CS 2015

Alveolar Ventilation and Factors Influencing It

Christian StrickerAssociate Professor for Systems Physiology

ANUMS/JCSMR - ANU

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

THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2015

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AimsAt the end of this lecture students should be able to

• outline different components of ventilation;

• discuss the components of physiological dead

space;

• explain how ventilation determines partial

pressure of CO2;

• recognise that RAW and compliance determine the

speed of gas exchange in alveoli; and

• illustrate how uneven ventilation of lung tissue can

arise.

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Contents• Review of

• Global ventilation– Physiological dead space and its elements

– Total ventilation, dead space ventilation

– Alveolar ventilation and alveolar CO2

– Speed of gas exchange

• Local ventilation– Larger compliance at base than apex

– Local RAW and CL

– Factors determining uneven pulmonary ventilation

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Review of Changes in

• Axis along bottom indicates distance from nose.

• At each step, ↓. Note notation.

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1. Global Ventilation

(over whole lung as an entity)

Total Pulmonary Ventilation =

Physiological Dead Space Ventilation

+ Alveolar Ventilation

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Total Ventilation• Total ventilation: (Volume per minute)

– Example: Breathing frequency (12 bpm) and tidal volume (TV; 0.5 L)

• Two components of air volume:– “dead space” (NO gas exchange) and

– alveoli (ONLY gas exchange).

• In a healthy person, “dead space” is called physiological

dead space.

• How big is ventilation of physiological dead space at TV

(“inefficiency”)? ¼ - ⅓ .

• How can physiological dead space be determined?

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Physiological Dead Space Ventilation

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Measuring Dead Space

• Single breath method (Christian Bohr, ~1900).

• Clinical relevance: Bronchiectasis, ventilation-perfusion

disturbances (obstruction by tumour, emboli, etc.).

Des

popo

ulos

& S

ilber

nagl

200

3

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Dead Space Ventilation ( ) • If TV increased from 0.5 to

0.7 L, as part of will be

smaller and vice versa.

• Consideration for snorkel:– Snorkel volume increases

physiological dead space.

– Volume must be limited (in

relation to TV): standards!

– Consequences for alveolar

gas pressures and/or

breathing work: CO2 retention.

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Physiological Dead Space (VD)• Two components

– Anatomical dead space: airways (nose → bronchioli)

– Functional dead space: ventilated lung parts, which

are not perfused (~0 for healthy person; next lecture).

• Roles of anatomical dead space:– Preparation for gas exchange (within first few cm):

• Cleaning of air (respiratory epithelia)

• Water saturation (100%)

• Temperature control (warming up)

– For particular , VD sets limits how much CO2 can be

breathed off: sets alveolar gas concentrations (FRC).

– Modulation of RAW (modulated by CO2).

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Functional Dead Space

• Functional dead space: ventilated parts of lung,

which are not perfused (see next lecture).– In a healthy human, physiological dead space is

equivalent to anatomical dead space; i.e. functional

dead space is very small (~ 0 L).

– Rises in pathology: atelectasis (“air free” areas).

• Problem with functional dead space:– Hypoxaemia ( ↓, ↑): no gas exchange – shunt.

– See next lecture (control of gas exchange under patho-

logical conditions: mixing of venous and arterial blood).

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[CO2] during Breathing Cycle

• [CO2] can be measured on-

line.

• At beginning of E, [CO2] = 0:

absolute dead-space.

• [CO2]↑ after delay.

• Steep rise in [CO2].

• [CO2]↑ linearly towards end

of E (CO2 delivery rate to

alveoli).

• At end of E, [CO2] = .

• During early I, rapid drop of

[CO2] to 0.

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Alveolar Ventilation

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Alveolar Ventilation(Physiologically relevant part of ventilation)

• Alveolar ventilation ( ) = total ventilation -

dead space ventilation (≈ const):

• Properties of – Under resting conditions, is ~ 70 - 75 % of .

– TV, VD and, therefore, VA are proportional to

body height, age, sex and ethnicity.

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Rate of Gas Renewal in Alveoli • TV = 350 mL

• FRC = 2300 mL (average ♂)

• With every TV, only 15% of gas volume

in FRC refreshed.

