Physics

INTRODUCTION
For the safe & efficient use of anaesthetic apparatus, the anaesthetist must have a clear concept of the physical aspects of the equipment in use.
Understanding of basic concepts may avert unnecessary accidents & near misses.

INTRODUCTION
Physics is the world in measurable terms and the physical laws apply to all states of matter (i.e. solids, liquids and gases).
As anaesthesiolgists we deal with liquids & gases under pressure at varying temperatures and volumes.
These inter-relationships are simple, measurable and their understanding ensures a safe outcome for the patient.

UNITS OF MEASUREMENT
Base SI units
- length (meter)
- mass (kilogram)
- time (second)
- current (ampere)
- temp (kelvin)
- luminous intensity (candela)
- amount of substance (mole)

UNITS OF MEASUREMENT
DERIVED UNITS
- temp in degrees celcius
- force (newton) - pressure (pascal)
- pressure (bar)
- energy (electron volt)
- power (watt)
- frequency (hertz)
- volume ( liter)

UNITS OF MEASUREMENT
UNITS NOT IN THE SI SYSTEM
- pressure (mmHg)
- pressure (cmh2o)
- pressure (std atmosphere)
- energy (calorie)
- force (kilogram weight)

UNITS OF MEASUREMENT
- 1 kilopascal = 7.5mmHg.
- 1 Bar = 750mmHg
- 1 kilopascal = 10.2cmH2O
- 1 std atmosphere = 101.325kPa
- 1 calorie = 4.18 J
- 1 kilogram weight = 9.81N
- Pounds / in2(PSI) -Atmospheric Pressure PATM=14.7 PSI)

PRESSURE
Force = mass x acceleration
= kgms -2 = Newton
Pressure = Force/Area
1 Pascal = I Newton acting over 1m2

PRESSURE
I Bar = 100kPa = Atmospheric pressure at sea level

PRESSURE
Normal thumb pressure on a syringe = 25N
2 ml syringe has an area of 5x10-5
Pressure is 500kPa – extravascular infusion easy

PRESSURE
With a 20 ml syringe, the pressure exerted is 100kPA = 6X SBP of 120 mmHg (16Kpa)
IVRA – rapid injection – pressure can exceed SBP or cuff pressure – decreased protection

PRESSURE
Bed Sores --- 20kg of patient mass supported on an area of contact of 100cm2
Force = 196N ( 20kg x 9.81 )
Pressure = 19.6 kPa
Normal SBP = 16kPa --- Risk of Ischemia

PRESSURE
Pressure relief valves and expiratory valves
Pressure in the circuit exerts a force on the diaphragm. If this force is greater that the force exerted by the valve, air escapes through the exp valve. They are typically low pressure valves(50Pa)

GAUGE AND ABSOLUTE PRESSURE
Full oxygen cylinder has a gauge pressure of 137 bar
Empty cylinder still has oxygen at atmospheric pressure
Absolute Pressure = 138 bar

GAUGE PRESSURE
Absolute P = Gauge P + Atmospheric P
Most times we ignore atmospheric P
Thus, ventilator pressures, gas cylinder pressures and arterial blood pressures are all gauge pressures

PRESSURE
For ideal gases (air, nitrogen, oxygen)-Full cylinder pressure = 2000 PSI -Full cylinder volume= 660 liters
1000 PSI --> 330 L500 PSI-->165 L
Volume remaining is proportional to pressure

FLUID FLOW
Flow = quantity of fluid/gas passing a point in unit time
Can be turbulent or laminar

LAMINAR FLOW
Flow moves in a steady manner with no eddies or turbulence
Flow is greatest in the centre
Zero flow at the wall
P/F = constant known as the resistance of the apparatus or tube
Hagen- Poiseuille Equation

LAMINAR FLOW
Flow = ∏Pd4/128ųl
P = Pressure
d = Diameter
Ų = viscosity
L = length

