Lung Compliance

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Introduction

Compliance is defined as the change in lung volume produced by a unit change in transpulmonary pressure. Lung compliance is represented by the gradient of the pressure–volume curve.

Normal Range: The total compliance of both lungs together in normal adult human being average is about 200 millilitres of air per centimetre of water trans-pulmonary pressure. That is every time the trans pulmonary pressure increases one centimetre of water the lung volume after 10 to 20 seconds will expand 200 milliliteres.

Compliance Diagram

The diagram relates the lung volume changes to changes in the trans pulmonary pressure. It differs for inspiration and expiration

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Compliance Curve

The two compliance curves are:

1. Inspiratory Compliance curve
2. Expiratory Compliance curve

Types of Compliance

Dynamic Compliance : This describes the compliance measured during breathing which involves a combination of Lung compliance and air resistance.

Static Compliance : This describes the pulmonary compliance when there is no air flow like an inspiratory pass.

Important Factors

The two important factors of Lung compliance  :

Elastic Fibers : More fibers in the tissue lead to ease in expand-ability and there for compliance.

Surface tension : It is decrease due to the production of surfactant to prevent collapse.

Hysteresis

Hysteresis is the term used to describe the difference between inspiratory and expiratory compliance. Lung volume at any given pressure during inhalation is less than the lung volume at any given pressure during exhalation.

Hysteresis is present in both static and dynamic lung compliance curves

Hysteresis develops due to:

• The effect of surfactant
• Relaxation of lung tissue
• Recruitment and derecruitment of alveoli
• Gas absrption during measurement
• Differences in expiratory and inspiratory air flow (for dynamic compliance)

Factors which affect compliance can be divided into chest wall factors and lung factors:

Factors which Affect Respiratory Compliance

Lung compliance	Chest wall compliance
Increased  lung compliance Lung surfactant Lung volume: compliance is at its highest at FRC Posture (supine, upright) Loss of lung conective tissue associated with age Emphysema	Increased chest wall compliance Ehler-Dahlos syndrome and other connective tissue diseases Rib resection Cachexia Flail segment rib fractures Open chest (eg clamshell)
Decreased static lung compliance Loss of surfactant (eg. ARDS) Decreased lung elasticity Pulmonary fibrosis Pulmonary oedema Decreased functional lung volume Pneumonectomy or lobectomy Pneumonia Atelectasis Small stature Alveolar derecruitment Alveolar overdistension Decreased dynamic lung compliance Increased airway resistance (eg. asthma) Increased air flow (increased resp rate)	Decreased chest wall compliance Structural abnormalities Kyphosis / scoliosis Pectus excavatum Circumferential burns Surgical rib fixation Functional abnormalities Muscle spasm, eg. seizure or tetanus Extrathoracic influences on chest/diaphragmatic excursion Obesity Abdominal compartment syndrome Prone position

Related articles

Lung volumes-physiopedia

Spirometry-physiopedia

Lung compliance

References

1. Iotti, G., & Braschi, A. (1999). Measurements of respiratory mechanics during mechanical ventilation. Rhäzüns, Switzerland: Hamilton Medical Scientific Library.
2. Harris, R. Scott. "Pressure-volume curves of the respiratory system." Respiratory care 50.1 (2005): 78-99.
3. Rahn, Hermann, et al. "The pressure-volume diagram of the thorax and lung." American Journal of Physiology-Legacy Content 146.2 (1946): 161-178.
4. https://www.us.elsevierhealth.com/guyton-and-hall-textbook-of-medical-physiology-9781455770052.html
5. https://www.ncbi.nlm.nih.gov/books/NBK554517/
6. https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/21%3A_Respiratory_System/21.6%3A_Factors_Affecting_Pulmonary_Ventilation/21.6B%3A_Factors_Affecting_Pulmonary_Ventilation%3A_Compliance_of_the_Lungs