MEASUREMENT OF FUNCTIONAL RESIDUAL CAPACITY AND RESIDUAL VOLUME
Residual volume and the
functional residual capacity cannot be measured by spirometer and can be
determined by three methods:
1. Helium dilution
technique
2. Nitrogen washout method
3. Plethysmography.
HELIUM DILUTION
TECHNIQUE
Procedure to Measure
Functional Residual Capacity
Respirometer
is filled with air containing a known quantity of helium. Initially,
the subject breathes
normally. Then, after the
end of expiration, subject breathes from respirometer. Helium from respirometer
enters the lungs and starts
mixing with air in lungs. After few minutes of
breathing, concentration of helium in the respirometer becomes
equal to concentration of helium in the lungs of subject. It is called the equilibration of helium. After equilibration of helium between
respirometer and lungs, concentration of
helium in
respirometer is determined. Functional residual capacity is calculated by
the
formula:
V (C1 – C2)
FRC =
C2
Where,
C1 = Initial concentration of helium in the
respirometer
C2 = Final concentration of helium in the
respirometer
V = Initial volume of air in the
respirometer.
Measured Values
For example, the following
data of a subject are obtained from the experiment:
1. Initial volume of
air in respirometer = 5 L (5,000 mL)
2. Initial concentration of helium in
respirometer = 15%
3. Final concentration of helium in
respirometer = 10%.
Calculation
From the above data, the
functional residual capacity of the subject is calculated in the following way:
V (C1 – C2)
FRC =
C2
5,000 (15/100 –
10/100)
FRC = mL
10/100
5,000 (5/100)
= mL
10/100
5,000 × 5
= mL
10
= 2,500 mL
Thus, the functional residual
capacity in this subject is 2,500 mL.
Procedure to Measure
Residual Volume
To determine functional
residual capacity, the subject starts breathing with respirometer after the end
of
normal expiration. To
measure residual volume, the subject should start breathing from the
respirometer
after forced expiration.
NITROGEN WASHOUT
METHOD
Normally, concentration of
nitrogen in air is 80%. So, if total quantity of nitrogen in the lungs is
measured, the volume of air present in lungs can be calculated.
Procedure to Measure
Functional
Residual Capacity
Subject is
asked to breathe normally. At the end of normal expiration, the subject
inspires pure oxygen through a valve and
expires into a Douglas bag. This procedure is repeated
for 6 to 7 minutes, until the nitrogen in lungs is displaced by oxygen. Nitrogen comes to the Douglas bag. Afterwards, following factors are
measured to calculate functional residual capacity.
Calculation
i. Volume of air collected
in Douglas bag
ii. Concentration of
nitrogen in Douglas bag.
By using the data, the functional
residual capacity is calculated by using the formula:
C1 × V
FRC =
C2
Where,
V = Volume of air collected
C1 = Concentration of nitrogen in the collected
air
C2 = Normal concentration of nitrogen in the
air.
Measured Values
For example, the following data are
obtained from the experiment with a subject:
i. Volume of air
collected = 40 L (40,000 mL)
ii. Concentration of nitrogen = 5%
in the collected air
iii. Normal concentration of = 80% nitrogen
in the air.
Calculation
From the above data, the functional
residual capacity of the subject is calculated in the following way:
C1 × V
FRC =
C2
5/100 × 40,000
FRC = mL
80/100
5 × 40,000
= mL
80
= 2,500 mL.
Thus, functional residual capacity in
this subject is 2,500 mL.
Procedure to Measure
Residual Volume
To measure the functional residual
capacity, the subject starts inhaling pure oxygen
after the end of normal expiration and to determine the residual volume, the subject
starts breathing pure oxygen after forceful expiration.
PLETHYSMOGRAPHY
Plethysmography is a technique
to study the variations in the size or volume of a part of the body such as
limb. Plethysmograph
is the instrument used for this purpose. Whole body plethysmograph
is the instrument used to measure the lung volumes including residual volume.
Plethysmography is based
on Boyle’s law of gas, which
states that the volume of a sample of gas is
inversely proportional to
the pressure of that gas at constant temperature.
Subject sits in an
airtight chamber of the whole body plethysmograph and breathes normally through
a
mouthpiece connected to a flow transducer called pneumotachograph.
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