During normal breathing, inspiratory muscles contract to inhale air actively, while exhaling is passive. Work of breathing is low at around 1-2% of total energy expenditure at rest and 3-5% during exercise. Work of breathing can be divided into compliance work against lung elasticity, tissue resistance work overcoming tissue friction, and airway resistance work overcoming friction in airways. Conditions that increase work of breathing include decreased lung compliance, increased airway resistance from obstruction, or increased tissue resistance from thoracic abnormalities.

1. Work of breathing
- During normal quiet respiration, work is done only during
inspiration (during which respiratory muscles contract), whereas
expiration is entirely a passive process.
- At rest, work of breathing is about 1-2% of the total body energy
expenditure. Even during heavy exercise, work is only about 3-
5% of the total energy expenditure of the body.

2. Work of breathing can be divided into:
1- Compliance work
(elastic work)
2- Tissue resistance work 3- Airway resistance work
65% of total work 7% of total work 28% of total work
- It is the work required to
expand the lung against its
elastic force.
- This can be calculated as
follow:
Compliance work = volume
x pressure needed to inflate
this volume
- It is the work required to
overcome the resistance of
non-elastic tissue of the
lungs and thoracic cage.
- It is the work required to
overcome airway resistance
during the
movement of air into the
lung.
- The major factor, affecting
airway resistance is the
diameter of the passage
ways. The larger the
diameter of the airway, the
less work done.

3. Conditions associated with increased work of breathing:
1- Increased compliance work:
- Decreased elasticity (fibrosis).
- Lung congestion (heart failure).
2- Increased tissue resistance work:
- Diseases affecting thoracic cage e.g. kyphoscoliosis.
- Muscle disease.
3- Increased airway resistance work:
- Obstruction of air way.
- Chronic bronchitis.
- Asthma.

4. • Inspiratory muscles provide the force necessary to overcome,
a. elastic recoil of the lungs and chest wall
b. frictional resistance
i. caused by deformation of lung tissue and thoracic cage → tissue resistance
ii. to air flow in the conducting airways → airway resistance

5. Resistance To Breathing
1. Elastic Resistance ~ 65%
2. Non-elastic resistance ~ 35%
i. Airflow ~ 80%
ii. Viscous ~ 20%

6. Elastic Resistance to Breathing
Elastic Recoil of the Lungs
• The tendency of elastic lung tissue to recoil from the chest wall results in a
sub-atmospheric intrapleural pressure.
• At FRC, the mean intrapleural pressure ~ 4-5 cmH20 sub-atmospheric
• The intrapleural pressure is normally estimated by an oesophageal balloon
catheter
• This is more accurate in measuring changes rather than absolute pressure,
due to interference from the weight of the heart

7. Compliance
Definition: The ability of the lungs to expand is expressed using a measure known as the lung
compliance. Lung compliance is the volume change per unit pressure change.
It is represented as C = ΔV/ΔP
Static Compliance
• It is the relationship between volume change of lung and the transpulmonary pressure change,
i.e. airway - intrapleural pressure change, measured under known static conditions (zero
airflow)
• Reflects the elastic resistance of the lung and chest wall.
• Given by CST = Corrected Vt/ (Plateau Pressure- PEEP)
• The normal value for a 70 kg adult ~ 200 ml/cmH20
• The value decreases as lung volume increases, due to the limitations of the non-elastic
components of the lung/chest wall system.

8. Work Of Breathing
Definition: It is the work required by the respiratory muscles to overcome the mechanical impedance to
respiration. It is the sum of work requires to overcome both elastic and airflow resistance.
• There are two categories that the physical work of breathing can be broken down into. One type is
resistance work in which an increase in resistance results in an increase in work.
• Compliance work is the other type of breathing work done. A decrease in compliance of the lungs
requires an increase in work of them.
Airway Resistance & Work of Breathing
• As given in the equation Raw = ΔP/V (Raw = Airway resistance
ΔP = PIP-Pplat
V= Flow )
• The pressure change ΔP, can be treated as the amount of work imposed on the patient.
Thus the work of breathing is directly proportional to the airway resistance and an increase in the
airway resistance increases the work of breathing.
• If the work of breathing remains constant then an increase in the airway resistance will decrease the
flow. In the clinical setting if the patient is unable to overcome the airway resistanc eby increasing
the work of breathing then hypoventilation may result in decrease of the minute ventilation of the
patient.

9. Compliance & Work of Breathing
• Since compliance is inversely related to pressure change, a decrease in compliance will result
in increase in the work of breathing. In the clinical setting, atelectasis is one of the most
frequent causes of increased work of breathing.
• If the change in pressure remains constant then the decrease in compliance will cause a
decrease in the tidal volume and minute ventilation.
• Thus in low compliance situations such as ARDS the decrease in minute ventilation is
characterized by low tidal volume and high respiratory rates
In summary, the work of breathing can be increased by
– increased airway resistance,
– reduced lung compliance, or
– reduced thorax compliance.

10. Work of Breathing during Normal Respiration
• During normal quite breathing, respiratory muscles work during inspiration to expand the
lungs, whereas expiration is a passive process.
• Normally lungs are highly compliant and airway resistance is low, so only 3% of total energy is
used by the body during quite breathing.
Clinical Application
Work of breathing may be increased:
1. When pulmonary compliance is decreased.– more work is required to expand the lung. eg
Pulmonary Fibrosis
2. When airway resistance is increased: more work is required to overcome the resistance. Eg.
COAD.

11. Work of Breathing during Normal Respiration
• During normal quite breathing, respiratory muscles work during inspiration to expand the
lungs, whereas expiration is a passive process.
• Normally lungs are highly compliant and airway resistance is low, so only 3% of total energy is
used by the body during quite breathing.
Clinical Application
Work of breathing may be increased:
1. When pulmonary compliance is decreased.– more work is required to expand the lung. eg
Pulmonary Fibrosis
2. When airway resistance is increased: more work is required to overcome the resistance. Eg.
COAD.