PHYSICS AND CLINICAL MEASUREMENTSPHYSICS AND CLINICAL MEASUREMENTS

DR.M.BILAL DELVIDR.M.BILAL DELVIMBBS,MD,DA,KSUF ARAB MBBS,MD,DA,KSUF ARAB

BOARDBOARD

ASSISTANT PROFESSOR ASSISTANT PROFESSOR OF ANAESTHESIAOF ANAESTHESIA

DEPT OF ANAESTHESIA DEPT OF ANAESTHESIA AND ICU AND ICU

COLLEGE OF MEDICINE COLLEGE OF MEDICINE

KING SAUD UNIVERSITYKING SAUD UNIVERSITY

BASIC PHYSICSBASIC PHYSICSGAS LAWSGAS LAWS

Boyle’s Law states that for a constant quantity of gas at a constant temperature, the absolute pressure is inversely proportional to the volume.

• Charles’s Law states that for a constant quantity of gas at a constant pressure, the absolute temperature of the gas is proportional to its volume.

• At a constant volume, the absolute pressure of a given mass of gas varies proportionately to its absolute temperature, or,

Henry's Law

• At a constant temperature, the amount of a given gas dissolved in a given liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Dalton's Law Of Partial Pressures

• in a mixture of gases, the pressure exerted by each gas is equal to the pressure which would be exerted if that gas alone were present.

• Graham's Law

• the rate of diffusion of a gas is inversely proportional to the square root of the molecular weight this only applies to simple models and is inaccurate when dealing with complex biological membranes

Avogadro’s Hypothesis states that equal volumes of gas at the same temperature and pressure contain the same number of molecules.

• One mole of gas occupies 22.4 dm3 at s.t.p. (273.15 K, 101.325 kPa).

These laws combine to give the relation:

PV=nRT

Where R is the universal gas constant.

Critical Temperature• is the temperature above which a gas cannot be

liquified by pressure alonei. N2O = 36.5 °Cii. O2 = -119 °CCritical Pressure• is the pressure at which a gas liquifies at its

critical Ti. N2O ~ 73 bar @ 36.5 °Cii. N2O ~ 52 bar @ 20.0 °C

Pseudo-Critical Temperature

For a mixture of gases at a specific pressure, the specific temperature at which the individual gases may separate from the gaseous phase

1. N2O 50% / O2 50% = - 5.5 °C for cylinders (most likely at 117 bar)

2. N2O 50% / O2 50% = - 30 °C for piped gas

Filling Ratio: = the mass of the gas in the cylinder

the mass of water which would fill the cylinder

N2O = 0.65 (UK)

Adiabatic Change• the change of physical state of a gas, without the

transfer of heat energy to or from the surrounding environment

• in rapid expansion, energy is required to overcome Van der Waal's forces of attraction, as this energy cannot be gained from the surroundings, it is taken from the kinetic energy of the molecules

SOLUBILITY

Bunsen Solubility Coefficient

• Def'n: the volume of gas, corrected to STP, which dissolves in one unit volume of the liquid at the temperature concerned, where the partial pressure of the gas concerned is 1 atmosphere

Ostwald Solubility Coefficient• Def'n: the volume of gas which dissolves in one

unit volume of the liquid at the temperature concerned

i. the temperature must be specifiedii. it is independent of pressure as the pressure

rises the number of molecules of gas in the liquid phase increases, however, when measured at the higher pressure the volume is the same

Partition Coefficient

• Def'n: the ratio of the amount of a substance present in one phase as compared with than in another, the two phases being of equal volume, the temperature must be specified and the phases in equilibrium.

DIFFUSION & OSMOSIS

Diffusion

• the spontaneous movement of molecules or other particles in solution, owing to their random thermal motion, to reach a uniform concentration throughout the solvent.

Fick's Law• the rate of diffusion of a substance across a unit

area is proportional to the concentration gradient for that substance further, the diffusion of gas across a membrane, or into or out of a liquid, is proportional to the gases solubility in the liquid.

• CO2 being more soluble than O2 diffuses far more rapidly across the alveolar membrane and into the RBC.

• N2O being far more soluble than N2 may diffuse into and expand closed cavities during induction of anaesthesia.

Exponential functions, Exponential functions, Integration andIntegration andDifferentiation.Differentiation.

Any process in which the rate of change of a quantity is proportional to the quantity is an exponential function. An example is the emptying of a bath: the rate of change of the volume of the bath (the plug-hole flow) is proportional to the volume remaining in the bath (which determines the pressure at the plug-hole):

Integrating with respect to time gives the exponential function:

• This situation is analogous to the elimination of a drug which demonstrates firstorder kinetics, such as the washout curve of an inhaled anaesthetic.

• It is also analogous to the natural expiration from the lungs where -k equals the rate constant of expiration, the reciprocal of the time constant (compliance times resistance).

• Integration is the derivation of a function which expresses the area under a function

y = f (x) from x = 0 to any value of x.

• Differentiation is the reverse process: deriving a function which expresses the rate of change of f (x).

Current : is the flow of charged particles resulting from a potential difference or changing magnetic field.

• Most commonly this is a flow of electrons through a metal or other conductor (such as graphite) which has freely mobile electrons.

• A current can also flow through solutions containing charged particles. All body fluids contain ions and so are capable of conducting current.

• The unit of current is the Ampere (1 Coulomb/second). Many quantities in monitoring devices are measured indirectly as electrical current.

• Nerve stimulators are calibrated to deliver a determined current through the tissue between the electrodes.

Solids : which do not contain many unbound electrons and solutions with few ions are poor conductors and are known as insulators.

Semi-conductors contain electrons which are loosely bound and may conduct a current if electrons are given enough energy to become unbound.

• This effect is seen in thermistors and photodetectors used in monitoring equipment. It is also the basis for transistors and silicon-based integrated circuits which are universally present in electronic equipment.

Potential difference is the difference in electrical energy between two points.

• Its unit is the Volt (1 Joule/Coulomb) and it generates a electromotive force which drives a current of charged particles.

Resistance is a measure of the electromotive force required to drive a current through a material.

