10. Lung Compliance Flashcards
Define lung compliance.
Compliance is the measure
of distensibility –
the ease at which something can be stretched.
Lung compliance (CL) is defined
as the change in lung volume (ΔV)
per unit change in
transpulmonary pressure (ΔP).
CL = ΔV/ΔP
> Specific compliance
> Specific compliance is compliance divided by FRC, thereby compensating for differing body sizes.
Elastance
> Elastance is the reciprocal of compliance
Draw a pressure–volume curve
of the lung.
Page 31
Vol Y
X Transpulmonary P
What do you understand by the term hysteresis?
Hysteresis is an important
phenomenon seen in P-V curves;
it represents ‘unrecoverable’ energy
because the lungs do
not act as a perfect elastic system
(i.e. a system in which any energy
that is put in is immediately returned).
At any given lung volume,
the pressure required to inflate the lung
is greater
than that required for deflation.
What is the difference between static and dynamic compliance?
Static compliance
Static compliance is the
lung compliance obtained
during ‘static’ conditions when
there is no gas flow
activity within the lungs.
Static compliance monitors
only elastic resistance
(i.e. the resistance offered by the
alveoli being stretched
and
the interstitium and
chest wall being moved).
The static compliance curve
can be used to select the
ideal level of PEEP during
mechanical ventilation.
Dynamic compliance
Dynamic compliance is the lung compliance
obtained under ‘dynamic’ conditions
when gas flow activity is
present during rhythmic breathing.
Dynamic compliance monitors
both elastic resistance and airway resistance
(which depends on gas viscosity and density, length and radius of lumen, gas flow rate and flow pattern).
How can static and dynamic compliance be measured?
Static compliance:
Static compliance:
This is obtained under ‘static’ conditions
when there is no gas flow
(e.g. during an inspiratory pause).
The subject breathes into a spirometer
to measure lung volumes
and an oesophageal pressure probe
is used to estimate intrapleural pressures.
The volumes and pressures measured
are then plotted to produce
a pressure–volume curve.
The compliance is then calculated
from the gradient of the curve
(usually measured near FRC).
The term ‘static’ is somewhat misleading because measurements have to be interrupted to allow the subject to breath and therefore the system never truly reaches static conditions (it is, therefore, sometimes called ‘quasi-static’ compliance).
How can static and dynamic compliance be measured?
Dynamic compliance:
Dynamic compliance:
This is obtained under ‘dynamic’ conditions
when there is gas flow
(i.e. during rhythmic breathing).
Again the subject breathes into a spirometer to measure lung volumes and an oesophageal probe is used to estimate intrapleural pressures.
The term ‘dynamic compliance’ is also somewhat confusing as the compliance is typically calculated during a tidal breath at the points of zero flow on the P-V loop (end-inspiration or end-expiration).
What factors affect lung compliance?
- > Lung volume
- > Lung elasticity
- Surface tension
> Lung volume
The slope of the P-V loop is not constant.
It is steepest around FRC but then reduces
at both low and high lung volumes
(note that FRC is affected by many factors including age, body posture and body size, which will all in turn affect lung compliance).
> Lung elasticity
> Lung elasticity –
this is due to the elastin and collagen present
in lung tissue.
With ageing there
is a gradual loss of elastic tissue
and this increases compliance of the lungs.
In emphysema,
loss of elastin from lung tissue
increases lung compliance.
Compliance is reduced in pulmonary fibrosis
due to increased collagen deposition
in lung tissue and pulmonary congestion
(e.g. oedema).
> Surface tension
this is the most important
determinant of lung compliance.
The water in alveolar fluid has a
high surface tension and
provides a force that
tries to collapse the alveolus.
Lung surfactant breaks up the
surface tension of the fluid,
increasing lung compliance
and making the alveolus less likely to collapse.
The effects of lack of pulmonary surfactant
are clearly evident in
conditions such as neonatal
respiratory distress syndrome.
How can you calculate the work of breathing?
What is most of the work done to overcome in quiet breathing
What is the normal meatbolic cost of breathing
increased by how much with what
Mechanical work of breathing
= Force × Distance
= Pressure × Volume
Thus, the work of breathing
= cumulative product of pressure ×
volume of air moved over time
= ΔP × ΔV/Δt
> During quiet breathing,
most of the work performed is required to
overcome elastic resistance (~65%).
This inflates the lung and provides a store of elastic energy that gets released during expiration and is therefore viewed as ‘useful’ work.
However,
overcoming non-elastic resistance
(e.g. airway resistance and viscosity, ~35%)
results in energy being dissipated
as heat and
is viewed as ‘wasted’ work.
> The normal metabolic
cost of breathing is
approximately 0.5–1.0 mL O2/L /min
but this may increase to
2–4 mL O2/L/min with hyperventilation.
Work increases with
increasing tidal volume,
increasing respiratory flow and
increasing airway resistance (e.g. COPD).
Fig. 10.2 Lung pressure-volume loop to show the work of breathing
pg 33
> Inspiration:
Work required to overcome the
elastic recoil of the chest wall and lungs,
airway resistance and viscosity (area AGBCD).
> Expiration:
Expiratory work returned (area BEADC).
This is passive under resting conditions
and active during stress conditions.
> Wasted work:
Area contained within the loop represents
total wasted
energy due to tissue and airway losses.
