FEV₆ as a surrogate for FVC in detecting airways obstruction and restriction in the workplace

The most widely used pulmonary function test is spirometry. However, spirometry is an effort-dependent test that requires careful instruction and the full cooperation of the test subject. The inability to perform acceptable and repeatable manoeuvres may be due to poor subject motivation, poor coaching techniques or failure to understand instructions 1. A prolonged exhalation effort is often needed to obtain a volume–time plateau to meet acceptable end-of-test (EOT) criteria. In subjects with airways obstruction, an acceptable EOT plateau may not occur even after 20 s of exhalation 2, 3. This can be physically exhausting and frustrating for both the subject and the technician, and occasionally results in subject syncope.

The results of spirometry tests are critically important in the occupational setting when used for screening and surveillance programmes 4. Poor quality tests increase the misclassification rates for restriction and airways obstruction. The use of forced expiratory volume in six seconds (FEV₆) as a surrogate for the forced vital capacity (FVC) has recently been found to be acceptable in clinical settings 5, 6. Spirometry results from four recently performed investigations were used to evaluate the possibility that use of FEV₆ would also be acceptable in workplace investigations.

METHODS

RESULTS

Only four out of the 1,143 workers had invalid spirometry results (test sessions where less than three acceptable manoeuvres were obtained), so their results were excluded from this analysis. The median (range) age of the remaining 1,139 workers was 37 yrs (18–71 yrs) and 51.4% were male (table 1⇓). Among them, 42.0% were current smokers, 15.4% were former smokers and 42.6% were never-smokers.

Interpretations based on the FEV₆ had a high agreement rate with those based on the FVC (Kappa = 0.90; p<0.001; table 2⇓). When the values of FEV₁/FEV₆ % were used, the sensitivity for detecting airways obstruction was 92% and specificity 98%. For detecting restriction, the sensitivity was 89% and specificity was 99%. For abnormal spirometry, the sensitivity was 94% and specificity was 98%. Positive and negative predictive values for abnormal spirometry were 92% and 98%, respectively.

The spirometry test results for 43 workers (3.8%) were classified differently by the two techniques, and all of them had observed values close to the LLNs. These inconsistently classified cases are found in the upper left and lower right quadrants of figure 1⇓. Among these 43 workers, pack-yrs of cigarette use were significantly higher (p = 0.009) than in rest of the group. Age, sex, race, test reproducibility, heights and ever-smoking did not differ between these two groups.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1—

a) Overall plot of difference between observed measures and the lower limit of normal (LLN) for both airways obstruction definitions. b) Detailed view of a) in the region of ±5% difference. Quadrant I (upper left): classified as not obstructive by forced expiratory volume in one second (FEV₁)/FEV₆ and obstruction by FEV₁/forced vital capacity (FVC) (1.1%); quadrant II (upper right): classified as not obstructive by FEV₁/FEV₆ and not obstructive by FEV₁/FVC (84.1%); quadrant III (lower left): classified as obstructive by FEV₁/FEV₆ and obstruction by FEV₁/FVC (13.4%); quadrant IV (lower right): classified as obstruction by FEV₁/FEV₆ and not obstructive by FEV₁/FVC (1.4%).

DISCUSSION

Clinical practice guidelines state that airways obstruction is defined as a low FEV₁/FVC 1, 9. Errors in the measurement of either the FEV₁ or the FVC will cause errors in the measurement of this ratio, which may cause misclassification of the results during the interpretation process, especially if the results are near the LLN range. Workers participating in a workplace screening or surveillance programme are generally healthy, but some are at risk for developing lung disease from their workplace exposures, so the median spirometry values are often closer to the LLN than in groups of patients with respiratory symptoms being referred to a pulmonary function laboratory for testing.

In order to avoid errors in the measurement of the FVC, the 1994 ATS recommendation stated that, to be considered acceptable, each manoeuvre should last until a plateau is achieved on the volume–time graph 1. This EOT criterion is defined as a <20-mL change in volume during the final 2 s of the manoeuvre. Patients with airways obstruction frequently fail to meet this EOT criterion 3, but, when coached to perform a “relaxed expiration”, they are somewhat more successful 10. However, spirometry reference equations were obtained from studies where the subjects were coached to perform with maximal exhalation efforts, so relaxed expirations cannot be recommended as a solution to this problem 11.

