Flow volume loop

flow volume loop and interpretation of lung function

1. Pulmonary Function Testing

2. Flow volume loop

3. (A) Normal. Inspiratory limb of loop is symmetric and convex. Expiratory limb is
linear. Flow rates at the midpoint of the inspiratory and expiratory capacity are
often measured. Maximal inspiratory flow at 50% of forced vital capacity (MIF
50%FVC) is greater than maximal expiratory flow at 50% FVC (MEF 50%FVC)
because dynamic compression of the airways occurs during exhalation.

4. (B) Obstructive disease (eg, emphysema, asthma). Although all
flow rates are diminished, expiratory prolongation predominates,
and MEF < MIF. Peak expiratory flow is sometimes used to estimate
degree of airway obstruction but is dependent on patient effort.

5. (C) Restrictive disease (eg, interstitial lung disease, kyphoscoliosis). The loop
is narrowed because of diminished lung volumes, but the shape is generally
the same as in normal volume. Flow rates are greater than normal at
comparable lung volumes because the increased elastic recoil of lungs holds
the airways open.

6. (D) Fixed obstruction of the upper airway (eg,
tracheal stenosis, goiter). The top and bottom of the loops are flattened
so that the configuration approaches that of a rectangle. Fixed
obstruction limits flow equally during inspiration and expiration, and MEF
= MIF.

7. (E) Variable extrathoracic obstruction (eg, unilateral vocal cord paralysis,
vocal cord dysfunction).
When a single vocal cord is paralyzed, it moves passively with pressure
gradients across the glottis. During forced inspiration, it is drawn inward,
resulting in a plateau of decreased inspiratory flow. During forced
expiration, it is passively blown aside, and expiratory flow is unimpaired.
Therefore, MIF 50%FVC < MEF 50%FVC.
Expiratory flow
unimpaired

8. (F) Variable intrathoracic obstruction (eg, tracheomalacia). During a forced
inspiration, negative pleural pressure holds the “floppy” trachea open. With
forced expiration, loss of structural support results in tracheal narrowing of the
trachea and a plateau of diminished flow. Flow is maintained briefly before
airway compression occurs.
Inspiratory flow
unimpaired

9. •How to interpret lung function test?

10. •Know the 3 aspects of lung function test:
1) spirometry
2) volumes
3) diffusion

11. Normal values
Spirometry:
•FEV1 and FVC >80% predicted.
•FEV1/FVC >80% predicted.
Volumes: 80-120%.
Diffusion: 75-125%.

12. •Low FVC suggest possible restriction but need
to look at TLC to confirm (TLC <80%)
•High FRC and TLC (>120% predicted) suggest
hyperinflation.
•High RV/TLC suggest gas trapping.

13. Stepwise approach
Step 1) look at the FEV1. ?obstruction
Step 2) look at FVC. If low, suggest restriction.
•If so, look at TLC to confirm restriction.
Step 3) look at FEV1/FVC.
Low- obstruction
High- suggest restriction. Look at TLC to confirm.

14. Step 4: Look for bronchodilator reversibility.
•Positive if improvement in FEV1 >12% and
>200mls.
Step 5: Look at lung volumes.
High RV/TLC suggest severe gas trapping.
FRC and TLC >120% suggest hyperinflation.

15. Step 6: Look at DLCO/KCO
Low DLCO, normal or low KCO may suggest obstruction
or restriction:
Dx: emphysema.
Low DLCO, very high KCO suggest restrictive lung dx:
Dx: lobectomy, severe pleural disease, kyphoscoliosis,
diaphragmatic paralysis.

16. •Step 7: Is there a mixed picture?
•Yes, if Low FEV1,FEV1/FVC BUT also Low TLC and
DLCO.
•Dx: Emphysema + coexisting pulmonary vascular
disease, anemia, pulmonary fibrosis.
•Dx: scleroderma if accompanied by severely low
DLCO.

17. Features suggestive of pulmonary vascular
disease (ie: Pulmonary embolism):
•Restrictive pattern (Normal FEV/FVC, low TLC)
•No bronchodilator response.
•Very low DLCO in a setting of a normal CXR.

18. •Features suggestive of obesity hypoventilation
syndrome:
•Restrictive pattern
•Obesity
•Hypercapnemia on ABG.(Type 2 respiratory
failure)