The Spirographic Kink as a Sign of Emphysema—A Re-Evaluation

Abstract

To test the validity of an earlier study's finding that the presence of a kink in the initial part of a forced expiratory spirogram indicated emphysema, we studied 158 patients with chronic obstructive pulmonary disease and/or asthma. We tested pulmonary function with a water-sealed spirometer and diffusing capacity with a bag-box system, a helium analyzer, and a carbon monoxide analyzer. The spirograms were reviewed by a person with no knowledge of the clinical diagnoses or PFT data. We used the same standard for significant emphysema that was used in the earlier study—a diffusing capacity (DL) less than 70% of predicted. 59% of the patients in our study, compared to 32% of those in the earlier study, had kinked spirograms. We also found that 64% of the patients with kinked spirograms also had unkinked spirograms on the same day. In our study, 45% of the patients with kink and 50% of those without kink had a low DL (emphysema), in contrast to 80% and 15%, respectively, in the earlier study. 55% of the patients with a kink versus 23% of those without a kink had a low flowrate (FEF25-75% less than 20% of predicted), and a low DL was found predominantly in patients in the lowest flowrate categories. Because we found the spirographic kink predominantly in the low flowrate groups, I believe the kink is simply a sign of adequate patient test effort. However, both our study and the earlier one were performed with a metal bell water-sealed spirometer, which can be affected by inertial resistance. Further studies are needed of spirograms from another type of spirometer to determine whether the kinks in our study and the earlier one occurred because inertia had caused an artifact in the spirometer system.