Respiration. Figure 22: Schematic representation of the respiratory system

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Emory Nicholson
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1 Respiration One of the seven characteristics of something which is living is respiration. Strictly speaking, respiration is the process that takes place at cellular level and is one of three different processes which are necessary in animals: Gas exchange: The movement of oxygen into an organism and carbon dioxide out of an organism. Breathing: The ventilation movements that are needed in some larger animals so that efficient gas exchange can take place. It involves airways, ribs, intercostal muscles, the diaphragm and the lungs. Collectively these are known as the respiratory system and Figure gives an overview of it. Respiration: The chemical process of releasing energy from (complex) organic compounds. It can be both aerobic (using oxygen) and anaerobic (not using oxygen) although only the former can be sustained for significant periods in most animals. Figure : Schematic representation of the respiratory system However, in medical engineering, the word respiration is also often used to mean breathing (which, after all, is the action of the respiratory system) and this is the aspect that we are interested in. The purpose of breathing is to enable gas exchange to take place. In a mammal, gases are exchanged between red blood cells and air in the alveoli of which each lung has approximately 300 million. These are very small sacks into which the air is inhaled and from which the air is exhaled. They have very thin walls ensuring that the numerous small blood vessels (capillaries) which surround them are very close to the inhaled air, maximising the potential for gas exchange and, in total, provide a surface area of approximately 75m

2 (roughly the area of a squash court) for his exchange to take place. When someone breathes in air enters the lungs and thereby the alveoli; when someone exhales air is forced out of the lungs and so the total volume of air in the lungs varies with inhalation and exhalation. Measuring Respiration There are two aspects of respiration that we may wish to measure: the frequency (or rate) of breaths and the depth of these breaths. Unlike the heart, the lungs do not generate electrical signals which can easily be detected and so an active device must be designed. Chest movement aside (which is not uniquely associated with breathing), there are two obvious physical effects correlated with breathing: air movement and a changes in the volume of air in the lungs. A summary of two techniques in use Air flow can be detected in a variety of ways almost all of which are obstructive. For example, in the thermal anemometer the patient s respired air is passed over an electrically heated wire. The current needed to maintain the wire at a constant temperature indicates the flow of respired air. However, this requires putting a mask over the patient s mouth and nose to channel the respired air. Alternatively, the chest volume (and its variations) can be measured using an inductive plethysmograph: a tight fitting vest containing wire coils is worn by the patient. As the patient breaths in and out the volume of their chest varies and the larger the chest volume the more the vest is stretched. This stretching varies the inductiveness of the coils. By measuring this is is possible to measure breathing. However, the tight fitting vest is considered uncomfortable, especially by those who are (or who feel) short of breath. What is needed is a technique that does not inhibit, or appear to inhibit, breathing in any way. Impedance properties of the lungs Fortunately, there exists a technique which measures variations in chest volume in a different way. From the description of the lungs above, it can be seen that the lungs consist of a large number of sacks of air surrounded by capillaries. The surrounding capillaries and flesh can be considered to be conductive with a low resistance. Whilst the air can be treated as an insulator at d.c., it is possible to consider the walls of the alveoli as also having capacitative coupling across the alveoli. It is therefore possible to model the impedance at a given point in time between two points on the upper body as shown in Figure 3 in which R A, R B and R C model the resistance of general body tissue whilst R L and model the impedance of the lungs. R L R A R B R C Figure 3: A simple model of the impedance between two point on the upper body In the model in Figure 3, the lungs are given a fixed capacitance. However, the capacitance of the lungs will be dependent on the volume of air in the lungs at any one time and it can

