7th February 2024

Pulmonary dynamics

Lung compliance

Compliance is defined as the unit change in volume per unit change in pressure. It is the inverse of elastance.

Compliance of the lung can be demonstrated using the compliance curve.

During inspiration, work is done to overcome (1) elastic recoil of the lungs and (2) resistance from airway and lung tissues sliding over each other. Work done to overcome the lung’s elastic recoil is stored as potential energy. This potential energy is released during expiration.

Hence, inspiration is active in normal, healthy patients, and expiration is passive.

This brings us to a discussion on work done during respiration.

Factors that alter compliance include (but are not limited to):

Increase compliance:

• Emphysema
• Increasing age (loss of pulmonary connective tissues)

Decrease compliance:

• Decreased lung compliance (pulmonary fibrosis, pulmonary oedema)
• Decreased chest wall compliance (obesity, burn to chest wall)

Work done during respiration.

Forgive this brief incorporation of physics. It will make sense, I promise. ( This will not be tested in the exam but will help understand the concepts below).

Work done = Pressure x Volume.

From the formula above, we can determine work done during respiration using the compliance curve. 

Work done during inspiration = purple + red + green

Work done during expiration = red (to overcome airway resistance)

Purple = work done loss as heat energy

In lung diseases, the compliance curve changes, and work done changes as well.

In restrictive lung disease, lung compliance decreases, and the inspiratory limb curves downwards and rightwards. This increases work done during inspiration.

In obstructive lung disease (i.e. COPD), the expiratory limb curves leftwards and upwards. This may curve outside of the purple zone. This area represents the additional work needed for active expiration.

Airway resistance

Airway resistance is affected by:

• Airway radius
• Lung volumes
• Phase of respiration
• Autonomic activity
• Elastic recoil of the lung
• Local PaCO2

Although the airway becomes narrower towards the lung periphery, the peripheral airway contributes little airway resistance due to multiple parallel airways. The upper respiratory tract contributes 30-50% of total airway resistance in a healthy person. The large-to-medium-sized bronchi are the location along the lower respiratory tract that contributes the most resistance.