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103 - Heart, Lungs, Blood
Theme 2: Lungs and Gas Exchange
T2 L8: Compliance and impedance in ventilation
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What are the risks of mechanical ventilation?
pneumothorax
lung damage
(barotrauma & volutrauma)
hypotension
&
reduced cardiac output
Main difference between mechanical ventilation & CPAP / BiPAP?
mechanical ventilation: tube from machine directly seals to
trachea
CPAP & BiPAP: mask covers
nose & mouth
How can you exhale with CPAP?
compliance
&
elastic recoil
What is BiPAP?
bi-level positive airway pressure
also called BPAP / NIV (
non-invasive ventilation
)
often used in
COPD exacerbation
(Type II Respiratory failure with respiratory acidosis)
reduces
work of breathing
as:
iPAP
(inspiratory positive airway pressure) higher
ePAP
lower
so air moves
in
What are the forces that oppose the ventilator trying to inflate the lungs?
low compliance
of lung and chest wall (
elastic recoil
)
airway resistance and
turbulence
surface tension
friction
sliding pleura
What is elastance?
recoil
high elastic recoil = low
compliance
= difficult to
stretch
How can lung compliance be altered in lung disease?
interstitial fibrosis:
lowers compliance
(
stiff
)
emphysema:
higher compliance
(
less stiff
)
What is impedance?
what it takes for a
ventilator
to
push air into lungs
What is 'work of breathing'?
Work required to:
stretch
elastic tissues of chest wall and lungs (compliance)
move
inelastic tissues (eg pleura sliding against chest wall)
push
air through tubes (resistance)
so amount of
energy required
to produce enough ventilation and respiration to meet metabolic demands of the body
What pressures are used to measure work?
Airway
pressure
Trans pulmonary
pressure
trans thoracic
pressure
trans respiratory
pressure
pleural
pressure
abdominal
pressire
alveolar
pressure
What is 'airway pressure'?
difference in
tracheal pressure
and
atmospheric pressure
(W)
What is 'trans-pulmonary pressure'?
difference in pressure between
alveolar space
and
pleural space
What is the 'trans thoracic pressure'?
difference in pressure between
pleural space
and
atmospheric pressure
What is 'trans respiratory pressure'?
difference in pressure between
alveolar space
and
atmosphere
What is 'trans pulmonary pressure'?
Pressure difference
between
alveoli
and pleural space.
What is 'trans-thoracic pressure'?
pressure difference between
alveoli
and
atmosphere
should be
negative
at rest
How does intrapleural pressure fluctuate during spontaneous tidal breathing?
negative
pressure overall
gets slightly
more negative
during inhalation
returns back to resting during exhalation
How does alveolar pressure fluctuate during spontaneous tidal breathing?
0 to
negative
to 0 during inhalation
0 to
positive
to 0 during exhalation
Ventilation (movement of air) occurs due to changes in what?
transrespiratory pressure
Breathing (expansion of lungs) occurs due to cycling changes in what?
transpulmonary pressure
What is 'peak transthoracic pressure'?
pressure required to overcome both
tissue-elastic and airway resistance
to oppose
recoil of lungs
pulling the chest inward
What is the effect of patients laying down (supine) on FRC (force respiratory capacity) and RV (residual volume)?
smaller
less volume
for breathing, less margin for error
as guts roll toward head and heart flops down onto lung
What is the effect of anaesthetics on FRC?
becomes
smaller
as
intercostal muscles relax
chest wall
becomes looser and more compliant
gravity
and
lung compliance
pull chest wall inward
At what point does the chest wall like to rest at naturally?
65% of TLC
What does a 'Static Relaxation-Compliance Curve' show?
at rest (FRC), chest and lungs pull in
opposite directions
and are in
equilibrium
What is 'dynamic compliance of the lungs'?
compliance of lung depends on how
inflated
it is
lung is less compliant at
higher volumes
due to
frictional resistance
and
turbulence
Why don't inhalation and exhalation PV curves overlay?
airway resistance
delay of
volume increase
during inhalation: due to pressure needed to pop
open collapsed small airways
delay of
volume decrease
during exhalation: due to
time
needed for air to
flow out
of small airways