Where they come from/basic calculations
Since computerized devices are generally very reliable, persons performing spirometry rarely have a need to hand calculate a spirometry tracing, however a basic understanding of where the data comes from will surely make you a better clinician.
The Flow Volume Curve
FVC liters (Forced vital capacity)
PEF (Peak exp flow) highly effort dependant
FEF 25 (Forced exp flow at 25% of the curve)
FIVC in (Forced inspiratory vital capacity)
PIF 50 (Peak inspiratory flow)
PIFR (Peak inspiratory flow rate)
Great curve provides instantly useful information.
Helps to differentiate normal-obstructive or possible restrictive defect. Also allows assessment of patient effort and inspiratory flow characteristics.
FEV1 l/s 2.9 L/S
FVC liters 4 L
FEV1/FVC (Ratio) : simple division
FET: Forced expiratory time (how long they exhaled, quality indicator)
FEV6: Forced expiratory volume in 6 seconds
FEF25-75 can be calculated from this curve
The volume / time curve is valuable to assess effort-- using FET, expiratory plateau, and evidence of back extrapolation error.
Persons familiar with this graphic can pick out obstruction.
- Maximal inspiration
- Blast the air out
- Continue exhalation for 6 seconds or ( 3 seconds if < 10 years old) or until expiratory plateau (no change in volume
Adequate test requires
Minimum of 3 acceptable FVC maneuvers
Acceptable repeatability: Largest FVC and next largest FVC is 0.150 L or less and the difference between the largest and next largest FEV1 is 0.150 L or less.
After 8 spirometry attempts-further attempts are not required.
Spirometry: Acceptable Maneuver Criteria
Assuming correct instruction and coaching, the subject should provide a maximal inspiration, with a good start, with a smooth continuous exhalation, with maximal effort and must meet the following conditions:
- Without a unsatisfactory start of expiration, no hesitation or false start
- Without coughing during the first second of the maneuver.
- Without early termination of expiration
- Without a Valsalva maneuver
- Without a leak
- Without an obstructed mouthpiece
- Without evidence of an extra breath being taken during the maneuver.
If 10 or less years old = 3 seconds minimum
Older that 10 years = 6 seconds minimum
After 3 acceptable spirograms have been obtained, apply the following tests:
- The two largest FVCs must be within 0.150 L of each other
- The two largest FEV1s must be within 0.150 L of each other
If both of these criteria have been met, the testing session may be concluded.
If both of these criteria are not met, continue testing unitl:
- Both of the criteria are met with analysis of additional acceptable spirograms OR
- A total of eight tests have been performed (optional)
- The patient /subject cannot or should not continue
Save, as a minimum, the 3 satisfactory maneuvers