Some everyday units are recognized by the system although they themselves are not true SI units. Examples include the litre 10"3 m3 , the minute 60 s , and the bar Pa. One litre is the volume occupied by 1 kg of water but was redefined in the s as being equal to cm3. Prefixes to the SI units In reality, many of the SI units are of the wrong order of magnitude to be useful.
For example, a pascal is a tiny amount of force imagine 1 newton about g acting on an area of 1 m2 and you get the idea. We, therefore, often use kilopascals kPa to make the numbers more manageable. The word kilo- is one of a series of prefixes that are used to denote a change in the order of magnitude of a unit.
The following prefixes are used to produce multiples or submultiples of all SI units. Interestingly, is known as a googol, which was the basis for the name of the internet search engine Google after a misspelling occurred. Although there is much more to mechanics as a topic, an understanding of some of its simple components force, pressure, work and power is all that will be tested in the examination. Force Force is that influence which tends to change the state of motion of an object newtons, N. Newton That force which will give a mass of one kilogram an acceleration of one metre per second per second or N kg:m:s"2.
When we talk about weight, we are really discussing the force that we sense when holding a mass which is subject to acceleration by gravity. The earths gravitational field will accelerate an object at 9. If we hold a 1 kg mass in our hands we sense a 1 kg weight, which is actually 9. Therefore, 1 N is 9.
Putting it another way, a mass of 1 kg will not weigh 1 kg on the moon as the acceleration owing to gravity is only one-sixth of that on the earth. The 1 kg mass will weigh only g. Pascal One pascal is equal to a force of one newton applied over an area of one square metre N. The pascal is a tiny amount when you realize that 1 N is equal to g weight.
For this reason kilopascals kPa are used as standard. Energy The capacity to do work joules, J. Work Work is the result of a force acting upon an object to cause its displacement in the direction of the force applied joules, J. Joule The work done when a force of one newton moves one metre in the direction of the force is one joule.
More physiologically, it can be shown that work is given by pressure volume. This enables indices such as work of breathing to be calculated simply by studying the pressurevolume curve. Power The rate at which work is done watts, W. Watt The power expended when one joule of energy is consumed in one second is one watt.
The power required to sustain physiological processes can be calculated by using the above equation. If a pressurevolume loop for a respiratory cycle is plotted, the work of breathing may be found. If the respiratory rate is now measured then the power may be calculated. The power required for respiration is only approximately mW, compared with approximately 80 W needed at basal metabolic rate.
Boyles law At a constant temperature, the volume of a fixed amount of a perfect gas varies inversely with its pressure. Charles law At a constant pressure, the volume of a fixed amount of a perfect gas varies in proportion to its absolute temperature. GayLussacs law The third gas law At a constant volume, the pressure of a fixed amount of a perfect gas varies in proportion to its absolute temperature.
Remember that water Boyles at a constant temperature and that Prince Charles is under constant pressure to be king. Perfect gas A gas that completely obeys all three gas laws. It is important to realize that this is a theoretical concept and no such gas actually exists. Hydrogen comes the closest to being a perfect gas as it has the lowest molecular weight. In practice, most commonly used anaesthetic gases obey the gas laws reasonably well. Avogadros hypothesis Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
The equation may be used in anaesthetics when calculating the contents of an oxygen cylinder. The cylinder is at a constant room temperature and has a fixed internal volume. Therefore, the pressure gauge can be used as a measure of the amount of oxygen left in the cylinder. The reason we cannot use a nitrous oxide cylinder pressure gauge in the same way is that these cylinders contain both vapour and liquid and so the gas laws do not apply. Laminar flow describes the situation when any fluid either gas or liquid passes smoothly and steadily along a given path, this is is described by the HagenPoiseuille equation.
HagenPoiseuille equation Flow. The most important aspect of the equation is that flow is proportional to the 4th power of the radius. If the radius doubles, the flow through the tube will increase by 16 times Note that some texts describe the equation as Flow. This form uses the diameter rather than the radius of the tube. As the diameter is twice the radius, the value of d4 is 16 times 24 that of r4. Therefore, the constant 8 on the bottom of the equation must also be multiplied 16 times to ensure the equation remains balanced 8 16 Viewed from the side as it is passing through a tube, the leading edge of a column of fluid undergoing laminar flow appears parabolic.
The fluid flowing in the centre of this column moves at twice the average speed of the fluid column as a whole. The fluid flowing near the edge of the tube approaches zero velocity. This phenomenon is particular to laminar flow and gives rise to this particular shape of flow. Turbulent flow describes the situation in which fluid flows unpredictably with multiple eddy currents and is not parallel to the sides of the tube through which it is flowing.
As flow is, by definition, unpredictable, there is no single equation that defines the rate of turbulent flow as there is with laminar flow. However, there is a number that can be calculated in order to identify whether fluid flow is likely to be laminar or turbulent and this is called Reynolds number Re. Reynolds number Re. If one were to calculate the units of all the variables in this equation, you would find that they all cancel each other out. Given what we now know about laminar and turbulent flow, the main points to remember are that viscosity is the important property for laminar flow density is the important property for turbulent flow Reynolds number of delineates laminar from turbulent flow.