• Its unit is the Ohm (1 Volt/Ampere). Thermistors display a change inresistance over a range of temperature and so with calibration the current flow for a specified voltage can be measured and used to determine temperature.

When a small change in resistance is to be measured, a Wheatstone Bridge circuit is commonly employed.

• Classically, R4 is measured by adjusting R3 until the galvanometer reads 0. In this situation, R1/R2 = R3/R4.

• In practice, a monitor is set up so that R1 and R4 vary together and inversely to R2 and R3.

• The deflection of the galvanometer is then read as output.

• Impedance is the resistance of a component or circuit with a specified characteristic current flowing.

• Resistance of many components (capacitors and inductors) varies with frequency of alternating current.

• In a surgical diathermy device, a capacitor is part of the circuit, providing low impedance at the high frequency typically used (1 MHz), but high impedance to low frequency currents likely to cause arrhythmias (50 Hz).

• Capacitance is a measure of the charge a device can hold. Its unit is the Farad (1 Coulomb/Volt).

• Defibrillators are based on a capacitor which is charged with a calibrated voltage to provide a determined energy output for DC reversion.

• The energy stored in a capacitor is 0.5 x charge x potential. A typical output of 360 J is usually produced by about 5000 V across about 150 mC.

• An inductor is used to slow the discharge of the defibrillator.

SI system of units.SI system of units.• Seven basic SI units from which all

other units are derivedMass kgtime sDistance mcurrent ATemperature Kluminous intensity cdamount of substance mol

• Derived SI units (some of them)• Temperature C K - 273.15• Force N kg m s-2• Pressure Pa N m-2• Energy J N m• Power W J s-1• Frequency Hz s-1• Volume l 10-3 m3

• Charge C A s• Potential V W A-1 or

J C-1• Capacitance F C V-1• Resistance V

A-1• Magnetic flux Wb V s• Radiation dose Gy J kg-1

water

Laws governing the behavior of Laws governing the behavior of fluids.fluids.

• Fluids are gases or liquids. They exhibit flow, which is defined as quantity (Q)moved per unit time (t):

Flow is characterized as laminar or turbulent. In laminar flow, fluid moves without eddies and flow is equal to pressure (P) over resistance (R):

In a cylindrical tube, resistance to flow is related to radius (r) and length (l) of the tube and viscosity (η) of the fluid, yielding the Hagen-Poiseuille equation:

• Above a critical speed, laminar flow changes to turbulent flow. For a smooth cylindrical tube, the transition occurs when Reynolds number is approximately 2000. For rough or bent tubes, the transition occurs at lower numbers. Reynolds number (RN) is defined in terms of speed (ν), density (ρ) and viscosity (η) of the fluid and diameter (d) of the tube:

Tension• Laplace's Law

• thus, for straight tubes,

• and, for spheres, • where, T = the tangential force in N/m, acting

along a length of wall h = the thickness of the wall (usually small)

• as the diameter of a vessel becomes smaller, the collapsing force becomes greater this can lead to vessel closure at low pressures, the critical closing pressure

• also seen in alveoli, leading to instability with small alveoli tending to fill larger ones

• however, due to the action of surfactant alveolar stability is maintained

ViscosityFor a given set of conditions, flow is inversely

proportional to viscosity blood viscosity increases with,

a. low temperaturesb. increasing agec. cigarette smokingd. increasing haematocrite. abnormal elevations of plasma proteins this

may be reduced with low MW dextran the viscosity of blood is anomalous due to the presence of cells, and its behavior is non-newtonian

The Bernoulli Principle Based on the principal of conservation of energy

the total energy of a fluid flow is given by,• E = PV + mgh + ½mv2

where, PV = the potential energy of pressure

mgh = the potential energy due to gravity

½mv2 = the kinetic energy of motion

• thus, as the velocity of flow increases passing though a narrowing and the velocity increases, so the pressure decreases also,

• for a system to work efficiently, laminar flow is important as turbulence would allow flow energy to be lost as heat this is the principal of operation of a venturi,

• where the opening of a side tube leads to the entrainment of another fluid the entrainment ratio

• ER is defined as, ER = Entrained Flow /

Driving Flow

• When there is no opening on the side of a narrowing in a tube, a region of low pressure is established and the stream tends to adhere to the wall

• If the tube then diverges, the stream may adhere to either wall, diverting flow to one or other lumen, the Coanda effect

• Valves can be constructed on this mechanism using fluid logic, a control nozzle being located just distal to the divergence of the lumen unfortunately these are wasteful and noisy

• Osmotic Pressure

• the pressure which would be required to prevent the movement of solvent across a semipermeable membrane, down a thermodynamic activity gradient for than solvent

• 1 mol of any solute dissolved in 22.4 litres of solution at 0°C will generate an osmotic

• pressure of 1 atmosphere

• Over 99% of the plasma osmolarity is due to electrolytes,

• The contribution of the plasma proteins being only ~ 1 mosmol/l normal rbc's lyse at osmolarities 200 mosmol/l as capillaries are relatively impermeable to protein,

• This generates an osmotic pressure difference between the plasma and the interstitial fluid,

• The plasma oncotic pressure ~ 26 mmHg

Osmolality• The number of osmotically active particles

(osmoles) per kilogram of solvent

• The depression of the freezing point of a solution is directly proportional to the osmolality,

• 1 mol of a solute added to 1 kg of water depresses the freezing point by 1.86°C the presence of increased amounts of solute also lowers the vapour pressure of the solvent.

HeatHeat• Def'n: a form of energy, being the state of

thermal agitation of the molecules of a substance, which may be transferred by,

i. conduction through a substance

ii. convection by a substance, and

iii. radiation as electromagnetic waves

Temperature• Def'n: is the physical state of a substance which

determines whether or not the substance is in thermal equilibrium with its surroundings,

• heat energy being transferred from a region of higher temperature to a region of lower temperature alterations in the temperature of a substance, through the addition or removal of heat energy,

• also leads to alterations of the physical properties of the substance thus, mercury expands when heated and this was used by Fahrenheit to construct the first temperature scale.

Kelvin• the SI unit of thermodynamic temperature• equal to 1/273.16 of the absolute temperature of

the triple point of water the temperature at which ice, water and water vapour are all in equilibrium

Celsius Scale• Temperature (K) = Temperature (°C) + 273.15• therefore, on the Celsius scale the triple point of

water is 0.01 °C

CLINICAL MEASUREMENTSCLINICAL MEASUREMENTS

BLOOD PRESSURE MONITORING

• NIBP: This is also known as a device for Indirect Non-Invasive Automatic Mean Arterial Pressure (DINAMAP).

History• Measurement of blood pressure was

first attempted by Hales in 1733. The blood pressure cuff was developed by Riva-Rocci in 1896.

• Cushing introduced the measurement of blood pressure into Anaesthetic practice in 1901.

• Harvey Williams Cushing (1869-1939). American neurosurgeon. Pioneered diathermy, advocated record keeping and monitoring in Anaesthesia. Described Cushing’s disease, syndrome, reflex and ulcer.

Mechanism Of Function • A microprocessor controls the sequence

of inflation and deflation of the cuff. The cuff is inflated to a pressure above the previous systolic pressure, it is then deflated incrementally.

• A transducer senses the pressure changes which are processed by the microprocessor. This has an accuracy of +/- 2%.

• The mean arterial pressure (MAP) corresponds to the maximum oscillation at the lowest cuff pressure.

• The systolic pressure corresponds to the onset of rapidly increasing oscillations.

• Diastolic pressure corresponds to the onset of rapidly decreasing oscillations. It is also calculated from the systolic and MAP (MAP= diastolic + 1/3rd pulse pressure)

The Cuff• The cuff should cover at least 2/3rds of the

upper arm. • The width of the cuff’s bladder should be

40% of the mid-circumference of the limb. • The middle of the cuff should overlay the

brachial artery. The device has a fast rate of inflation and a slow cuff deflation. This avoids venous congestion and allows time to detect arterial pulsation.

Sources of error• If the cuff is too small the blood pressure

over-reads. Similarly, if too large then the blood pressure under-reads, (greatest error is seen with an undersized cuff).

• Systolic pressure over-reads at low pressures (<60mmHg) and under reads at high systolic pressures.

• Arrhythmias such as AF affect accuracy. • External pressure on the cuff (e.g. the

surgeon!) can cause inaccuracies.

Complications• Frequent, repeated inflations can cause

ulnar nerve palsy and petechial haemorrhage of the skin underlying the cuff.

Invasive Blood Pressure Monitoring

• Invasive arterial pressure monitoring provides beat-to-beat information about the perfusion of patient’s vital organs.

Equipment

• An indwelling Teflon arterial cannula (20/22G) is used. The cannula may be sited in the radial, ulnar, brachial, posterior tibial, femoral or dorsalis pedis artery. The radial artery is the preferred site. The cannula has parallel walls to minimise turbulent flow.

Allen’s TestBefore cannulating the radial artery, Allen’s test should be carried out:(Inadequate collateral flow exists in 3% of hospitalised patients)

Causes of Inadequate Ulnar Circulation

[i] Normal anatomic variant [ii] Hypothenar Hammer Syndrome

• The cannula is connected to a transducer (a transducer converts one form of energy to another) via a column of heparinised saline at a pressure of 300mmHg.

• The saline passes through a drip chamber adjusted to allow a flow of 4ml/hour. This continuously flushes the tubing and cannula.

• The ideal solution for use is dextrose, since a non-electrical conducting fluid avoids current passing down the catheter into the heart. 

• The transducer should be at the level of the right atrium. Raising or lowering the transducer results in errors.

• Mechanism • The saline column moves back and forth

with the pulsation. • This causes the diaphragm to move. This

movement results in a change in resistance and current flow (V=IR) through the transducer.

• The transducer is connected to a Wheatstone bridge.

• The heparinised saline allows flushing of the cannula and prevents backflow.

• The transducer is a strain gauge variable transducer.

• If a wire is stretched it becomes longer and thinner and thus its resistance increases.

• This is known as a strain gauge.

• This is connected to an amplifier and oscilloscope.

Wheatstone Bridge

Sir Charles Wheatstone (1802–1875), British physicist and inventor.

• Null deflection of the galvanometer implies R1/R2=Rv/Ru

• The Wheatstone bridge is an electrical circuit for the precise comparison of resistances.

• It consists of a common source of electrical current and a galvanometer that connects two parallel branches, containing four resistors, three of which are known.

• One parallel branch contains one known resistance and an unknown (Ru).The other parallel branch contains resistors of known resistances.

• In order to determine the resistance of the unknown resistor, the resistances of the other three are adjusted and balanced until the current passing through the galvanometer decreases to zero.

• The Wheatstone bridge is well suited also for the measurement of small changes of a resistance and, therefore, is also suitable to measure the resistance change in a strain gauge.

• The strain gauge transforms strain applied to it into a proportional change of resistance.

• Thus changes in resistance and current are measured, then electronically converted and displayed as systolic, diastolic and mean arterial pressures.

• Most pressure transducers contain four strain gauges which form the four resistances in the Wheatstone bridge.

• The system is designed so that the resistances of two of the strain gauges at opposite sides of the bridge increase while the resistances of the other two decrease.

• This results in a larger potential change at the galvanometer connections. The potential is then amplified before display.

• Damping is caused by dissipation of stored energy.

• Anything which takes energy out of the system results in a progressive diminution of amplitude of oscillations.

• Increased damping lowers the systolic pressure and elevates the diastolic pressure. MAP is unaltered.

• Damping can result from air bubbles, blood clots, soft diaphragm or soft tubing.

• Resonance occurs when the driving force frequency coincides with the resonant frequency of the system.

• This may occur if the tube or diaphragm is too stiff or non-compliant.

• The resonant frequency (or natural frequency) is the frequency at which the monitoring system itself resonates and amplifies the signal.

• It should be at least 10 times the fundamental frequency.

• If the natural frequency is less than 40Hz it falls within the range of the blood pressure and a sine wave will be superimposed on the blood pressure wave.

• The fundamental frequency of this system is the heart rate.

• This is also known as the first harmonic. • The first ten harmonics contribute to the

waveform seen.

INFORMATION DEDUCED FROM THE INFORMATION DEDUCED FROM THE ARTERIAL WAVE FORMARTERIAL WAVE FORM

[i] Arterial blood pressure: The mean pressure is the average pressure throughout the cardiac cycle.

• Because systole is shorter than diastole, the MAP is slightly less than the value halfway between systolic and diastolic pressure.

• MAP can be determined by integrating a pressure signal over the duration of one cycle.

• The mean pressure is then given by the value of this integral divided by time.

ii] The slope of the upstroke of the wave reflects myocardial contractility (dP/dt)

iii] The stroke volume can be calculated by measuring the area from the beginning of the upstroke to the dicrotic notch. If this is multiplied by the heart rate then cardiac output can be estimated.

iv] The position of the dicrotic notch on the down stroke. A low dicrotic notch is seen in hypovolaemic patients

• v] The slope of the diastolic decay indicates resistance to outflow. A slow fall is seen in vasoconstriction.

Specific Complications

Thromboembolism/vasospasm/thrombosis resulting in:

[i] Compromise of circulation leading to blanching and possible necrosis or gangrene of tissues and extremities.

[ii] Emboli.

[iii] Damage to peripheral nerves.

ELECTROCARDIOGRAMELECTROCARDIOGRAM

• This is the recording and display of cardiac electrical activity. First performed in 1887.

• Potentials from the heart are transmitted through the tissues and can be detected by electrodes which produce an ECG recording.

• Silver and silver chloride forms a stable electrode combination. They are separated from the skin by a foam pad soaked in conducting gel.

• The ECG signal is boosted by an amplifier which also filters out noise. The amplified ECG signal is then displayed on an oscilloscope.

Monitoring mode• This mode has a frequency response of

0.5-40Hz. • All ECG monitors use filters to narrow the

bandwidth in an attempt to reduce environmental artefacts.

• The high-frequency filters reduce distortions from muscle movement, mains current and electromagnetic interference from other equipment.

• The low-frequency filters help provide a stable baseline by reducing respiratory and body movement artefacts.

Diagnostic mode

• This mode monitors the S-T segment and there is a greater need for filtering of the signal. Thus there is a wider frequency response range of 0.05-100Hz.

• The high-frequency limit of 100Hz ensures that tracings allow assessment of QRS morphology and tachy-arrhythmias.

• The low-frequency limit allows representation of P and T-wave morphology and ST-segment analysis.

Electrode Configurations• Lead II is best for detecting arrhythmias.

CM5 detects 89% of S-T segment changes due to left ventricular ischaemia.  (Right arm electrode on manubrium, left arm electrode on V5 and indifferent lead on left shoulder).

• CB5 useful in thoracic anaesthesia. Right arm electrode over the centre of the right scapula and left arm electrode over V5.

Sources of errorElectrical interference. • This is the distortion of a biological signal

by capacitance effects or inductance effects.

• Any electrical device, powered by AC can act as one plate of a capacitor and the patient acts as the other plate. This may cause a current with AC frequency to flow in the ECG leads.

• Interference may also result from high frequency diathermy. Shielding of cables and leads, differential amplifiers and filters help reduce such interference.

• The shielding consists of woven material which is earthed.

• Interference currents are induced in the metal screen and not in the monitoring leads. The screening layer may often be covered by a second layer of insulation. Shivering can produce artefacts. Thus aim to place electrodes over bony prominences.

Differential Amplifiers• The differential amplifier measures the

difference between the potential from two different sources.

• Hence if there is interference common to the input terminals (e.g. mains frequency) it can be eliminated since it is only the difference between the terminals that is amplified by the differential amplifier.

• This is known as common mode rejection. • The ratio of the output signal amplitude to

the input signal amplitude is known as the gain of the amplifier.

MONITORING AND CLINICAL MONITORING AND CLINICAL MEASUREMENTS RELATED TO MEASUREMENTS RELATED TO

ANAESTHESIAANAESTHESIA

Spectrophotometric Analysis

Principle of shining radiation through a sample and determining quantity of radiation absorbed

There are two important laws:

Beers Law: absorption of a given thickness of sol. of given conc. is the same as twice the thickness of half conc.

Lambert’s Law: each layer of equal thickness absorbs an equal fraction of radiation which passes through it.

• The light absorbed by blood depends upon the quantities of deoxy– and oxy Hb present and the wavelength of light.

• The points where the absorbances for the two forms of Hb are identical are known as ‘Isobestic Points’ and are dependent upon Hb concentration.

The Pulse Oximeter • Two light emitting diodes, a red (660nm)

and infrared (940nm) shine through the finger and the photocell detects the transmitted light.

• The output is processed electronically to give a pulse waveform and the arterial oxygen saturation.

• Diodes are switched on in sequence with a pause where both diodes are off.

• This allows for the microprocessor to compensate for ambient light.

• The diodes are switched off hundreds of times a second– thus the processor can detect cyclical changes due to arterial blood flow. the non-pulsatile component is disregarded.

Errors and problems 

• In the following situations the pulse oximeter readings may not be accurate:

• A reduction in peripheral pulsatile blood flow produced by peripheral vasoconstriction (hypovolaemia, severe hypotension, cold, cardiac failure, some cardiac arrhythmias) or peripheral vascular disease.

• These result in an inadequate signal for analysis. 

• Venous congestion, particularly when caused by tricuspid regurgitation, may produce venous pulsations which may produce low readings with ear probes.

• Venous congestion of the limb may affect readings as can a badly positioned probe.

• When readings are lower than expected it is worth repositioning the probe.

• In general, however, if the waveform on the flow trace is good, then the reading will be accurate. 

• Bright overhead lights in theatre may cause the oximeter to be inaccurate, and the signal may be interrupted by surgical diathermy.

• Shivering may cause difficulties in picking up an adequate signal. 

• Pulse oximetry cannot distinguish between different forms of haemoglobin.

• Carboxyhaemoglobin is registered as 90% oxygenated haemoglobin and 10% desaturated haemoglobin - therefore the oximeter will overestimate the saturation.

• The presence of methaemoglobin will prevent the oximeter working accurately and the readings will tend towards 85%, regardless of the true saturation

• When methylene blue is used in surgery to the parathyroids or to treat methaemoglobinaemia a shortlived reduction in saturation estimations is registered. 

