Physics Viva Flashcards
Name the base units
Length metre time second temperature kelvin mass kilogram amount mole luminosity candela Current amper
Derived but not named units
Area m2 Volume m3 Density Kg/m3 velocity m/s acceleration m/s2
Named derived units
Newton - force Pascal - pressure Joule - energy Watt - power Coulomb - charge Volt - potential Farad - capacitance Ohm - resistance Weber - magnetic flux Tesla - flux density Henry - inductance Celsius - temperature
Boyles law
P ~ 1/V
Critical temperature of oxygen and N2O
Oxygen -119 degrees
N2O 36.5
What is an “ideal gas”
A gas that where individual molecules behave as individual particles that move in random manner independent of each other and other intermolecular forces
Charles law
V ~ T
Gay-Lussacs law
P ~ T
Ideal gas law
PV= nRT
n = number of moles R = universal gas constant
What is avogadro’s hypothesis
Equal volumes of gases at a given temperature and pressure contain the same number of molecules
One mole of gas at STP will occupy 22.4 litres and contain 6.02 x 10”22 molecules
How do we calculate N2O and O2 volumes that are in cylinders
N2O
- Use avogadros number and the weight of N2O and molecular weight of N2O
O2
- P1V1 = P2V2
Daltons law
The total pressure exerted by a gaseous mixture is equal to the SUM of each individual gas partial pressures
How is oxygen manurfactured
Fractional distilation of liquified air (commercial)
Oxygen concentrators - zeolite absorbers absorb nitrogen
Poynting effect
The liquidation and separation of gas components when below the pseudocritical temperature
eg entonox less than -5.5 degrees
Oxygen cylinder features
State: Gas
Colour: black/white
Cylinder pressure: 137 bar
Critical temperature: -119
N2O cylinder features
State: vapour
Colour: blue
Cylinder pressure: 52 bar
Critical temperature: 36.5
Air cylinder features
State: gas
Colour: black/black-white
Cylinder pressure: 137 bar
Critical temperature: -141
CO2 cylinder features
State: vapour
Colour: grey
Cylinder pressure: 50 bar
Critical temperature: 31
Entonox cylinder features
State: gas mix
Colour: blue/blue-white
Cylinder pressure: 137
Critical temperature: -5.5
Heliox cylinder features
State: gas
Colour: brown/brown-white
Cylinder pressure: 137
Critical temperature: x
O2 supply hospital
VIE
Cylinder manifold (size J)
Define force
A vector quantity that can cause an object with mass to accelerate
1 newton will accelerate 1 kg at 1m/s2 in a vaccum
Units of pressure
1 bar 1 atmosphere 14.5 lb/in (psi) 30 inches of Hg 101 kPa 760 mmHg (Tor) 1020 cmH2O
What is a gauge pressure
A pressure above or below atmospheric pressure
what is absolute pressure
Pressure measurement incorporating atmospheric pressure `
Different types of pressure regulators
Direct
Indirect
Two stage
Slave
Hagen-Poiseuille equation
Flow = Pi x Pressure change x radius”4 / 8 x L x Viscosity
Laminar flow
steady flow no eddies or turbulence pressure gradient must exist flow is proportional to this difference flow rate greatest at centre Reynolds number <2000 Viscosity of fluid important
Turbulent flow
chaotic eddies and swirls fluid velocity varies across the tube Resistance is not constant density of fluid is important
Flow is proportional to square root of pressure and radius squared
flow is inversely proportional to square root of tube length and square root of fluid density
What is the reynolds number
a number that predicts the onset of turbulent flow
(velocity x density x tube diameter)/viscosity
Example of clinical application of Reynolds number
HELIOX
Critical velocity
The velocity above which the flow of a fluid within a given tube is likely to change from laminar to turbulent
How can flow be measured
Wright respirometer
Pneumotachograph - constant orifice, variable pressure
Rotameter - constant pressure, variable orifice
Describe the Bernoulli, venturi and coanda effect
x
What is viscosity
the tendency of a fluid to resist flow
What is turbulent flow proportional to
radius”2
Square root of pressure
Inversely proportional to length and density
Tube vs orifice …
Tube = laminar flow
Orifice = tube where the diameter exceeds the length
Explain resistance
The opposition to flow of DIRECT CURRENT
Unit Ohm
V=IR
Explain reactance
The opposition to flow of ALTERNATING CURRENT caused by the inductance and capacitance in a circuit rather than by resistance