The Bernoulli principle An increase in the flow velocity of an ideal fluid will be accompanied by a simultaneous reduction in its pressure. The Venturi effect The effect by which the introduction of a constriction to fluid flow within a tube causes the velocity of the fluid to increase and, therefore, the pressure of the fluid to fall. These definitions are both based on the law of conservation of energy also known as the first law of thermodynamics.
The law of conservation of energy Energy cannot be created or destroyed but can only change from one form to another. Put simply, this means that the total energy contained within the fluid system must always be constant. Therefore, as the kinetic energy velocity of the fluid increases, the potential energy pressure must reduce by an equal amount in order to ensure that the total energy content remains the same.
The increase in velocity seen as part of the Venturi effect simply demonstrates that a given number of fluid particles have to move faster through a narrower section of tube in order to keep the total flow the same. This means an increase in velocity and, as predicted, a reduction in pressure. The resultant drop in pressure can be used to entrain gases or liquids, which allows for applications such as nebulizers and Venturi masks. The Coanda effect The tendency of a stream of fluid flowing in proximity to a convex surface to follow the line of the surface rather than its original course.
The effect is thought to occur because a moving column of fluid entrains molecules lying close to the curved surface, creating a relatively low pressure,. As the pressure further away from the curved surface is relatively higher, the column of fluid is preferentially pushed towards the surface rather than continuing its straight course. The effect means that fluid will preferentially flow down one limb of a Y-junction rather than being equally distributed.
Heat The form of energy that passes between two samples owing to the difference in their temperatures. Temperature The property of matter which determines whether heat energy will flow to or from another object of a different temperature. Heat energy will flow from an object of a high temperature to an object of a lower temperature. An object with a high temperature does not necessarily contain more heat energy than one with a lower temperature as the temperature change per unit of heat energy supplied will depend upon the specific heat capacity of the object in question.
Triple point The temperature at which all three phases of water solid, liquid and gas are in equilibrium at A change in temperature of 1 K is equal in magnitude to that of 1 8C. Kelvin must be used when performing calculations with temperature. For example, the volume of gas at 20 8C is not double that at 10 8C: 10 8C is To convert absolute temperatures given in degrees celsius to kelvin, you must add For example 20 8C Resistance wire The underlying principle of this method of measuring temperature is that the resistance of a thin piece of metal increases as the temperature increases.
This makes an extremely sensitive thermometer yet it is fragile and has a slow response time. Draw a curve that does not pass through the origin. Over commonly measured ranges, the relationship is essentially linear. The slope of the graph is very slightly positive and a Wheatstone bridge needs to be used to increase sensitivity. Thermistor A thermistor can be made cheaply and relies on the fact that the resistance of certain semiconductor metals falls as temperature increases. Thermistors are fast responding but suffer from calibration error and deteriorate over time.
Draw a smooth curve that falls as temperature increases. The curve will never cross the x axis. Although non-linear, this can be overcome by mathematical manipulation. At the junction of two dissimilar metals, a voltage will be produced, the magnitude of which will be in proportion to the temperature difference between two such junctions. Thermocouple The thermocouple utilizes the Seebeck effect. Copper and constantan are the two metals most commonly used and produce an essentially linear curve of voltage against temperature. One of the junctions must either be kept at a constant temperature or have its temperature measured separately by using a sensitive thermistor so that the temperature at the sensing junction can be calculated according to the potential produced.
Each metal can be made into fine wires that come into contact at their ends so that a very small device can be made. This curve passes through the origin because if there is no temperature difference between the junctions there is no potential generated. It rises as a near linear curve over the range of commonly measured values. The output voltage is small 0.
The term humidity refers to the amount of water vapour present in the atmosphere and is subdivided into two types: Absolute humidity The total mass of water vapour present in the air per unit volume kg. Relative humidity The ratio of the amount of water vapour in the air compared with the amount that would be present at the same temperature if the air was fully saturated. The main location of hygroscopic mediums is inside heat and moisture exchange HME filters.
The humidity graph is attempting to demonstrate how a fixed amount of water vapour in the atmosphere will lead to a variable relative humidity depending on the prevailing temperature. It also highlights the importance of the upper airways in a room fully humidifying by the addition of 27 g. You will be expected to know the absolute humidity of air at body temperature. These points must be accurate. The graph demonstrates that a fixed quantity of water vapour can result in varying RH depending on the temperature concerned.
Not all heat energy results in a temperature change. In order for a material to change phase solid, liquid, gas some energy must be supplied to it to enable its component atoms to alter their arrangement. This is the concept of latent heat. Latent heat The heat energy that is required for a material to undergo a change of phase J. Specific latent heat of fusion The amount of heat required, at a specified temperature, to convert a unit mass of solid to liquid without temperature change J.
