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Part 2

Mechanical Properties of the Lung and Chest Wall: Static and Dynamic

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Section I

STATIC LUNG MECHNICS:

The mechanical properties of a lung whose volume is not changing with time

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1. Pulmonary Volume and Capacity

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Pulmonary Volumes• Tidal volume ( 潮气量）

– Volume of air inspired or expired during a normal inspiration or expiration (400 – 500 ml)

• Inspiratory reserve volume （补吸气量）– Amount of air inspired forcefully after inspiration of nor

mal tidal volume (1500 – 2000 ml)

• Expiratory reserve volume （补呼气量）– Amount of air forcefully expired after expiration of norm

al tidal volume (900 – 1200 ml)

• Residual volume （残气量， RV ）– Volume of air remaining in respiratory passages and lung

s after the most forceful expiration (1500 ml in male and 1000 ml in female)

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Pulmonary CapacitiesA Capacity is composed of two or more volumes

• Inspiratory capacity （深吸气量）– Tidal volume plus inspiratory reserve volume

• Functional residual capacity （功能残气量 , FRC ）– Expiratory reserve volume plus the residual volume

• Vital capacity （肺活量 , VC ）– Sum of inspiratory reserve volume, tidal volume, and exp

iratory reserve volume

• Total lung capacity （肺总量 , TLC ）– Sum of inspiratory and expiratory reserve volumes plus t

he tidal volume and residual volume

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RV/TLC

• Normally less than 0.25

• Increase by the obstructive pulmonary

disease (RV)

• Increase during the restrictive lung disease

(TLC)

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Minute and Alveolar Ventilation

• Minute ventilation: Total amount of air moved into and out of respiratory system per minute

• Respiratory rate or frequency: Number of breaths taken per minute

• Anatomic dead space: Part of respiratory system where gas exchange does not take place

• Alveolar ventilation: How much air per minute enters the parts of the respiratory system in which gas exchange takes place

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Dead Space

• Area where gas exchange cannot occur

• Includes most of airway volume

• Anatomical dead space (=150 ml)

– Airways

• Physiological dead space

= anatomical + non functional alveoli

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Basic Structure of the LungBasic Structure of the Lung

VVDD

A tube = Airway A tube = Airway (Trachea – Bronchi – Bronchioles) (Trachea – Bronchi – Bronchioles)

NO GAS EXCHANGENO GAS EXCHANGE

DEAD SPACEDEAD SPACE

A thin walled Sac = AlveolusA thin walled Sac = Alveolus

Blood VesselsBlood Vessels

GAS EXCHANGEGAS EXCHANGEOCCURS HEREOCCURS HERE

VVAA

Formula: Total Ventilation = Dead Space + Alveolar SpaceFormula: Total Ventilation = Dead Space + Alveolar Space V VTT = V = VD D + V+ VA A

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Physiological =Physiological = Anatomical Dead SpaceAnatomical Dead Space Dead Space Dead Space + +

Similar Concept: Physiological Dead SpaceSimilar Concept: Physiological Dead Space

Diseased Diseased lungs:lungs:

Healthy Lungs:Healthy Lungs:

BlockedBlockedVesselVessel

Additional Dead SpaceAdditional Dead Space

• Anatomical Dead SpaceAnatomical Dead Space = Airways (constant) = Airways (constant)

VVAA

VVDD

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2. Lung Compliance

• Lung compliance (CL)

• is a measure of the elastic properties of t

he lung,

• is a reflection of lung distensibility

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Elasticity• Tendency to return to initial size after

distension.

• High content of elastin proteins.

– Very elastic and resist distension.

• Recoil ability.

• Elastic tension increases during inspiration and

is reduced by recoil during expiration.

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Compliance

• Distensibility (stretchability):– Ease with which the lungs can expand.– The compliance is inversely proportional to elastic

resistance

• Change in lung volume per change in transpulmonary pressure.

V/P• 100 x more distensible than a balloon.• Specific compliance ( 比顺应性） : the complia

nce per unit volume

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0

100

50

0 30

Lungvolume(%TLC)

Transpulmonary pressure (cmH2O)

Static lung compliance C = V/P

inflation

deflation

normal breathing

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Section II DYNAMIC LUNG MECHANICS

Aspects of mechanics

that studies the lung in motion

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1. Dynamic Compliance

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Volume LVolume L

RVRVPleural PressurePleural Pressure00

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00 - 30 cm H- 30 cm H22OO- 15- 15

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Normal (with surfactant)Normal (with surfactant)Saline FilledSaline Filled

Without surfactant

Volume-pressure curves of lungs filled with saline and with air (with or without surfactant)

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Dynamic lung compliance

• Is always less than static compliance

• Increase during exercise

• Increase during sighing and yawning

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2. Airflow in Airways

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Types of Flow

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Laminar flow

• … is when concentric layers of gas flow parallel t

o the wall of the tube.

• The velocity profile obeys Poiseuille’s Law

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Poiseuille and Resistance

• Airway Radius or diameter is KEY.

radius by 1/2 resistance by 16 FOLD - think

bronchodilator here!!

