Work of Breathing

Describe the work of breathing and its components

Work of breathing is the energy used by the muscles for respiration. It is defined as:
$Work = Pressure \times Volume$, measured in Joules.

• This gives the work for a single respiratory cycle
  Energy expenditure over time is better described as the "power of breathing".
• It does not take into account respiratory rate or flow rate
  These factors have a significant effect on energy requirement.
  • This would be given by the rate of work, or power, where:
    $Power = {Work \over Time}$, measured in Watts.
  • Tidal breathing is efficient and uses < 2% of BMR
• The oxygen requirement of breathing at rest is ~2-5% of VO₂, or ~3ml.min^-1

Determinants of Work of Breathing

Work of breathing is divided into:

• Elastic work
  About 65% of total work, and is stored as elastic potential energy. Energy required to overcome elastic forces:
  • Lung elastic recoil
  • Surface tension of alveoli
• Resistive work
  About 35% of total work, and is lost as heat. This is due to the energy required to overcome frictional forces:
  • Between tissues
    • Increased with increased interstitial lung tissue
  • Between gas molecules
    • Increased at high flow rates
    • Increased with turbulent flow
      • High respiratory rates
      • Upper airway obstruction
      • Increased airway density
        • Hyperbaric
        • Diving
    • Increased with decreased airway radius
      • Low lung volume
        • Inadequate PEEP
        • Decreased respiratory muscle tone
      • Bronchoconstriction
      • Dynamic airway compression
        Effort-independent expiration.
      • Apparatus
        • Endotracheal Tube
        • HME filters
      • Airway resistance varies depending on airway division:
        • Resistance peaks at the 3rd airway division (lobar bronchi)
        • Falls with increasing airway divisions due to increased cross-sectional area
          

Graphing Work of Breathing

Work of breathing can be evaluated with a dynamic lung compliance curve:

• If there were no resistive forces, then this curve would be a straight line
  • The triangular area is the elastic work done
• The resistive work of breathing causes the deviation of the inspiratory and expiratory lines:
  • The area between the compliance line and the inspiratory line is additional resistive inspiratory work done
  • The area between the compliance line and expiratory line is additional resistive expiratory work done
    • This work is typically done by elastic recoil of the lungs
    • If this area falls within the area of elastic work of breathing, it is a purely passive process, using the stored elastic potential energy of inspiration
    • If part of this area falls outside the area of elastic work of breathing, it demonstrates additional active work of expiration which may occur in obstructive lung disease or when minute ventilation is high

Active expiratory work:

Minimising Work of Breathing

Work of breathing can be minimised by optimising the determinants:

• Elastic work
  • PEEP
    Keep lung volume at FRC and maximise number of ventilated alveoli.
  • Positioning
    Optimise lung volume.
  • Surfactant
    Minimising surface tension.
  • Optimise respiratory rate
    Elastic work of breathing typically decreases with increased respiratory rate.
• Resistive work
  • Decrease respiratory rate
    Respiratory rate is directly proportional to resistive work.
  • Increase laminar flow
    Laminar flow is more efficient than turbulent flow. Laminar flow can be increased by:
    • Reducing gas density
      Heliox.
  • Increase Radius
    • Increase lung volume
    • Bronchodilators

Derivation

Work is defined as:
$W = FD$, where:

• $W$ = Work in Joules
• $F$ = Force in Newtons
• $D$ = Distance in Metres

Additionally, pressure is defined as:
$P = {F \over A}$, where:

• $P$ = Pressure in Pascal
• $A$ = Area in Meters squared

Therefore:
$F = PA$

Substituting:
$W = PAD$
$W = PV$, where:

• $V$ = Volume

Therefore:
$Work = Pressure \times Volume$

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
2. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.