Chapter 9 Breathing systems - UiO

Medisinsk-teknisk avdeling, Rikshospitalet
Fysisk institutt, UiO
Chapter 9 GAS
INSTRUMENTATION av Sverre Grimnes 2008
Introduction In each cell the complex mechanisms of life are based upon the simple use
of two gases: oxygen supplied - carbon dioxide produced. In the lungs
these two gases are separated by the gas/blood membrane but transported as
gas on the ventilation side and as liquid (dissolved blood gases) on the
blood side. Most tissues of the body do not contain gas in the gas phase;
gas bubbles in the small blood vessels are dangerous because they act as
emboli hindering blood flow. The guts and the lungs are the only organs
where gases are to be found normally. Gas instrumentation in medicine
serves first and foremost the lungs, and both for diagnosis (gas
analysers); therapy (aerosol nebulizers) and support (ventilators,
anaesthesia workstations). We can not be without lung ventilation for many
minutes; therefore support instrumentation is critical equipment with
respect to technical malfunction and wrong use.
. Table 1 Content of air Volume %, equal to kPa if the barometric pressure is 100 kPa.
| |dry |saturated |
| | |37oC |
|nitrogen |78.1 |73.4 |
|oxygen |20.9 |19.6 |
|argon | 0.9 | 0.8 |
|carbon dioxide |0.04 | 0.04 |
|water vapor | 0 | 6.3 |
Table 1 shows the content of air. Notice the influence of water vapour.
Oxygen and carbon dioxide are called blood gases, together with nitrogen
these gases are dissolved in the blood and therefore are also in the liquid
phase. Nitrogen is not used by the body, so there is no net nitrogen
transport across the lung membrane. In blood most of the oxygen transport
is performed by oxygen chemically bound to haemoglobin (bluish) forming
oxyhaemoglobin (reddish). If anaesthetic drugs in gas or vapour phase (e.g. N2O or sevoflurane) they
are supplied through the ventilation. Scavenging systems remove such gases
before they reach the operating room ambiance.
Airway and lung anatomy The lower airways below the throat comprise the trachea and the bronchi.
The trachea is split into the two main bronchi, at the distal part they end
in the alveoli (Fig.1). Here the air and the blood meet but separated by
the very thin membrane of the air sac (alveolus). On the tissue side blood
capillaries envelop an alveolus.
Oxygen is transported as O2 gas molecules in the trachea down to the
alveoli, as dissolved gas and chemically bound to haemoglobin in the blood,
and in the end diffuses the last tenths of a millimetre from blood
capillaries through the extracellular liquids up to the living cells.
Oxygen supply is from outside of the body, and it is therefore a
concentration gradient with falling values from the mouth to the cells.
Carbon dioxide is produced in the cells, diffuses to the blood capillaries
and is then transported by blood to the lung capillaries, diffuses through
the lung membrane and is expelled from the body as CO2-gas through the
airways. The CO2 gradient is therefore with the highest values in the cells
and lowest in the mouth. The gas exchange takes place in the alveoli. Most textbooks present alveoli
as a bunch of grapes, but pulmonary alveoli are prismatic or polygonal in
shape, i.e. their walls are flat. There are about 600 millions of them in
our two lungs. The membrane surface in an adult healthy person is about 160
m2 and this assures a very effective gas exchange between the air and the
blood. The exchange is as a gradient driven diffusion process through the
membranes, tissue and the walls of the blood capillaries. Lung volumes, lung capacitance The total volume of both lungs of an adult healthy person at maximum
inspiration is about 6L, fig.2. The residual minimum volume at maximum
expiration is about 1L: it is impossible to empty the lungs completely all
the way to collapse. The difference (5L) is the vital capacity. The tidal
volume is the normal inspiration or expiration volume under quiet
breathing, for instance 0.5L.
Lung compliance, pneumothorax Each lung is enclosed in a gas-tight pleural volume by the double-walled
lung sac membrane. The outer membrane is fixed to the thorax cage, the
inner to the lungs. Because of the surface tension of the liquid films a
lung tends to contract and reduce its volume. Therefore the intermembrane
volume has a negative pressure of about -4 cmH2O with respect to
atmospheric pressure. During inspiration the diaphragm pulls the lower
surfaces of the pleural volume down increasing the lung volume and thereby
increasing the negative pressure in the alveoli. A puncture of the lungs
destroying the negative pressure is critical for the patient. The lungs
will collapse and the patient will not be able to breath (pneumothorax).
