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Competency

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No. PY6.2 (Respiratory Physiology)

CompetencyDescribe the alveolar surface tension, compliance and airway resistance.

D/L/Core K/KH/Y

AM Written/Viva voce

Integration No

Imp. Concept Compliance & Elastance, law of laplace

Phy. Trivia Application of Young-laplace law

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Everything about mechanical ventilation can be discussed in terms of flow, volume, pressure, resistance and compliance.

Two Imp Properties of RS

Stretchability – Compliance

The ability of the lungs to expand

Elasticity – Elastance

This is a measure of the resistance of a

system to expand.

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Elastance = 1/Compliance = Pressure change / Volume change

⧍V/ ⧍P

⧍P/ ⧍V

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WORK OF BREATHINGResistance during breathing

• 65% of work is elastic work due to elastic forces of lung &

thorax. (Tissue and surface tension)

• 35% of work is non elastic work due to

Viscous resistance: 7%

Airway resistance: 28%

• Work done is calculated by ΔP/ΔV curve (W = P × V)

• Value- during quiet breathing ranges from 0.3-0.8

kg.Mt/min. 7

Factor affecting airway resistance

1. Rate of gas flow;

Greater the rate of gas flow, greater is the resistance.

2. Airway diameter (R = 1/r4 );

It is the most important factor. Smaller the diameter of the

airway, greater is the resistance.

Air way resistance is more during forced expiration

3. Length of airway; is not an important factor.

4. Types of air flow; Turbulence flow > Laminar flow

Factors……contWhile airway resistance is ↓ by ↑ in cross sectional

area of the respiratory passage (like in exchange zone).

Approximately 80% of the total airway resistance is

offered by airways from o to 7th generation

Means maximum resistance is offered by medium

sized airways

Viscosity and density of inspired gas (O2-He mixture

is used in deep sea diving)

Normal Airway Resistance (R=⧍P/F) Is about 1-3 cm of H2O/L/Second

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Maximum resistance is offered by medium sized airways (0-7)

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OUTLINE

Static compliance

Definition and normal value

Significance and determinants

Measurement

Factors affecting

Clinical

Specific compliance

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DEFINITION & NORMAL VALUE

• Definition;

Compliance means the ability to stretch (distensibility) or the

ability to recoil (elastic recoil). Lung compliance can be

assessed from pressure-volume curve.

Compliance = ∆V/∆P

• Normal Values;

Lung alone = 0.22 L/cm of H2O

Total compliance (lungs & thorax) = 0.13 L/cm of H2O

1/CTOT = 1/CL + 1/CCW14

Significance and Determinants

• Significance;

Compliance is a measure of stretchability or expansibility of

lung and chest wall

• Determinants;

Elastic forces

Surface force

other determinants- Airway & Viscous resistance

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MEASUREMENT

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Total Respiratory Compliance

MEASUREMENT...

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Pulmonary Compliance

Hysteresis’ curve.line is curved bothduring inspirationand expirationbecause of tworesistances- Viscous andairway resistance.

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Air Vs Saline Filled Lung20

FACTORS AFFECTING

• Phase of respiratory cycle; E > I

• Effect of gravity; Apex < Base

• Compliance inversely proportional to the Elastance

• Surfactant

• Lung volume (Adult > Children)

• Age (old age > Adult)

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CLINICAL

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• Decreased lung compliance and compliance curve

is shifted to downwards and right:

Restrictive lung disease,

Pulmonary edema.

• Increased lung compliance and compliance curve

is shifted to upwards and left:

Obstructive lung disease; (Emphysema)

Old age ; (Physiological)

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OLD

RLD

SPECIFIC COMPLIANCE

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• SC = Static compliance/FRC

• SC for both intact lung;

= 0.22/2.2

=0.1 L/cm H2o

• SC for one lung (After pneumonectomy);

= 0.11/1.1

= 0.1 L/cm H2o

SPECIFIC COMPLIANCE - Significance

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• Importance (Advantage) is in individuals with one lung only,

lung compliance is approximately half of the normal

compliance. This is in spite of the fact that the remaining lung

may be healthy, with normal ‘distensibility’.

• Similarly in children, because of smaller lung volume,

compliance will be below normal in spite of normal

distensibility. This fallacy is removed with specific

compliance since ‘FRC’ is proportionately reduced and

specific compliance remains essentially constant.

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PULMONARY ELASTANCE

• Elastance means “recoil” tendency

• Elastance of thoracic cage

• Elastance of lungs;

Tissue force

Surface force

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SURFACE TENSION

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LAW OF LAPLACE

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Definition

Compositions

Source: Formation and secretion

Mechanism of action

Functions

Factor affecting

Clinical

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DEFINITION

Surfactant means any surface acting agent that

spread over the fluid at interface & decrease its

surface tension.

Surfactant present in the fluid lining alveoli is

called pulmonary surfactant that ↓es surface

tension of the fluid lining alveoli & thus collapse

tendency of alveoli.

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COMPOSITION

i) 90% lipids (specially phospholipid):

In surfactant 77% are phospholipids

(main is DPPC 62% dipalmitoyl phosphatidyl

choline),

13% neutral lipids,

ii) 8% Proteins:

Apoprotein A, B, C & D

iii) 2% carbohydrates and ions specially Ca++

MECHANISM OF ACTION

• It form a monomolecular film or barrier over the

fluid at interface and ↓ surface tension from 50

to 5-25 dyne/cm2.

• Apoprotein B and ca++ speed up the spread of

Surfactant over the fluid.

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FUNCTIONS

i) Prevent collapsing tendency of alveoli

ii) Prevent pulmonary edema

iii) ↓ work of breathing

2/3 of elastic work is due to surface tension.

(iv) Role in inflation of lungs at

birth lungs.

According to law of LaPlace

Distending Pressure = 2 x Wall

tention / radius

So at birth small alveoli with

greater collapsing pressure

specially needs surfactant to

inflate

(v) Role in stabilization of different size of alveoli-

Smaller alveoli have more collapsing tendency (P=2T/R) &

Tendency to deflate into larger But surfactant form more

concentrated film in smaller alveoli and prevent it.

(vi) Defense & ↓ inflammation ( apoprotein A & D)

Factors affecting

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• Hormonal factors: Thyroid, Insulin, Glucocorticoid

• Quantity and quality of proteins present in

surfactant

• Stretching of lungs: Deep breathing after surgery

improves lung functions

• Pharmacological agents: Beta-2 Agonist, calcium

• Exercise

IRDS (infant respiratory distress syndrome

or hyaline membrane disease )

Causes

- Premature babies,

- Deficiency of thyroid hormones,

insulin or cortisol (mother)

Management

- Positive pressure ventialtion

- Inhalational phospholipids

- bovine surfactant given through inhalation

- Glucocorticoid

i) Long term inhalation of

100% O2.

ii) Occlusion of main

bronchus.

iii) Occlusion of one

pulmonary artery.

iv) Cigarette smoking

v) Cutting both vagi.

vi) Aspiration (trauma)

ARDS (adult respiratory distress syndrome)-

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