PULMONARY COMPLIANCE

The water molecules on the surface of water (at air-water interphase) have an especially strong attraction for one another. This results in the water surface to contract to produce surface tension.

Thus the water molecules lining the alveoli produce an elastic contractile force due to surface tension which causes the alveoli to collapse.

This surface tension force of water molecules is broken by a surfactant (a mixture of phospholipids, proteins, and ions, most important phospholipids of which is dipalmitoylphosphatidylcholine-DPCC). It is secreted by type II alveolar epithelial cells and lines the alveoli.

The phospholipid molecules have a hydrophilic head and two parallel hydrophobic ‘tails’. Thus only the head part of the molecule dissolves in the fluid lining the alveolar surface and the hydrophobic tails face the alveolar lumen. This new surface thus formed of the surfactant has a significantly reduced surface tension than the water molecules.

By reducing the surface tension, surfactant serves two purposes:

• Prevents the alveoli_, from collapsing

• Prevents pulmonary edema.

In COPD, compliance is increased and the pressure volume curve is shifted upward and to the left. In a highly compliant lung, as in emphysema, the elastic tissue has been damaged, usually due to their being overstretched by chronic overinflation. Patients with emphysema have a very high lung compliance due to the poor elastic recoil, they have no problem inflating the lungs but have extreme difficulty exhaling air. In this condition extra work is required to get air out of the lungs.

Ref: Review of Medical Physiology by William ganong, 22nd edn/page 654

The low surface tension when the alveoli are small is due to the presence of surfactant in the fluid lining the alveoli. It is secreted by type II alveolar epithelial cells and lines the alveoli. Surfactant is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace.

The water molecules lining the alveoli produce an elastic contractile force due to surface tension which causes the alveoli to collapse. This surface tension force of water molecules is broken by surfactant. In spherical structures like an alveolus, the distending pressure equals two times the tension divided by the radius (P = 2T/r); if T is not reduced as r is reduced, the tension overcomes the distending pressure. Surfactant also helps to prevent pulmonary edema.

Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 34. Introduction to Pulmonary Structure and Mechanics. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

B i.e. Chronic bronchitis (Answer of exclusion)

Change in lung volume per unit change in distending (airway) pressure of lung (if enough time is allowed to reach equilibrium) is k/a lung compliance. So the volume change per unit pressure change or (volume change resulting from 1 cm H₂0 change in pressure) or the slope of pressure-volume curve is k/a compliance.

It is normally measured in the pressure range where the relaxation pressure curve is steepest. Because of hysteresis caused by surfactant, the deflation P-V curve is used for measurement. And lung compliance is the slope of line b/w any two points on deflation limb of pressure volume loop.

–  Unit of compliance is mL (or liter) per cm H₂0. The compliance of normal human (both) lung is 0.2i/cm H₂0 (200 mL/Cm H20,) which means every time the transpulmonary pressure increases 1 cm H₂0, the lung volume will expand 200 ml (0.2 L) after 10-20 seconds.

Lung compliance (C_L) is a measure of the elastic properties (or stretchability) of lung and hence its total capacity (TLC). It is a measure of how easily the lung is distended. High value refers to a lung that is readily distended and low compliance means a stiff lung that is not easily distended.

–                       Lung’s elastic behaviour (i.e its tendency to return to its resting volume after distension) or compliance is due to geometrical (interwoven) arrangement of elastin and collagen fibers and elastic forces caused by surface tension. The compliance is a static measure of lung and chest recoil. Whereas resistance of lung & chest (i.e. pressure difference required for a unit of air flow) is a dynamic rather than static measurement that takes into account the resistance to airflow in airways.

– However, compliance also depends on lung volume; and patient with one lung has approximately half compliance of normal (i.e. half AV for a given AP). Expiratory compliance (measured during deflation) is slightly greater than inspiratory compliance (measured during inflation) . The compliance of respiratory system (combined lung & thorax) is exactly half of the compliance of lung alone (i.e. 110 ml/cm H20).

