Helium dilution technique
Encyclopedia
The helium dilution technique is the way of measuring the functional residual capacity
of the lung
s (the volume left in the lung
s after normal expiration
).
This technique is a closed-circuit system where a spirometer is filled with a mixture of helium
(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. 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:
C1×V1 = C2×V2
C1×V1 = C2×(V1+FRC)
FRC = V1×(C1-C2)/C2
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).
Note: To measure FRC we connect the patient 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 we 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
with diagnosis of ALI
/ARDS according to a cross-sectional study . The results show a good correlation [EELV(He)=208+0.858xEELV(CT), r=0.941, p<0.001] between 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).
Functional residual capacity
Functional Residual Capacity is the volume of air present in the lungs, specifically the parenchyma tissues, at the end of passive expiration...
of the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s (the volume left in the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s after normal expiration
Expiration
Expiration is an independent feature film written, directed and starring Gavin Heffernan. It was the winner of the Grand Jury Prize and Best Film at the Canadian Filmmakers' Festival....
).
This technique is a closed-circuit system where a spirometer is filled with a mixture of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads 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. 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:
C1×V1 = C2×V2
C1×V1 = C2×(V1+FRC)
FRC = V1×(C1-C2)/C2
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).
Note: To measure FRC we connect the patient 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 we 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
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...
with diagnosis of ALI
Acute lung injury
Acute lung injury is a diffuse heterogeneous lung injury characterized by hypoxemia, non cardiogenic pulmonary edema, low lung compliance and widespread capillary leakage...
/ARDS according to a cross-sectional study . The results show a good correlation [EELV(He)=208+0.858xEELV(CT), r=0.941, p<0.001] between 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).