Describe the pressure and flow-volume relationships of the lung, chest wall and the total respiratory system

Describe the measurement and interpretation of pulmonary function tests, including diffusion capacity.

Pulmonary function tests are performed with a spirometer, which measures either volume or flow (integrated for time) to quantify lung function.

Basic spirometry can be used to quantify:

  • Lung volumes and capacities
    All except residual volume (and therefore FRC and TLC).
  • Dynamic measurements
    • FEV1
      Volume of air forcibly exhaled in one second.
    • FVC
      Forced vital capacity.
    • PEFR
      Peak expiratory flow rate.
    • Flow-volume loop

Additional testing can be performed to measure:

  • Residual volume
    FRC and TLC can therefore be calculated.
  • Diffusion capacity

Basic Spirometry

Basic spirometry includes:

  • Forced spirometry
    Patient forcibly exhales a vital capacity breath, producing a exponential (wash-in) curve. This calculates:
    • PEFR from the gradient at time 0 (assuming maximal effort)
    • FEV1 is the volume expired in 1s
      Normal is > 80% of predicted.
    • FVC is the total volume exhaled.
    • The FEV1/FVC ratio
      Normal is > 0.7.
    • These values also quantify disease severity:
      • In obstructive airways disease:
        • FEV1 <80% predicted
        • FEV1/FVC ratio
      • Restrictive disease:
        • FEV1 <80% predicted
        • FVC
        • FEV/FVC ratio >0.7
          The ratio is normal as the FEV1 and FVC fall proportionally.

  • Volume-Time Graph (also known as a spirograph or spirogram)
    Quantifies static lung volumes by having a patient perform:
    • Normal tidal breathing
    • Vital capacity breath
    • Vital capacity exhalation

Flow-Volume Loops

  • Normal
    • Peak expiratory flow of ~8L.s-1
      Initial flow is highest as the increased lung volume increases the calibre of lung airways, reducing airways resistance.
      • This is called the effort dependent part of the curve
    • Flow tails off later in expiration
      • Lungs collapse, and airway calibre falls
      • Small airways are compressed
        Any increase in expiratory pressure will increase airway resistance proportionally.
        • This is called dynamic airways compression, and results in a uniform flow rate that is independent of expiratory effort
          This is therefore labeled the effort independent part of the curve.

  • Obstructive lung disease
    • RV and TLC are increased due to gas trapping
    • Peak flow is limited
    • Effort-independent portion becomes concave

  • Restrictive lung disease
    • TLC is reduced, but residual volume is unchanged
    • Peak flow may be reduced (as seen here)
      However, this reduction is proportional to the decrease in volume, such that the FEV1:FVC ratio is normal. If peak flow is preserved, the FEV1:FVC ratio will be increased.
    • Effort independent part is linear

  • Fixed upper airway obstruction
    Describes an upper airway obstruction that does not change calibre during the respiratory cycle.
    • Peak inspiratory and expiratory flow rates are limited by the stenosis

  • Variable extrathoracic obstruction
    Variable as the obstruction changes during the respiratory cycle:
    • During (negative pressure) inspiration the lesion is pulled into trachea, reducing inspiratory flow
    • During expiration the lesion is pushed out of the trachea
      The way to remember this is an extrathoracic obstruction impedes inspiration
    • The reverse effect occurs in positive pressure ventilation

  • Variable intrathoracic obstruction
    The opposite to extrathoracic obstruction.
    • During inspiration the airway calibre increases and inspiratory flow is unimpeded
    • During expiration the airway calibre falls and expiratory flow is reduced


  1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
Last updated 2021-08-23

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