The helium dilution technique is the way of measuring the

functional residual capacity Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...

of the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s (the volume left in the

s after normal

expiration Expiration or expiration date may refer to: Expiration Expiration may refer to: *Death *Exhalation of breath, breathing out *Expiration (options), the legal termination of an option to take an action *Shelf life, or the time after which a product ...

). This technique is a closed-circuit system where a spirometer is filled with a mixture of

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

(He) and oxygen. The amount of He in the spirometer is known at the beginning of the test (concentration × volume = amount). The patient is then asked to breathe (normal breaths) in the mixture starting from FRC (functional residual capacity), which is the gas volume in the lung after a normal breath out. The spirometer measures helium concentration. The helium spreads into the lungs of the patient, and settles at a new concentration (C2). Because there is no leak of substances in the system, the amount of helium remains constant during the test, and the FRC is calculated by using the following equation:

C_1V_1=C_2V_2

C_1  V_1 = C_2  (V_1 + FRC)

FRC = \frac- V1

V2 = total gas volume (FRC + volume of spirometer) V1 = volume of gas in spirometer C1 = initial (known) helium concentration C2 = final helium concentration (measured by the spirometer)

Measure

Note to measure FRC the patient is connected to the spirometer directly after a normal breath (when the lung volume equals FRC), if the patient is initially connected to the spirometer at a different lung volume (like TLC or RV) the measured volume will be the initial volume started from and not FRC. In patients with obstructive pulmonary diseases the measurements of the helium dilution technique are not reliable because of incomplete equilibration of the helium in all areas of the lungs. In such cases it is more accurate to use a body plethysmograph. A simplified helium dilution technique may be used as an alternative to quantitative CT scans to assess end-expiratory lung volumes (EELV) among patients who are on

mechanical ventilation Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV), is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air ...

with diagnosis of

ALI ʿAlī ibn Abī Ṭālib ( ar, عَلِيّ بْن أَبِي طَالِب; 600 – 661 CE) was the last of four Rightly Guided Caliphs to rule Islam (r. 656 – 661) immediately after the death of Muhammad, and he was the first Shia Imam ...

ARDS Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin color ...

according to a

cross-sectional Cross-sectional data, or a cross section of a study population, in statistics and econometrics, is a type of data collected by observing many subjects (such as individuals, firms, countries, or regions) at the one point or period of time. The anal ...

study. The results show a good correlation ELV(He)=208+0.858xEELV(CT), ''r''=0.941, ''p'' < 0.001between the two methods, and the helium dilution technique offers the advantages of lower cost, decreased transportation of critically ill patients, and reduced radiation exposure. This study's results may have limited generalizability due to its specificity to the ALI/ARDS population and its small sample size (21 patients).

References

{{DEFAULTSORT:Helium Dilution Technique Respiratory system procedures Respiratory therapy Pulmonary function testing