Pulmonary Function Tests

Preoperative assessment for thoracic surgery is covered under thoracic principles.

Pulmonary function tests are used to:

Includes tests of:

Testing

May involve testing:

  • Ventilation
  • Gas exchange
  • Other

Ventilation Tests

Include:

  • Peak flow
  • Spirometry
  • Total body plethysmography

Peak Flow

Patient forcefully exhales from a vital capacity breath into a peak flow meter:

  • Measures peak expiratory flow rate
  • Cheap and easy to perform
  • Used in monitoring course of obstructive disease
    Asthma and COAD.

Spirometry

Process:

  • Patients breaths in and out of a flowmeter
    • Difficult to coordinate in children under 5
  • Allows all volumes except residual volume to be measured, i.e.
    • Tidal volume
    • Expiratory reserve volume
    • Inspiratory reserve volume
  • From this, capacities calculated include:
    • Vital capacity
    • Inspiratory capacity
  • Forced vital capacity can also be measured
    Tests ability of lungs to act as a bellows ,and is reduced by restrictive disease.
    • FEV1 can be derived
      Indicates how effectively a patient can cough, and is reduced in obstructive disease.
    • FEV1FVC/ ratio
      Allows differentiation between obstructive and restrictive disease.
      • Normal is ~80%
  • May be performed before-and-after bronchodilator use
    Assesses reversibility of disease.

Interpretation:

  • Validity
    Multiple best-effort measurements should be taken, and FEV1 and FVC measurements should be within 0.2L
  • FEV1FVC ratio of <70% (adults) or <85% (children) of predicted
    Indicates an obstructive defect.
    • Reversibility demonstrated by >12% ↑ following bronchodilator
      Asthma is usually reversible, COPD is not.
    • Multiple grading systems, the GOLD 2008/NICE 2010 system:
      • FEV1 ⩾80% predicted
        Mild.
      • FEV1 50-79% predicted
        Moderate.
      • FEV1 30-49% predicted
        Severe.
      • FEV1 ⩽30% predicted
        Very severe.
  • FVC <80% predicted
    Indicates a restrictive defect.
  • Presence of both a reduced FVC and reduced FEV1FVC/ ratio indicates a mixed defect

Total Body Plethysmography

Allows measurement of volumes and capacities that are not measured with spirometry, including:

  • Residual volume
  • Functional residual capacity
  • Total lung capacity

Gas Exchange Testing

Include:

  • Transfer factor (TLCO)

Transfer Factor (TLCO/DLCO)

Measures the diffusion capacity of the lungs for carbon monoxide, which can be used to calculate the functional surface area of the lungs. Involves:

  • Laboratory test
  • Involves:
    • Single-breath of 10% helium with 0.3% CO
      • Helium is minimally absorbed and so can be used to calculate the initial concentration of CO
      • CO is highly bound by Hb and so the partial pressure in the blood remains low, such that the primary determinant of CO absorption is the diffusing capacity of the lungs
    • Patient holds breath for 10-20s
    • First 750mL of gas is discarded
      To ensure dead space removed.
    • Subsequent litre is analysed
    • Residual CO can be used to calculate how much CO has been absorbed
    • The amount of CO absorbed allows diffusing capacity to be calculated
  • Reduced by:
    • ↑ Thickness
      Fibrosis.
    • ↓ area
      Lung resection, emphysema.
    • Reduced ability to bind to Hb
      Anaemia.
  • Artificially ↑ by:
    • Alveolar haemorrhage
      Blood in alveolus takes up CO.

Other Tests

Include:

  • Maximal breathing capacity
  • Cardiopulmonary exercise testing
  • 6-minute walk test

Maximal Breathing Capacity

Maximal volume of air that can be breathed when the subject hyperventilates as forcefully as possible:

  • ⩽40% predicted indicates high perioperative risk

Cardiopulmonary Exercise Testing

Provides a functional assessment of cardiopulmonary reserve:

  • Involves:
    • Exercising at ↑ intensity on an exercise bike
    • Inspired and expired O2 and CO2 are measured
    • Aims to calculate VO2 max and anaerobic threshold

References

  1. Portch D, McCormick B. Pulmonary Function Tests and Assessment for Lung Resection. Update in Anaesthesia.
  2. Gould G, Pearce A. Assessment of suitability for lung resection. Contin Educ Anaesth Crit Care Pain. 2006 Jun 1;6(3):97–100.
  3. Johnson JD, Theurer WM. A Stepwise Approach to the Interpretation of Pulmonary Function Tests. AFP. 2014;89(5):359-366.