CS 2015

Mechanical Properties of Lung and Chest Wall

Christian StrickerAssociate Professor for Systems Physiology

ANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Mechanics.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2015

CS 2015

Aims

At the end of this lecture students should be able to

• explain different types of air flow conditions;

• identify determinants of airway resistance (RAW);

• illustrate the concepts of static and dynamic

compliance and how these are measured;

• demonstrate why a small lung volume is harder to

inflate than a larger; and

• point out how surfactants increase compliance.

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Contents

• Airway resistance– Air flow conditions

– Locations and determinants of RAW

– Transmural pressure

• System compliance and its elements– Static & dynamic compliances

– Alveolar surface tension

– Laplace’ law and alveolar pressure

– Surfactants and compliance

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Determinants of RAW

• Air flow conditions

• Locations and determinants of RAW

• Transmural pressure

• Modulation of RAW

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Air Exchange

• Conducting airways: blood supply via bronchial artery.

• Bronchioles: no skeleton; exposed to transmural pressure.

• Respiratory unit = physiological unit, where O2 and CO2

are exchanged; blood supply via pulmonary artery.

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Consequences for Air Flow

• Flow over vocal cords is biggest and decays later to small

values in alveolar airways.

• Functional consequence: ~ turbulent flow over vocal cords;

but ~ laminar flow in alveolar airways.

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Flow Conditions in Airways• Since airways are bifurcated, turbulence arises at

bifurcation points.

• Flow in airways is transitional (in between laminar

and turbulent).

• Ohm’s law is used to determine RAW (airway and

tissue deformation):

• Contribution to RAW:

Boron & Boulpaep, 2003

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Determinants of RAW

• Under laminar flow conditions,

with η viscosity, l length and r radius.

• Normally, viscosity is constant (air); altered

with pressure (altitude, diving) & gas mixtures.

• Elements of RAW (around TV)

– Rvisc ~ 40% (dynamic parameter; flow dependent).

• Laminar and turbulent conditions (80%)

• Tissue resistance (“friction” between elastic fibres; 20%)

• Inertia (very little)

– Relast ~ 60% (static parameter; volume dependent).

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RAW and Lung Volume

• Lung volume affects airway diameter, particularly airways

without skeleton: during E, tension release (alveolar size ↓)

and positive pressure on bronchioli → r ↓; during I, vice

versa.

• It is easier to breath in than out (air trapping…).

• COPD: r↓ → RAW↑. To maintain ventilation, flow↑.

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Transmural Pressures

• Affects bronchioles

• During forced I, positive

transmural pressure keeps

small airways open.

• During forced E, when Ppl >

0, transmural pressure can

become ≤ 0; i.e. airways

collapse.

• Can be seen in flow-volume

loop: airway closure.

Modified from Hlastala & Berger 2001

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Modulation of RAW

• Smooth muscle tone– Parasympathetic: bronchial constriction and mucus production ↑.

– Sympathetic: β2-action (smooth muscle relaxation, secretion ↓).

• With ↑ → local airway dilation; ↓→ local airway constriction.

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Compliance of Breathing System

• Static & dynamic compliances

• Alveolar surface tension- Laplace’ law and alveolar pressure

- Surfactants and compliance

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Compliance of Breathing System

Static compliance: no flow, volume fixed

Dynamic compliance: both flow, volume change

CT = total compliance (breathing system)

CL = lung compliance

CCW = thorax (chest wall) compliance

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How to Measure Compliances• Shown with body

plethysmograph.– Required for Cdyn.

– Not necessary for Cstatic (no

flow…).

• Cstatic with valve and

spirometer only.– Measured during expiration

(see later).

– PA and ΔVL measured

simultaneously after halting

flow (= Poral): at each

volume, PA measured.

Modified from Boron & Boulpaep, 2003

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Static Lung Compliance (No Flow)

• Total system compliance (CT) can be

measured after breathing out (“relaxation

curve”); linear within range of TV.

• Both lung (CL; fibrosis – too small;

emphysema – too large) and chest-wall

compliance (CCW; skoliosis) are needed

clinically.

• CT is related to CCW and CL via

• Requires that Ppl be measured with each

volume change.

• Within TV, CL ~ CCW ~ 2 CT, ~ 0.1 L/cm

H2O.Modified from Hlastala & Berger 2001

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Static CL and Pathology

• Static CL important in

pathophysiology.