Explain impedance
The total resistance to flow in ALTERNATNG CURRENT from both resistance and reactance
Explain a capacitor
A device that can store charge
2 conducting plates separated by an insulator
Amount of stored charge depends on:
- size of plates
- separation gap
- dielectric material
It blocks DC but passes AC
Capacitative reactance decreases with increase frequency - SO.. diathermy with high frequency 1.5MHz will be conducted but mains electricity at 50Hz will not be conducted
Charge = capacitance x voltage
Energy stored by a capacitor
E=1/2 x charge x voltage
Or
E=1/2 x capacitance x voltage”2
Unit of capacitance
Farad
Explain an inductor
A device that resists a change in electric current
A wire coiled around a ferrous core - when current passes a electromagnetic field is generated
This blocks AC but allows DC
Used in transformers and to isolate equipment from earth (floating circuits) and in defibrillators to smooth and lengthen current pulse
Explain a transformer
A device that transfers electrical energy from one circuit to another
Used to step up or down the voltages of alternative current in electrical applications
Essentially they are two inductors placed close to one another so the EM field in one generates a current in the other
Voltage generated in the second circuit calcultaed from FARADAYS law of induction `
Diode or rectifier
Allows current to flow in one direction only
What is a battery
A collection of galvanic cells that convert stored chemical energy to electrical energy
Two half cells (positive and negative) and conducting electrolyte
Oxidation occurs at the anode and reduction at the cathode
Amplifier
Makes the input signal larger for easier interpretation
Types of electrical filter
Common mode rejection
Amplifying only set frequency ranges
Draw the defibrillator
Eurgh..
Explain the defibrillator
Delivers DC shock
Uses 5000 volts - required step up transformer
Charges CAPACITOR (which have low reactance to AC and high resistance for DC)
When shock delivered runs through an INDUCTOR to prolong duration of current discharge
-> DC shock from 30A for 3ms with 5000V
thoracic impedance ~ 50-150 ohms
External monophasic defib - 360J
External biphasic defib - 150J
ICD - 20-50J
Difference between DC cardioversion and defibrilation
Cardioversion synchronised to prevent R on T phenomenon which can cause VF
What are the risks with defibrillation
Burns Igniting flammable materials Interference with electrical components (ICD/pacemaker) Precipitation of VF Electrocution of staff
How do we minimise risks of defibrillation
Training "stand clear" Checking device Audible alarm - "charging" "ready to shock" Dry patient surroundings Removing oxygen
Describe mains electricity
AC current 50Hz at 240V
AC used to allow it to be transmitted over large distances with limited power loss
UK mains electric wires
brown - live
blue - neutral
yellow/green - earth
Explain the earth wire
Safety feature - connected to any exposed conducting parts of electrical appliance
If live wire came into contact with this part current would flow down earth wire and not into person touching
BUT BUT BUT
When using diathermy patients must be PROTECTED FROM EARTH WIRE - an earth wire would provide an alternative route for the diathermy current to flow potentially causing severe burns
What do the adverse effects of electrocution depend on
Current type (AC worse than DC) Magnitude of current (V=IR) Current density (current/area) Current duration (greater duration = greater heat) Tissues through which it flows (heart vulnerable to VF)
Effects of different current amplitude
0-5 mA = tingling
5-10 mA = pain
10-50 mA = Muscle spasm ("cant let go")
50-100 mA = Respiratory muscle spasm & VF
5A - tonic contraction of myocardiumMacro vs micro shock
macro - through whole body mains -> earth
- dangerous at mA level
Micro - direct to myocardium
- dangerous at microA level
How is electrical equipment classified
Class I - earthed casing
Class II - double insulated casing (doesn’t need to be earthed)
Class III - battery operated
Type B - class I, II, III - low leakage current. Safe for connection to patient but not heart
Type BF - similar to B but patient is isolated via a floating circuit.
