Spirometry tests were originally recorded by a pen trace on kymograph paper as a volume-time (VT) curve. Because the changes in the contour of a volume-time curve that occur with various lung disorders are very subtle it is a less than ideal tool for interpreting results. Flow-volume loop (FVL) contours however, are often closely associated with specific lung disorders and for this reason spirometry is most often graphically represented by these instead of volume-time curves.

In order for flow-volume loops to be assessed accurately they need to be reported with sufficient size for details and features to be visible.  Thumbnail versions are inadequate for this purpose and should be avoided.  In addition the tidal loop should be included whenever possible.

It should be noted that the ATS/ERS spirometry standards encourage the selection of the largest FVC and FEV1 even when these come from different tests.  There are however, no guidelines for selecting the most appropriate flow-volume loop.  It would seem that the flow-volume loop associated with the selected FEV1 should be reported since the FEV1 is primarily associated with flow rates. Whether this occurs however, will depend on the reporting software’s abilities and settings, and/or manual overrides. Moreover, since the FVC may be selected from another effort the selected flow-volume loop may show flaws specific to this choice.  

Ideally, an interpreter would be able to review all flow-volume loops from a single testing session, but this may not be an option.  It is therefore the responsibility of the interpreter to familiarize themselves with way in which flow-volume loops are reported.

The basic FVL elements are:

Because there is no time axis in a flow-volume loop it is not possible to determine FEV1 or FEV6 from a flow-volume loop unless the software includes a marker for these values.

Although the ATS/ERS standards for spirometry includes an illustration of a flow-volume loop when discussing normal flow-volume loops, there is no official definition of what constitutes normality. Frequently however, this is considered to be the presence of what has been variously termed a “knee” or “shoulder” in the expiratory contour.

This type of flow-volume loop is typically found in younger subjects and more often in females than in males. Longitudinal studies have shown that this feature often disappears with increasing age even when spirometry results remain normal.

Signs of airway obstruction in flow-volume loops:

Flow-volume loops typically become more concave both with increasing age and with an increase in the severity of airway obstruction.  This concavity has been variously termed “scooping” or “coving”.

When airway obstruction is severe, this inward concavity can develop a distinct inflection point. This contour has been termed the “airway collapse pattern” by researchers and is usually associated with emphysema.

When severe airway obstruction is due to chronic bronchitis, there is generally a severe decrease in Peak Flow and relatively mild concavity.

Signs of restriction in flow-volume loops:

When restriction is due to fibrosis peak flow is often preserved until lung volumes are significantly reduced. Because the vital capacity is reduced this appears as a tall, narrow flow-volume loop.

There are multiple causes for restriction however, and peak flow is not preserved in all of these.  An individual with restrictive lung disease can have a normal looking flow-volume loop that is just reduced in overall size.

Other airway disorders:

Certain types of airway disorders can also affect the flow-volume loop contour.  Paralyzed vocal cords or enlarged goiter or a tumor pressing against the airway can limit flow rates.  A variable flow limitation will only appear in either the expiratory or inspiratory portion of the flow-volume loop, and which portion of the flow-volume loop the limitation appears will indicate whether the flow limitation is intrathoracic or extrathoracic.

Intrathoracic flow limitations will appear only during exhalation.

Extrathoracic flow limitations only appear during inhalation.

When a plateau is present on both inspiration and expiration then the obstruction is a fixed component of the subject’s airway.

Reproducibility is critical when assessing plateaus.  Inspiratory plateaus often occur because of reduced patient effort.

Tracheomalacia can cause the largest airways to collapse during exhalation.  Although this can be said to cause an expiratory plateau the pattern has some differences in that the amount of collapse tends to increase during exhalation making it somewhat variable.

A flow-volume loop can have a sawtooth profile.  This can occur during exhalation or inhalation.  The sawtooth pattern is usually generated either by airflow disturbances in the upper airway or from tremors of the respiratory muscles. It is associated with sleep apnea, obesity, upper airway injury and some neurological disorders but is not a reliable diagnostic sign of these disorders nor does its presence or absence indicate the severity of the disorder. 

Spirometry quality:

A rounded peak flow can be an indication of an inadequate subject effort

An abrupt drop in expiratory flow usually indicates an early termination of exhalation.

A notch in a flow-volume loop indicates a cough or other pause in exhalation.  Since the FEV1/FVC ratio is usually between 0.70 and 0.85, if this notch appears during the first three-quarters of the exhalation then the pause likely affects the FEV1.

A side-ways hop at the beginning of exhalation is an indication of a starting hesitation and usually indicates the presence of excessive back-extrapolation.

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A delayed peak flow can also be an indication of excessive back-extrapolation.

Summary:

The contour of a flow-volume loop is a useful and often critical component in the interpretation process. It is also useful in assessing test quality. A flow-volume loop can sometimes be diagnostic by itself but it is most often useful as a way to confirm or fine-tune a diagnosis that is evident from the numerical results.  

A normal flow-volume loop has usually been considered as one that has a convex or straight profile between peak flow and the end of exhalation, but this is too simplistic. Because the flow-volume loop normally becomes more concave with increasing age the age of the subject also needs to be considered.  A concave flow-volume loop in a young individual likely indicates the presence of airway obstruction whereas in an elderly individual the same flow-volume loop may well be normal.

 

PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License