Specific latent heat of vaporization The amount of heat energy required, at a specified temperature, to convert a unit mass of liquid into the vapour without temperature change J. Note that these same amounts of energy will be released into the surroundings when the change of phase is in the reverse direction. Heat capacity The heat energy required to raise the temperature of a given object by one degree J.
Specific heat capacity The heat energy required to raise the temperature of one kilogram of a substance by one degree J. Specific heat capacity is a different concept to latent heat as it relates to an actual temperature change. There is an important graph associated with the concept of latent heat. It is described as a heating curve and shows the temperature of a substance in relation to time.
A constant amount of heat is being supplied per unit time and the main objective is to demonstrate the plateaus where phase change is occurring. At these points, the substance does not change its temperature despite continuing to absorb heat energy from the surroundings. Heating curve for water.
The curve crosses the y axis at a negative value of your choosing. Between the plateaus, the slope is approximately linear. The plateaus are crucial as they are the visual representation of the definition of latent heat. The first plateau is at 0 8C and is short in duration as only kJ. The next plateau is at 8C and is longer in duration as kJ.
An isotherm is a line of constant temperature and it forms part of a diagram that shows the relationship between temperature, pressure and volume. The graph is gas specific and usually relates to nitrous oxide. Three lines are chosen to illustrate the volumepressure relationship above, at and below the critical temperature. Nitrous oxide isotherm. Liquid and vapour Draw this outline on the diagram first in order that your other lines will pass through it at the correct points. Once all vapour has been liquidized, the line climbs almost vertically as liquid is incompressible, leading to a rapid increase in pressure for a small decrease in volume.
This climbs from right to left as a rectangular hyperbola with a small flattened section at its midpoint. This is where a small amount of gas is liquidized. It climbs rapidly after this section as before. The pressure doubles as the volume halves. As it is above the critical temperature, it is a gas and obeys the gas laws.
Henrys law The amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid. Grahams law The rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. Ficks law of diffusion The rate of diffusion of a gas across a membrane is proportional to the membrane area A and the concentration gradient C1 C2 across the membrane and inversely proportional to its thickness D. Blood : gas solubility coefficient The ratio of the amount of substance present in equal volume phases of blood and gas in a closed system at equilibrium and at standard temperature and pressure.
Oil : gas solubility coefficient The ratio of the amount of substance present in equal volume phases of oil and gas in a closed system at equilibrium and at standard temperature and pressure. Bunsen solubility coefficient The volume of gas, corrected to standard temperature and pressure, that dissolves in one unit volume of liquid at the temperature concerned where the partial pressure of the gas above the liquid is one atmosphere. Ostwald solubility coefficient The volume of gas that dissolves in one unit volume of liquid at the temperature concerned. The Ostwald solubility coefficient is, therefore, independent of the partial pressure.
Osmolarity The amount of osmotically active particles present per litre of solution mmol. Osmolality The amount of osmotically active particles present per kilogram of solvent mmol. Osmotic pressure The pressure exerted within a sealed system of solution in response to the presence of osmotically active particles on one side of a semipermeable membrane kPa.
One osmole of solute exerts a pressure of Colligative properties Those properties of a solution which vary according to the osmolarity of the solution. These are: depression of freezing point. The freezing point of a solution is depressed by 1. Raoults law The depression of freezing point or reduction of the vapour pressure of a solvent is proportional to the molar concentration of the solute.
Osmometer An osmometer is a device used for measuring the osmolality of a solution. Solution is placed in the apparatus, which cools it rapidly to 0 8C and then supercools it more slowly to "7 8C. This cooling is achieved by the Peltier effect absorption of heat at the junction of two dissimilar metals as a voltage is applied , which is the reverse of the Seebeck effect. The solution remains a liquid until a mechanical stimulus is applied, which initiates freezing. This is a peculiar property of the supercooling process.
The latent heat of fusion is released during the phase change from liquid to solid so warming the solution until its natural freezing point is attained. Plot a smooth curve falling rapidly from room temperature to 0 8C. After this the curve flattens out until the temperature reaches "7 8C.
Cooling is then stopped and a mechanical stirrer induces a pulse. The curve rises quickly to achieve a plateau temperature freezing point. Electrical resistance is a broad term given to the opposition of flow of current within an electrical circuit. However, when considering components such as capacitors or inductors, or when speaking about resistance to alternating current AC flow, certain other terminology is used. Resistance The opposition to flow of direct current ohms,! Impedance The total of the resistive and reactive components of opposition to electrical flow ohms,!
All three of these terms have units of ohms as they are all measures of some form of resistance to electrical flow. The reactance of an inductor is high and comes specifically from the back electromotive force EMF; p. It is, therefore, difficult for AC to pass. The reactance of a capacitor is relatively low but its resistance can be high; therefore, direct current DC does not pass easily. Reactance does not usually exist by itself as each component in a circuit will generate some resistance to electrical flow. The choice of terms to define total resistance in a circuit is, therefore, resistance or impedance.