THE ANESTHESIA MACHINE
The resistance to flow is a function of the viscosity of the gas, the length of the pipe and the radius of the pipe to the fourth power
Called Poiseuille’s Law
A 7.0 ETT has almost twice the resistance of an 8.0 ETT

THE ANESTHESIA MACHINE

TURBULENT FLOW
Swirls or eddies present
Resistance is higher than laminar flow
Reynold’s Number = vpd/µ (velocity x pressure x density)/viscosity
Re Number > 2000 = Turbulent Flow
ET connector/ Kinked ETT
Use of Helium reduces the density

TURBULENT FLOW
Most important property is density which is mass/volume

CRITICAL FLOW
Critical flow for a typical anesthetic gas has approx the same numerical value as the diameter of the airway concerned
9mm ETT has a critical flow of 9L/min
Above 9L/min = turbulent flow

CRITICAL FLOW
Air has a lower density than Nitrous Oxide – laminar flow prevails
Air flow through the smaller airways is slower – laminar flow predominates
Corrugated surfaces induces turbulence at low flow rates

CRITICAL FLOW
Although the bronchi and smaller air passages are narrower than the trachea, the air flow through them is slower.
Laminar flow is usual in the LRT

TENSION
Tension is a tangential force in Nm acting on a length of wall
A balance must be present between the pressure caused by the smooth muscle and elastic tissue and the fluid pressure in the tube to prevent the tube progressively distending or collapsing

TENSION
A fall in pressure in an arteriole tends to distend it less and so would reduce its radius but the smooth muscle in the wall maintains tension.
Ratio of tension to radius is increased and pressure across the wall is raised (La Place’s law)

SURFACE TENSION
Pressure = 2T/R ( wall of a sphere)
Surfactant decreases surface tension lining the alveoli – makes surface tension variable
Tension decreases as the alveoli contract and increase as the alveoli distend

SURFACE TENSION
On the surface of a liquid, some of the forces of attraction between molecules act in a direction parallel to the surface
Also forces between molecules and the walls – results in a meniscus
Water – concave meniscus
Mercury – convex meniscus

THE ANESTHESIA MACHINE
Tension in the wall of the bag equals Pressure x Radius x ½ ( T = PxR /2 )
For a cylindrical structure such as an artery, the wall tension = PxR

BERNOULLI PRINCIPLE
Fall in pressure at a narrowing of a tube
Gas/Fluid has potential energy in the form of its pressure and kinetic energy associated with its flow
At the narrowing – increase in fluid velocity – increase in kinetic energy

BERNOULLI PRINCIPLE
Therefore decrease in potential energy
If this pressure falls below atmospheric pressure, can entrain gas/fluid via the side hole at the constriction
Example is a nebulizer and the oxygen mask

THE GAS LAWS
Boyles Law
Charles Law
Third Perfect Gas Law
Dalton’s Law of Partial Pressures
Universal Gas Constant

BOYLES LAW
At constant temp, V ∞ 1/P
How much oxygen is available at atmospheric pressure in a tank?
Internal capacity of cylinder = 10L
Absolute Pressure = 138 bar
Therefore, volume = 1380L
P1V1 = P2V2
13800kPa x 10L = 100kPa x 1380L

CHARLES LAW
At constant pressure, V∞Temp
Gases expand when heated

THIRD PERFECT GAS LAW
At constant volume, P ∞ Temp
STP – 273.15K and 101.325kPa

ADIABATIC CHANGE
The three gas laws describe the behaviour of a gas when one of the three variables (pressure, temp or volume) is constant.
For these conditions to apply, heat energy is required to be added or be taken from a gas
The state of a gas can also be altered without allowing the gas to exchange heat energy with its surroundings

ADIABATIC CHANGE
The state of a gas can be altered without allowing the gas to exchange heat energy with its surroundings
An example is the use of the cryoprobe
If a gas cylinder connected to an anesthetic machine is turned on quickly, the pressure of gas in the connecting pipes and gauges rises rapidly

