Work of Breathing

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  • Work of breathing - the work required to move the lung and chest wall
  • Work = pressure x volume (measured in joules)


  • CFADC is the work done on the lung
  • CBADC is the work required to overcome elastic forces
  • CFABC is the work overcoming airway/tissue resistance
  • Higher airway resistance/inspiratory flow rate makes CFABC area larger
  • Higher breathing rates lead to faster flow rates and larger area CFABC
  • CBAEC is work done by stored elastic energy from inspiration, and CBADC - CBAEC is potential energy lost as heat
  • As respiratory frequency increases, flow rates increase and non-elastic area ABCEA increases
  • As tidal volume increases, elastic work area increases
  • Patients with poor compliance take small, rapid breaths
  • Patients with severe airway obstruction breathe slowly, reducing work done on lungs

Total Work of Breathing:

  • Hard to measure, but estimated using paralyzed artificially ventilated patients in an iron-lung ventilator
  • Total work can also be calculated by measuring O2 cost of breathing and assuming a figure for efficiency
  • Efficiency % = ( Useful work/total energy expended ) x 100
  • Efficiency is normally 5-10%
  • O2 cost of quiet breathing is very small - < 5% of total resting O2 consumption
  • Increases to 30% with voluntary hyperventilation
  • In obstructive lung disease, O2 cost of breathing may limit exercise ability