Lung function tests

Techniques

• • Lung volumes

    • Spirometry

  • Flow rates

    • Spirometry

    • Peak flow

  • Maximal respiratory pressures

    • Maximal pressure that can be produced by the patient trying to breath through a blocked mouthpiece

    • Useful in following the course of patients with neuromuscular disorders.

  • Diffusing capacity

    • Single-breath diffusing capacity for carbon monoxide (DLCO)

  • Oxygen desaturation during exercise

    • The six-minute walk test is a good index of physical function and therapeutic response

  • Arterial blood gases

Spirometry

• • Volume-time curve

    • Volume (liters) along the Y-axis and time (seconds) along the X-axis

• Flow-volume loop

  • • Rate of airflow on the Y-axis and the total volume inspired or expired on the X-axis

Parameters

• • There are four lung volumes and four lung capacities

  • A lung capacity consists of two or more lung volumes

  • The lung volumes are:

    • tidal volume (V_T)

    • inspiratory reserve volume (IRV)

    • expiratory reserve volume (ERV)

    • residual volume (RV)

  • The four lung capacities are:

    • total lung capacity (TLC)

    • inspiratory capacity (IC)

    • functional residual capacity (FRC)

    • vital capacity (VC)

Obstructive Disease

• • In obstructive diseases FEV₁ is diminished because of increased airway resistance to expiratory flow

  • The FVC may be decreased as well, due to the premature closure of airway in expiration

    • Generally not in the same proportion as FEV₁

  • Therefore the FEV1/FVC ratio, also called Tiffeneau index, falls

  • e.g. asthma, COPD, chronic bronchitis, emphysema

  • The diagnosis of COPD is made when:

    • the FEV₁/FVC ratio is less than 70% after bronchodilator medication has been given

    • FEV1% is less than 80% of predicted

Restrictive Disease

• • The FEV₁ and FVC are both reduced proportionally

  • Thus the FEV1/FVC value may be normal or even increased as a result of decreased lung compliance

  • e.g. pulmonary fibrosis

Pictures