8. Respiratory Dead Space Flashcards
Define dead space as applied to the respiratory system.
Respiratory dead space is the volume of inspired gas that does not take part in gas exchange.
It is divided into
anatomical
and
alveolar dead space.
> Anatomical dead space
What is it
How many gens
What it include
Whats the volume /kg of it
> Anatomical dead space:
• Constitutes the conducting airways
(Weibel classification –
airway generations 1–16:
trachea,
bronchi,
bronchioles
and terminal bronchioles)
• Includes the
mouth,
nose and
pharynx
• Equates to 2 mL/kg
Table 8.1
Factors affecting anatomical dead space
Anatomical dead space increased by:
Anatomical dead space
increased by:
1 Sitting up
2 Neck extension and Jaw protrusion
3 Increasing age
4 Increasing lung volume
Anatomical dead space decreased by:
Anatomical dead space decreased by:
General anaesthesia
Hypoventilation
Intubation
Tracheostomy
> Alveolar dead space:
> Alveolar dead space:
• Constitutes alveoli that are
ventilated but not perfused
and, therefore,
no gas exchange occurs
• Can be significantly affected by
physiological and pathological
processes
> Physiological dead space:
• Represents the combination of
anatomical and alveolar dead space
How is anatomical dead space
measured?
Fowler’s method is used to measure
anatomical dead space.
It is a technique that uses
single-breath nitrogen washout
utilising a rapid nitrogen gas analyser.
Decribe Fowlers method
1
A nose clip is placed on the subject,
and the subject breathes air in and
out through their mouth via a mouthpiece.
2
> From the end of a normal expiratory breath
(i.e. FRC) the subject takes a
maximal breath of 100%
O2 to vital capacity.
3
> Subject then exhales maximally
at a slow
and
constant rate to residual volume.
4
> During exhalation the expired gas
passes through the rapid nitrogen
analyser and so
nitrogen concentration
is measured against volume.
5
> Four distinct phases are seen in
expired nitrogen concentration.
Describe the phases seen in Fowlers method graphically seen
> Phase I:
Initial expired gas from the
conducting airways
containing 100%
O2 and no N2.
> Phase II:
Nitrogen concentration increases
as alveolar gas begins to mix
with anatomical dead space gas.
> Phase III:
Alveolar plateau phase –
exhalation of alveolar gas containing
N2 from the alveoli.
Oscillations can be seen in phase 3,
which are caused by
interference from the heartbeat.
> Phase IV:
Represents closing capacity.
During expiration airways at the
lung bases close as the lung
approaches residual volume,
so phase 4 expired gas comes mainly
from the upper lung regions.
During normal inspiration the lung
bases are preferentially ventilated
therefore the lung apices receive
less of the 100% O2 breath.
At closing volume N2 from the
lung apices is expired causing the
phase 4 rise in
expired N2 concentration.
> Anatomical dead space is
found by dividing phase 2
so that areas A and B are equal
and measuring from the start of exhalation.
How is physiological dead space measured?
The Bohr equation is used to
derive physiological dead space
(anatomical + alveolar).
V D.PHYS PaCO2 – PeCO2
________ = ______________
Vt PaCO2
.
Where:
VD.PHYS Physiological dead space
VT T idal volume – measured with a spirometer
PaCO2 A rterial partial pressure of CO2 – measured from an arterial
blood gas
PECO2 M ixed expired partial pressure of CO2 – measured from
end-tidal CO2.
Any of the situations previously
mentioned that
increase anatomical dead space will
consequently increase
physiological dead space.
Alveolar dead space is
increased by most lung diseases
(especially pulmonary embolus),
general anaesthesia,
positive pressure ventilation and
positive end expiratory pressure.
Under such circumstances
VD.PHYS/VT may approach 70% (normally 35%), which has obvious implications for CO2
removal.
