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Distribution of FEV1 and VC

This is an unusual diagram. The first decision is usually whether FEV1%VC is below the 5th percentile limit, and if so the severity of airway obstruction. The larger or smaller the VC for age, height and gender, the larger or smaller in general the FEV1. Hence the use of FEV1%VC, a derived index which no longer holds all information contained in the numerator and denominator. The X-Y diagram displays the primary variables VC and FEV1 (expressed in standard deviation scores = z-scores); they give rise to a scatter diagram. The FEV1/VC ratio forms an implicit yet invisible part of the diagram. The ellipse delimits 95% of the healthy population. If the VC and FEV1 of a patient are within the perimeter of the oval, the findings cannot be distinguished from those of healthy subjects. A note of caution, however: in elderly and very short individuals the predicted values for VC and FEV1 are not very accurate.

Relationship between standardized FEV1 and standardized FVC

The X-Y diagram occasionally leads to different interpretation of findings than if you consider the z-scores (= standard deviation scores) of FEV1 and VC separately. In the latter case not all information is used: the relationship between the two is not taken into account. The lower perimeter of the z-score (which takes into account both age and height standardized VC and FEV1) extends to lower values that the individual 5th percentiles of FEV1 or VC. After all, the 5% of the population ‘excluded’ on the basis of FEV1 is not the same as the 5% delineated on the basis of both FEV1 and VC.

The explanation of the red figures (1-7) is given below. To facilitate the explanation we fictitiously adopt a 5th percentile for both FEV1 and VC z-score –2, making it easier to follow from inspection of the grid. Please be aware that the choice of a -2RSD cutoff point is just arbitrarily chosen for convenience: you can easily identify that line in the illustration, but in practice the 5-percentile will be somewhere near -1.64RSD.

Case 1 - The VC and FEV1 are within the ellipse, which delineates the 95% confidence interval of healthy subjects. Similarly, the standard deviation scores of both FEV1 and VC, evaluated individually, are within the normal range.

Case 2 - FEV1 and VC combined are outside the 95% reference interval, whereas FEV1 and VC separately are within the limit of 2 z-scores adopted for this illustration. Apparently the FEV1 is low compared with the (relatively large) VC, giving rise to a low FEV1/VC ratio in this subject.

Case 3 - The FEV1 and VC combined are within the 95% reference interval for healthy subjects. If each index is assessed separately, however, the FEV1 is too low (according to the adopted lower limit of -2 for the z-score), whilst the VC is within normal limits.

Case 4 - The z-score of VC is way beyond -2 (the lower limit adopted for this illustration); it is even unusually large for the VC. The z-score for FEV1 is within normal limits. However, FEV1 and VC combined are outside the 95% reference interval for healthy subjects. We are dealing with a normal FEV1and an FEV1/VC ratio below normal limits combined with an exceptionally large VC. This is an unusual finding. It is generally assumed that this is of no consequence, in particular if the findings are unaffected by bronchodilator drugs.

Case 5 - The z-score of the VC is larger than -2 (the fictitious lower limit adopted for this illustration), and therefore appears to be within the normal range. The score for the FEV1 is clearly too low; also the combination of FEV1 and VC is way beyond the 95% reference interval for healthy subjects. We are dealing with a patient with clear-cut airway obstruction but a VC within normal limits. This is a normal finding in mild to moderate airway obstruction.

Case 6 - According to any of the criteria FEV1 and VC are outside the normal range. The VC is more disturbed than the FEV1; hence the FEV1/VC ratio is this patient is unusually high. In a non-clinical setting restrictive ventilatory defects are very rare, so that the primary concern is to make sure that the VC is truly small rather than being caused by incorrect performance of the maneuver. A restrictive disorder cannot be demonstrated from spirometric findings, but it can be excluded (normal VC) or at best be suspected. A restrictive ventilatory defect requires confirmation from clinical findings, physical examination, chest X-ray and/or assessment of the total lung capacity.

Case 7 - According to all criteria FEV1 and VC are grossly abnormal, a condition which occurs in severe airway obstruction.

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