The Forced Vital Capacity (FVC) is defined as the maximal amount of air that can be exhaled with a maximally forced exhalation following a maximal inhalation. The FVC is far more prone to being underestimated than being overestimated and is this usually due to:

• Suboptimal or erratic patient effort
• Pulmonary disorders that limit full exhalation
• Trade-offs in software algorithms
• Patient leaks

The accuracy of the FVC is usually assessed by the expiratory time and the expiratory flow rate at the end of the test.  However additional clues can be gained by comparison with the inspiratory volume or by position of the tidal loop and the apparent IC and ERV.

Expiratory Time:

Lung elastic recoil, respiratory muscle strength and airway conductance all place limits on how fast it is possible for air to be exhaled. These factors are near their maximum values at full inhalation (TLC) and decrease dynamically during exhalation. Exhalation time is a function of these factors and when exhaled volume is plotted against time it approximates an exponential decay curve.

The normal expiratory time has been determined empirically and the ATS/ERS spirometry standards state that subjects should exhale

“for ≥3 s(econds) in children aged < 10 years and for 6 s(econds) in subjects aged > 10 years.”

It also states that:

“For patients with airways obstruction of older subjects, exhalation times >6 s(econds) are frequently needed. However exhalation times of >15 s(econds) will rarely change clinical decisions.”

In one sense, an adequate expiratory time can therefore be immediately determined by the reported expiratory time. All spirometry systems have been computer based for decades however, and the way in which expiratory time is determined is not specifically defined by the ATS/ERS standards. This means that the end of an expiratory effort is determined is by proprietary software algorithms written by different spirometer manufacturers with varying degrees of success. For this reason the accuracy of the reported expiratory time should always be verified by inspection of the volume-time curve. The cause of an inadequate expiratory time can usually be determined, sometimes from the volume-time curve and sometimes from the flow-volume loop.

An early termination of exhalation is probably the commonest reason for a low expiratory time.

The end of an expiratory effort is usually determined when an inspiratory effort is detected. However, there has to be a threshold that inspiratory volume or flow has to pass before an exhalation is automatically ended and this is because patients frequently cough or exhale erratically towards the end of an expiratory effort.

An inspiratory threshold however, can cause the FVC and expiratory time to be underestimated due to a cough or other form of hesitation.  In one sense this example should not be reported because of low test quality but for a given patient, particularly for one with a chronic cough, this may be the best test they were capable of.

The need for an inspiratory threshold can also cause expiratory time to be artificially prolonged when a patient stops exhaling for any reason, but does not inhale.

A short expiratory time does not necessarily mean that the FVC is significantly underestimated.  Because expiratory flows are highest when young and decrease with increasing age, there is a relationship between age and expiratory time. The probability that FVC is underestimated based on expiratory time alone changes with age.

Expiratory flows also decrease with increasing airway obstruction. For this reason when the degree of airway obstruction worsens the probability that FVC is underestimated increases even when the ATS/ERS criteria for expiratory time is met.

End of test conditions:

A full exhalation is usually accompanied by a gradual decrease in expiratory flow rates. An abrupt ending to an expiratory effort is therefore an indication that it has been ended earlier than it should have been. The ATS/ERS standards for the end-of-exhalation (EOT) address this:

“The volume-time curve shows no change in volume (<0.025 L) for ≥ 1 second”

Expiratory efforts with an elevated EOT volume is an indication they were ended earlier than they should have been.  However, the EOT volume will usually meet the ATS/ERS criteria when a patient stops exhaling and does not inhale.

Clues from inspiration:

Patient leaks can be difficult to detect but can be suspected when there is a mismatch between inspiratory and expiratory volumes. Because expiratory time is usually longer than inspiratory time, leaks in either the patient or the testing system will tend to affect expiration more than inspiration.

It should be noted that some individuals with severe airway obstruction are prone to early airway closure and gas trapping and this can also appear as an inspiratory-expiratory mismatch.  However, when the inspiration following the expiratory effort has a larger volume this likely means that the initial inspiratory effort was likely inadequate.

Clues from IC and ERV:

Another clue that an initial inspiration was inadequate comes from the position of the tidal loop within the maximal flow-volume loop. Over a relatively broad range of ages and heights the Expiratory Reserve Volume (ERV) is between one-third to one-quarter of the FVC (or alternately one-half to one-third the size of the Inspiratory Capacity (IC)).  

The IC will decrease due to expiratory flow limitation when there is severe airway obstruction and this is an indication of gas trapping. There is also often a mild decrease in IC in patients with low BMI’s.  

A tidal loop that shows a reduced IC and an elevated ERV without the presence of significant airway obstruction or a low BMI strongly suggests that the initial inspiration was inadequate.

In addition, whenever the FVC is underestimated due to an inadequate inspiration the FEV1 will be underestimated as well.

Overestimation:

The FVC is much less frequently overestimated but when this occurs it is usually due to:

• Spirometer is uncalibrated or miscalibrated
• Errors in Zero offset for flow-based systems

These errors can be difficult detect and are usually apparent when results don’t agree with the patient’s prior results, the patient’s apparent condition or reality. An elevated signal gain in a poorly calibrated spirometer will increase the measured volume and this can occur in both volume and flow-based test systems. A zero offset error in a flow-based system will cause the circuitry to add an apparent flow signal when no flow is present.

Finally, an incorrect FVC, regardless of the reason, has implications not just for itself but also for the FEV1/FVC ratio as well. 

The amount by which the FVC is under- or over-estimated can only be guessed.  For this reason an interpretation has to consider the reported results in terms of the probability they are correct rather than assuming they are.

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