Work of breathing

Definitions

Work and power

Work

In respiration,

Work
= Pressure x Volume

Units for work:

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

Power

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

Normal values

Metabolic cost of work of breaking
= 3mL of O2 per minute
= 60.44 J/min
* <2% of basal metabolic rate
* Can increase to 30% in hyperventilation
* Alternative unit: 0.5mLs of O2 L^-1min^-1

 

NB:

• 1 mL of O2 ~ 20.15J
  * 4.82 cal/mL of O2 consumed [WG21:p284]
  * 1 calorie = 4.1868 J
• Efficiency of respiratory muscles
  = Useful work/total expenditure
  = 5-10%

 

Work of breathing

Components of work of breathing

1. Chest
   * Elastic work
   * Resistance work (viscous)
2. Lung
   * Elastic work
   --> 65%
   * Resistance work
   --> 35%

Lung component

1. Elastic work
   * i.e. Work against elastic recoil
2. Resistance work
   * i.e. Work against non-elastic resistance (mainly frictional)

1. Elastic work

• Work is stored as potential energy
• Eventually work is dissipated as heat

NB:

• Elastic recoil is due to
  * Surface tension
  * Intrinsic elasticity of tissue fibres

2. Resistance work

[WG21:p659]

Work is required to overcome:

• Airway resistance
  * 28%
• Tissue resistance
  * aka pulmonary resistance
  * i.e. viscous forces within tissues as they slide over each other
  * 7%

NB:

• ??KB's lecture say it's 15% tissue resistance and 20% airway resistance

Factors influencing elastic work

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 pressure required for inflation
  * Laplace law

3. Other factors

Lung volume

Large lung volume
--> More stretched fibres
--> Higher recoil

NB:

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

Respiratory rate

Give the same minute volume,

Increased RR
--> Decreased work due to recoil

Factors influencing resistance work

1. Airway resistance

AWR is increased by:

• Reduced lung volume
  * Opposite to elastic recoil
• Increased bronchial smooth muscle tone
• Increased density and/or viscosity of gas
• Increased turbulent flow
• Increased respiratory rate
  * Given the same minute volume
  --> increased RR INCREASES work due to airflow resistance

2. Viscous tissue resistance

Probably inherent.

• May be affected by ?pulmonary hypertension

Inspiration vs expiration

Inspiration

• 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

NB:

• According to KB and WG21, 65% is stored

Expiration

Normal expiration during tidal breathing is a passive process.

Thus,

• There is no active muscular contraction
• Energy is still required, and is provided by the elastic potential energy stored during inspiration.

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, resistance work is increased (due to increased airway resistance)
• At high lung volume, elastic work 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 optimal RR which minimises the total work required.

NB:

Way to remember this:

• Increase in RR
  --> Decrease in tidal volume
  --> Increase in AWR
• RR vs Work of breathing due to AWR is opposite to Volume vs AWR curve

Deviation from optimal respiratory rate

• RR > Optimal rate
  --> Decreased tidal volume
  --> Increased work due to AWR
• RR < Optimal rate
  --> Increase work due to elastic recoil

Changes to optimal respiratory rate

• When there is increased elastic resistance
  --> Optimal RR increases
• When there is increased air flow resistance
  --> Optimal RR decreases
• ##20050306(1) - "Work of breathing vs Respiratory rate"

Effects of disease

Restrictive lung disease

In restrictive lung disease
--> Increase work due to elastic recoil
--> Optimal RR increases

Obstructive lung disease

In obstructive lung disease
--> Increase work due to increase airway resistance
--> Optimal RR decreases

Additional notes