ADIABATIC CHANGE
Thus, the gas is compressed adiabatically and a large temp rise with the associated risk of fire can occur

DALTON’S LAW OF PARTIAL PRESSURES
In a mixture of gases, the pressure exerted by each gas is the same as that which it would exert if it alone occupied the container

AVOGADRO’S NUMBER
States that equal volumes of gases at the same temp and pressure contain equal number of molecules
Avogadro’s number = 6.022 x 1023
One mole of any gas occupies 22.4L at STP

BREATHING SYSTEMS
Breathing Circuitsa)Open (non-rebreathing) •Simple face mask or nasal cannula (CO2 diffuses away from the face) •Bag-Valve-Mask system (Ambu®): uses 3 valves to allow either spontaneous or controlledventilation while preventing rebreathing b)Semi-Open (Mapleson / Bain)•Most efficient removal of CO2 for a given gas flow when the "pop off" valve is nearest the source of the ventilatory power -Spontaneous ventilation: Mapleson A -Controlled ventilation: Mapleson D•However, the "A" system is very inefficient (requires high gas flows) to prevent rebreathing during controlled ventilation, while the "D" system is reasonably efficient for both controlled and spontaneous ventilation, so the "D" is preferred for most applications. -Bain circuit is a coaxial Mapleson Dc)Semi Closed Circle System •Patient gas uptake < fresh gas flow < minute ventilation •Some rebreathing of exhaled gas (following removal of CO2 by absorber)d)Closed System •Gas inflow = Patient Uptake •If using sidestream agent / CO2 analyzer, must route exhaust back into circuit •Starting values-O2: 3-4 ml/kg/min

AVOGADRO’S NUMBER
Typical Nitrous cylinder has 3.4kg of Nitrous Oxide
Molec wt = 44 ( 1 mole)
1 mole occupies 22.4L at STP
3400g occupies 22.4 x 3400/44 = 1730L

UNIVERSAL GAS CONSTANT
PV = nRT
In a cylinder, the volume and temp is constant
Therefore, P is ∞ n
Implies that the pressure gauge acts as a contents gauge if the cylinder contains a gas

CRITICAL TEMP
Defined as the temp above which a substance cannot be liquefied however much pressure is applied
Critical Pressure is the vapour pressure at the critical temp
Critical temp for nitrous is 36.5 degrees – it is a gas if the temp is above 36.5

CRITICAL TEMP
Critical temp for oxygen is -119 degrees
Impossible to turn oxygen into its liquid form at room temp

SOLUBILITY
When a liquid is placed in a closed container, an equilibrium is eventually established at the surface between the vapour of the liquid and the liquid itself.
In this equilibrium state, the partial pressure exerted by the vapour is known as saturated vapour pressure

SOLUBILITY
Saturated Vapour Pressure
Henry’s Law – states that at a particular temp, the amt of a given gas dissolved in a given liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid
As a liquid is warmed, less gas dissolves in it --- may see bubbles ( Blood warmer)

SOLUBILITY
The effect of high pressure on the solubility of nitrogen is particularly relevant to deep sea divers as nitrogen if breathed under pressure passes into solution in the tissues
If a return to atmospheric pressure is made too rapidly, the nitrogen comes out of solution as small bubbles in the joints and elsewhere

SOLUBILITY
Ostwald Solubility Coefficient is the volume of gas which dissolves in one unit volume of the liquid at the temp concerned
Independent of pressure

SOLUBILITY
Partition Coefficient is defined as the ratio of the amount of substance present in one phase compared with another, the 2 phases being of equal volume and in equilibrium

SOLUBILITY
Ether has the highest Ostwald Solubility Coefficient (12). Halothane is 2.3 and Nitrous is 0.47
Ether carried away more rapidly from the lungs – conc of ether builds up more slowly in the alveoli --- slower induction of anesthesia

SOLUBILITY
Second Gas effect
Diffusion hypoxia

SECOND GAS EFFECT
During the inspiration of a gas mixture containing nitrous oxide, the N2O is absorbed into the bloodstream faster than the oxygen or nitrogen.
So at peak inspiration, when the pressure in the alveoli has equalized with the ambient pressure, there is a net surplus of oxygen and N2 molecules

DIFFUSION HYPOXIA
At the end of an anesthetic using N2O, the N2O diffuses faster into the alveoli diluting the gases there ---- leads to a fall in oxygen concentration

SOLUBILITY
Fat is an impt constituent of tissue
Oil is therefore used for measurements
Agents with the highest oil solubility have the greatest potency
Halothane = 224
Nitrous Oxide = 1.4
Sevoflurane = 55
Desflurane = 18.7

SOLUBILITY
High solubility = lower MAC values
Anesthetics tend to interfere with the molecular configuration of the long fatty acid at a critical point within the neurones
Attachment to the chain is loose and readily reversible with Van der Waals forces

SOLUBILITY
Also attach to the long carbon chain molecules present in rubber and plastics

DIFFUSION AND OSMOSIS
Diffusion is the process by which the molecules of a substance transfer through a layer such as the surface of a solution
What is more likely to happen if diffusing capacity in the lungs are decreased?

DIFFUSION
Pulmonary Diffusing Capacity
Rate at which CO leaves the alveoli is dependent on the rate of diffusion through the membrane and not on pulmonary blood flow
Sarcoidosis, Asbestosis

EFFECT OF MOLECULAR SIZE
Grahams Law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight
Example – Injection of local anesthetics – inject as close as possible to the nerve because diffusion only allows limited penetration of the LA into the tissues

OSMOLARITY
Is the sum total of the molarities of the solutes in a solution
RL has an osmolarity of 278mosm/l (Na 131, K 5, Cl 111, Ca 2, Lactate 29
Plasma has an osmolarity of 300 >99% due to Na, Cl, HCO3. Plasma proteins account for 1 mosm/l

OSMOLARITY
If a patient is transfused hypotonic fluids – get changes in the osmotic pressure gradient across cell membranes – fluid diffuses into cells.
Albumin and globulin give rise to an oncotic pressure of 26mmHg
Decreased oncotic pressure – decreased gradient at the venous end - edema

OSMOLARITY
Number of osmoles per kg of water or clear solution
Avoids the effect of temp which affects volume

HEAT CAPACITY
Specific Heat Capacity is defined as the amount of heat required to raise the temp of 1kg of a substance by 1 kelvin. J per kg per kelvin
Heat Capacity is the amount of heat required to raise the temp of a given object by 1 kelvin.

HEAT CAPACITY
Body temp = 36 degrees
Shivers – increases heat production 4 fold to 320W. Basal level of heat production is 80 W
An extra 240W = 14.4 kJ/min
245kJ needed to increase temp by 1 degree . (total heat capacity = 3.5 x 70kg)

HEAT CAPACITY
This patient will need to shiver for 17 minutes to produce the heat required to do this

HEAT CAPACITY
Specific Heat Capacity of blood = 3.6kJ/kg/C
Transfuse 2L of blood at 5 degrees
Warmed to 35 degrees in the patient

HEAT CAPACITY
Heat Required = 216kJ (2x3.6x30)
Heat Capacity of 70kg person = 245kJ/C
Therefore temp must fall by 1 degree

THE ANESTHESIA MACHINE

THE ANESTHESIA MACHINE
N2O is stored in the tank as a liquid in equilibration with the N2O gas above it.
As we draw N2O gas from the tank, it is replaced with gas that boils off from the liquid below it.
The heat required for this phase transition is drawn from the cylinder which draws it from the air around the tank
Results in the cylinder cooling

THE ANESTHESIA MACHINE
Why does the pressure go down as the gas cools?

THE ANESTHESIA MACHINE
The pressure in the tank reflects the force of the molecules bouncing off every part of the wall tank.
Pressure = Force of the molecules as they bounce off the surface divided by the surface area of the tank.
Force of each molecule reflects the thermal energy of the molecule.

THE ANESTHESIA MACHINE
Thermal energy is taken out of the nitrous tank by heat of vaporization.
Therefore, the collision between each molecule and the wall of the tank is less energetic ---- therefore the pressure drops
Gay- Lussac’s Law – pressure is inversely related to temperature when volume is constant

THE ANESTHESIA MACHINE
As oxygen is drawn from the tank, both the temp and pressure drop over time.
The pressure drops because we are removing gas from the tank
The temp drops because the released molecules take their thermal energy with them – also contributes to the loss of pressure

CIRCULATION
Ohm’s Law
Pressure = Flow x Resistance
Voltage = Current x Resistance
Resistance = Pressure/Flow

CIRCULATION
SVR = (MAP – CVP)/CO
Poiseulle’s equation – Resistance to flow is proportional to 1/r4
Arterial blood pressure is measured
- by the auscultatory method
- by the oscillometric method
- invasively

CIRCULATION
P = 2T/R
Failing heart – Increase in R and therefore a decrease in P. Unable to increase T
Normal Heart – Increase in R due to increased VR. Also get an increase in T (Frank Starling). Therefore no change in P

CIRCULATION
MAP dependent on SVR and CO
Patient with a decreased SVR, a high BP indicates an increased CO
In a patient with an increased SVR, a high BP indicates a decreased CO

AUSCULTATORY METHOD
Based on the Korotkoff sounds
The systolic and diastolic pressures are determined and the mean is calculated
MAP = DBP +1/3PP
Not readily calibrated

OSCILLOMETRIC METHOD
Based on pressure waveform in an air filled cuff coupled to the arterial pulse
Primarily determines MAP which is the point of maximum oscillation
Systolic and diastolic is inferred from the MAP

INVASIVE MONITORING
Transducer is a strain gauge that linearly converts pressure to electrical resistance
The monitor measures the electrical resistance and calculates the corresponding pressure
Compensate for atmospheric pressure by exposing the back side of the strain gauge to air

INVASIVE MONITORING
Strain Gauge – movements of the diaphragm alter the tension in the resistance wire – changes resistance – changes current flow – amplified and displayed on an oscilloscope

INVASIVE MONITORING
Wheatstone Bridge
4 resistors, a source and a galvanometer
Variable resistor can be zeroed – adjusted until there is a null deflection on the galvanometer
Strain gauge resistor

INVASIVE MONITORING
What does it mean to “zero” the transducer?

INVASIVE MONITORING
The act of zeroing the transducer tells the monitor the electrical resistance that should be considered zero pressure
What you are correcting for is
- minor imperfections in the calibration of the strain gauge
- the column of water between the left atrium and the transducer

INVASIVE MONITORING
Column of water between the point that is opened to air and the transducer itself
Force that this water exerts on the strain gauge is subtracted from the subsequent force measured by the transducer
Establishes a net pressure in the blood vessel relative to this point

CARDIAC OUTPUT
Gold standard for measuring cardiac output is by applying Fick’s Law to oxygen flow
Net flow of oxygen into blood as it courses through the lungs = 200mls/min
Net flow of oxygen out of the lungs = cardiac output x (Arterial oxygen content – Mixed venous oxygen content)

PULSE OXIMETRY
Beer Lambert Law
Absorption of light = Concentration x Thickness x extinction coefficient
Has two diodes
At 660nm, little absorption by oxyhemoglobin
At 940nm, little absorption by deoxyhemoglobin

PULSE OXIMETRY
Beer’s Law states that the absorption of radiation by a given thickness of a solution of a given concentration is the same as that of twice the thickness of a solution of half the concentration

PULSE OXIMETRY
Lambert’s Law states that each layer of equal thickness absorbs an equal fraction of radiation which passes through it

PULSE OXIMETRY
The diodes alternate at about 100 times a second between 660nm, 940nm and off
A single photocell on the opposite side of te tissue records the transmitted signal and sends it to the microprocessor