Ohms law The strength of an electric current varies directly with the electromotive force voltage and inversely with the resistance. The equation can be used to calculate any of the above values when the other two are known. A capacitor consists of two conducting plates separated by a non-conducting material called the dielectric. Capacitance The ability of a capacitor to store electrical charge farads, F. Farad A capacitor with a capacitance of one farad will store one coulomb of charge when one volt is applied to it. One farad is a large value and most capacitors will measure in micro- or picofarads Principle of capacitors Electrical current is the flow of electrons.
When electrons flow onto a plate of a capacitor it becomes negatively charged and this charge tends to drive electrons off the adjacent plate through repulsive forces.
When the first plate becomes full of electrons, no further flow of current can occur and so current flow in the circuit ceases. The rate of decay of current is exponential. Current can only continue to flow if the polarity is reversed so that electrons are now attracted to the positive plate and flow off the negative plate. The important point is that capacitors will, therefore, allow the flow of AC in preference to DC. Because there is less time for current to decay in a highfrequency AC circuit before the polarity reverses, the mean current flow is greater. The acronym CLiFF may help to remind you that capacitors act as low-frequency filters in that they tend to oppose the flow of low frequency or DC.
Graphs show how capacitors alter current flow within a circuit. The points to demonstrate are that DC decays rapidly to zero and that the mean current flow is less in a low-frequency AC circuit than in a high-frequency one. These curves would occur when current and charge were measured in a circuit containing a capacitor at the moment when the switch was closed to allow the flow of DC.
Current undergoes an exponential decline, demonstrating that the majority of current flow occurs through a capacitor when the current is rapidly changing. The reverse is true of charge that undergoes exponential build up. Capacitor in low-frequency AC circuit. Base this curve on the previous diagram and imagine a slowly cycling AC waveform in the circuit. When current flow is positive, the capacitor acts as it did in the DC circuit. When the current flow reverses polarity the capacitor generates a curve that is inverted in relation to the first.
The mean current flow is low as current dies away exponentially when passing through the capacitor. When the current in a circuit is alternating rapidly, there is less time for exponential decay to occur before the polarity changes. This diagram should demonstrate that the mean positive and negative current flows are greater in a high-frequency AC circuit. Inductor An inductor is an electrical component that opposes changes in current flow by the generation of an electromotive force.
An inductor consists of a coil of wire, which may or may not have a core of ferromagnetic metal inside it.
A metal core will increase its inductance. Inductance Inductance is the measure of the ability to generate a resistive electromotive force under the influence of changing current henry, H.
Henry One henry is the inductance when one ampere flowing in the coil generates a magnetic field strength of one weber. Electromotive force EMF An analogous term to voltage when considering electrical circuits and components volts, E. Principle of inductors A current flowing through any conductor will generate a magnetic field around the conductor.
If any conductor is moved through a magnetic field, a current will be generated within it. As current flow through an inductor coil changes, it generates a changing magnetic field around the coil. This changing magnetic field, in turn, induces a force that acts to oppose the original current flow. This opposing force is known as the back EMF.
In contrast to a capacitor, an inductor will allow the passage of DC and lowfrequency AC much more freely than high-frequency AC. This is because the amount of back EMF generated is proportional to the rate of change of the current. It, therefore, acts as a high-frequency filter in that it tends to oppose the flow of high-frequency current through it.
A graph of current flow versus time aims to show how an inductor affects current flow in a circuit. It is difficult to draw a graph for an AC circuit, so a DC example is often used. The key point is to demonstrate that the back EMF is always greatest when there is greatest change in current flow and so the amount of current successfully passing through the inductor at these points in time is minimal.
Current Draw a build-up exponential curve solid line to show how current flows when an inductor is connected to a DC source. On connection, the rate of change of current is great and so a high back EMF is produced. What would have been an instantaneous jump in current is blunted by this effect. As the back EMF dies down, a steady state current flow is reached.
This explains how inductors are used to filter out rapidly alternating current in clinical use. Defibrillator circuit You may be asked to draw a defibrillator circuit diagram in the examination in order to demonstrate the principles of capacitors and inductors. When charging the defibrillator, the switch is positioned so that the V DC current flows only around the upper half of the circuit. It, therefore, causes a charge to build up on the capacitor plates. When discharging, the upper and lower switches are both closed so that the stored charge from the capacitor is now delivered to the patient.
The inductor acts to modify the current waveform delivered as described below. Defibrillator discharge The inductor is used in a defibrillation circuit to modify the discharge waveform of the device so as to prolong the effective delivery of current to the myocardium. The unmodified curve shows exponential decay of current over time. This is the waveform that would result if there were no inductors in the circuit. The modified waveform should show that the waveform is prolonged in duration after passing through the inductor and that it adopts a smoother profile.
Both resonance and damping can cause some confusion and the explanations of the underlying physics can become muddled in a viva situation. Although the deeper mathematics of the topic are complex, a basic understanding of the underlying principles is all the examiners will want to see. Resonance The condition in which an object or system is subjected to an oscillating force having a frequency close to its own natural frequency.
