Lung Compliance

Current · V5 (2025) → C3.iii Historical · V4 (2023) → F3.ii 6 exam appearances

2022A Q03

Exam question

Define respiratory compliance, include its components and their normal values (25% marks). Explain the factors that affect respiratory compliance (75% marks).

CICMWrecks answer

Master answer

Compliance \; = \; {{\Delta \, V} \over {\Delta \, P}}

\Delta \, P \; = \; Alveolar \; – \; Intrapleural \; pressure

{{1} \over {Respiratory \; Compliance}} \; = \; {{1} \over {Lung\; Compliance}} + {{1} \over {Chest \; Wall\; Compliance}}

{{1} \over {100}} \; = \; {{1} \over {200}} + {{1} \over {200}}

Integrated respiratory system pressure-volume curve — CICMWrecks original diagram

Lung compliance

Change in lung volume per unit change in transmural pressure
Normal lung compliance ~200ml/cmH2O
Static compliance (Cs)
- ” Compliance of lung measured when lung held at constant lung volume”
- no pressure componet due to resistance
- is a function of: Elastic recoil of the lung, Surface tension of alveoli
Dynamic compliance (Cd)
- ” Compliance of lung measured during cycles of inspiration and expiration”
- includes the pressure required to generate flow by overcoming resistance forces
- is a function of: respiratory rate
Dynamic Compliance usually < Static Compliance due to the degree of time dependence in the elastic behaviour of a particular lung
Specific compliance
- Compliance per unit volume lung
- Used to compare different lungs

C_S \; = \; {{C_{Tot}} \over {FRC}}

Hysteresis
- any process where the future state of a system is dependent on its current and previous state
- Compliance of the lung is different in inspiration and expiration
- In dynamic compliance curves:
  - Airways resistance is a function of flow rate. Flow rate (therefore resistance) is maximal at the beginning of inspiration and end-expiration.
- In static compliance curves:
  - There is no resistive component. Hysteresis is due to viscous resistance of surfactant and the lung.

Lung pressure-volume hysteresis and compliance — CICMWrecks original diagram

Factors Affecting Compliance

Lung Compliance

Surfactant
- increases lung compliance
- decreases surface tension at alveolar air – water interface
- prevents small alveoli from collapsing
- accounts for most of hysteresis in intact lungs
Lung volume
- Lung Compliance decreases at higher lung volumes
- Specific compliance (Compliance/FRC) remains constant
  - Elephants have greater lung compliance than mice!
Pulmonary blood volume
- Increased PBV decreases lung compliance
- Pulmonary venous congestion from L heart failure or mitral regurgitation
  decreases lung compliance
Bronchial smooth muscle tone
- Increased bronchial smooth muscle tone decreases compliance
- Decreased dynamic lung compliance by 50% in animal models of methacoline
  challenge
Disease
- ARDS, pneumonia decreases lung compliance
- pulmonary fibrosis → Impraired elasticity → decreases lung compliance
- Asthma, Emphysema increases lung compliance

Chest Wall Compliance

Chest wall restriction – reduced chest wall compliance
- Obesity
- Spastic paralysis of chest wall musculature
- Ossification of costal cartilages
- Kyphosis/scoliosis
- Scarring/constriction (e.g. circumferential burns)
Position: Prone (60% reduced compliance)
Collagen Disorders: Increased Chest Wall Compliance

Measurement of Compliance

Dynamic Compliance

C_d \; ∼ \; {{\Delta volume} \over {initial \; \Delta transmural \; pressure}}

= \; {{V_{max} \; – \; V_{min}} \over {P_{max} \; – \; P_{min}}}

” Compliance of lung measured during cycles of inspiration and expiration”
Measure:
- Measure with spirometer
- Using Volume/Pressure curve during normal rhythmic breathing at points of no flow
- Equation above then gives dynamic compliance

Static Compliance

” Compliance of lung measured when lung held at constant lung volume”
Volume vs Pressure loops during inspiration and expiration demonstrate Hysteresis
Measure:
- @ no flow in or out of the lung, AND time for pressure to equlibrate across the lung (>10sec).
- Patient exhales in steps holding the volume with open glottis
- Intrapleural pressure measured as oesophageal pressure
- Measured compliance secondary to viscoelastic properties and accounts for both short and long time-constant alveoli.

Specific Compliance

C_S \; = \; {{Static \; Compliance} \over {Lung \; Volume}}

Measure of the average compliance across all lung units
Children = Adults ~ 0.05 cmH₂O^-1

Gladwin / Sakurai / JC 2020

Reusable content

Formulae used in this answer

Dynamic Compliance

CWF-0049

C_d \; ∼ \; {{\Delta volume} \over {initial \; \Delta transmural \; pressure}}

Lung compliance

CWF-0057

C_S \; = \; {{C_{Tot}} \over {FRC}}

Resp Mechanics – Compliance

CWF-0071

Compliance \; = \; {{\Delta \, V} \over {\Delta \, P}}

Resp Mechanics – Compliance

CWF-0072

\Delta \, P \; = \; Alveolar \; - \; Intrapleural \; pressure

Resp Mechanics – Compliance

CWF-0074

{{1} \over {Respiratory \, Compliance}} \; = \; {{1} \over {Lung\, Compliance}} + {{1} \over {Chest \, Wall\, Compliance}}

Specific Compliance

CWF-0076

C_S \; = \; {{Static \; Compliance} \over {Lung \; Volume}}

Answer by Gladwin, JC, Sakurai

Past papers

Exam appearances

6 appearances

Exam	Exact exam wording	Candidate success
2022A Q03	Define respiratory compliance, include its components and their normal values (25% marks). Explain the factors that affect respiratory compliance (75% marks).	27%
2019B Q17	Explain respiratory compliance and outline the factors that affect it.	51%
2017A Q14	Define respiratory compliance (20% of marks). Describe the factors that affect it (80% of marks).	54%
2014A Q15	What is lung compliance and how is it measured?	34%
2011B Q07	Define lung compliance (30% marks). Describe how is it measured (70% marks).	36%
2007B Q13	Briefly describe the factors that affect lung compliance.	14%