Work of breathing

Describe the determinants of work of breathing in an adult human at rest. Explain how to minimise work of breathing. (01B2, 00A5, 1991)

 

Work

In respiration,

Work = Pressure x Volume

=> Units for work:

• 1 Joule (J)
• = 1 newton meter (Nm)
• = 1 litre kilopascal (LkPa)

Power = Work / time

=> Units for Power

• 1 watt
• 1 joule per second

 

Work of breathing should really be power of breathing unless we are talking about the work associated with one single breath.

 

Components of work of breathing

1. Work against elastic recoil
2. Work against non-elastic resistance (mainly frictional)

1. Work against elastic recoil

All work is stored as potential energy, and no work is dissipated as heat.

2. Work against non-elastic resistance

Additional pressure (and thus work) is required to overcome:

• frictional resistance to gas flow
  i.e. airway resistance
• tissue resistance (aka pulmonary resistance)
  i.e. viscous forces within tissues as they slide over each other

Tissue resistance is about 20% of total resistance.

 

[See diagram 20050306(2) - "Work of breathing"]

Inspiration vs expiration

Inspiration is an active process requiring work.

About half of the work is dissipated during inspiration to overcome the frictional forces, the other half is stored as potential energy in deformed elastic tissues.

Normal expiration during tidal breathing is a passive process.

=> There is no active muscular contraction

=> energy is still required, and is provided by the elastic potential energy stored during inspiration.

 

Factors influencing elastic recoil

The higher the elastic recoil

=> the more work required to overcome elasticity

1. Intrinsic elasticity of fibres

• Age
  => elastic recoil reduces as age increases
• Pathology
  => e.g. emphysema reduces elastic recoil

2. Surface tension

• Surfactant is responsible for 70% of elastic recoil
  => Increased in surface tension increases recoil
• Size of alveoli
  => Larger alveoli reduce surface tension

3. Lung volume

The higher the lung volume, the more stretched fibres are,

=> the greater the recoil

NB. At low lung volume, compliance is reduced, but it has nothing to do with recoil.

4. Respiratory rate

Give the same minute volume,

=> Increased RR

=> Decreased work due to recoil

Factors influencing non-elastic resistance

1. Airway resistance AWR is affected by:

• Lung volume
  => The greater the lung volume, the lower the AWR
  NB. Opposite to elastic recoil
• Bronchial smooth muscle tone
  => increased tone increases AWR
• Density and viscosity of gas
• Respiratory rate
  => given the same minute volume
  => increased RR INCREASES work due to airflow resistance

2. Viscous resistance is probably inherent.

• May be affected by ?pulmonary hypertension

 

Oxygen cost of normal tidal ventilation at rest

About 3mLs O2/min, or (0.5mLs O2L^-1min^-1)

=> <2% of body's O2 consumption

Can increase to 30% in hyperventilation

 

Efficiency of tidal breathing

Efficiency = useful work/total expenditure = 5-10%

 

Minimising work of breathing

Among the factors influencing work of breathing, two factors are important in minimising work:

1. Lung volume (at FRC)
2. Respiratory rate

Lung volume

Work of breathing is minimised at FRC, because

• High pulmonary compliance (on steep part of the pressure-volume curve)
  => elastic work is low
• Low airway resistance
  => resistance work is low (but not lowest)
• Partial inflation and being at a volume above the closing capacity
  => no work required to open collapsed parts of the lung or closed airways
• At low lung volume, work due to non-elastic resistance is increased (due to increased airway resistance)
• At high lung volume, work due to elastic resistance is increased (due to already stretched fibre)

Respiratory rate

Given the same minute volume,

• increasing RR increases work due to air flow resistance
• decreasing RR increases work due to elastic recoil

There is a particular RR which minimises the total work required.

Changes to optimal respiratory rate

When there is increased elastic resistance
=> optimal RR increases

When there is increased air flow resistance
=> optimal RR decreases

 

[diagram 20050306(1) - "Work of breathing vs Respiratory rate"]

 

Additional notes

Diaphragm

The diaphragm descends about 1cm during tidal inspiration.

During a vital capacity breath, total movement is about 10cm.

Diaphragmatic contraction is responsible for 70% of the tidal volume

Respiratory muscles

[Need to add]

NB: During tidal breathing, peak alveolar pressure is +1cmH2O. Mouth pressure is atmospheric.

?need to add respiratory muscles

Examiner's comment

• Definition, with correct units (Joules), also how work is derived (Pressure x volume)
• 2 components: elastic and non-elastic
  elastic - deforming elastic tissues, and overcoming surface tension
  non-elastic - overcoming airway resistance, and viscous forces
• Work is performed for both inspiration and expiration, but energy for expiration is acquired during inspiration
• Effect on work due to changes in compliance, airway resistance, and respiratory rate
• Pressure-volume graph, and demonstrate elastic and non-elastic/resistance work, potential energy stored elastically for passive expiration, and energy dissipated as heat
• ????? Overcoming elastic forces, airway resistance, tissue viscous resistance - contribution to overall work as %.
• A figure for O2 consumption by respiratory muscles
• graph for work and respirate rate, plus effects of increases resistance and increased elastic work
• FRC and role of surfactant
• Poiseuille's equation and Reynold's number
• common mistake - not mentioned potential energy is stored during inspiration and used for expiration
• common mistake - too detailed account on compliance, airway resistance, and pathophysiological conditions