From the Guidelines
Helium dilution is a method used to measure functional residual capacity (FRC) of the lungs, and the calculation is based on the principle of conservation of mass, using the formula FRC = V × (C₁ - C₂)/C₂, where V is the initial volume of the circuit, C₁ is the initial helium concentration, and C₂ is the final equilibrium concentration. To perform this calculation, you first need a closed breathing circuit with a known volume and a known initial concentration of helium. The patient breathes normally through this circuit until the helium concentration equilibrates between the circuit and the lungs. For example, if your circuit volume is 4 liters with an initial helium concentration of 10%, and after equilibration the concentration drops to 5%, then FRC = 4 × (10% - 5%)/5% = 4 liters.
The test typically takes 5-7 minutes to complete and requires the patient to maintain a good seal on the mouthpiece throughout the procedure to prevent helium leakage, which would invalidate the results 1. Equilibration is considered to be complete when the change in helium concentration is less than 0.02% for 30 seconds. This method is particularly useful for patients with restrictive lung diseases but may underestimate lung volume in patients with severe obstructive diseases due to poorly ventilated areas not fully equilibrating with helium.
Some key considerations for the test include:
- The patient must be able to breathe normally through the circuit
- The circuit must be closed with a known volume and initial helium concentration
- The patient must maintain a good seal on the mouthpiece to prevent helium leakage
- Equilibration is complete when the change in helium concentration is less than 0.02% for 30 seconds
- The formula used is FRC = V × (C₁ - C₂)/C₂, where V is the initial volume of the circuit, C₁ is the initial helium concentration, and C₂ is the final equilibrium concentration.
It's also important to note that the helium dilution technique may not be suitable for all patients, particularly those with severe obstructive diseases or those who are unable to perform the required respiratory maneuvers 1. In such cases, alternative methods such as body plethysmography may be used. However, the choice of method depends on the individual patient's needs and the availability of equipment.
In terms of reference values, the European Respiratory Society has published prediction equations for lung volumes in children and adolescents, which can be used to interpret the results of helium dilution tests 1. These equations take into account factors such as age, sex, and body size, and can help to identify abnormal lung function. However, it's essential to use the most recent and highest-quality studies to guide clinical decision-making, as the field of respiratory medicine is constantly evolving.
From the Research
Helium Dilution Technique
The helium dilution technique is a method used to measure lung volumes, particularly total lung capacity (TLC) and residual volume (RV). This technique involves breathing in a known concentration of helium and then measuring the dilution of the helium in the lungs.
Calculation of Lung Volumes
The calculation of lung volumes using the helium dilution technique is based on the principle of conservation of mass. The initial volume of helium in the lung is equal to the final volume of helium in the lung, assuming no helium is absorbed or eliminated during the measurement period. The formula for calculating TLC using the helium dilution technique is: TLC = (Vi x Ci) / Cf where Vi is the initial volume of the lung, Ci is the initial concentration of helium, and Cf is the final concentration of helium.
Studies on Helium Dilution Technique
- A study published in 2010 2 compared plethysmographic and helium dilution lung volumes in patients with chronic obstructive pulmonary disease (COPD). The study found that plethysmographic measurements overestimated lung volume compared to helium dilution measurements.
- Another study published in 2016 3 compared lung volumes measured by body plethysmography and helium dilution methods in COPD patients. The study found that the differences between the two methods increased with the severity of COPD and were negatively correlated with FEV1%pred and DLCO%pred.
Limitations of Helium Dilution Technique
The helium dilution technique has some limitations, including:
- It may not be accurate in patients with severe airflow obstruction, as the technique assumes that the helium is evenly distributed throughout the lungs.
- It may not be suitable for patients with certain lung diseases, such as bullous lung disease, as the technique assumes that the lungs are relatively homogeneous.
Note: The studies referenced 4 and 5 are not relevant to the topic of helium dilution technique for lung volume measurement.