Page 127 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Dead Space, Residual Volume, Airway indirect techniques such as helium dilution
Resistance (! B). Helium (He) is a poorly soluble inert gas.
The test subject is instructed to repeatedly in-
The exchange of gases in the respiratory tract hale and exhale a known volume (V Sp) of a
occurs in the alveoli. Only a portion of the tidal helium-containing gas mixture (e.g., F He 0 = 0.1)
volume (VT) reaches the alveoli; this is known out of and into a spirometer. The helium dis-
as the alveolar part (VA). The rest goes to dead tributesevenly in the lungs (VL) and spirometer
space (not involved in gas exchange) and is (! B) and is thereby diluted (F He x ! F He 0 ). Since
therefore called dead space volume (VD). The the total helium volume does not change, the
oral, nasal, and pharyngeal cavities plus the knowninitialheliumvolume(V Sp ! F He O )isequal
trachea and bronchi are jointly known as phys- to the final helium volume (V Sp + V L) ! F He x . VL
iological dead space or conducting zone of the can be determined once F He x in the spirometer
airways. The physiological dead space (ca. has been measured at the end of the test (! B).
0.15 L) is approximately equal to the functional VLwillbeequivalenttoRVifthetestwasstarted
dead space, which becomes larger than physi- after a forced expiration, and will be equivalent
ological dead space when the exchange of to FRC if the test was started after normal ex-
gases fails to take place in a portion of the alve- piration, i.e. from the resting position of lung
Respiration purify (! p. 110), humidify, and warm inspired ures gases in ventilated airways only.
and chest. The helium dilution method meas-
oli (! p. 120). The functions of dead space are
to conduct incoming air to the alveoli and to
Body plethysmography can also detect
ambient air. Dead space is also an element of
gases in encapsulated spaces (e.g., cysts) in the
5 the vocal organ (! p. 370). lung. The test subject is placed in an airtight
chamber and instructed to breathe through a
The Bohr equation (! A) can be used to esti-
mate the dead space. pneumotachygraph (instrument for recording
Derivation: The expired tidal volume VT is equal to the the flow rate of respired air). At the same time,
sum of its alveolar part VA plus dead space VD (! A, respiration-dependent changes in air pressure
top). Each of these three variables has a characteris- in the subject’s mouth and in the chamber are
tic CO 2 fraction (! p. 376): FE CO 2 in VT, FA CO 2 in VA, continuously recorded. FRC and RV can be
and FI CO 2 in VD. FI CO 2 is extremely small and therefore derived from these measurements.
negligible. The product of each of the three volumes
and its corresponding CO 2 fraction gives the volume Such measurements can also be used to de-
of CO 2 for each. The CO 2 volume in the expired air termine airway resistance, RL, which is defined
(VT ! FE CO 2 ) equals the sum of the CO 2 volumes in its as the driving pressure gradient between the
two components, i.e. in VA and VD (! A). alveoli and the atmosphere divided by the air
Thus, three values must be known to determine flow per unit time. Airway resistance is very low
the dead space: VT, FE CO 2 and FA CO 2 . VT can be under normal conditions, especially during in-
measured using a spirometer, and FE CO 2 and FA CO 2 can spiration when (a) the lungs become more ex-
be measured using a Bunte glass burette or an in- panded (lateral traction of the airways), and (b)
frared absorption spectrometer. FA CO 2 is present in
the last expired portion of VT—i.e., in alveolar gas. the transpulmonary pressure (PA-P pl) rises
This value can be measured using a Rahn valve or (! p. 108). PA-P pl represents the transmural
similar device. pressure of the airways and widens them more
The functional residual capacity (FRC) is the and more as it increases. Airway resistance
amount of air remaining in the lungs at the end may become too high when the airway is nar-
of normal quiet expiration, and the residual rowed by mucus—e.g., in chronic obstructive
volume (RV) is the amount present after forced pulmonary disease, or when its smooth muscle
maximum expiration (! p. 112). About 0.35 L contracts, e.g. in asthma (! p.118).
of air (VA) reaches the alveolar space with each The residual volume (RV) fraction of the total lung
breath during normal quiet respiration. There- capacity (TLC) is clinically significant (! p. 112). This
fore, only about 12% of the 3 L total FRC is re- fraction normally is no more than 0.25 in healthy
newed at rest. The composition of gases in the subjects and somewhat higher in old age. It can rise
to 0.55 and higher when pathological enlargement
alveolar space therefore remains relatively
114 of the alveoli has occurred due, for example, to
constant. emphysema. The RV/TLC fraction is therefore a rough
Measurement of FRC and RV cannot be per- measure of the severity of such diseases.
formed by spirometry. This must be done using
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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