Normally a pressure change of as little as -1cmH2O (+1 cmH2O during
expiration) in the alveoli is sufficient for a quiet respiration. When the
patient is breathing spontaneously the inhalation is caused by the work of
the lung muscles resulting in the alveolar negative pressure. During
expiration little muscle work is done, it is the relaxation process of the
stretched tissue which brings the air out. During forceful ventilation also
the rib rise increases the pleural volume and increases the negative
pressure. Then also special muscle groups actively compress the pleural
volume during expiration.
[pic] The lungs may be soft and easy to fill, meaning that a relatively large
inspiration volume is obtained with only a small negative pleural pressure
change. Compliance is a much used parameter to describe the expansibility
of the lungs, compliance C is defined as: Equation 1 Compliance C = ?V / ?P [L/Pa, L/cmH2O] The compliance of the normal lungs and thorax is about 0,13 [L/cmH2O].
Reduced compliance makes the patient more difficult to ventilate. A therapy
is the use of surfactants, substances which lowers the surface tension at
the inside alveoli surfaces. A near ideal zero compliance closed volume is
a gas supply bottle, a near ideal maximum compliance volume is the closed
volume spirometer. Flow resistance, gas viscosity The trachea is equipped with cartilage rings so that it will not collapse
at negative pressure. The basic model for flow resistance in tubes is based
upon the law of Poiseuille[1], describing the resistance R to flow through
a tube of radius r and length L under the influence of gas viscosity ( [Pa
s]: Equation 2 Poiseuille [pic] [Pa/m3/s = pressure / flow rate]
Validity
1) Laminar flow in a straight tube geometry
2) Gases and liquids (fluids), but better model for gases than for liquids
3) Flow rate in [m3/s], not [mol/s]
4) Gas viscosity is increasing with temperature (in contrast to liquids).
It is pressure independent, and R is therefore independent on the mean
pressure level in the tube [pic]Thus, the resistance is not dependent on friction between the fluid
and the walls, only on the internal friction in the fluid. At the walls the
velocity is zero, increasing to maximum at the centre of the tube. Fig.3
illustrates the Poiseuille ideal flow model in a tube, the flow profile is
parabolic. The frictional forces between layers of the fluid are forces
parallel to the flow, they are shear forces. Ohms law for electrical
parameters is (V=RI where (V is voltage difference [V] and I current flow
[A] through R. As a parallel to Ohms law (P=RQ where (P is pressure
difference [Pa] at the wall and Q is mean flow [m3/s]. In spite of the
variable velocity illustrated in Fig.3, R is therefore related to the mean
velocity. By measuring (P we have a gas mean velocity sensor, see
subchapter on gas sensors.
The extreme dependence on the tube radius shown in Eq.2 has very
important consequences e.g. with catheters and syringes for the injection
or aspiration of fluids. It is also important in obstructed airways (airway
resistance work, asthma). It is a very effective regulating mechanism in
the body when the arterial blood vessel walls are equipped with muscles to
contract and reduce the radius of the vessel.
Turbulence When the velocity of a fluid is increased beyond a threshold value, the
flow modus changes from laminar to turbulent. The resistance to flow is
increased and Poiseuilles law no longer describes the process correctly.
The flow resistance is not determined so much by the fluid viscosity as by
fluid density.
Turbulence is important in many parts of the body, both in the airways
and in the blood stream, in particular at bifurcations and around heart
valves. The laminar model is useful, necessary and important, but its
validity range must always be kept in mind.
.
Gas physics Water has a very low compressibility because of the strong polar bonds
between the molecules. The molecular bonds in oil are somewhat weaker, and
oil is therefore slightly compressible. Air at room temperature and 1 bar
has a density of about 1,25 kg/m3, about thousand times lower than that of
water, and the distance between the molecules is accordingly roughly 10
times larger than in water. A gas is very compressible, but if a gas is at
a temperature higher than its critical temperature,