3 more completely be modelled a variable capacitance. However, one can also simplify the circuit by combining the various resistances as shown in Figure 4. R R Figure 4: A simplified model of upper body impedance Equivalent circuit for a measurement system At a specific frequency, the reactance of a capacitor is given by the formula: Z f C One can therefore measure a capacitance by measuring a reactance at a known frequency and applying the relationship C! f Z. This results in the basic principle of impedance plethysmography: a small current at a known frequency is passed through the body allowing changes in physiology which have alter reactance to be measured. This allows us to simplify the model still further, as shown in Figure 5 R R V Figure 5: At a specific frequency, the variable capacitance can be modelled as a variable reactance and the circuit again simplified By combining this simplified model with the electrode model from Figure the overall equivalent circuit for the tissue (including the lungs and air) and the electrode systems can be determined. This is given in Figure 6 in which R L is the reactance resulting from the air present in the lungs. Determining R L will allow the volume of the air in the lungs to be calculated. Application of this equivalent circuit In order to determine the value of the capacitance giving rise to the reactance in Figure 6 it is necessary to determine the values of R T, R, R, R, R and the reactances resulting from C and C. We can simplify this circuit as shown in Figure 7 and, if R T, Z and Z were constant for any given patient and application of electrodes, then reference readings could be taken at different frequencies and their values determined. However, the air in the lungs is not the only thing that will result in variations in these values. Other factors include:

4 Tissue Resistance Electrode electrolyte interface C R T E R R R L E R C R Figure 6: Equivalent circuit for the lungs and electrode system R T E Z R L E Z Figure 7: A simplified view of the equivalent circuit in Figure 6 Blood Flow: Blood can broadly be considered to be a suspension of molecules and cells in water. Much as adding impurities to water will alter its conductivity, the chemicals in the blood will affect the conductivity of blood. However, a much more significant effect is due to the red blood cells which are non conductors. In flowing blood the blood cells will be oriented (by the shear forces applied by the fluid on them) with the long axis parallel to the direction of flow; in stationary blood there will be no ordering of orientation. The resistivity of blood therefore varies with blood flow and therefore synchronously with the rhythm of the heart (ie there is a pulsatile variation in resistivity of blood.) Blood Volume: The resistivity of blood is different from that of the various organs and bones which the upper body is composed of. The volume of blood in the upper body varies over the course of the cardiac cycle. This is a further cause of pulsatile variations in the resistivity of the upper body. Electrode Contact Impedance: In the discussion of electrode impedance earlier in these notes, it was noted that the ionic electrode past and the silver metal of the electrode form a local solution at the electrode skin interface into which silver dissolves, producing Ag + ions. However, over the course of time, this solution will dry out, reducing the number of dissolved Ag + ions and increasing the resistance of the electrode. It is therefore necessary to determine R T, Z and Z dynamically and to subtract them from the measured overall impedance to know R L at any point in time.

5 # # The need for 4 electrodes It was pointed out above that by taking measurements at different frequencies, the different reactances in a circuit can be characterised. By using two impedance measurers operating at different frequencies and connected to independent pairs of electrodes, it becomes possible to determine two different combined impedances (using Figure 6 and defining Z = X + jy): Z electrode pair R T X X j Y Y Y L Z electrode pair R T X X j Y Y Y L We can take the Y values to be purely capacitative on the basis of the discussions so far then we can define the two measured resistances as follows: R electrode pair R T X X C C f C C R electrode pair R T X X C C f C C f f If we assume that the electrodes have identical impedances then these equations simplify considerably: R electrode pair R T X e f R electrode pair R T X e f f f If we subtract these two values: R R electrode pair $ R electrode pair f f $ f $ f which gives: $ R f f f f This is usable in practise assuming that can be estimated to sufficient accuracy (the closer it is to the more accurate the estimate must be). A brief summary Breathing results in changes of air volume in the lungs which alters the capacitance of the lungs; by measuring this capacitance we can infer the volume of air in the lungs. We can measure changes in this this capacitance by measuring changes in the impedance presented at a known frequency. However, if we wish to know the absolute value of this capacitance it is necessary to measure the impedance at two frequencies so that the resistances and reactances of other parts of the body can be disambiguated from the impedance presented by the lungs. Rate versus volume As has been shown, to measure the volume of air inhaled and exhaled, a 4 electrode system is necessary. If, however, only the rate (ie frequency) of respiration is required, then a two %

6 electrode system (ie a single measurement of impedance) can be used. The output of such a system is shown in Figure 8 from which it can be seen that the frequency of the patient s breaths can be determined from the trace. Figure 8: An impedance plethysmography trace showing 0 breathing cycles &

GASEOUS EXCHANGE 17 JULY 2013

GASEOUS EXCHANGE 17 JULY 2013 Lesson Description In this lesson we: Discuss what is gaseous exchange? Consider requirements of an efficient gaseous exchange surface. Look at diversity in gas exchange systems.