Interpreting Spirometry

FEV1 and its importance

This is the forced expiratory volume in one second, reflecting airway narrowing which is relatively independent of effort unlike PEFR which is a measurement of flow over ten milliseconds.

FVC

This is the forced vital capacity.

Normally FEV1 = 70-80% of the FVC

Airflow obstruction

  • FEV1 decreases
  • FVC decreases to a lesser extent
  • Therefore FEV1 is <65% of the FVC

Diseases affecting the lung parenchyma

  • FEV1 and FVC decrease to the same extent
  • Therefore FEV1 is 70-80% of the FVC but the absolute values are lower

Uses of FEV1 and FVC

  1. To differentiate between obstructive and restrictive patterns.
  2. Those with obstructive patterns are more likely to respond to bronchodilator therapy. The FEV1 can then be used to assess the response.
  3. To diagnose asthma (15-20% variability in FEV1), to assess the severity or the condition and to monitor response to therapy. It is recommended that the FEV1 is measured yearly.
  4. To monitor the severity of COPD (if the FEV1 is 20-50% of normal the predicted survival is 5 years). In 15-20% of smokers there is an accelerated decline in FEV1 which predicts the likelihood of progression to severe disease.
  5. To assess the suitability of patients for oxygen therapy. E.g. FEV1 of <1.5 L, FVC <2 L plus chronic arterial hypoxaemia.

Examples

  • Chronic Bronchitis
    When the small airways are involved the FEV1 is normal. Severe airways obstruction which may respond to bronchodilator therapy is due to an accelerated decline in FEV1. This decline of 50-7Omls per year in 15-20% of smokers compares with a decline of 30mls per year in non smokers. Cessation of smoking returns the decline in FEV1 to that of non smokers thus preserving lung function.
  • Emphysema
    FEV1/FVC is <50%
  • Asthma
    FEV1 is reversible by 15-20% and is a more reliable measurement than PEPR. If the FVC declines this is due to air trapping indicating severe asthma.
  • Occupational Asthma
    Exposure to the allergen leads to increased airways sensitivity, the FEV1 decreases 8 hours after exposure. With increasing exposure airways sensitivity also increases.
  • Extrinsic Allergic Alveolitis e.g. Farmers lung
    Restrictive pattern with decreased FVC. FEV1 decreases only if asthma or chronic respiratory failure is present.
  • Sarcoidosis, silicosis, pneumoconiosis, cryptogenic fibrosing alveolitis
    All have a restrictive pattern.

Uses of spirometry in practice

  1. Asthma Clinics
    Measure FEV1 at the yearly check.
    Use FEV1 in reversibility tests at diagnosis.
    If the patient is not producing his maximum effort or you suspect his not then measure the FEV1.
  2. Well woman clinics
    Measure FEV1 in smokers. Reduction by 30 mls per year normal, if reduction is 50 mls or more per year Dr or nurse education on the high risk they have of serious lung disease unless they stop smoking.
  3. COPD
    Measure FEV1/FVC: if this is <0.7 check reversibility.
    If FEV1 is <1.5L and FVC is <2L refer back to Dr. If the patient no longer smokes then it may be appropriate to check the arterial blood gases. Chronic arterial hypoxaemia may benefit from oxygen therapy.
  4. Breathlessness of unknown cause
    Measure FEV1/FVC.
    This will help to differentiate a restrictive from an obstructive cause and helps to decide on the next course of action by the Doctor.

Restrictive ventilatory pattern

  • FVC reduced <80%
  • FEV1 reduced
  • FEV1% normal

Obstructive ventilatory pattern

  • FEV1 reduced <80%
  • FVC normal or reduced
  • FEV1% reduced <70%

Reversibility tests

Significant reversibility is defined as a rise in FEV1 that is both greater than 200ml and 15% of the pre-test value. Substantial reversibility (>500ml) indicates asthma:

  • in response to bronchodilator
  • in response to steroid trial
    • Prednisolone 30mg for 2w (will miss 5%)
    • beclomethasone 500mcg/day for 6 weeks
  • with time (diurnal variability with PEF meter)

Classification of COPD

National guidelines vary:

FEV1 as a % of predicted

  • <80% = mild
  • <60% = moderate
  • <40% = severe
  • <80% = mild
  • <50% = moderate
  • <30% = severe
    (NICE guidelines)
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