following steps and considerations are taken into account:

• The helium dilution method involves breathing a gas mixture with a high helium concentration and then rebreathing from an air-filled spirometer until the helium concentration equilibrates 2.
• The FRC is calculated from the helium equilibration concentration reached during this process 2.
• The technique can be modified for use in small and lively newborn animals, such as lambs and piglets, under flexible laboratory conditions using relatively simple equipment 2.
• In the compartmental model analysis, the helium concentration equilibrium curve is composed of one compartment and does not have a polyphasic structure 3.
• Each segment of the helium concentration equilibrium curve is evaluated using univariate and multivariate regression analyses to determine the relationship between the segments 3.
• The helium concentration equilibrium curve decreases exponentially over the time course of the analysis 3.

Key Factors Influencing FRC Calculation

Some key factors that influence the calculation of FRC from helium dilution include:

• The initial and middle periods during at rest ventilation, which can affect the concentration change seen in the peripheral lung 3.
• The ventilation volume of the peripheral lung, which does not significantly affect the latter parameter 3.
• The use of a closed helium dilution bag-in-box technique, which allows accurate FRC measurement with good repeatability during both partial support ventilation and pressure-controlled ventilation 4.
• The comparison of helium dilution and nitrogen washout measurements of FRC, which yield accurate, reproducible, and comparable results in normal, healthy infants and very young children, and in those with respiratory disease 5.

Studies Supporting FRC Calculation

Several studies support the calculation of FRC from helium dilution, including:

• A study published in 2016, which analyzed the helium concentration equilibrium curves of healthy patients and clarified the determinants of the equilibrium concentration 3.
• A study published in 2007, which evaluated the accuracy of FRC measurement using the oxygen washout technique and compared it to helium dilution and body plethysmography 6.
• A study published in 2007, which validated a bag-in-box closed helium dilution technique for measuring FRC during partial support ventilation and pressure-controlled ventilation 4.
• A study published in 1992, which devised an accurate method for measuring FRC in small and lively newborn animals using a modified helium dilution technique 2.
• A study published in 1992, which compared measurements of FRC by helium dilution and nitrogen washout in a lung model of known volumes and in infants and very young children 5.