–  Compliance of a lung depends on its size (volume). Similarly lung is stiffer (less distensible or has smaller compliance) at high lung volumes (and high expanding pressures) as shown by the flatter slope of curve. For this reason compliance per unit volume of lung (k/a specific compliance) is measured to know intrinsic elastic properties of lung tissue.

Compliance Decreased i.e. curve shifted                                                  Compliance Increased i.e curve shifted Pulmonary interstitial fibrosisQ

Interstitial lung disease                                                             – EmphysemaQ

Alveolar edema (prevents inflation of some alveoli)                     – Normal aging lung

Pulmonary congestion (increased pulmonary venous                     – During an asthma attack

pressure)Q

If lung remains unventilated for long period especially if lung volume is low

Decreased surfactantQ (causing increased surface tension and alveolar atelectasis)

Deformities of thorax eg kyphosis, scoliosis

–                                                                        Paralysis of respiratory muscles

Pleural effusion pneumo/hydro/hemo-thorax

B i.e. Chronic bronchitis (Answer of exclusion)

Change in lung volume per unit change in distending (airway) pressure of lung (if enough time is allowed to reach equilibrium) is k/a lung compliance. So the volume change per unit pressure change or (volume change resulting from 1 cm H₂0 change in pressure) or the slope of pressure-volume curve is k/a compliance.

It is normally measured in the pressure range where the relaxation pressure curve is steepest. Because of hysteresis caused by surfactant, the deflation P-V curve is used for measurement. And lung compliance is the slope of line b/w any two points on deflation limb of pressure volume loop.

–  Unit of compliance is mL (or liter) per cm H₂0. The compliance of normal human (both) lung is 0.2i/cm H₂0 (200 mL/Cm H20,) which means every time the transpulmonary pressure increases 1 cm H₂0, the lung volume will expand 200 ml (0.2 L) after 10-20 seconds.

Lung compliance (C_L) is a measure of the elastic properties (or stretchability) of lung and hence its total capacity (TLC). It is a measure of how easily the lung is distended. High value refers to a lung that is readily distended and low compliance means a stiff lung that is not easily distended.

–                       Lung’s elastic behaviour (i.e its tendency to return to its resting volume after distension) or compliance is due to geometrical (interwoven) arrangement of elastin and collagen fibers and elastic forces caused by surface tension. The compliance is a static measure of lung and chest recoil. Whereas resistance of lung & chest (i.e. pressure difference required for a unit of air flow) is a dynamic rather than static measurement that takes into account the resistance to airflow in airways.

– However, compliance also depends on lung volume; and patient with one lung has approximately half compliance of normal (i.e. half AV for a given AP). Expiratory compliance (measured during deflation) is slightly greater than inspiratory compliance (measured during inflation) . The compliance of respiratory system (combined lung & thorax) is exactly half of the compliance of lung alone (i.e. 110 ml/cm H20).

–  Compliance of a lung depends on its size (volume). Similarly lung is stiffer (less distensible or has smaller compliance) at high lung volumes (and high expanding pressures) as shown by the flatter slope of curve. For this reason compliance per unit volume of lung (k/a specific compliance) is measured to know intrinsic elastic properties of lung tissue.

Compliance Decreased i.e. curve shifted                                                  Compliance Increased i.e curve shifted Pulmonary interstitial fibrosisQ

Interstitial lung disease                                                             – EmphysemaQ

Alveolar edema (prevents inflation of some alveoli)                     – Normal aging lung

Pulmonary congestion (increased pulmonary venous                     – During an asthma attack

pressure)Q

If lung remains unventilated for long period especially if lung volume is low

Decreased surfactantQ (causing increased surface tension and alveolar atelectasis)

Deformities of thorax eg kyphosis, scoliosis

–                                                                        Paralysis of respiratory muscles

Pleural effusion pneumo/hydro/hemo-thorax

Answer is D (Emphysema):

Emphysema is associated with an increased static compliance but decreased dynamic compliance.

`The effect of emphysema on compliance is unique in that it results in an increase in static compliance but a decrease in dynamic compliance.

