Interpretation of the Flow-Volume Loop
We will only discuss the interpretation of the most important test (Forced Vital Capacity).
The flow-volume shape can take on a few distinguishable shapes that correspond to a certain type of pathology:
- normal spirometry
- pathological spirometry
- obstructive lung disease
- restrictive lung disease
- mixed lung disease
- obstruction of the large airways
- exercise induced asthma
A normal flow-volume loop:
A normal Flow-Volume loop begins on the X-axis (Volume axis): at the start of the test both flow and volume are equal to zero. After the starting point the curve rapidly mounts to a peak: Peak (Expiratory) Flow.
After the PEF the curve descends (=the flow decreases) as more air is expired. A normal, non-pathological F/V loop will descend in a straight or a convex line from top (PEF) to bottom (FVC).
The forced inspiration that follows the forced expiration has roughly the same morphology, but the PIF (Peak Inspiratory Flow) is not as distinct as PEF.
A normal volume-time curve:
Another way of representing the spirometry test is through the volume-time graph. The start is at coordinates 0-0 (at time 0, flow is 0). Since most air is expired at the beginning, when the patient empties his large airways, the graph rapidly rises. About 80% of total volume is expired in the first second. As the lungs are emptied the rise in expired volume gets lower and lower to end in a horizontal level.
If spirometry values are too low they may indicate a problem in the airways or lungs. There are several ways to compare spirometric values with predicted values.
Percentage of predicted
Spirometry values have always been compared to predicted values. If the spirometry values were lower than 80% of predicted values, the values were considered to be too low.
This is true for all parameters except the ratios, like FEV1-ratio (or Tiffeneau index). Since FEV1 ratio is a percentage (FEV1/FVC%) it did not make sense to compare this value to a predicted value, in stead it was said FEV1 ratio was too low if it was less than 70%.
Interpretation of spirometry data is based on the best FVC and best FEV1 of all the reproducible tests (these are also used to calculate FEV1-ratio). All the other parameters are taken from the best individual test of the session. The best test is defined as the test that has the highest sum of FEV1 and FVC.
This means that FEV1, FVC and all other parameters do not necessarily come from the same test. Consider the following situation:
|test 1||test 2||test 3||best|
For interpretation the best FEV1 (test 1) and best FVC (test 2) should be used. All other parameters need to come from the best test (highest FEV1+FVC: test 3). The calculated FEV1-ratio (4.86/6.42 = 75.7) is a value that is not found in the individual tests!
Lower Limits of Normal
For years it was known that using a fixed cut-off point across the entire range of ages did not seem to be the best way to assess the spirometry values.
The Lower Limits of Normal (LLN) seem to be a better way to assess spirometric values than the fixed 80% rule. LLN is the lower fifth percentile of the Gaussian bell curve: 95% of healthy people can blow better than the LLN value. (Note that this means there is still a 5% chance of false positives!)
LLN is calculated for every parameter and takes into account age, ethnicity, gender and height. If a spirometry value is lower than the LLN it is considered to be abnormal. This also applies to the FEV1/FVC ratio or Tiffeneau index: according to the new interpretation method a FEV1/FVC ratio of 71% can be too low for a young adult where a FEV1-ratio of 68% can be perfectly normal for an elderly person.
Another way of describing the LLN is the Z-score or Standard Score: a Z-score is the number of standard deviations a certain value is above the mean value of the data set (the Z-score will be negative if the value is lower than the mean). A spirometry value is considered too low if it is more than -1.64 standard deviations from the predicted value (which is the same as the lower 5 percentile).
The advantage of Z-score is that it permits comparison of values between different populations.
In patients with obstructive lung disease, the small airways are partially obstructed by a pathological condition. The most common forms are asthma and COPD.
A patient with obstructive lung disease typically has a concave F/V loop.
is concave, FEF25-75 too low, FVC normal
The air in the large airways usually can be expired without problems, so PEF may be normal.
When all the air is expired from the large airways, air from the smaller airways will be expired. With obstructive lung disease, these airways are partially blocked, so the air will come out slower (you can simulate this by blowing out through a straw!).
This will result in a lower flow and a (more or less) sharp fall in the flow-volume .
FEV1 and FEF25-75 will be too low.
Typically the patient will have a normal FVC at the early stages of his condition.
The FET (Forced Expiratory Time) will be higher due to the lower flow but equal volume.
Historically a Tiffeneau index (FEV1/FVC x 100) less than 70% was considered to be very suggestive for obstructive lung disease. Nowadays the value is compared to LLN.
A bronchodilator test will than be performed to assess reversibility.
Restrictive lung disease means that the total lung volume is too low. Although an accurate diagnoses of total lung volume is not possible with spirometry (residual lung volume cannot be measured with a spirometer) spirometry results can be very suggestive for a restrictive lung disease.
Since the airways are normal, the flow volume loop will have a normal shape: the curve will descend in a straight line from the PEF to the X-axis.
shape normal, FVC low
Total lung volume is low, which results in a low FVC. PEF can be normal or low.
FEV1 is equally lowered than FVC, so the Tiffeneau index will be normal or even raised.
FEV1 too low, FET normal
Often patients will show signs of both obstructive and restrictive lung disease. The flow-volume loop will have characteristics of both syndromes.
FVC, FEV1 and FEF25-75 too low
A typical shape of the flow-volume loop is seen in cases of obstruction of the large airways.
Three different shapes of flow-volume loops can be distinguished.
Variable Extrathoracic Obstruction
Typically the expiratory part of the F/V-loop is normal: the obstruction is pushed outwards by the force of the expiration.
During inspiration the obstruction is sucked into the trachea with partial obstruction and flattening of the inspiratory part of the flow-volume loop.
This is seen in cases of vocal cord paralysis, extrathoracic goiter and laryngeal tumors.
Variable Intrathoracic Obstruction
This is the opposite situation of the extrathoracic obstruction. A tumor located near the intrathoracic part of the trachea is sucked outwards during inspiration with a normal morphology of the inspiratory part of F/V-loop.
During expiration the tumor is pushed into the trachea with partial obstruction and flattening of the expiratory part of the F/V loop.
Fixed Large Airway Obstruction
This can be both intrathoracic as extrathoracic.
The flow-volume loop is typically flattened during inspiration and expiration.
Examples are tracheal stenosis caused by intubation and a circular tracheal tumor.
A small number of patients are never able to blow reproducible flow-volume loops, even with the best instructor next to them, because of a poor understanding of the test or bad coordination. There is however another reason why patients are not able to produce two reproducible f-v loops: exercise-induced asthma. The forced manoeuvre of the FVC can cause an asthma attack in reactive patients. The results of every following flow-volume loop will be worse than the previous trial. It can be tricky to spot this but one should always be aware of this possibility.
The first (red) loop seems ok,
the second (black) shows signs of obstruction,
the third (blue) even more so.