• Exponential decay of concentration:

time constant (τ).

• For normal ventilation, τ is ~23 s.– if is halved, then τ is doubled and vice

versa.

• Slow replacement of alveolar air– prevents sudden changes in and

.

– stabilises feed-back mechanisms for

respiratory control ( ).Modified from Guyton & Hall, 2001

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and

• ↑ causes ↓ and vice versa (40 mmHg = 5.3

kPa).

• Relevance: Mountain climbing, diving, many clinical

conditions.

• Consequence of ↑: → ↓ → ↑ → ↑M

odifi

ed a

fter

Ber

ne e

t al.,

200

4

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Alveolar Ventilation in Exercise

• Linear increase in with increasing exercise.

• Alveolar gasses remain largely identical with increasing

exercise: central control of respiration (see that lecture).

• Gas exchange rate ↑ with exercise (τ becomes shorter).

• increases more than : improvement with exercise.

Guy

ton

& H

all,1

2. e

d., 2

011

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2. Local Ventilation

(between different alveoli

and lung segments)

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Local Ventilation Differences

• Radioactive gas inhaled to track rate of local

ventilation via radiation counting (scintigraphy).

• Finding to explain: upper lung areas are less

ventilated than the lower ones.

Mod

ified

afte

r W

est,

6. e

d., 2

003

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Distribution of Ventilation

• For same ΔPL, ΔV at base bigger than at apex (CL larger).

• Ventilation is smallest at apex, biggest at base (CL).

• Difference largely disappears when laying down or in zero

gravity (space): Lay down when lung function is poor…

• Emphysema starts at apex of lung…

Ber

ne e

t al.,

200

4

• If upright, PL (= PA - Ppl)

biggest at top and

smallest at base

(“hanging from the top” -

due to gravity): apex is

more inflated than base

as PL tracks volume.

• Lung at apex is more

inflated than at base (PL).

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Ventilation between Lobuli

• How fast can an alveolus

equilibrate after a volume change?

• Alveolar filling takes time due to

small flow in terminal bronchioli.

• BOTH, compliance (local CL) and

resistance (local RAW) determine

time constant of filling (local

ventilation).

• RAW↑ and CL↑ → slower filling.

• RAW↓ and CL↓ → faster filling.

Berne et al., 2004

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Ventilation between Lobuli• Ideally, time constant of filling is

i.e. product of RAW and CL.

• Consequence: uneven alveolar

ventilation if local RAW and CL vary

within lung areas.

• Consequence for – ventilation: uneven and in

different lung areas; and

– perfusion (see next lecture…).

• Application: in COPD, asthma,

emphysema, tumours, foreign body

aspiration (peanut), etc.

Berne et al., 2004

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Take-Home Messages• Two parts of physiological dead space:

anatomical and functional; latter normally small.

• Alveolar ventilation ≈ 0.7 of total ventilation.

• Gas exchange is slow (TV vs FRC ≈ 15%): stabilises and .

• is inversely proportional to ventilation.

• RAW and CL determine extent of ventilation: if

increased, exchange is longer and vice versa.

• Ventilation is not uniform across lung: worse at top; better at bottom.

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pH

A ↓ ↓ ↓

B ↓ ↑ ↑

C ↓ ↑ ↓

D ↑ ↑ ↓

E ↑ ↓ ↑

MCQDuring strenuous exercise, O2 consumption and CO2 formation

can increase up to 20-fold. Ventilation increases almost exactly

in step with this increase in O2 consumption. Which of the

following statements best describes the changes of ,

and arterial pH in a healthy athlete during such exercise?

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That’s it folks…

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pH

A ↓ ↓ ↓

B ↓ ↑ ↑

C ↓ ↑ ↓

D ↑ ↑ ↓

E ↑ ↓ ↑

MCQDuring strenuous exercise, O2 consumption and CO2 formation

can increase up to 20-fold. Ventilation increases almost exactly

in step with this increase in O2 consumption. Which of the

following statements best describes the changes of ,

and arterial pH in a healthy athlete during such exercise?