Interpreting Spirometric Loops and Curves

Interpreting Spirometric Loops and Curves

Warren S. Goff, DO
7-4-2011

The Flow Volume Loop patterns are the most important representations of performance in Spirometry. Combined with the Volume Time Curve, the adequacy of effort can be ascertained. In addition, characteristic patterns define functional pathology. Finally, the response to therapy can be observed. These graphic recordings are confirmatory and complimentary when one interprets the numeric results as well.

Normal Flow Volume Loops

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There is a Sharp Immediate Increase to the Peak Expiratory Flow Rate (PEFR)
within the first 100 milliseconds from Total Lung Capacity.
This is the effort dependant portion of the curve.

Flow decreases in a linear fashion until one reaches Residual Volume at Flow = 0.
Both ends of the Expiratory and Inspiratory loops start and end on 0 flow.

The portion of the Expiratory Limb between 25% and 75% of Expiration is the FEF 25-75 and is
NOT effort dependant and reflects small airways function.

The Inspiratory Limb should end at 0 Flow and TLC, where Expiration started +/- 5%.

If it falls below TLC this indicates that Expiration was not performed from the actual TLC.

The midpoint of volume (X-axis) represents the FEF50% (MEF50) and FIF50%. (MEF50)
The FEF50% must be greater than the FIF50% (e.g. FEF50/FIF50 >= 1.0)

otherwise, a Variable Extrathoracic Upper Airway Obstruction is suggested.

Not to be confused with a Variable Intrathoracic Upper Airway Obstruction.

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Normal Volume Time Loop

The Spirobank G USB print out displays the data as labeled above.
The FVL and VTC are combined.

Effort and Quality Assessment Flow Volume Loop

Sometimes during tidal respiration we see Inspiration begin while
Expiration is still happening especially in the setting of Severe Airways Obstruction.
This is the pathophysiology of Air Trapping or Dynamic Hyperinflation and
is the etiloogy of Auto-PEEP in ventilated patients.

You will note a still normal shape of the FVL in this instance of coughing.
Sometimes, especially when malingering is suspect, you can have the patient cough
throughout the expiratory manuver. A normal configuration argues against deliberate poor effort.

Effort and Quality Assessment Flow Volume Loop

Actually, one may substitute the FEV at 6 seconds (FEV6) for the FVC in this instance,
as people as obstructed as this will have trouble completing 15 seconds or more expiration.
This is an example of marked airflow obstruction.

Children and sometimes young aduls can empty their lungs in 2-4 seconds.
It is important though, to maintain the effort at zero flow for 6 seconds at least.