Natural frequency The frequency of oscillation that an object or system will adopt freely when set in motion or supplied with energy hertz, Hz. We have all felt resonance when we hear the sound of a lorrys engine begin to make the window pane vibrate. The natural frequency of the window is having energy supplied to it by the sound waves emanating from the lorry. The principle is best represented diagrammatically. The curve shows the amplitude of oscillation of an object or system as the frequency of the input oscillation is steadily increased.
Start by drawing a normal sine wave whose wavelength decreases as the input frequency increases. Demonstrate a particular frequency at which the amplitude rises to a peak. By no means does this have to occur at a high frequency; it depends on what the natural frequency of the system is. Label the peak amplitude frequency as the resonant frequency. Make sure that, after the peak, the amplitude dies away again towards the baseline.
This subject is most commonly discussed in the context of invasive arterial pressure monitoring. Damping A decrease in the amplitude of an oscillation as a result of energy loss from a system owing to frictional or other resistive forces. A degree of damping is desirable and necessary for accurate measurement, but too much damping is problematic. The terminology should be considered in the context of a measuring system that is attempting to respond to an instantaneous change in the measured value.
This is akin to the situation in which you suddenly stop flushing an arterial line while watching the arterial trace on the theatre monitor. Damping coefficient A value between 0 no damping and 1 critical damping which quantifies the level of damping present in a system. Zero damping A theoretical situation in which the system oscillates in response to a step change in the input value and the amplitude of the oscillations does not diminish with time; the damping coefficient is 0. The step change in input value from positive down to baseline initiates a change in the output reading.
The system is un-damped because the output value continues to oscillate around the baseline after the input value has changed. The amplitude of these oscillations would remain constant, as shown, if no energy was lost to the surroundings. This situation is, therefore, theoretical as energy is inevitably lost, even in optimal conditions such as a vacuum. The system is unable to prevent oscillations in response to a step change in the input value. The damping coefficient is The step change in input value from positive to baseline initiates a change in the output reading.
The system is under-damped because the output value continues to oscillate around the baseline for some time after the input value has changed. It does eventually settle at the new value, showing that at least some damping is occurring. Over-damped The system response is overly blunted in response to a step change in the input value, leading to inaccuracy. This time the curve falls extremely slowly towards the new value.
Given enough time, it will reach the baseline with no overshoot but clearly this type of response is unsuitable for measurement of a rapidly changing variable such as blood pressure. Critical damping That degree of damping which allows the most rapid attainment of a new input value combined with no overshoot in the measured response. The damping coefficient is 1. The response is still blunted but any faster response would involve overshoot of the baseline.
Critical damping is still too much for a rapidly responding measurement device. Optimal damping The most suitable combination of rapid response to change in the input value with minimal overshoot. The damping coefficient is 0. Draw this curve so that the response is fairly rapid with no more than two oscillations around the baseline before attaining the new value. This is the level of damping that is desirable in modern measuring systems. There are a number of equations and definitions associated with the principles behind the working of the pulse oximeter.
Beers law. The absorbance of light passing through a medium is proportional to the concentration of the medium. Draw a line that passes through the origin and which rises steadily as C increases. The slope of the line is dependent upon the molar extinction coefficient! Note that if emergent light I is plotted on the y axis instead of absorbance, the curve should be drawn as an exponential decline. Lamberts law. The absorbance of light passing through a medium is proportional to the path length. The line is identical to that above except that in this instance the slope is determined by both!
Again, if emergent light I is plotted on the y axis instead of absorbance, the curve should be plotted as an exponential decline. Both laws are often presented together to give the following equation, known as the BeerLambert law, which is a negative exponential equation of the form y a. In the pulse oximeter, the concentration and molar extinction coefficient are constant. The only variable becomes the path length, which alters as arterial blood expands the vessels in a pulsatile fashion.
Haemoglobin absorption spectra The pulse oximeter is a non-invasive device used to monitor the percentage saturation of haemoglobin Hb with oxygen SpO2. The underlying physical principle that allows this calculation to take place is that infrared light is absorbed to different degrees by the oxy and deoxy forms of Hb. Two different wavelengths of light, one at nm red and one at nm infrared , are shone intermittently through the finger to a sensor. As the vessels in the finger expand and contract with the pulse, they alter the amount of light that is absorbed at each wavelength according to the BeerLambert law.
The pulsatile vessels, therefore, cause two waveforms to be produced by the sensor. If there is an excess of deoxy-Hb present, more red than infrared light will be absorbed and the amplitude of the red waveform will be smaller. Conversely, if there is an excess of oxy-Hb, the amplitude of the infrared waveform will be smaller. It is the ratios of these amplitudes that allows the microprocessor to give an estimate of the SpO2 by comparing the values with those from tables stored in its memory.