• Nail varnish may cause falsely low readings. However the units are not affected by jaundice, dark skin or anaemia.

CO2 MONITORING• The excretion of CO2 is the final common

pathway of metabolism. As such it provides a useful global indication that all is well.

• Ventilation must be sufficient to carry oxygen into the lungs oxygen is being transported to the mitochondria (cardiovascular function) aerobic metabolism is consuming this oxygen and producing carbon dioxide CO2 is being transported to the lungs (cardiovascular function) CO2 in the expired air gives an indication of adequate ventilation

Risk management

• The Dutch looked at preventable anaesthetic deaths in healthy people having minor surgery. They demonstrated on the way that there is no such thing as minor anaesthesia!

• They found the commonest problems were with the airway and that the earliest detection would be with CO2 analysis.

• Their response was to make CO2 analysis compulsory in 1980. Their work has been demonstrated again by the AIMS data.

• The college of anaesthesia in Australia is taking an interesting approach. They have recommended that a CO2 analyser is available for every intubated AND ventilated patient.

MECHANISM OF FUNCTION• A light is shone through the expired air and

the degree of absorption of a certain frequency of infra-red light is proportional to the concentration of CO2.

• The light may be split with half passing through a reference cell.

• The light may also be 'chopped' so that it is not continuously heating the gas in the reference cell.

• The analyser may be placed in one of 2 places: in-line, and out of circuit at the end of a sampling tube.

In-line analyser• Advantages/ Disadvantages• No sampling tube to block • bulky needs to be heated • windows fogging up• can't be used on non-intubated

patients

Sampling• The sampling variety has the big

disadvantage of a sampling tube which tends to get blocked with condensation.

• Some companies offer a special tube that allows water to escape. They are an expensive gimmick.

Arterial pCO2• The plateau is essential for accurate

analysis.The normal end tidal value is about 40 mm Hg or 5%.

• In the absence of significant cardiac shunts there is no significant alveolar to arterial CO2 gradient so what you are seeing is also the arterial CO2 concentration.

CO2 Trends• There are two main graphs that we look at

which are a function of the sweep speed.These are waveforms and trends.

• At slower speed we get a trend image of the peeks and troughs of expired CO2.

• The trend of end tidal CO2 is the most useful graph to watch to follow ventilation. and there are a few patterns that are diagnostic.

• In a patient with chronic airways disease the slope may be increased and the end tidal value somewhat higher.

• This is useful to record at the beginning of an anaesthetic to prevent over ventilation. It is surprising how easy it is to hyperventilate an elderly patient.

• The standard 15 ml/kg 12 breaths /Min will drop their CO2 to 25 mm Hg. This will make them slow to breath at the end of the case.

• The other real danger is the cerebral vasoconstriction caused by the low CO2. Perhaps that has been the reason that granny is sometimes 'not quite herself' after the operation.

CO2 Waveforms (Capnograms)

• There are two main graphs a function of the sweep speed: namely waveforms and trends.

• At high sweep speed we get a wave form of the CO2 from each breath which is known as the capnogram. There is only one normal shape.

Intubation• First of all if you get a

capnogram you can be sure you have intubated the trachea.

• It is said you may get three or four breaths of CO2 from the stomach.

• There are case reports of oesophageal intubation where a supervising consultant performed all the known tests to verify tube placement.

Poor plateau• Kinked tube

• Herniated cuff

• Bronchospasm ie Any obstruction that limits expiration

'Curare cleft'• Usually seen with

high CO2

• It is a diaphragmatic twitch pulling some fresh gas past the sampling tube

• Not to be confused with cardiogenic oscillations

Cardiogenic oscillations• Caused by the

beating of the heart against the lungs (c/f. helicopters and HFV)

• Said to be more readily seen as relaxant wears off and tone returns to chest and abdominal walls and diaphragm.

• It is more common in paediatrics because the heart takes up relatively more space in the chest.

Camel Capnogram

• Unequal emptying of lungs

• Lateral position

• Tube touching carina

Slow decrease in CO2

• Hyperventilation

• Fall in body temperature

• Falling lung or body perfusion

A sudden drop in end tidal CO2 to zero.

• Spontaneous breathing and ventilated patients

• Kinked ET tube

• Kinked or disconnected sampling tube

• Patient extubated

• Total anaesthetic circuit disconnect

• In a ventilated patient The ventilator has

failed

A sudden drop in end tidal CO2 but not to zero

• Leak in circuit eg deflated cuff

• Obstruction eg acute broncho-spasm.

• Leak in sampling tube drawing in room air

A sudden rise in baseline

• Stuck valve in circle absorber system

• Exhausted CO2 absorber

• Calibration error in monitor

An exponential decrease in CO2

• Circulatory arrest: cardiac or hypovolaemic

• Embolism: air or clot • Sudden severe

hyperventilation• On the other hand,

when all other monitors fail, it is comforting to see CO2 coming out. It can be seen to rise in external cardiac massage.

Gradual increase in CO2

• Hypoventilation

• Absorption of CO2 from peritoneal cavity

• Rapidly rising body temperature

Sudden increase in CO2

• Injection of sodium bicarbonate

• Release of tourniquet

• Sudden increase in blood pressure

OXYGEN MEASUREMENT• under normal conditions, the oxygen

cascade results in an interstitial PO2 between 20-40 mmHg and an intracellular PO2 ~ 20 mmHg

• mitochondrial enzyme systems are designed to function at a PO2 ~ 3 mmHg, therefore there is usually an excess of oxygen

• Hypoxia could therefore be defined as a mitochondrial PO2 < 3 mmHg in the classic study of Comroe & Botelho (1947), after 7,204 observations,

• It was found that trained observers were unable to detect any degree of cyanosis until the arterial SaO2 < 85%

• For the detection of cyanosis ~ 5 gm of reduced Hb must be present with a normal haematocrit this corresponds to a SaO2 ~ 60-70% in the presence of anaemia, the saturation must be considerably lower

Oxygen Content• Def'n: volume of oxygen, in ml, contained in 100

ml of blood at 1 atmosphere, at 37°C = volume percent

• CaO2 ~ (1.37 x [Hb] x SaO2) + (0.0034 x PaO2) ~ 20 vol%

• The ideal value for the carriage of oxygen by Hb of 1.39 ml/g is not reached in vitro.