PULSE OXIMETRY
Two parts to the waveform
- static component which represents the absorption of the tissue, venous blood, nail polish, etc
- oscillating component which represents the absorption by arterial blood

PULSE OXIMETRY
On the assumption that the tissue thickness is the same for both oxyHb and deoxyHb, the microprocessor solves two simultaneous equations for the relative concentrations of oxyHb and deoxyHb

PULSE OXIMETRY

PULSE OXIMETRY
Unable to distinguish more that two types of Hb
Cannot identify carboxyHb --- to the pulse oximeter, it appears to be 90% oxyHb, 10% deoxyHb
MetHb is interpreted as 85% oxyHb and 15% deoxyHb
A proper blood gas machine has 9 to 13 wavelengths to distinguish multiple Hb moieties

ELECTRICAL SAFETY
Two major hazards
- burns
- arrhythmias

ELECTRICAL SAFETY
Three types of electrical current
- macroshock
- microshock
- radiofrequency currents

ELECTRICAL SAFETY
A power station supplies electricity at very high voltage to a substation where the voltage is reduced by a transformer.
Current the passes to the hospital along 2 wires – live and neutral. The neutral wire is connected to earth at the substation
Mains electric sockets in the hospital provide connections to the live and neutral conductors and also to a third conductor

ELECTRICAL SAFETY
Third conductor is connected to earth at the hospital.
If a person touches a live wire at the hospital, an electric current can be completed through the body, through the earth and back to the substation

ELECTRICAL SAFETY
1 mA = tingling sensation on touching the live parts of the apparatus
Current which flow through the anesthetist depends on the impedence presented to this flow
Impedence of antistatic footwear and the floor is about 240kohms
Current is therefore less than 1mA

ELECTRICAL SAFETY
If you are wearing non standard footwear and standing in a pool of saline on the floor while in contact with faulty equipment, a higher current flows.
A current of greater than 1mA flows. A current of 24 mA would result in being unable to release your hand from the equipment.

ELECTRICAL SAFETY
Most of the impedence now occurs at the points of contact with the skin and the feet with the shoes. May be around 5kohms
Current = 120 volts/ 5000ohms x1000
= 24mA

ELECTRICAL SAFETY
Risk of VF
Risk is much greater if the current passes through the heart during repolarization – early T wave of the EKG
Mains alternating current of 50 Hz is more dangerous than high frequency current of 1kHz or greater
Underlying myocardial disease

CLASS 1 EQUIPMENT
Any conducting part that is accessible to the user, such as the metal case of an instrument, is connected to an earth wire which becomes the third wire connected via the plug to the mains supply socket
If a fault occurs, a high current flows which melts a protecting fuse and disconnects the circuit

CLASS 2
Double insulated equipment
All accessible parts are protected by 2 layers of insulation or reinforced insulation
An earth wire is not required

CLASS 3
Internally powered equipment
Has its own power source located within the equipment
Although the risk of electric shock may still be present, the particular risks associated with mains electricity are avoided

ISOLATED PATIENT CIRCUITS
Some equipment requires electrical connections be made to the patient (monitors)
A deliberate attempt is made to reduce the impedance at the junction between the electrode and the skin
Decreases the protection that the skin might otherwise offer

ISOLATED PATIENT CIRCUITS
To counteract this, use an isolated patient circuit or a floating circuit
The electrical circuit is divided into two parts – a mains part which contains a power supply driven directly by the mains and an isolated part which is separated from the mains part by an electrical barrier

ISOLATED PATIENT CIRCUIT
Intended to provide protection should a fault develop in the mains part and to reduce flow of mains leakage currents in the patient circuit.

LEAKAGE CURRENT STANDARDS
Electromedical equipment is classified according to the maximum leakage current permissible for particular applications
CF – electrodes which may contact the heart directly. Indicates cardiac use and a floating circuit. Leakage current should be less than 50uA

LEAKAGE CURRENT STANDARDS
B or BF it it has a floating circuit. Leakage current of 500uA
All new equipment in a hospital is subjected to an acceptance test which will verify leakage currents.

