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Can combined defects be diagnosed on spirometry alone?
  1. Postgraduate Institute of
  2. Medical Education and Research
  3. 1136/15B Chandigarh 160015, India
    1. Department of Medicine for the Elderly
    2. Department of Respiratory Medicine
    3. Wirral Hospital, Arrowe Park, Upton, Wirral Merseyside L49 5PE, UK
      1. JOHN VERDIN,
      1. Department of Medicine for the Elderly
      2. Department of Respiratory Medicine
      3. Wirral Hospital, Arrowe Park, Upton, Wirral Merseyside L49 5PE, UK

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        Sir,I read with interest the article on hospital doctors' assessment of baseline spirometry, by Stephenson and colleagues, in a recent issue of this journal.1 It is indeed a matter of great concern that only 12% of the respondents could accurately interpret all the five vitalographs given to them. What is still more disturbing is the fact that the correct interpretation for the third vitalograph was considered as ‘combined defect’. This vitalograph, in a 77-year-old man, shows a mild reduction in forced vital capacity (FVC) (63% of predicted) and a severe reduction in both the forced expiratory volume in the first second (FEV1) and the FEV1/FVC ratio (30% and 38%, respectively). The volume–time curve provided is also flattened, indicating diminished expiratory flow. All these features are consistent with a obstructive rather than a combined defect. As per standard guidelines on interpretation of spirometry,2 an obstructive defect should be diagnosed when the decrease in FEV1 is out of proportion to any decrease in vital capacity, as in this case. One must resist the temptation of diagnosing a combined defect based on spirometry alone. It is well known that FVC may be reduced in patients with airway obstruction, especially in those with severe airflow limitation. This is largely related to air-trapping and a consequent increase in residual volume (RV). Estimation of RV, total lung capacity (TLC) and RV/TLC ratio is necessary to diagnose such an abnormality and to look for any coexisting restriction.

        I therefore believe that, based on the database provided in this vitalograph, one can diagnose only a severe obstructive defect. Additional information is needed before one can consider this defect to combine obstructive and restrictive defects. The results of this study indicate that 50 of 78 respondents incorrectly interpreted this vitalograph, the number of incorrect responses being far more than in any of the other four records. It would be interesting to know the number of respondents who diagnosed an obstructive defect in this record. Such spirometry reports often cause some confusion during interpretation; however, with adequate knowledge of currently recommended interpretative strategies,2 one can usually reach a correct conclusion and ask for additional investigations in case of any doubt.


        This letter was shown to the authors who responded as follows:

        Sir,Dr Aggarwal has some very valid comments to make, including the fact that a diagnosis of a combined defect should, strictly speaking, not be based on spirometry alone and that further investigations, including measurements of TLC, should be requested.

        A restrictive pattern means that lung volumes are small and the primary criteria for this diagnosis is a reduction in TLC. TLC cannot be measured by spirometry but a reduced VC may be used to suggest the presence of restriction. If there is any doubt about the cause of a reduced VC, TLC should be measured.

        However, in the context of this study, looking at doctor's interpretation of baseline spirometry with no other information available, we believe it was justified to ask whether the vitalograph showed a combined defect in the differential interpretation. The vitalograph had kindly been provided by the Cardiorespiratory Department as examples of normal, restrictive, obstructive and combined defects and validated independently by two respiratory physicians. Figure 3 did show a reduced FEV1, consistent with an obstructive defect, however, the vital capacity was reduced, raising the question of a combined defect. We appreciate that measurement of TLC would be necessary to confirm this (although not a single participant raised this issue).

        We fully concur with the author that, with adequate knowledge, interpretation of spirometry should lead to a correct conclusion. The problem, however, as the study has illustrated, is that hospital doctors do not have adequate knowledge and 72% felt that they had not had adequate teaching. We should not be complacent that doctors can interpret spirometry and address the issue of adequate training and wider dissemination of the current guidelines on interpretation of spirometry.