• For the measurement of content three variables must be known, SaO2, PaO2 and [Hb]

• The mixed venous oxygen roughly reflects global tissue oxygenation

• the normal value corresponds with

• i. Cv'O2 = 12-15 vol.%

• ii. Pv'O2 = 40-46 mmHg

• iii. Sv'O2 = 72-78 %

• however, different vascular beds have different extraction ratios and the mixed venous PO2 does not reflect regional ischaemia

SaO2 is a function of PaO2 as expressed by the Hb-O2 dissociation curve three key points on this standard curve are,

• i. 90% 60 mmHg• ii. 75% 40 mmHg• iii. 50% 26.2 mmHg =

P50

The curve is displaced to the right by 4 factors,

1. increasing [H+] (decreasing pH)

2. increasing temperature

3. increasing CO2

4. increasing 2,3-DPG • it is displaced to the left

by Hb-F, metHb and CO-Hb

Oxygen Delivery - Flux• Def'n: O2 Flux = CO x CaO2 x 10 ml O2/min• The normal CO is taken from the cardiac index,

CI = CO/BSA ~ 3.0-3.4 l/min/m2 • This gives an average O2 flux ~ 640 ml/m2/min• The average BSA for a 70 kg male = 1.8 m2

CO ~ 5.75 l/min O2 flux ~ 1150 ml/min• The normal MRO2 is stable for a given

individual at rest and ranges from 115-165 ml/m2/min

• Measurement of PO2

• in 1956, Leyland Clarke developed the polarographic oxygen electrode for measuring the partial pressure of oxygen prior to this the PO2 had not been measured.

electronics & display

ammeter

+-

0.7 v

Platinumcathode

Ag/AgClanode

Thermistor

KClelectrolyte

O-ring

Plastic membrane

Blood sample

Temperaturecompensation

• The Severinghaus CO2 electrode was developed in 1958 and arterial blood gas analysis was revolutionized PO2 may also be measured by,

i. fuel cell

ii. paramagnetic analysis

iii. the optode

iv. mass spectrometry

O-ring

Plastic membrane

Blood sample

mesh

bicarbonate

H+ sensitiveglass

electrodes

• Clarke Electrode

the circuit consists of,

a. DC voltage source (0.6 V)

b. ammeterc. platinum

cathoded. silver/silver

chloride anode

e. Following reaction takes place at the platinum cathode, O2 + 2H2O + 4e- 4OH

• The current flow being in direct proportion to the consumption of oxygen

• The platinum electrode cannot be inserted directly into the blood stream as protein deposits form an affect its accuracy

Oxygen Fuel Cell• circuit consists of, a. ammeter b. gold mesh cathodec. lead anoded. compensating thermistore. electrolyte solution (KCl)

and O2-permeable membrane the same reaction takes place at the cathode, O2 + 2H2O + 4e- 4OH

• Current flow depends upon the uptake of oxygen at the cathode the reaction at the anode is as follows,

• Pb + 2(OH-) PbO + H2O + 2e unlike the Clarke electrode, the fuel cells requires no external power source,

• acting as an oxygen dependent battery like other batteries, the fuel cell will eventually expire the output is affected by temperature, as is that of the Clarke electrode,

• however compensation may be achieved by means of a parallel thermistor the typical response time is ~ 20-30 s

• Cell Reactions. Cell reactions are described as follows:

• O2 + 2H2O + 4e 4OH cathode reaction

• 2Pb + 4OH 2PbO + 2H2O +4e anode

reaction

• The overall cell reaction is as follows:

• O2 + 2Pb 2PbO

Paramagnetic Oxygen Analysis• oxygen is paramagnetic and is therefore

attracted into a magnetic field this is due to the unpaired outer shell electrons of the oxygen molecule most other gases, such as N2, are weakly diamagnetic and are repelled from a magnetic field

• actually measures oxygen concentration

• Most common systems use deflection of nitrogen containing glass spheres.

• Arranged in a dumbbell or similar these indicate either by direct rotation of a pointer or deflection of light, or may be arranged in a null deflection system.

• They require calibration before use with 100% N2 and 100% O2

• The presence of water vapour biases the result, therefore gases should be dried through silica gel before analysis.

PO2 Optode

• based on the principle of photoluminescence quenching

• when light shines on luminescent material, electrons are excited to higher energy states and on their return emit light at characteristic wavelengths

• Measurement of Hb-Saturation

• CaO2 was originally measured volumetrically by the method of Van Slyke and Neill oxygen saturation is defined as the CaO2 / the oxygen capacity, expressed as a percentage this includes contributions from Hb-O2 and dissolved O2.

• Normal adult blood contains four species of Hb,

1. O2-Hb

2. Hb

3. met-Hb

4. CO-Hb• the later two are normally found only in low

concentrations, except in disease states, and are ineffective in the transport of oxygen .

• Functional SaO2 = O2-Hb x 100 %

Measurement of pH

• pH is defined as the negative logarithm to the base 10 of the hydrogen ion activity (~ []'n) at 37°C, the normal blood pH = 7.4 ± 0.04 the circuit consists of,

a. a capillary tube of pH sensitive glass V

b. a reference buffer solution the other side of the glass + a silver/silver chloride electrode

c. an electrolyte solution (KCl) in contact with blood + mercury/mercury chloride electrode

d. a surrounding water jacket at 37°C

e. a voltmeter

The electrodes are metal/metal chloride, which are then in contact with electrolyte containing Clto maintain their stability

• the pH difference across the glass produces a potential in proportion to the [H+] difference

• temperature control is important as acids/bases dissociate at higher temperatures altering the pH

• This is described approximately by the formula by Rosenthal, pH ~ T°C x -0.015

• Before use pH meters should be calibrated with two buffer solutions.

Measurement of PCO2

• the normal PaCO2 = 40 mmHg (5.3 kPa) measurements are based on pH, due to the dissociation of carbonic acid the PCO2 is therefore related to the [H+] the Severinghaus CO2 Electrode provides a direct measure of PCO2 from the change is pH.

