S p i r o m e t e r
Interpretation of PFT Data
6
®
Lung Volume vs. Time
Lung Volume is the constantly-changing amount of air in the lungs. A spirometer
can measure how it changes, but cannot directly measure its absolute value. Lung
volume can be calculated if one of two specific absolute volumes, residual volume
(RV) or functional residual capacity (FRC), is known or estimated.
Residual volume (RV) is the amount of air remaining in the lungs at the end of a
forced expiration. A typical RV is 1.2 L.
Functional Residual Capacity (FRC) is the amount of air left in the lungs after a
normal expiration (FRC = RV + ERV). A typical FRC is 2.5 liters.
The formula for Lung Volume is:
Lung Volume = V
0
- TotalFlow
V
0
is the lung volume when the subject first starts to breath through the Spirometer.
Set V
0
equal to the subject's FRC, or 2.5 L. Once you have acquired data, you may
need to adjust V
0
so that the graph reflects the subject's known or estimated FRC or
RV.
*
TV
ERV
IRV
FEV
1.0
IC
VC
1 sec.
RV
FRC
TLC
*See the Online Help
for instructions on
entering and graphing
this calculation.