In order to calculate the amount of oxy-Hb or deoxy-Hb present from the amount of light absorbance, the absorbance spectra for these compounds must be known. Oxy-Hb Crosses the y axis near the deoxy-Hb line but falls steeply around nm to a trough around nm. It then rises as a smooth curve through the isobestic point where it flattens out. This curve must be oxy-Hb as the absorbance of red light is so low that most of it is able to pass through to the viewer, which is why oxygenated blood appears red. Deoxy-Hb Starts near the oxy-Hb line and falls as a relatively smooth curve passing through the isobestic point only.
Compared with oxy-Hb, it absorbs a vast amount of red light and so appears blue to the observer. You will be expected to be familiar with capnography. The points to understand are the shape and meaning of different capnograph traces and the nature of the reaction taking place within the CO2 absorption canister. Capnometer The capnometer measures the partial pressure of CO2 in a gas and displays the result in numerical form. Capnograph A capnograph measures the partial pressure of CO2 in a gas and displays the result in graphical form. A capnometer alone is unhelpful in clinical practice and most modern machines present both a graphical and numerical representation of CO2 partial pressure.
Normal capnograph. Assume a respiratory rate of 12 min! From zero baseline, the curve initially rises slowly owing to the exhalation of dead space gas. Subsequently, it rises steeply during expiration to a normal value and reaches a near horizontal plateau after approximately 3 s. Inspiration causes a near vertical decline in the curve to baseline and lasts around 2 s.
The main difference when compared rebreathing with the normal trace is that the baseline is not zero. If the patient is spontaneously breathing, the respiratory rate may increase as they attempt to compensate for the higher PETCO2. Inadequate paralysis. The bulk of the curve appears identical to the normal curve. However, during the plateau phase, a large cleft is seen as the patient makes a transient respiratory effort and draws fresh gas over the sensor.
Usually seen when the respiratory rate is slow. The curve starts as normal but the expiratory pause is prolonged owing to the slow rate. Fresh gas within the circuit is able to pass over the sensor causing the PCO2 to fall. During this time, the mechanical pulsations induced by the heart force small quantities of alveolar gas out of the lungs and over the sensor, causing transient spikes. Inspiration in the above example does not occur until point A. In this example, the respiratory rate has increased so that each respiratory cycle only takes 3 s.
Rarely seen. The excess CO2 is generated from the increased skeletal muscle activity and metabolic rate, which is a feature of the condition. Acute loss of cardiac output. With hyperventilation, the fall would be slower. Any condition that acutely reduces cardiac output may be the cause, including cardiac arrest, pulmonary embolism or acute rhythm disturbances.
If the PCO2 falls instantly to zero, then the cause is disconnection, auto-calibration or equipment error. Breathing system disconnection 5. Following a normal trace, there is the absence of any further rise in PCO2. You should ensure that your x axis is long enough to demonstrate that this is not simply a result of a slow respiratory rate. Obstructive disease. Instead of the normal sharp upstroke, the curve should be drawn slurred.
This occurs because lung units tend to empty slowly in obstructive airways disease. The respiratory rate is reduced such that each complete respiratory cycle takes longer. This is usually a result of a prolonged expiratory phase, so it is the plateau that you should demonstrate to be extended. Carbon dioxide is absorbed in most anaesthetic breathing systems by means of a canister that contains a specific absorbing medium. This is often soda lime but may also be baralime in some hospitals.
However, if the granules are too small then there will be too little space between them and the resistance to gas flow through the canister will be too high. Chemical reaction You may be asked to describe the chemical reaction that occurs when CO2 is absorbed within the canister. Heat is produced at two stages and water at one. This can be seen and felt in clinical practice. Note that NaOH is reformed in the final stage and so acts only as a catalyst for the reaction. The compound that is actually consumed in both baralime and soda lime is Ca OH 2. The Fick principle The total uptake or release of a substance by an organ is equal to the product of the blood flow to the organ and the arterio-venous concentration difference of the substance.
Thermodilution and dye dilution A marker substance is injected into a central vein. A peripheral arterial line is used to measure the amount of the substance in the arterial system. A graph of concentration versus time is produced and patented algorithms based on the StewartHamilton equation below are used to calculate the cardiac output.
When dye dilution is used, the graph of concentration versus time may show a second peak as dye recirculates to the measuring device. This is known as a recirculation hump and does not occur when thermodilution methods are used. A special form of the equation used with thermodilution is. Draw a curve starting at the origin that reaches its maximum value at around 5 s. The curve then falls to baseline but is interrupted by a recirculation hump at around 15 s. This is caused by dye passing completely around the vasculature and back to the sensor a second time.
Demonstrate that the semi-log plot makes the curve more linear during its rise and fall from baseline. The recirculation hump is still present but is discounted by measuring the area under the curve AUC enclosed by a tangent from the initial down stroke. This is the AUC that is used in the calculations.
The actual graph of temperature versus time for the thermodilution method would resemble the one below. Demonstrate that the thermodilution curve has no recirculation hump when compared with the dye dilution method. Otherwise the line should be drawn in a similar fashion. For reasons of clarity, the graph is usually presented with temperature decrease on the y axis so that the deflection becomes positive.
The semi-log transformation again makes the rise and fall of the graph linear. Note that this time there is no recirculation hump. As the fall on the initial plot was exponential, so the curve is transformed to a linear fall by plotting it as a semi-log. The AUC is still used in the calculations of cardiac output. The Doppler effect is used in practice to visualize directional blood flow on ultrasound, to estimate cardiac output and in some types of flow meter.
Doppler effect The phenomenon by which the frequency of transmitted sound is altered as it is reflected from a moving object. It is represented by the following equation: V. Principle Sound waves are emitted from the probe P at a frequency F0. They are reflected off moving red blood cells and back towards the probe at a new frequency, FR. The phase shift can now be determined by FR F0.
The angle of incidence! If a measurement or estimate of the cross-sectional area of the blood vessel is known, flow can be derived as area multiplied by velocity m2. This is the principle behind oesophageal Doppler cardiac output monitoring. P Skin F0 FR. It is also possible to calculate the pressure gradients across heart valves using the Doppler principle to measure the blood velocity and entering the result into the Bernoulli equation.
Bernoulli equation DP 4v 2 where DP is the pressure gradient and v is the velocity of blood. This topic tests your knowledge of the physics and physiology behind the use of neuromuscular blocking drugs NMBDs. You will benefit from a clear idea in your mind about what each type of nerve stimulation pattern is attempting to demonstrate. Single twitch A single, supra-maximal stimulus is applied prior to neuromuscular blockade as a control. The diminution in twitch height and disappearance of the twitch correlates crudely with depth of neuromuscular block. Supra-maximal stimulus An electrical stimulus of sufficient current magnitude to depolarize all nerve fibres within a given nerve bundle.
Notice that you are being asked to describe the output waveform of the nerve stimulator. The axes must, therefore, be time and current as shown. Each stimulus is a square wave of supra-maximal current delivered for 0. The train of four TOF is delivered at 2 Hz so there is one stimulus every ms. This means that if the TOF starts at time 0, the complete train takes ms. Tetanic stimulus A supra-maximal stimulus applied as a series of square waves of 0. Notice now that you are being asked to describe the response to a TOF stimulus. The axes are, therefore, changed to show time and percentage response as shown.
It is important to realize that each twitch is still being delivered at the same current even though the response seen may be reduced. Partial depolarizing neuromuscular block causes an equal decrease in the percentage response to all four stimuli in the TOF. Non-depolarizing block train of four. Initial TOF should demonstrate each successive twitch decreasing in amplitude: this is fade. The second TOF should still demonstrate fade but the twitches as a group should have increased amplitude. This is posttetanic potentiation. The TOFR is used for assessing suitability for and adequacy of reversal.
Draw four twitches at 0. Explain that this patient would be suitable for reversal as all four twitches are present. Assessment of receptor site occupancy Twitches seen. Double-burst stimulation Two bursts of three stimuli at 50 Hz, each burst being separated by ms. In double-burst stimulation, the ratio of the second to the first twitch is assessed. Demonstrate two clusters of three stimuli duration 0. The heights of both clusters are identical. If questioned, the current should be greater than 60 mA for the same reasons as when using the TOF. Residual neuromuscular block ms. Demonstrate the two clusters with the same time separation.
Post-tetanic count A post-tetanic count is used predominantly where neuromuscular blockade is so deep that there are no visible twitches on TOF. The post-tetanic twitch count can help to estimate the likely time to recovery of the TOF twitches in these situations. The meaning of the count is drug specific.
Draw a 5 s period of tetany followed by a 3 s pause. Next draw single standard twitches at a frequency of 1 Hz: 20 stimuli are given in total. Using atracurium, a single twitch on the TOF should appear in approximately 4 min if there are four post-tetanic twitches evident. Phase 1 and phase 2 block Phase 1 Cause. The principle behind the use of surgical diathermy is that of current density.
Physics, Pharmacology and Physiology for Anaesthetists: Key Concepts for the FRCA: Medicine & Health Science Books @ ykoketomel.ml Buy Physics, Pharmacology and Physiology for Anaesthetists: Key Concepts for the FRCA 1 by Matthew Cross (ISBN: ) from Amazon's Book.
When a current is applied over a small area, the current density is high and heating may occur. If the same current is applied over a suitably large area then the current density is low and no heating occurs. For unipolar diathermy, the apparatus utilizes a small surface area at the instrument end and a large area on the diathermy plate to allow current to flow but to confine heating to the instrument alone. Bipolar diathermy does not utilize a plate as current flows directly between two points on the instrument.
Frequency The safety of diathermy is enhanced by the use of high frequency 1 MHz current, as explained by the graph below. Note that the x axis is logarithmic to allow a wide range of frequencies to be shown. The y axis is the current threshold at which adverse physiological events dysrhythmias etc. The highest risk of an adverse event occurs at current frequencies of around 50 Hz, which is the UK mains frequency.
