It’s all about FEV1, except when it isn’t.

A number of physicians and researchers I’ve known and respected have said that in spirometry it always comes down to FEV1 since it is the primary indicator for airway obstruction. Certainly FVC and the FEV1/FVC ratio are important but because patients can stop exhaling early for any number of reasons FVC can be underestimated which in turn can cause the FEV1/FVC ratio to be overestimated so they are not quite as reliable as FEV1.

There are, of course, a number of factors that can cause FEV1 to be mis-estimated. It can be underestimated due to cough or glottal closure and it can be overestimated because of excessive back-extrapolation. Nevertheless, I think that overall the FEV1 tends to be the most accurate and reliable number obtained from spirometry.

This spirometry report came across my desk this morning: 

  Observed: % Predicted: Predicted:
FVC (L): 5.01 114% 4.39
FEV1 (L): 3.86 117% 3.30
FEV1/FVC: 77 103% 75
PEF (L/sec): 4.91 55% 8.99 

Because a reduced FEV1 is a reliable indicator of airway obstruction doesn’t that mean that a normal or as in this particular case, a slightly elevated FEV1 rules it out? Well, actually no, it doesn’t.

At first glance this looked like a very normal spirometry test, although I was a bit curious about the low PEF. For this reason I pulled up the raw test results and although I saw they were very reproducible one look at the flow-volume loop told me what was going on.

Tracheomalacia_loop 

The Cardio-Thoracic Surgery division of my hospital has an active tracheal reconstruction and stenting program. For this reason we see a fair number of patients with tracheomalacia. Tracheomalacia occurs for a wide variety of reasons and can be congenital or caused by intubation or it can be secondary to cystic fibrosis, relapsing polychondritis, emphysema and a number of other lung diseases. What it basically means is that the trachea has lost some or all of its rigid cartilagenous support and has become floppy and easily collapsible. When a patient with tracheomalacia performs a Forced Vital Capacity, the trachea collapses and limits expiratory flow. This is shown by the plateau on the flow-volume loop.

When you suspect tracheomalacia on the basis of a flow-volume loop one key element that needs to be confirmed is whether the loops are reproducible or not. Performing an FVC maneuver with maximal effort always causes airways to undergo some degree of narrowing and it is this factor that tends to define the maximal flow-volume loop envelope. When an FVC maneuver is performed with a relaxed rather than a maximal effort, airways will not narrow as much and, depending on the underlying condition, the FEV1 can be significantly larger than one from a maximal effort. This is a reason that we use Peak Flow as one of the criteria when selecting which spirometry effort will be reported.

Relaxed_vs_Forced_Effort 

Most importantly, however, when a patient performs an FVC sub-maximally it is very difficult to do this reproducibly. Like anybody else, an individual with tracheomalacia will have a certain amount of variability in their spirometry efforts, nevertheless there will be a threshold flow rate that their efforts should not exceed. When efforts are maximal, the maximal flow rates in the flow-volume loops should cluster near the threshold.

Threshold 

It also helps if there appears to be a true plateau, as if the effort was cut off at the threshold, but the narrowing that occurs from tracheomalacia can be variable so this isn’t a given. In this case, the patient’s flow-volume loops were somewhat erratic, but did pretty much met the threshold criteria. The only loop that didn’t was from an exceptionally erratic effort where the peak flow, if that is what it was, occurred late in the effort.

This patient had a slightly elevated FEV1 and FEV1/FVC ratio and yet still has significant airway obstruction. The patients that we see with tracheomalacia usually have a reduced FEV1 so there usually isn’t a question that some level of airway obstruction is present. You can’t say that this patient’s FEV1 was a lie because his FEV1 really was 117% of predicted, but this is a case where FEV1 alone is misleading. The reduced PEF is a clue, but in a normal flow-volume loop peak flow occurs only momentarily so although suggestive it is not definitive in any way. In fact, none of the numerical results that come from an FVC maneuver can tell you that an expiratory plateau is present (it might be possible to use the MEF25%, MEF50% and MEF75% for this purpose but these are only single points on the flow-volume curve and I’ve haven’t seen any research that links these values to an expiratory plateau). I doubt that there are any computer algorithms that would have caught this and I also suspect that there are many places where spirometry is regularly performed where it would not be recognized. 

Tracheomalacia is a consequence of many lung diseases. It is often unrecognized because it is either relatively minor or because other forms of airway obstruction predominate. The more advanced cases we see because of our hospital’s Cardio-Thoracic Surgery’s tracheomalacia clinic are less common, but there is a distinct flow-volume loop signature for tracheomalacia that anybody performing or interpreting spirometry should be aware of. 

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

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