Type CF - considered safe for direct connection to the heart because the leakage of current is very small (less than 10 microA) - used for thermodilution catheters, ECG leads and pressure transducers)
Measures in theatre to prevent electrical injury
Anti-static floors Relative humidity of 50% (inhibits build up of static) Circuit breakers non-sparking switches and plugs Regular checks and maintenance
Classify solid materials according to their electrical conductivity
Conductors
Insulators
Semi-conductors
Types of magnet
Permeant and electromagnets
Features of magnets
All have north and south pole
Magnetic flux is the flow of energy from north to south pole (Webers)
Magnetic flux density is the amount of magnetic flux per unit area (tesla)
Strong magnets used in MRI are 1.5 tesla
One ampere
One unit of charge (coloumb) per second
What are the causes of electrical injury
Resistive coupling (faulty equipment and leakage) and capacitive coupling (person acts as one side of capacitor)
Features of diathermy
AC current with high frequency - uses heating effect of passing current through tissue with high impedance to burn or vaporise tissues
heating effect depends on current density and duration
"Cutting" = alternating sine wave pattern "coagulation" = pulsed sine wave pattern
monopolar - single probe and diathermy plate. Power 100-400W
Bipolar - double probe so local electrical field. Power 40W
Hazards of diathermy
Burns
Shocks
Pacemaker interference
Monitor interference
What is heat capacity
The energy required to increase the temperature of a material by 1 degree
Specific HC = 1 kg
Molar HC = mole
What is boiling
The point at which the SVP equals the surrounding ambient pressure
Raoults law
The depression of SVP of a solvent is proportional to the molar concentration of the solute present
What is temperature
Refers to the thermal state of a substance - it is the degree of “hotness” and reflects its potential for heat transfer
How is temperature measured
Non-electrical, electrical and infra-red
NON-ELECTICAL
- Liquid expansion thermometers (mercury/alcohol)
- Gas expansion thermometers (bourdon gauge dial)
- Bimetallic strip dial thermometer
- Chemical thermometer
- strip of small cells containing a mixture that melts over a range of temperatures to produce temperature dependent colour change
ELECTRICAL
- Thermocouple - Two metallic strips, constant temp strip, galvanometer
- Resistance thermometers - strip of metal, resistance proportional to temp
- Thermistor - uses semi-conductor, resistance inversely proportional to temp
INFRA-RED
- All objects emit IR radiation depending on their temperature
- tympanic temp use tympanic membrane as reference of core temp
Use pyroelectric or thermopile sensors
pyroelectric sensor
contain crystals that change polarity in given temperature
thermoile sensors
numerous thermocouples connected in series and allows continuous measurement to be taken
Classify types of scavenging
Active and passive
Passive - requires no external power , gas movement to the exterior is due to the pressure generated by the patient during expiration. Ventile system
Active - utilises an external power source such as a vacuum pump to generate negative pressure which propels gas into external atmosphere
Problems with passive and active scavenging
Passive
- excess positive or subatmospheric pressures at outlet can be caused by wind or air movement at the outlet
- outlets at roof height - dense molcules can fall down
Active
- excess positive pressure can lead to barotrauma
- excess negative pressure can deflate reservoir bag of the breathing system and lead to re-breathing
Methods of measuring oxygen concentration
Clark electrode
Galvanic fuel cell
Paramagnetic O2 analyser
Mass spectrometer
Photoacoustic spectroscope
Raman spectroscope
Chemical (Haldane apparatus)
Describe the Clark electrode
Measures PO2 in solution
Main features
- Platinum cathode
- Silver anode
- KCl solution
- External power source
- Blood sample with thin teflon membrane
2 equations
4Ag + 4KCl -> 4AgCl + 4K+ + 4e-
4e- + O2 + H2O -> 4OH-
Electrons go from silver to platinum
The cathode uses the electrons - the amount of which is proportional to amount of O2 so the current is dependent on amount of oxygen
Temperature dependent
Describe the fuel cell
Measures PO2 in GAS mixture. Similar to the Clarke electrode
Main features:
- lead anode
- gold cathode
- KOH solution
- NO external power source
- thermistor
Electrons are produced at the lead anode:
Pb + KOH -> PbO + H2O + e-
These electrons react with oxygen from gas sample with gold cathode acting as catalyst
O2 + 4e- + 2H2O -> 4OH-
Again electrons produced proportional to amount of avaliable oxygen and therefore current proportional
OH- ions return to anode and continue reaction
The thermistor is incorporate to allow temperature compensation
Lifespan dependent on lead anode
Slow response time
Paramagnetic analyser
Oxygen and nitric oxide are both paramagnetic - they have unpaired electrons on outer ring
Rate of diffusion of a gas dependent upon
Concentration gradient
Temperature
Molecular weight
Rate of diffusion across a membrane
Membrane surface area and thickness
Factors effecting the solubility of a gas in liquid
Temperature
Partial pressure
Chemical nature - ie polarity
