Outside the pulmonary lab there is this notion that spirometry is supposed to be so easy that anyone can do it. You just tell the patient to take a deep breath in and blow out fast and to keep blowing until they’re empty. What’s so hard about that?
Sheesh. GIGO. I keep finding ways in which the patient, their physiology and our equipment can conspire in ways to produce errors that even experienced technicians can miss. I’ve been paying a lot of attention to flow-volume loops lately and maybe it’s for this reason that I’ve seen a steady stream of spirometry tests that have something wrong with the FVC volume.
What I’ve been seeing are flow-volume loops where the end of exhalation is to the left of either the start of the FVC inhalation or of the tidal loop. Taken at face value this means that the patient did not exhale as much as they inhaled (and that the FVC is therefore underestimated) but there are several reasons why this happens and it takes a bit of detective work to figure out the cause and what to do about it.
The simplest reason is a short expiratory time. Flow-volume loops however, do not show time, only flow and volume. Sometimes when a patient stops exhaling abruptly it’s easy to see that the effort is short.
Other times it isn’t as clear:
and you need to look at the volume-time curve as well.
When Tiffeneau described the FEV1 in 1947 and Gaensler the timed FVC and FEV1/FVC ratio in 1950 this opened up an entirely new territory for pulmonary investigators to explore. Numerous new measurements were rapidly mapped out with often conflicting titles. The situation became confusing enough that the British Thoracic Society met in 1956 specifically to standardize terminology. At this time only the volume-time curve was available for measurement purposes (usually a pen trace on kymograph paper) and this fact helped determine what measurements it was possible for researchers to make.
These measurements were in somewhat common use for the first decades of modern spirometry. They have since mostly passed into disuse and have largely been forgotten either because they were superceded by the flow-volume loop or because they never established any particular clinical value. Even so most of these measurements are included as reporting options in current spirometry testing systems. Despite being of questionable value they are still interesting if for no other reason than that they highlight the incredible number of ways that a volume-time curve can be analyzed.
In retrospect the use of the FEV1 and FEV1/FVC ratio to assess airway function seems inevitable but in the early decades it wasn’t clear what timed measurements were optimal. Like the FEV1 the forced expired volume in 0.5 seconds can be measured from the volume-time curve. The FEV0.5 is considered more reproducible than the Peak Expiratory Flow and has been used to assess cough. Although normal values for the FEV0.5 were included in the reference equations from the 1961 VA-Army and Kunudson’s 1976 spirometry study this measurement has rarely been used in adults but has instead found extensive use when measuring and reporting spirometry in infants and children.
I’ve discussed the issue of inserting a predicted FVC into the predicted lung volumes several times now. At the risk of beating this issue to death I’d like to put to rest the notion that an FVC and an SVC are the same thing.
A Forced Vital Capacity (FVC) maneuver is designed to measure the maximum expiratory flow rates, in particular the expired volume in 1 second (FEV1). It has long been recognized that the effort involved in the FVC maneuver can cause early airway closure, even in individuals with normal lungs, and that for this reason the vital capacity can be underestimated due to gas trapping. This effect is usually magnified with increasing age and in individuals with obstructive lung disease.
A Slow Vital Capacity (SVC) maneuver is designed to measure the lung volume subdivisions Inspiratory Capacity (IC) and Expiratory Reserve Volume (ERV), and to maximize the measured volume of the vital capacity. Due to the more relaxed nature of the SVC maneuver there is significantly less airway closure and for this reason the SVC volume is usually larger than the FVC, again even in individuals with normal lungs.
Comparing individual reference equations can be difficult but in general the reference equations for SVC and FVC agree with this. Taking the available SVC and FVC reference equations (unfortunately limited to Caucasian because there are almost no SVC equations for other ethnicities) it is apparent that the average predicted SVC is larger than the average predicted FVC, and that the magnitude of this difference increases with age: