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
- Chest
* Elastic work
* Resistance work (viscous)
- Lung
* Elastic work
--> 65%
* Resistance work
--> 35%
Lung component
- Elastic work
* i.e. Work against elastic recoil
- 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:
- Lung volume (at FRC)
- 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
