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Specific compliance is reduced in the following except: (AIPG 2011)

A Pulmonary congestion

B Reduced functional pulmonary surfactant

C Chronic bronchitis

D Pulmonary fibrosis

Ans. C Chronic bronchitis.

1. Lung compliance refers to the ability of lungs to stretch. However, many normal factors affect lung compliance and it is best represented by a whole pressure-volume curve.

2. Thus, Lung compliance is the change in lung volume for a given change in pressure.

Compliance = ΔV / ΔP.

3. The normal compliance of human lungs and chest wall is about 0.2 L/cm H2O.

4. Decreased pulmonary compliance due to lung edema, lung hemorrhage, or loss of surfactant. Compliance is reduced in restrictive lung disease.

Lung compliance

I. Compliance

a. the slope of the pressure-volume curve at a particular lung volume ΔV => i.e. volume change per unit of pressure change (mL/cmH2O)

b. normal value = 200mLs/cmH2O

c. Lower compliance = more effort of breathing

II. Specific compliance:

a. = compliance divided by FRC (/cmH2O)

b. normal value = 0.05/cmH2O

c. similar values in both sexs and all ages including neonates

d. measurement of the intrinsic elastic property of the lung tissue

III. Dynamic compliance vs static compliance:

a. Static compliance is the compliance measured when there is no gas flow into or out of the lung.

IV. Lung compliance and chest wall compliance:

a. Compliance is made up of lung compliance and chest compliance

b. => 1/Ct = 1/Cl + 1/Ccw

c. (Ct = total compliance, Cl = lung compliance, Ccw = chest wall compliance)

V. Factors affecting compliance:

a. Lung elastic recoil

b. Lung volume

c. Pulmonary blood flow

d. History of recent ventilator

e. Bronchial smooth muscle tone

f. Diseases

a. Lung's elastic recoil

Due to:

i. Surface tension in the alveoli

ii. Stretched elastic fibres in the lung parenchyma

iii. Surface tension accounts for 70% of the elastic recoil

b. Lung volume

i. The slope of the P-V curve is not constant across different lung volumes.

ii. At high lung volumes, compliance is reduced because more pressure is required to stretch the already stretched elastic tissues further.

iii. At very low volumes, compliance is reduced because of closed airway and collapsed alveoli

1) increased surface tension

2) increased pressure is needed to re-open the airway/alveoli

3) reduced compliance

iv. At the base of the lung, at very low volumes, compliance is even more reduced because of positive intrapleural pressure

v. Posture affects compliance by affecting the lung volume.

vi. Restriction of chest expansion also affect lung volmen and chest wall compliance.

c. Pulmonary blood flow

i. Contributes to stiffness of the lung, especially in the case of pulmonary congestion

d. History of recent ventilation

i. Prolonged periods of hypoventilation without periodic deep breath may lead to reduced compliance.

ii. May be related to atelectasis.

e. Bronchial smooth muscle tone

i. In animal studies, increased bronchocontriction can lower time constant and reduced dynamic compliance.

ii. Static compliance is probably not affected.

f. Disease

i. In diseased lungs, where time constant for the alveolis are different, units with higher time constants are slow to fill and empty.

ii. With higher respiratory rate, the problem worsens:

1) units with high time constant hypoventilates

2) less lung units participate in volume changes

3) dynamic compliance reduced.

iii. With collapsed alveoli

1) greatly increased surface tension

2) very high pressure is required to re-open airway/alveoli.

Diseases that REDUCE compliance


Pulmonary hypertension/congestion

Alveolar atelectasis

Reduced surfactant (increased surface tension) e.g. artificial ventilation, prematurity

Diseases that INCREASE compliance:

Pulmonary emphysema (alteration in elastic tissue) -> static compliance is Increased but dynamic compliance is reduced.

Normal ageing (alteration in elastic tissue)

Asthma (reason unknown).