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Transcript of CS 2015 Alveolar Ventilation and Factors Influencing It Christian Stricker Associate Professor for...
CS 2015
Alveolar Ventilation and Factors Influencing It
Christian StrickerAssociate Professor for Systems Physiology
ANUMS/JCSMR - ANU
[email protected]://stricker.jcsmr.anu.edu.au/Ventilation.pptx
THE AUSTRALIAN NATIONAL UNIVERSITY
CS 2015
CS 2015
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.
CS 2015
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
CS 2015
Review of Changes in
• Axis along bottom indicates distance from nose.
• At each step, ↓. Note notation.
CS 2015
1. Global Ventilation
(over whole lung as an entity)
Total Pulmonary Ventilation =
Physiological Dead Space Ventilation
+ Alveolar Ventilation
CS 2015
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?
CS 2015
Physiological Dead Space Ventilation
CS 2015
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
CS 2015
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.
CS 2015
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).
CS 2015
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).
CS 2015
[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.
CS 2015
Alveolar Ventilation
CS 2015
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.
CS 2015
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
CS 2015
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
CS 2015
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
CS 2015
2. Local Ventilation
(between different alveoli
and lung segments)
CS 2015
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
CS 2015
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).
CS 2015
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
CS 2015
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
CS 2015
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.
CS 2015
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?
CS 2015
That’s it folks…
CS 2015
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?