At diathermy frequencies, the threshold for an adverse event is massively raised. Cutting diathermy This type of diathermy is used to cut tissues and is high energy. It differs from coagulation diathermy by its waveform. When activated, the instrument delivers a sustained high-frequency AC waveform. Current density is high at the implement and local heating causes tissue destruction. The sine wave continues until the switch is released.
Coagulation diathermy. When activated, the instrument delivers bursts of high-frequency AC interrupted by periods of no current flow. Local tissue heating still occurs but is not sustained and, therefore, causes less destruction than cutting diathermy. Maintaining cleanliness and sterility is involved in everyday practice but, for the most part, is not under the direct control of anaesthetists. Nevertheless, a familiarity will be expected with the main definitions and methods of achieving adequate cleanliness. Cleaning The process of physically removing foreign material from an object without necessarily destroying any infective material.
Disinfection The process of rendering an object free from all pathogenic organisms except bacterial spores. Sterilization The process of rendering an object completely free of all viable infectious agents including bacterial spores. Decontamination The process of removing contaminants such that they are unable to reach a site in sufficient quantities to initiate an infection or other harmful reaction. The process of decontamination always starts with cleaning and is followed by either disinfection or sterilization. The MeyerOverton hypothesis is the theory of anaesthetic action which proposes that the potency of an anaesthetic agent is related to its lipid solubility.
Potency is described by the minimum alveolar concentration MAC of an agent and lipid solubility by the oil:gas solubility coefficient. The MeyerOverton hypothesis proposed that once a sufficient number of anaesthetic molecules were dissolved in the lipid membranes of cells within the central nervous system, anaesthesia would result by a mechanism of membrane disruption. While an interesting observation, there are several exceptions to the rule that make it insufficient to account fully for the mechanism of anaesthesia.
MeyerOverton graph of potency versus lipid solubility. After drawing and labelling the axis note the slightly different scales , draw a straight line with a negative gradient as shown. Make sure you can draw in the position of the commonly used inhalational agents. Note that the line does not pass directly through the points but is a line of best fit, and also that although isoflurane and enflurane have near identical oil:gas partition coefficients they have different MAC values and, therefore, this relationship is not perfect.
The concentration effect The phenomenon by which the rise in the alveolar partial pressure of nitrous oxide is disproportionately rapid when it is administered in high concentrations. Nitrous oxide N2O , although relatively insoluble, is 20 times more soluble in the blood than nitrogen N2. The outward diffusion of N2O from the alveolus into the blood is therefore much faster than the inward diffusion of N2 from the blood into the alveolus.
Consequently, the alveolus shrinks in volume and the remaining N2O is concentrated within it. This smaller volume has a secondary effect of increasing alveolar ventilation by drawing more gas into the alveolus from the airways in order to replenish the reduced volume. Graphical demonstration The above concept can be described graphically by considering the fractional concentration of an agent in the alveolar gas FA as a percentage of its fractional concentration in the inhaled gas FI over time.
Nitrous oxide Desflurane Sevoflurane. After drawing and labelling the axis draw a series of build-up negative exponential curves with different gradients as shown. The order of the curves is according to the blood:gas partition coefficients. The more insoluble the agent, the steeper the curve and the faster the rate of onset.
The exceptions to this are the N2O and desflurane curves, which are the opposite way round. This is because of the concentration effect when N2O is administered at. The second gas effect The phenomenon by which the speed of onset of inhalational anaesthetic agents is increased when they are administered with N2O as a carrier gas. This occurs as a result of the concentration effect and so it is always useful to describe the concentration effect first, even if being questioned directly on the second gas effect.
If there is another gas present in the alveolus, then it too will be concentrated by the relatively rapid uptake of N2O into the blood. Isomerism is a subject which can easily become confusing due to the myriad of definitions and nomenclature it involves. Remembering a schematic diagram such as the one below often helps to focus the mind as to where each type of isomer fits. Isomerism The phenomenon by which molecules with the same atomic formulae have different structural arrangements.
Isomers are important because the three-dimensional structure of a drug may determine its effects. Tautomerism The dynamic interchange between two different forms of a molecular structure depending on the environmental conditions. Stereoisomerism Identical chemical formulae and bond structure but different three-dimensional configuration. Enantiomers Compounds that have a single chiral centre and form non-superimposable mirror images of each other. Diastereoisomers Compounds containing more than one chiral centre or which are subject to geometric isomerism and, therefore, have more than just two mirror image forms.
Geometric isomerism Two dissimilar groups attached to two atoms that are in turn linked by a double bond or ring creates geometric isomerism because of the reduced mobility of the double bond or ring. Chiral centres encountered in anaesthetics usually have carbon or quaternary nitrogen as the chiral centre.
Any compound which contains more than one chiral centre is termed a diastereoisomer by definition.
New to eBooks. How many copies would you like to buy? Cross , Emma V. Add to Cart Add to Cart. Add to Wishlist Add to Wishlist. The FRCA examination relies in part on a sound understanding of the basic sciences physics, physiology, pharmacology and statistics behind anaesthetic practice. It is important to be able to describe these principles clearly, particularly in the viva section of the examination.