Spirometry

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Spirometry

• D Acharya
• Department of Thoracic Medicine
• Crawley and East Surrey Hospitals

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Measurements

• The manoeuvre involves the subject inspiring to
  total lung capacity and then forcibly exhaling to
  residual volume .
• Four measurements are commonly made
• Total vol. exhaled when done slowly (VC)
• Total vol. exhaled when done rapidly (FVC)
• Volume of FVC exhaled in the 1st sec (FEV1)
• FEV1/FVC ratio

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Equipment

• Calibrated spirometer
• Nose Clip
• Selection of mouth pieces
• Filters
• Technician with good motivational skills
• Printer
• Height scale, Predicted values

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Procedures

• Entirely volitional exercise on the part of the
  subject.
• Maximum inspiratory effort followed by exhaling
  completely (for at least six seconds)
• Three times at a slow pace (VC)
• Followed by three times at a rapid pace (FVC and
  FEV1 ).

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Interpretation

• Comparison of values measured with values
  expected based on age, height, sex and race of
  the subject.
• The range of normal VC or FVC is 82 to 118 of
  predicted (SD of 9). If it is reduced it is
  described as restrictive lung disease.

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Interpretation (contd.)

• FEV1 is evaluated in comparison
• to the values predicted for age, height, sex and
  race
• and as a ratio of the FVC (FEV1/FVC)
• A reduced FEV1 (lt 80) and reduced FEV1/FVC ratio
  (lt70) are features of obstructive lung disease.

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Flow volume loops

• Additional information for evaluating airflow
  limitation.
• Expiratory flow volume loop helps diagnose small
  airways asthma ( or FEF25-75) and distinguishes
  between asthma (volume dependent airflow
  limitation) and COPD (pressure dependent airflow
  limitation).
• Inspiratory flow volume loop helps investigate
  extra-thoracic airways obstruction.

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Peak Expiratory Flow Rate

• More dependent on appropriate voluntary effort
  than spirometry.
• High negative predictive value for airways
  obstruction.
• Low positive predictive value for airways
  obstruction.
• Good tool for self monitoring.

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