In this article we will discuss about:- 1. Definition of Respiratory Dead Space 2. Types of Respiratory Dead Space 3. Measurement.
Definition of Respiratory Dead Space:
The respiratory tract is so designed that during inspiration its upper part upto the level of terminal bronchiole is filled with atmospheric air.
The first part of the expired air, therefore, is also atmospheric air from the non-respiratory part of the lungs, the second part is a mixture in which the percentage of atmospheric air gradually decreases and that of the alveolar air from the respiratory part of the lungs gradually increases and the last part of the expired air is pure alveolar air.
The air which remains confined in the upper respiratory tract with each inspiration and is not available for gaseous interchange constitutes what is known as ‘dead space air’.
Types of Respiratory Dead Space:
I. Anatomical Dead Space:
It can be measured by making a plaster cast of airways from the mouth to the terminal bronchioles in the dead space and measuring its volume. It amounts roughly to 150 ml.
Functions of Anatomical Dead Space:
i. Inspired air is saturated by water vapour before it reaches the alveoli of the lungs.
ii. Removes the particulate matter in sizes more than 2.0 µm from the inspired air before it is delivered to the alveoli. But the particles about 2.0 µm entering the alveoli are removed by lymphatics.
Factors Influencing the Anatomical Dead Space:
i. Reflex broncho contraction due to stimulation of the parasympathetic nerves diminishes the anatomical dead space.
ii. Adrenaline dilates the lower airways by relaxing smooth muscles and upper airways by vasoconstriction which shrink the nasal mucosa.
II. Physiological Dead Space:
In an ideal lung all the alveoli are evenly ventilated and adequately perfused with exactly the required amount of blood. But such an ideal condition is seldom obtained. In fact, in normal subjects the apical alveoli are ventilated adequately but have got a poor blood supply. Most of the ventilation in these alveoli, therefore, is wasted and produced partial dead space effect.
In diseased conditions the situation may be worse-there may be many alveoli which are hyperventilated but have very poor blood flow. These are, therefore, partially dead space areas. Measurement of dead space in these cases will give a value much higher than anatomical dead space and constitutes what is known as physiological dead space. In normal subjects the volumes are very nearly the same.
In some disease of the lungs the physiological dead space may amount to 1 to 2 litres producing great respiratory insufficiency. The normal ratio of dead space to tidal volume is in the range 0.2 to 0.35 during breathing at rest. This ratio increases with age but decreases on exercise.
According to Fowler’s method, physiological dead space is measured with the volume of conducting airways down to the level where rapid dilution of inspired gas occurs with gas already in the lungs. But Bohr’s method measures the volume of the lungs which does not eliminate CO2.
Measurement of Total Dead Space (Anatomical and Physiological):
This can be done provided a sample of alveolar air is first collected and its CO2 content is determined by the method to be described subsequently and the partial pressure of CO2 in the alveolar air (PACO2) is calculated. Expired air is a mixture of dead space air which is practically CO2 free and of the perfused alveoli. Its CO2 tension (PECO2) will always be lower than that of PACO2. If VD represents the ‘dead space volume, VT represents the tidal volume.
The ratio of the dead space volume to tidal volume is given by the equation:
VD/VT = [(PACO2 – PECO2/PACO2]
If PACO2 = 40 mm Hg and PECO2 = 30 mm Hg
VD/VT = [(40 – 30)/40] – [(10/40) 0.25]
In other words ¼th of the tidal ventilation will remain in the dead space. The inspired air is humidified brought to the body temperature and filtered during its passage through the dead space. The anatomical dead space can be reduced by 50% by tracheostomy, which is beneficial for patients with weak respiration and diminished tidal volume
The subject takes a deep breath of oxygen and then expires through a N2 meter which records graphically the percentage of N2 in the expired air from moment to moment. The first part of expired air contains ‘zero’ % of N2 because it is pure O2 from dead space. The second part of the curve shows rising N2 concentration because it is a mixture of O2 from the dead space and N2 from alveolar air.
In the last part of the tracing the N2 percentage level off and remains more or less steady because it is pure alveolar air and in an ideal lung all the alveoli are contracting synchronously unloading their N2 content in the expired air at a uniform rate. In Fig. 8.17 the steady N2 level is shown to be at 60%.
If the total volume of expired air be 500 ml and the dotted area represents the alveolar air and the hatched area be dead space air –
Let us assume that the area of dots = 70 sq. cm and the area of hatching = 30 sq. cm. Since the total volume of expired air = 500 ml, the dead space air = 30/30 + 70 or 150 ml.