U01.16.010 Chest Wall Mechanics

The interaction between the lungs and chest wall plays a crucial role in respiration. The balance between the elastic recoil of the lungs and the outward pull of the chest wall determines lung volumes and pressures during inspiration and expiration.

Lung Volumes and Pressures

Parameter	Inspiration	Expiration
Lung Volume (L)	VT	FRC
Alveolar Pressure (cm H₂O)	-2	+2
Intrapleural Pressure (cm H₂O)	-8	-4
Transpulmonary Pressure (cm H₂O)	-10	-6

Lung Volumes and Pressures

VT (Tidal Volume): The amount of air inhaled or exhaled during normal breathing.
FRC (Functional Residual Capacity): The volume of air remaining in the lungs after normal expiration.
Alveolar Pressure: Pressure within the alveoli, changes during breathing to drive airflow.
Intrapleural Pressure: Pressure in the pleural cavity, remains negative to keep lungs inflated.

Compliance and Elastic Recoil

Elastic Recoil:
- The lungs have a natural tendency to collapse inward.
- The chest wall has a tendency to spring outward.
- At FRC, the inward pull of the lungs is balanced by the outward pull of the chest wall, keeping the system pressure at atmospheric level.
- Pulmonary vascular resistance (PVR) is minimized at FRC.
Compliance (ΔV/ΔP):
- Definition: The change in lung volume for a given change in pressure.
- High Compliance: Lungs are easier to fill (e.g., in emphysema, aging).
- Low Compliance: Lungs are harder to fill (e.g., in pulmonary fibrosis, pneumonia, ARDS, pulmonary edema).
- Hysteresis: The phenomenon where lung inflation follows a different pressure-volume curve compared to lung deflation due to surface tension forces.

Compliance Comparison	Condition	Effect
High Compliance	Emphysema, Aging	Easier lung inflation
Low Compliance	Pulmonary fibrosis, Pneumonia, ARDS, Pulmonary edema	Harder lung inflation

Compliance and Elastic Recoil

Points to Remember

FRC: Critical for maintaining open airways and minimizing the work of breathing.
Intrapleural Pressure: Negative pressure is essential for preventing lung collapse (atelectasis).
Compliance: Reflects lung and chest wall flexibility; influenced by various pathological conditions.
Hysteresis: Important in understanding lung mechanics, especially in diseases affecting surfactants.

Bibliography

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USMLE Step 1 Review Series

Curriculum

U01.16.010 Chest Wall Mechanics

Lung Volumes and Pressures

Compliance and Elastic Recoil

Points to Remember

Bibliography

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