The circuit consists of,

a. a closed cylinder of pH sensitive glass in the centre

b. 2 electrodes, one inside, the other outside the cylinder

c. a surrounding solution of sodium bicarbonate

d. a thin film of bicarbonate impregnated nylon mesh covering the end of the cylinder

e. a thin, CO2 permeable membrane covering the end of the electrode at the end of the electrode CO2 diffuses from the blood sample through the membrane into the nylon mesh and by the formation of carbonic acid lowers the pH of the bicarbonate solution this change in pH alters the V across the glass

O-ring

Plastic membrane

Blood sample

mesh

bicarbonate

H+ sensitiveglass

electrodes

NEUROMUSCULAR MONITORING

Measurement Of Temperature- • Non-electrical

a. mercury thermometers accurate, reliable, cheap readily made in maximum reading form

• easily made into a thermostat low coefficient of expansion and requires 2-3 mins to reach thermal equilibrium

• unsuitable for insertion in certain orifices

Measurement - Electrical

a. resistance thermometer

• electrical resistance of a metal increases linearly with temperature

• frequently use a platinum wire resistor, or similar

• accuracy improved by incorporation in a Wheatstone bridge

• calibration may be changed by exposure to severe temperatures, eg. sterilization

b. alcohol thermometers cheaper than mercury useful for very low temperatures, mercury solid at -39°C unsuitable for high temperatures as alcohol boils at 78.5°C expansion also tends to be less linear than mercury

• c. bimetallic strips• d. Bourdon guage pressure

b. Thermistor made from a small bead of metal oxide unlike normal metals,

• the resistance falls exponentially with temperature

c. Thermocouple based on the Seebeck effect at the junction of two dissimilar metals a small voltage is produced,

• the magnitude of which is determined by the temperature metals such as copper and constantan (Cu+Ni) requires a constant reference temperature at the second junction of the electrical circuit may be made exceeding small and introduced almost anywhere

Body Temperature• Humans, like all mammals and birds are

homeothermic and control their body temperature within a narrow range = 37 ± 0.5 °C

• normal circadian rhythm varies temperature by 0.4 °C, being lowest in the early am. and highest in the evening

a. central core ~ 37 °C b. intermediate zone c. shell ~ 2.5 cm ~ 32-53 °C

Heat Production• In the average male under resting conditions ~ 50

W.m-2, or 80 W• Total increases of the BMR occur after food,

with exercise etc. also, the BMR rises when there is an increase in the core temperature

• There is no mechanism for a reduction in heat production to compensate for overheating increased

• Heat production can be achieved by shivering and voluntary muscular activity

Heat Loss• there are four routes of heat loss from the

body,

a. radiation ~ 40%

b. convection ~ 30%

c. evaporation ~ 20%

d. respiration ~ 10% - humidification 8% - heating of air 2%

• Conduction is not an important means of heat loss in humans as gases are poor conductors.

• Radiation is predominantly in the infrared spectrum and is determined by the temperature difference between the body and surrounding objects

• the amount of heat loss by evaporation may be increased up to 10 fold by sweating all of these mechanisms depend upon the surface area of skin exposed to the environment thus, if this area is reduced heat loss is minimized

Specific Heat Capacity

• the heat required to raise the temperature of 1 kg of a substance by 1 K (J/kg/K)

i. water SHC = 4.18 kJ/kg/K or, 1 kcal/kg/K

ii. blood SHC = 3.6 kJ/kg/K

• infusion of 2000 ml of blood at 5°C, requiring warming to 35°C, would therefore require, 2 kg x 3.6 kJ/kg/°C x (35-5)°C = 216 kJ this would result in the person's temperature falling by ~ 1°C

Specific Latent Heat

• the heat required to convert 1 kg of a substance from one phase to another at a given temperature= latent heat of vapourization

• therefore, the lower the temperature the greater the latent heat required as temperature rises, the latent heat falls until ultimately it reaches zero at a point which corresponds with the critical temperature

Latent Heat In Anaesthesia• Vaporisation of ethyl chloride skin

cooling and local anaesthesia• Vaporisation of volatile anaesthetics results

in cooling & lowering of saturated vapour pressure.

• Compensatory mechanisms are then required to ensure a constant vapour pressure

• Rapid emptying of a N2O cylinder results in cooling and a steady decrease in the cylinder pressure this returns to 52 bar if the cylinder is closed and allowed to reheat.

• Carbon dioxide and cyclopropane are also stored as liquids but the rate of use is too slow to significantly reduce the liquid temperature.

• Liquid oxygen is stored in containers at about -160°C as its critical temperature is -119°C.

• The pressure inside the vessel is set at ~ 7 bar which is the vapour pressure of oxygen at -160°C.

• This is then passed through a superheating coil and regulated to a pipeline pressure of ~ 4.1 bar.

HUMIDFICATION• Absolute Humidity• the mass of water vapour (g) present in a given

volume of air (m3), numerically = mg/l• Relative Humidity• the ratio of the mass of water vapour in a given

volume of air to the mass required to fully saturate that volume of air at a given temperature (%)

• NB: fully saturated air at 20°C contains ~ 17 mg/l 37°C contains ~ 44 mg/l although relative humidity is expressed in terms of mass, as mass is directly proportional to the number of moles present, then by the ideal gas equation it becomes evident that,

• Relative humidity = actual vapour pressure /saturated vapour pressure

Measurement of Humidity1. Hair Hygrometer• based on the principle that hair elongates as the

humidity rises• very simple and cheap• only really accurate over the range 30-90%

2. Wet & Dry Bulb Hygrometer• the temperature of the wet bulb is reduced due

to evaporation• the lower the humidity the greater the

evaporative cooling and the greater the temperature difference tables relating T to % humidity air must be flowing over the wet bulb to prevent a local rise in the humidity

3. Regnault's Hygrometer• uses the principle that condensation occurs when

the air is fully saturated at a given temperature = the dew point

• air is blown through a silver test tube containing ether, reducing the temperature by evaporation

• the dew point is noted and from tables both the relative and absolute humidity can be established,

• Relative humidity = s.v.p. at dew point /s.v.p. at ambient temp.

