Pulmonary Function Tests

Background

  • These can be useful in evaluating the nature and severity of respiratory disease BUT must always be interpreted in context of the patient history and examination

Indications

  • To evaluate presentations of lung disease (most commonly dyspnoea and hypoxia, but also for others e.g. hypercapnia, cyanosis, wheezing etc)
  • To assess progression of lung disease (NB more commonly this is done using peak flow monitoring but PFTs can also be a more formal measure, particularly if you suspect more than one underlying cause)
  • To evaluate patients pre-operatively

Components

  • Spirometry
    • Measures the air movement in and out of the lungs during respiratory maneouvres e.g. forced inspiration and expiration
      • It can measure volumes and speeds of air movement (i.e. FEV1)
      • Most useful at measuring the FVC, FEV1 and FEV1/FVC ratio

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  • Spirometry can also be used to produce a flow-volume loop (showing the same thing but volume is plotted on the x-axis and flow is plotted on the y-axis.  These can be useful in estimating the kind of obstruction.
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Note that volume is plotted backwards on this volume loop

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  • Diffusing capacity
    • A further PFT which uses labelled Carbon monoxide to measure the ability of the lungs to diffuse and transfer gases- which can subsequently be used as a measure of functional lung surface area and thus total lung volume.
    • It is most commonly used to evaluate restrictive pattern diseases or spirometry results that have not been responsive to bronchodilators.
    • Common causes of reduced capacity include
      • **Anaemia (remember non-respiratory causes- take into context)
      • Emphysema, ILD, Po Oedema, Po vascular disease

Interpretation

  1. Is the FEV1/FVC ratio less than the lower limit of normal (0.70 is the normal cut off for obstructive disease; >0.75 is regarded normal)?
    1. Obstructive or not?
  2. Is the FVC less than the lower limit of normal? (NB Normal FVC is calculated based on age, size/weight/height etc)
    1. Any restrictive component or not?
  3. Is the total lung capacity known?
    1. Reduced in true restriction and normal in pseudo-restriction
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