Resp Mechanics – Work of Breathing

Master answer

Work of Breathing

• WOB = Energy used by the respiratory muscles during ventilation
• Work = Pressure x Volume (measured in Joules)
• represented as area on pressure-vol loop (dynamic compliance curve)
• Tidal breathing at rest: <2% BMR; O2 requirement = 3ml/min

Components of Work of Breathing

WOB can be divided into:

1. Inspiratory work
   • Active
   • Elastic + resistive work
2. Expiratory work
   • Usually passive (uses potential energy stored during inspiration)
   • Resistive work

Inspiratory elastic work

• ~65% of total work
• work done on inspiration to overcome:
  • surface tension of lung (70%)
  • elastic properties of lung/ lung elastic recoil (30%)
• stored as elastic potential energy → used on expiration

Inspiratory + expiratory resistive work

• ~35% of total work; lost as heat
• energy required to overcome frictional forces: includes
  • Airway resistance: between gas molecules
    • depends on type of flow + airway calibre
    • turbulent flow (↑RR; upper airway obstruction, ↑airway density)
      → ↑ airway resistance than laminar flow
    • ↓ airway calibre (radius) (e.g. bronchoconstriction, dynamic airway compression, ETT)
      → ↓ calibre → ↑ WOB
  • Tissue resistance e.g. interstitial lung disease
• Usually potential energy stored from insp work is sufficient to perform expiratory resistive work → expiration can become active in pathological conditions e.g. COPD, asthma

Total work and efficiency of breathing

• O2 cost of breathing:
  • During a normal TV breath at rest, the total work of breathing is low – The
    inspiratory muscles consumes 3 mL O2/min, which is ~1% of the body’s total
    O2 consumption at rest (VO2 = 250 mL O2/min)
  • O2 consumption of breathing increases by up to 30% with hyperventilation (due to an increase in either minute volume or RR)
• Total work and efficiency of breathing:
  • Difficult to measure – It is calculated by measuring O2 cost of breathing, then
    assuming efficiency as:

Normal efficiency is estimated at 5-10%

Increased WoB

• Elastic Work:
  • Breathing large lung volumes: increases total work of inspiration by extending the line AEC to a higher volume above FRC, increasing area 0AECD0
  • Poorly compliant lungs (e.g. restrictive disease surfactant deficiency): increases total work of inspiration by reducing slope of AEC, increasing area 0AECD0
• Resistive Work
  • e.g. turbulent flow with rapid RR, lung disease associated with increased AWR
  • Increases total work of inspiration by producing a larger area ABCEA (i.e. it bulges)
  • This also causes AECFA to bulge out and increase area, such that when stored elastic potential is insufficient to overcome this tissue resistance force, expiration becomes an active process requiring work as expiratory muscles are recruited

Minimizing WoB

• Elastic work:
  • PEEP: keep lung vol at FRC and maximise number of ventilated alveoli
  • Positioning: optimise lung volume
  • Surfactant: minimise surface tension
  • Optimise RR: elastic WOB ↓s with ↑ RR
• Resistive work
  • ↓RR: RR directly proportional to resistive work
  • ↑laminar flow: more efficient than turbulent flow; can be ↑ by ↓gas density e.g. heliox
  • ↑radius: ↑lung vol; bronchodilators

Kerr / Bianca