• Emphysema (“overblown”

lung) has large

compliance at FRC: loss

of recoil (elastance; 1/CL).

• Conversely, fibrosis

reduces CL and FRC: too

much recoil …Modified from Boron & Boulpaep, 2003

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Dynamic Compliance• Example for TV

• Hysteresis (CCW move)

• Cdyn at end of E > than at

beginning of I.– For both I and E, smaller at

beginning than at end.

– Elastic recoil > at end of I

which helps at start of E

• Cstat ≈ average Cdyn (which

is typically a bit smaller).

• Effort sets width of

hysteresis.Modified from Despopoulos & Silbernagl 2003

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Compliances in Disease• Emphysema with a high static

compliance and a wide

dynamic hysteresis (work! -

recoil lost).

• Asthma increases

compliance; TV at FRC↑;

large expiratory work

(increased RAW).

• RDS has low static and

dynamic compliance and TV

at high pressures.Modified from Koller, 1979

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Alveolar Surface Tension

Laplace’ law

Surfactants

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Surface Tension and Compliance

• CL↑ when lung filled with

saline - but finite.

• Surface tension is largest

factor determining CL:

– Laplace’ law.

• How to minimise surface

tension?– Detergents (soap)

– Surfactants …

Modified from Boron & Boulpaep, 2003

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What Every Child Knows…

• What is the hardest part to

blowing up a balloon?– Initial volume change…

– Becomes easier as you inflate…

– Ultimately so easy, it can be

blown apart…

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Laplace’ Law

•

• Precoil in B is 2 x that in A.

• If A and B are coupled in series,

what happens?– B blows A up.

• To counter this, alveoli are– interdependent: physically

interconnected with each other; and

– lined with surfactant.

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Surfactants and Surface Tension• Surfactant (surface-active agent)

• Reduces surface H2O and hence

surface tension: it is an attractive force

of surface molecules that tends to

minimise surface area.

• Combination of dipalmitoylphosphatidyl-

choline and apoproteins (SP-A/B/C/D).

• Secreted by alveolar type II cells

• Can easily be destroyed with O2.

• Produced shortly before birth; problem

in premature babies (respiratory

distress syndrome).– Steroid priming for 2-3 d can initiate

surfactant expression.

Modified from Boron & Boulpaep, 2003

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Surface Expression• Surfactants form micelles.

• Dynamic system:– During I, as alveolar surface

increases and [surfactant]

decreases, surfactant from

micelles is recruited to surface.

– During E, alveolar surface de-

creases, [surfactant] is higher

and micelles re-form.

• Role:– Reduction in surface tension:

keeps alveoli “open”.

– Keeping alveoli dry.Modified from Hlastala & Berger 2001

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Ventilation and Surfactants

• Rapidly expanding alv. →

[surfactant]↓ → CA↓ →

ventilation↓.

• Slowly expanding alv. →

[surfactant]↑ → CA↑ →

ventilation↑.

• Homeostatic principle to

open alveoli to ~ similar volume.

Modified from Boron & Boulpaep, 2003

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Take-Home Messages• Flow in bronchi is transitional, in alveoli

laminar.

• RAW is volume dependent; is neurally

modulated.

• CL is ~2 x CT; is linear in range of TV.

• A small alveolus requires a larger pressure to increase its volume than a large one;

• Hysteresis in V-P loop is result of surface tension and Laplace’ law; and

• Surfactants reduce surface tension and ease alveolar ventilation.

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MCQAnna May, a 43 year-old female, has an extensive lung

function analysis. As she exhales under static conditions from

FRC + 1 L to FRC, her oesophageal pressure changes from -

10 to -5 cm H2O and the alveolar pressure from 5 to 0 cm

H2O. What is the best estimate of her static lung compliance?

A. 0.5 L / cm H2O

B. 5.0 cm H2O / L

C. 0.1 L / cm H2O

D. 2.0 cm H2O / L

E. 0.2 L / cm H2O

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That’s it folks…

CS 2015

MCQAnna May, a 43 year-old female, has an extensive lung

function analysis. As she exhales under static conditions from

FRC + 1 L to FRC, her oesophageal pressure changes from -

10 to -5 cm H2O and the alveolar pressure from 5 to 0 cm

H2O. What is the best estimate of her static lung compliance?

A. 0.5 L / cm H2O

B. 5.0 cm H2O / L

C. 0.1 L / cm H2O

D. 2.0 cm H2O / L

E. 0.2 L / cm H2O