Introduction

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.

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Table 1 Content of air

Volume %, equal to kPa if the barometric pressure is 100 kPa.

	dry	saturated 37^oC
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.

Figure 1 Airways with larynx, trachea and bronchi

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.

Figure 2 Lung volume parameters

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 = ΔV / ΔP [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 Poiseuille¹, 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 [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
   

Figure 3 Parabolic velocity profile in accordance with the Poiseuille model.
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.

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Figure 4 Flow lines with local hindrance and a back eddy (non-laminar zone)

urbulence 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.

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