The current authors understand that a normal FVC rules out restriction 12, and that a low FVC poorly predicts a low total lung capacity (TLC; true restriction), requiring confirmation by static lung volume tests, which include the TLC, the residual volume and the functional residual capacity 9; however, static lung volume tests are rarely performed in occupational settings. Failure to obtain FVC manoeuvres with acceptable EOT plateaus is relatively common in the occupational and primary care settings, in some cases due to time constraints, lack of adequate technician training or dedication (leading to poor coaching), poor motivation of the subjects to keep blowing out, a high prevalence of subjects with severe airway obstruction, or a faulty spirometer that prematurely terminates data collection. These short FVC manoeuvres cause underestimations of the true FVC, making a healthy subject's FVC more likely to fall below the LLN, a falsely positive result mimicking ‘restriction.’ At the same time, the FEV₁/FVC ratio is falsely increased, so that subjects with mild airways obstruction are more likely to have a falsely negative result. These short exhalation times should be detected either by the quality control software of modern spirometers or by the physician interpreting the results. Performance of slow vital capacity (SVC) manoeuvres may demonstrate a vital capacity in the normal range, thereby ruling out restriction, but SVC manoeuvres are rarely performed outside of hospital-based pulmonary function laboratories.

Unfortunately, many spirometers that are currently in use either lack manoeuvre quality-control checks and messages, or they are turned off or ignored. Physicians interpreting the spirometry tests may miss the short exhalation times, the lack of EOT plateaus, or may view only the numeric results. Both types of misclassification of spirometry interpretations have negative consequences in the workplace setting. A potential method for reducing this misclassification rate in the “real-world” (nonresearch) setting, where short manoeuvres are common, is to utilise reference equations based on 6-s manoeuvres.

Glindmeyer et al. 13 noted that short FVC manoeuvres result in underestimates of the FVC, and considered recommending shorter spirometry manoeuvres (allowing subjects to stop after 5, 6 or 7 s). However, the ability to use shorter manoeuvres without high misclassification rates awaited the publication of reference equations for these variables from shorter manoeuvres. More than a decade later, the NHANES III study finally provided reference equations for the FEV₆ and FEV₁/FEV₆ 8. The National Lung Health Education Program quickly recommended that 6-s spirometry manoeuvres and the NHANES III reference equations should be used for detecting airways obstruction in adult cigarette smokers being seen by general practitioners who provide primary care 14. Some new models of office spirometers provide NHANES III reference equations for the FEV₆ and FEV₁/FEV₆, and base their automated interpretations on these values.

Swanney et al. 6 showed that using the FEV₁/FEV₆ to detect airways obstruction in 337 patients referred to a hospital-based pulmonary function laboratory in New Zealand resulted in a low misclassification rate when compared with the traditional FEV₁/FVC. Enright et al. 5 then showed that the FEV₁/FEV₆ predicted the subsequent 5-yr decline in FEV₁, as well as the FEV₁/FVC in 5,887 adult smokers.

There is currently some minor confusion regarding the nomenclature for measurements of the vital capacity from 6-s spirometry manoeuvres. The 1999 NHANES III study defined the FEV₆ as the volume in the spirometer at exactly 6 s after the manoeuvre had begun (using the standard back-extrapolation technique to define the start of the exhalation manoeuvre). The air collected inside volume spirometers, such as the ones used by the NHANES III study, cools throughout the FVC manoeuvre (especially when the ambient temperature is relatively low); therefore, in some healthy young subjects who rapidly exhale almost all of their air within 6 s, the cooling and contraction of exhaled air in the spirometer causes the volume–time graph to “droop” before the 6-s mark. Such slight downward slopes of the volume–time tracing mimic the effect caused by a leak in the spirometer, and make the FEV₆ slightly lower than the largest volume obtained at any time during the first 6 s (recently defined as the FVC₆). Fast-responding temperature sensors inside the volume spirometer (with real-time body temperature, ambient pressure, saturated with water vapour corrections) can eliminate this artefact, but this degree of sophistication is not currently available in any commercial volume spirometers (and is not used in NIOSH field studies). A re-analysis of the raw data from the NHANES III study recently provided reference equations for the newly defined FVC₆ and FEV₁/FVC₆ 15. However, the mean differences in the FEV₆ and FVC₆ are very small (<0.05 L) and do not occur when flow-sensing spirometers are utilised. A recent study from Spain provides an alternate set of FEV₆ and FEV₁/FEV₆ reference equations for older adults 16.

The current results confirm that the forced expiratory volume in six seconds can be used as a surrogate for forced vital capacity in detecting airways obstruction and restriction in workers, although with some misclassification when compared with obtaining American Thoracic Society-acceptable manoeuvres of longer duration. Additional studies are recommended to determine if the current results are generalisable for those using flow-sensing spirometers in occupational settings.