Work of Breathing

• 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