Static compliance increases due to destruction of elastic tissues in the alveolar walls. The reduction in elastic tissue makes it easier for the lung to expand in response to distending pressure, However the loss of elastic tissue in the walls of smaller airways makes them vulnerable to dynamic airway compression. This increases the airway resistance with consequent decrease in dynamic compliance – ‘Principles of Medical physiology’ by Sircar 1″ (2008)/320

Common causes of Rhonchi include:

• Bronchial Asthma
• COPD — Chronic bronchitis, Bronchiectasis, Emphysema
• Cardiac Asthma (Pulmonary Edema)
• Foreign body (Fixed monomorphic rhonchi – localized)
• Tumor (Fixed monomorrphic rhonchi — localized.)

Static Compliance

• Static compliance, indicates compliance when the lungs are at rest
• Static compliance values reflect true lung compliance alone
• Static compliance reflects elasticity of the lung and is not affected by airway resistance because there is no flow

Reduced static compliance indicates rigid lung tissue

• Interstitial fibrosis (Fibrosing al veolitis)
• Atelectosis
• Pulmonary edema
• Pneumonia
• ARDS

Dynamic Compliance

• Dynamic compliance indicates compliance when air flow is occurring
• Dynamic compliance values reflect lung compliance along with airway resistance
• Dynamic compliance is affected by both elasticity of the lung and airway resistance

Reduced Dynamic compliance (without change in static compliance) indicates Ted airway resistance

• Obstruction
• Bronchospasm (Allergic Asthma)
• Collapse of small airways etc

Dynamic Compliance

• Dynamic compliance indicates compliance when air flow is occurring
• Dynamic compliance values reflect lung compliance along with airway resistance
• Dynamic compliance is affected by both elasticity of the lung and airway resistance

Reduced Dynamic compliance (without change in static compliance) indicates Ted airway resistance

• Obstruction
• Bronchospasm (Allergic Asthma)
• Collapse of small airways etc
• Reduced static compliance is usually also associated with a decreased dynamic compliance due to associated airway resistance

Increased static compliance indicates easily expandible lung

• Emphysema
• Emphysema is associated with an increased static compliance but decreased dynamic compliance due to associated increased airway resistance.

Reduced Dynamic compliance with reduced static  compliance

• Most conditions where static compliance is reduced are also associated with reduced dynamic compliance due to associated increase in airway resistance.

Answer is B (COPD)

Pulmonary Compliance is increased in Emphysema (COPD)

Pulmonary Congestion, decreased surfactant and pulmonary fibrosis are all associated with reduced pulmonary Compliance.

Ans. B i.e. Alveoli

Pulmonary surfactant

• It is a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells.
• The proteins and lipids that comprise the surfactant have both a hydrophilic region and a hydrophobic region.
• By adsorbing to the air-water interface of alveoli with the hydrophilic head groups in the water and the hydrophobic tails facing towards the air
• The main lipid component of surfactant, dipalmitoyl phosphatidyl choline (DPPC), reduces surface tension.

Alveolar Lining Epithelial Cells

Two types

• Type I cells – the primary lining cells of the alveoli (squamous epithelium); Helps in gas exchange
• Type II cells – granular pneumocytes produce surfactant
• Type II cells make 60% of epithelial cells in alveoli but type I cells form 95% of the total surface area because of its flat surface.
• Type II cells are stem cells that help in replenishing damaged Type 1 pneumocytes in conditions like drug-induced pulmonary toxicity by amiodarone, bleomycin, etc. 

Ans. is ‘a’ i.e., 200 ml/cm water

The lungs and thoracic cage are both elastic structures. Hence they display a constant relationship between distending pressure and change in volume.

The change in volume per unit change in pressure is called “compliance”.

The total compliance of both lungs together in the normal adult human being is about 0^.2 L/cm water.

That is, every time the transpulmonary pressure increases by 1 centimeter of water, the lung volume will expand 0^.2 L (200 ml).

Compliance is a measure of distensibility.