In general, the word ‘ventilation’ refers to circulation of replacement of air or gas in a space. In respiratory physiology, ventilation is the rate at which air enters or leaves the lungs. Ventilation in respiratory physiology is of two types:
1. Pulmonary ventilation
2. Alveolar ventilation.
PULMONARY VENTILATION
Pulmonary ventilation is
defined as the volume of air moving in and out of respiratory tract in a given unit
of time during quiet breathing. It is
also called minute
ventilation or
respiratory
minute volume (RMV).
Pulmonary ventilation is a cyclic
process, by which fresh air enters the lungs and an equal volume of air
leaves the lungs.
NORMAL VALUE AND
CALCULATION
Normal value of pulmonary ventilation
is 6,000 mL (6 L)/minute. It is the product of tidal volume (TV) and
the rate of respiration (RR). It is
calculated by the formula:
Pulmonary ventilation
= Tidal volume × Respiratory rate
= 500 mL × 12/minute
= 6,000 mL/minute.
ALVEOLAR VENTILATION
Alveolar ventilation is the amount of
air utilized for gaseous exchange every minute.
Alveolar ventilation is different from
pulmonary ventilation. In pulmonary ventilation, 6 L of air moves
in and out of respiratory tract every
minute. But the whole volume of air is not utilized for exchange of
gases. Volume of air subjected for
exchange of gases is the alveolar ventilation. Air trapped in the respiratory passage
(dead space) does not take part in gaseous exchange.
„ NORMAL VALUE AND
CALCULATION
Normal value of alveolar ventilation
is 4,200 mL (4.2 L)/ minute.
It is calculated by the formula:
Alveolar ventilation
= (Tidal volume – Dead space) x
Respiratory rate
= (500 – 150) mL × 12/minute
= 4,200 mL (4.2 L)/minute.
DEAD SPACE
Dead space is defined
as the part of the respiratory tract, where gaseous exchange does not take place. Air present in the dead
space is called dead space air.
TYPES OF DEAD SPACE
Dead space is of two types:
1. Anatomical dead space
2. Physiological dead space.
Anatomical Dead Space
Anatomical dead space extends from
nose up to terminal bronchiole. It includes nose, pharynx, trachea,
bronchi and branches of bronchi up to
terminal bronchioles. These structures serve only as the
passage for air movement. Gaseous
exchange does not take place in these structures.
Physiological Dead
Space
Physiological dead space includes
anatomical dead space plus two additional volumes.
Additional volumes included in
physiological dead space are:
1. Air in the alveoli, which are non-functioning. In some respiratory
diseases, alveoli do not function because of dysfunction or destruction of
alveolar membrane.
2. Air in the alveoli, which do not
receive adequate blood flow. Gaseous exchange does not take place
during inadequate blood supply. These
two additional volumes are generally considered as wasted ventilation. Wasted
ventilation and wasted air Wasted ventilation is the volume of air that
ventilates
physiological dead space. Wasted air
refers to air that is not utilized for gaseous exchange. Dead space air is generally
considered as wasted air.
NORMAL VALUE OF DEAD
SPACE
Volume of normal dead space is 150 mL.
Under normal conditions, physiological dead space is equal to
anatomical dead space. It is because,
all the alveoli are functioning and all the alveoli receive adequate blood flow
in normal conditions.
Physiological dead space increases during respiratory
diseases, which affect
the pulmonary blood flow or
the alveoli.
MEASUREMENT OF DEAD
SPACE – NITROGEN WASHOUT METHOD
Dead space is measured by single
breath nitrogen washout method. The subject respires normally for few minutes.
Then, he takes a sudden inhalation of pure oxygen.
Oxygen replaces the air in dead space
(air passage), i.e. the dead space air contains only oxygen and it
pushes the other gases into alveoli. Now,
the subject exhales through a nitrogen meter.
Nitrogen meter shows the concentration
of nitrogen in expired air continuously.
First portion of expired air comes from
upper part of respiratory tract or air passage, which contains only
oxygen. Next portion of expired air
comes from the alveoli, which contains nitrogen. Now, the nitrogen meter shows
the nitrogen concentration, which rises sharply and reaches the plateau soon.
By using data obtained from nitrogen meter, a graph is plotted. From this
graph, the dead space is calculated.
The graph has two areas, area without
nitrogen and area with nitrogen. Area of the graph is measured
by a planimeter or by computer. Area
without nitrogen indicates dead space air.
It is calculated by the formula:
Area
without N2 Volume of
Dead space = × expired air Area Area
with N2
+ without N2
For example, in a subject:
Area with nitrogen = 70 sq cm
Area without nitrogen = 30 sq cm
Volume of air expired = 500 mL
Dead space = × 500
70 + 30
30
= × 500
100
= 150 mL.
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