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The gas flow in the larger airways (nose,

mouth, glottis, and bronchi) is turbulent

Gas flow in the smaller airway is laminar

Breath sounds heard with a stethoscope

reflect the turbulent airflow

Laminar flow is silent

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3. Gas Flow Resistance

• Elastic Resistance

• Inelastic Resistance

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Elastic Resistance

Caused by the elastic tissue of the lung and the thoracic wall surface tension of the fluid that lines the inside w

all of the alveoliThe elastic resistance caused by surface tension

are much more complex. accounts for about two thirds of the total elastic resistanc

e

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Inelastic Resistancecomprises

1.airway resistance (friction)

2.pulmonary tissue resistance (viscosity, and inertia).

the airway resistance account for 80%-90% of the inelastic resistance

the more important both in health and disease.

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Airway Resistance

• Airway resistance is the resistance to flow of

air in the airways

• due to :

1) internal friction between gas molecules

2) friction between gas molecules and the

walls of the airways

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Factors that influence airway resistance

• Airway diameter• asthma ( 哮喘） and parasympathetic stimulation:

Narrowing airways.• Emphysema ( 肺气肿） : decreases small airway d

iameter during forced expiration

• Turbulence air flow • Rapid breathing:

• Density and viscosity of the inspired gas

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Control of Airway Smooth Muscle

• Neural control– Adrenergic beta receptors causing dilatation

– Parasympathetic-muscarinic receptors causing constriction

– NANC nerves (non-adrenergic, non-cholinergic)• Inhibitory release VIP and NO bronchodilitation

• Stimulatory bronchoconstriction, mucous secretion, vascular hyperpermeability, cough, vasodilation “neurogenic inflammation”

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Control of Airway Smooth Muscle

• Local factors

– histamine binds to H1 receptors-constriction

– histamine binds to H2 receptors-dilation

– slow reactive substance of anaphylaxis ( 过敏反应） - constriction-allergic response to pollen

– Prostaglandins （前列腺速） E series - dilation

– Prostaglandins （前列腺素） F series - constriction

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Control of Airway Smooth Muscle (cont)

• Environmental pollution

– smoke, dust, sulfur dioxide, some acidic

elements in smog

• Elicit constriction of airways

– mediated by:

• parasympathetic reflex

• local constrictor responses

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4. Measurement of Expiratory Flow - FVC

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FVC - forced vital capacity

• Defines maximum volume of exchangeable air in

lung (vital capacity)

– forced expiratory breathing maneuver

– requires muscular effort and some patient

training

• Initial (healthy) FVC values approx 4 liters

– slowly diminishes with normal aging

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• Significantly reduced FVC suggests damage to lung tissue– restrictive lung disease (fibrosis ，纤维化 )– constructive lung disease– loss of functional alveolar tissue

• Intra-subject variability factors– age– sex– height– ethnicity

FVC - forced vital capacity(cont)

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FEV1 - forced expiratory volume (1 second)

• maximum air flow rate out of lung in initial 1 second interval

– forced expiratory breathing maneuver

– requires muscular effort and some patient training

• FEV1/FVC ratio

– normal FEV1 about 3 liters

– FEV1 needs to be normalized to individual’s vital capacity (FVC)

– typical normal FEV1/FVC ratio = 3 liters/ 4 liters = 0.75

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• Standard screening measure for obstructive lung disease – FEV1/FVC reduction trend over time (years) is key indic

ator– calculate % predicted FEV1/FVC (age and height normal

ized)• Reduced FEV1/FVC suggests obstructive damage to lung ai

rways– episodic, reversible by bronchodilator drugs

• probably asthma ( 哮喘 )– continual, irreversible by bronchodilator drugs

• probably COPD （ chronic obstructive pulmonary disease ，慢性阻塞性肺病）

FEV1 - forced expiratory volume (1 second)

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Vol

um

e (l

itre

s)

Time (sec)

Forced Vital Capacity - FVC

Total Lung Capacity

Residual Volume

Spirometry

Forced Expiratory Volume in 1 sec - FEV1

1 sec

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eg fibrosis / pulmonary oedema

Assessment of RESTRICTIVE Lung Diseases

These are diseases that reduce the effective surface area available for gas exchange

Normal Lung Volume Lung Volume in Restrictive Disease

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REDUCED

Vol

um

e (l

itre

s)

Time (sec)

Vital Capacity

Total Lung Capacity

Residual Volume

Spirometry

RESTRICTIVE lung disease

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eg asthma / bronchitis

Assessment of OBSTRUCTIVE Lung Diseases

These are diseases that reduce the diameter of the airways and increase airway resistance -

remember Resistance increases with 1/radius 4

Normal Airway Calibre Airway Calibre in Obstructive Disease

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Forced Vital Capacity - FVC

Forced Expiratory Volume in 1 sec - FEV1

FEV1 > 80% of FVC

is Normal

or in words - you should be able to forcibly

expire more than 80% of your vital capacity in

1 sec.

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Forced Vital Capacity - FVC

Vol

um

e (l

itre

s)

Time (sec)

Total Lung Capacity

Residual Volume

Spirometry

Forced Expiratory Volume in 1 sec - FEV1

1 sec

FEV1 < 80% of FVC

OBSTRUCTIVE lung disease

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