ELECTRICAL SAFETY
Electrical currents flow in circuits
A path must exist from the electrical source to the patient and another path from the patient back to the electrical source for a shock hazard to exist
Ohm’s Law I = V/R
Current density is the amount of current flowing per unit area

ELECTRICAL SAFETY
Standardized voltage is about 120V
The “120” is the root mean square voltage
Alternating current cycles at 60 Hz
Average voltage is therefore 0
If one squares all the voltages and the takes the average, the result is 120V
The peak voltage is about 150V – is the potential driving energy

ROOT MEAN SQUARE
If all the values of the sine wave are squared, all the amplitudes are converted to positive numbers
By taking the mean of this, a value is obtained which is related to the amplitude of the wave
Square root of this figure is the equivalent DC value

MACROSHOCK
Potential for both burns and arrhythmias
Current must flow through the thorax
In the thorax it is split between the chest wall and the great vessels – delivers the current density to the myocardium.

MACROSHOCK- FACTORS FOR ELECTROCUTION
Patient unclothed and wet
Patient is on a large metal table, frequently electrically operated
Patient is surrounded by electrical devices. These electrical devices are exposed to spilled fluids and operator abuses
Anesthetized patient is unable to respond or withdraw from an electric shock

MACROSHOCK
How much current can we deliver to the anesthetized patient?

MACROSHOCK
Patient may receive 150 volts with direct contact
The current he receives will depend on the resistance to flow ( I = V/R )
Main resistance to flow is the skin
Resistance of dry skin is 50000 ohms
Current through dry skin is 150V/50 000 which is 0.003A or 3 mA

MACROSHOCK
Current required to produce VF is 80mA
Therefore 3mA will not cause VF
Resistance of wet skin is about 500 – 1000 ohms ( this is also about the resistance of EKG electrodes)
Current could therefore be 300mA ----- VF is a major risk

MACROSHOCK
What is the voltage required to produce an 80mA current across wet skin?
V = I x R ----- 500 x 80 = 40 volts

MACROSHOCK
How could a patient come into contact with 40 volts in the OR?

MACROSHOCK

MACROSHOCK
Hot and neutral leads power the device
Ground lead connects to the chassis of the device to return any leaking current back to the ground
If the chassis is properly grounded, then current will flow through the ground wire which has very little resistance – current will be high and a fuse will blow

MACROSHOCK

MACROSHOCK
To avoid helping electrocute the patient, no properly functioning modern monitoring device will complete a circuit between the patient and ground
The ground plate on the electrocautery unit is merely the return electrode and not a true ground
NOT GROUNDING PATIENTS IS AN IMPORTANT ASPECT OF ELECTRICAL SAFETY IN THE OR

MACROSHOCK
Equipment must be designed so the the hot wire cannot easily short out with the chassis
Every chassis must be grounded
Ground wires must be regularly inspected
Patient should not be connected to potential grounds
Line Isolation Monitors

LINE ISOLATION TRANSFORMER
Simple device that prevents a circuit from being completed by connection to ground

LINE ISOLATION TRANSFORMER

LINE ISOLATION TRANSFORMER
How do you monitor a line isolation transformer to see if there is any connection between both wire and ground?

LINE ISOLATION TRANSFORMER

LINE ISOLATION MONITOR
Resistor has a resistance of about 150 000 ohm’s so that the maximum current that can pass through the circuit is 1mA
When the resistance detected by the Lim falls to less than 75 000 ohm’s, a warning is signalled ---- should the other line come into full contact with ground, a current of 2mA could flow

MICROSHOCK
Refers to currents delivered directly to the myocardium via intracardiac electrodes or catheters.
Minimum fibrillation threshold is 10 microamps

MICROSHOCK
How much safety does the isolation transformer provide against microshock hazard?