4. Other Methodsi. electrical transducers - both resistance &

capacitance

ii. mass spectrometry

iii. UV absorption spectroscopy

Types of Nebulizersa. cold water bubble through

b. condenser

c. hot water bath

d. heated Bernoulli nebulizer and anvil

e. ultrasonic nebulizer

NB: * these are in order of increasing efficiency

GAS CHROMATOGRAPHY

• Chromatography is now used as a general term for analytical procedures that separate a mixture into its components

• As the mixture passes through a column the system has a stationary phase and a mobile phase for gaseous mixtures,

• the stationary phase of the column is frequently a material such as fine silica-alumina coated with polyethylene glycol or silicone oil

• through this column a flow of carrier gas is passed, such as argon or helium sample gases are then entered into the stream, and the speed with which they pass through the column is determined by their differential solubility between the two phases as solubility is temperature dependent,

• the apparatus is maintained at a constant temperature this system is often termed gas liquid chromatography

• as the gases leave the column they pass through some form of detector, which may be either a,

a. flame ionization detector - organic vapourb. thermal conductivity detector - inorganic

vapourc. electron capture detector - halogenated

vapours

• NB: ** none of these detectors allows absolute identification of the component gasses, and some knowledge of the substituents is necessary prior to analysis

• the time between entry of the sample and the appearance of the component is the retention time

• most samples will have numerous peaks with varying retention times with appropriate calibration the area of a peak can be used to calculate the quantity of the gas present in the mixture

• if the portal of entry of the sample is heated then injected liquids will be vapourized and these can also be analyzed

Clinical Uses - Gas Chromatography

a. volatile anaesthetic agents

b. barbiturates

c. benzodiazepines

d. phenothiazines

e. steroids

f. catecholamines• It is useful for measuring very low

concentrations of either gases or liquids• Continuous analysis is not possible and some

knowledge of the sample must be available

MASS SPECTROMETER

• the sample is passed through a molecular leak into an ionizing chamber

• the ionized particles are then accelerated and focussed into a beam

• which directed though a strong magnetic field depending upon their charge/mass ratio,

• different molecules describe different arcs of travel these separated beams are then detected depending upon their position by varying the accelerating voltage,

Ultra-Violet Analysis• halothane absorb light in the UV spectrum

therefore the concentration of halothane may be measured in accordance with Beer's law,

• for end-tidal CO2 a reference is obtained with a beam splitter and a second chamber the sample and reference cells have quartz windows as glass absorbs UV light

Piezoelectric Gas Analysis • “Emma” a quartz crystal is coated with oil • Gasses are absorbed into the oil in proportion

to their gas:oil partition coefficients and in accordance with Henry's law.

• The presence of the gas alters the resonant frequency of the crystal which can be measured.

• Electronically these analyzers are not agent specific and will respond partially to water vapour.

MEASUREMENT OF CARDIAC OUTPUTDEFINITION• The volume of blood pumped by the heart

per minute is called the cardiac output (CO) and is the product of the stroke volume (SV) and the heart rate (HR).

• At rest in the average adult the cardiac output is 5l/min and in exercise it may rise to 35l/min.

• The CO is often corrected for body surface area (cardiac index=2.5-4.2l/min/m2).

• The CO is pivotal in maintaining arterial BP (BP=CO X SVR) and oxygen delivery.

Methods of measuring CO

1) Invasive

a) The Fick principle

Fick in the 19th century realised that the following relationship is true:- Q = M / (V - A)

• Where Q is the volume of blood flowing through an organ in a minute

• M the number of moles of a substance added to the blood by an organ in one minute

• V and A are the venous and arterial concentrations of that substance.

• This principle can be used to measure the blood flow through any organ that adds substances to, or removes substances from, the blood.

• The heart does not do either of these but the cardiac output equals the pulmonary blood flow and the lungs add oxygen to the blood and remove carbon dioxide from it.

2. Thermodilution:

• 5-10ml cold saline injected through port of a pulmonary artery catheter.

• Temperature changes are measured by a distal thermistor.

• Plot of temperature change against time gives a similar curve to the dye curve (but without the second peak).

• Calculation of Co is achieved using the Stewart-Hamilton equation.

 

2) Non-Invasivea) Doppler techniques• These machines transmit an ultrasonic

vibration into the body and record the change in frequency of the signal that is reflected off the red blood cells

• so Doppler techniques measure velocity, not flow;

• the flow could be obtained by integrating the signal over the cross sectional area of the vessel.

b) Transoesophageal echo (TOE) can measure the length of blood in the ascending aorta in unit time.

• This is multiplied by the cross-sectional area of the aorta to give SV.

• Consequently, the results of these devices should be assessed critically.

Limitations:• The main disadvantages of the method

are that a skilled operator is needed • the probe is large and therefore heavy

sedation or anaesthesia is needed, • the equipment is very expensive and • the probe cannot be fixed so as to give

continuous cardiac output readings without an expert user being present.

c) Transthoracic impedance• Can be measured across externally

applied electrodes. Impedance changes with the cardiac cycle (changes in blood volume).

• The rate of change of impedance is a reflection of CO.

• Thought to be useful in estimating changes but not for absolute measurements.

Limitations:• Contraction of the heart produces a

cyclical change in transthoracic impedance of about 0.5%,

• unfortunately giving a rather low signal to noise ratio.

b) Dilution techniques

1. Dye dilution: A known amount of dye is injected into the PA, and its concentration is measured peripherally.

• Indocyanine green is suitable due to its low toxicity and short half-life.

• A curve is achieved which is replotted semi-logarithmically to correct for recirculation of the dye.

• CO is calculated from the injected dose, the area under the curve (AUC) and its duration. (Short duration indicates high CO).

What is represented in this Figure?

Central Venous Pressure Monitoring

• CVP is an index of the circulating blood volume and preload to the right ventricle

• the pulsatile characteristics of the CVP are a function of the uninterrupted return of venous blood to the right atrium, right atrial size and compliance, intrathoracic pressure, and the mechanical properties of the tricuspid valve and right ventricle .  

Pulmonary Floatation Catheter and Monitoring

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