Canonical Question
Resp Mechanics – Compliance
Master answer
\[ Compliance \; = \; {{\Delta \, V} \over {\Delta \, P}} \]
\[ \Delta \, P \; = \; Alveolar \; – \; Intrapleural \; pressure \]
\[ {{1} \over {Respiratory \; Compliance}} \; = \; {{1} \over {Lung\; Compliance}} + {{1} \over {Chest \; Wall\; Compliance}} \]
\[ {{1} \over {100}} \; = \; {{1} \over {200}} + {{1} \over {200}} \]
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.
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
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2007B Q13 | Briefly describe the factors that affect lung compliance. | historical_member | — |
| 2019B Q17 | Explain respiratory compliance and outline the factors that affect it. | historical_member | — |
| 2011B Q07 | Define lung compliance (30% marks). Describe how is it measured (70% marks). | historical_member | — |
| 2014A Q15 | What is lung compliance and how is it measured? | historical_member | — |
| 2017A Q14 | Define respiratory compliance (20% of marks). Describe the factors that affect it (80% of marks). | historical_member | — |
| 2022A Q03 | Define respiratory compliance, include its components and their normal values (25% marks). Explain the factors that affect respiratory compliance (75% marks). | historical_member | — |