MICROSHOCK
Ground wire should be intact
LIM signals a warning if the resistance between the ground and either wire is less than 75 000 ohm’s, which corresponds to a 2mA current running through the ground wire

MICROSHOCK
In the presence of a LIM, it takes two shorts to the chassis of two devices, with both ground wires broken, to detect a macroshock hazard
A single short to the chassis of the device with a broken ground can create an undetected microshock hazard

ELECTROCAUTERY
Current density = current flow per unit area
Explains the heating effect of the electrosurgical equipment
Passage of direct current or low frequent alternating current may cause physical sensation, stimulate muscular contraction and gives a risk of VF

ELECTROCAUTERY
These effects become less as the frequency of the current increases being small above 1 kHz and negligible above 1 MHz. The burning and heating effect can occur at all frequencies

ELECTROCAUTERY
Electrosurgical equipment is used to pass a current of a high frequency ( 1Mhz) through the body to cause cutting and coagulation by local heating of the tissues
Degree of heating depends on the current density

ELECTROCAUTERY
Two connections – neutral or patient plate and the active or cutting electrode
The same current flows through both plates
At the cutting electrode, heating or burning occurs because of the small area
No burning should occur at the neutral plate due to the large surface area

ELECTROCAUTERY
If the neutral plate is not properly applied, the area of contact can be reduced so that burns can now result at points where the plate is in contact with skin.
If the neutral plate is completely detached, the current may return to the cautery unit via any point at which the patient is in contact with an earthed metal object

ELECTROCAUTERY
Current density can reach a hazardous value when the electrosurgical current flows through parts of the body that have small cross sections
Current density can be increased by metallic prosthesis
Bipolar cautery

ELECTROCAUTERY
Frequencies of 500 00- to 2000 000 Hz are used by electrocautery
Too high to fibrillate the heart
Major concern is burn protection

ELECTROCAUTERY
The grounding plate does not ground the patient to ground
It is the return electrode to the electrocautery unit
Ground plates should not be placed over metallic prosthesis

CAPACITANCE
Is a measure of the ability of an object to hold electric charge
Charge is the measure of the amount of electricity
Coulomb = amperes x seconds

DEFIBRILLATOR
Is an example of an instrument in which electric charge is stored and then released in a controlled fashion.

BREATHING SYSTEMS
Open (non-rebreathing)
Simple face mask or nasal cannula (CO2 diffuses away from the face)
Bag-Valve-Mask system (Ambu®): uses 3 valves to allow either spontaneous or controlled ventilation while preventing rebreathing

BREATHING SYSTEMS
Semi-Open (Mapleson / Bain)
Most efficient removal of CO2 for a given gas flow when the "pop off" valve is nearest the source of the ventilatory power
Spontaneous ventilation: Mapleson A
Controlled ventilation: Mapleson D

BREATHING SYSTEMS
However, the "A" system is very inefficient (requires high gas flows) to prevent rebreathing during controlled ventilation, while the "D" system is reasonably efficient for both controlled and spontaneous ventilation, so the "D" is preferred for most applications.
Bain circuit is a coaxial Mapleson D

BREATHING SYSTEMS
Semi Closed Circle System
Patient gas uptake < fresh gas flow < minute ventilation
Some rebreathing of exhaled gas (following removal of CO2 by absorber

BREATHING SYSTEMS
Closed System
Gas inflow = Patient Uptake
If using sidestream agent / CO2 analyzer, must route exhaust back into circuit
Starting values-O2: 3-4 ml/kg/min

CO2 ABSORPTION
CO2 Absorption Granules
Small enough to have large surface area but large enough to avoid “channeling”
Typically 4-8 mesh

CO2 ABSORPTION
Composition
Sodalime: NaOH, Ca(OH)2
Baralyme: KOH, Ca(OH)2, Ba(OH)2 -More likely to react with anesthetics to form CO (desflurane) or compound A (sevoflurane)