What is a vaporuiser
A device that allows accurate mixing of FGF and volatile anaesthetic agent
Classified:
in circuit - draw over, rely on subatmospheric pressure
Out of circuit - rely on PPV from a FG supply eg plenum
Safety features of a vapouriser
Wicks - increase SA
Baffles - direct FGF onto surface of anaesthetic
Heat reservoir - heat conductive metal
Temperature compensation - bimetalic strip/med rod
Other
- High internal resistance - prevents back flow
- Maximum filling mark
- Long high resistance outlet path
- Anti-spill mechanism
- Selectatec system (interlock bar system)
- geometric/colour specific devices
Cylinder sizes
C 170l CD 460l Most small sued D 340l E 680l anaesthetic F 1360l Ambulance G 3400l J 6800l cylinder manifold
Formula for energy
Energy (work) = force x distance
Or
Work = pressure x volume
What is power
Power is the rate of energy use
WATT (joules/second)
(or work per time unit)
MAP formula
DBP + 1/3(SBP-DBP)
Non invasive BP measurement
Intermittent manual
- sphygamometer
- oilometer
- Von reckinhouse ossilometer
Intermittent automatic
- DINAMAP
Continuous
- Penaz/finaprez
How can CO2 be measured
Capnography
Severinghaus electrode
Describe how CO2 is measured by infrared spectroscopy
CO2 absorbs infrared light
- Absorbed by any molecule containing 2 or more types of atom
- Different molecules absorb different wavelengths
- By seeing how much of the wavelength is absorbed we can work out how much CO2 is present
Components:
- Hot wire emitting IR radiation
- Passes through a filter that only allows a certain wavelength through
- Passes through chamber made of sapphire
- reflected onto photo detector
- parallel there is photodector
As stated by the Beer-Lambert law its absorption is proportional to the amount present and distance
Types of suction
Active and passive
Types of scavenging
Active and passive
What methods are there to measure fresh gas flow
Most commonly a ROTAMETER
(CONSTANT pressure and VARIABLE orifice)
Made of bobbin, tapered transparent tube and needle valve
Flow of gas pushes up bobbin along tube with associated scale
- Initial flow is laminar
- becomes more turbulent as gap widens
So initially flow inversely proportional to viscosity and once turbulent flow predominates proportional to density
Mini-wright flow meter
Pneumotachograph
Measurement of gas volumes
Commonly measured by measuring flow and multiplying by time
Other clinical scenarios:
- Benedict Roth spirometer
- Vitalograph
What is an EEG
An electroencephalogram
Electrodes at the skin measure the electrical potentials from brain electrical activity
Indications for EEG
Diagnostic: Seizures, encephalitis
Neuro-prognostication
Depth of anaesthesia monitoring
Tell me about the EEG waveform
Complex waveform generated by pyrimidal cells in cortical layers 3 & 4
EEG uses 16 scalp electrodes with the resulting electrical signal having very low amplitude
- 10-50 microvolts
To reduce noise:
- differential amplifiers used
- high and low pass filters
EEG waveform classifications
Beta - 13-35 Hz
Alpha 8-13 Hz
Theta 4-8 Hz
Delta 0-4 Hz
How is EEG used to monitor anaesthesia
Used in depth of anaesthesia monitoring:
- Delta and theta waves likely to represent surgical anaesthesia
- Beta waves likely to represent awake patient
- Burst suppression likely to represent unnecessary depth
Processed EEG used as 16-electrode EEG impractical. A 4 electrode frontal EEG creates a dimensionless index of depth of anaesthesia from 0-100
Compare ECG. EEG and EMG monitoring
Electrode - Origin - amplitude - frequency
ECG - cardiac myocyte - 1-5 mV - 0.05-20 Hz
EEG - pyrimidal cells - 10-50 microV - 0.1 -30 Hz
EMG skeletal muscle - 0.1-1 mV - 40-3000 Hz
Other than EEG monitoring, how can depth of anaesthesia be monitored
- Clinical signs
- Motor response to stimulation
- Population parameters (MAC)
- Rogue ones..
- Skin conductance - sweat gland activity
- Frontalis EMG - decreased activity means deeper
- Isolated arm technique - tourniquets..
- Lower oesophageal contractility - smooth muscle
- Evoked potentials
What is electric charge
The physical property of matter that causes it to experience a force when placed in an EM field
Charge = current x time
Couloumb (SI unit)
1 C = quantity of charge which passes a point when a current of 1 Ampere flows for 1 second
What is capacitance
A measure of the ability of an object to hold charge
SI unit = farad
1 farad = potential difference of 1 volt is applied across a capacitors plates when they hold a charge of 1 coulomb
Q = V x C
Properties of a capacitor that affect its ability to store charge and equation for energy stored
Distance between plates
Surface area of plates
Properties of insultator used
Energy = 1/2 x Q x V
= 1/2 x charge x voltage
Components of defibrilator
Charging circuit and discharging circuit
Differences between monophasic and biphasic defib.
Monophasic - only one direction with single discharge from capacitor
Biphasic - current flows forwards and backwards from two consecutive pulses of current. More successful at lower energy
Transthoracic impedance
The impedance present by the patient during cardioversion
- lower impedance = greater charge reaches the heart
Factors that effect TTI:
- paddle size
- electrode coupling with skin (use of gel pads)
- Paddle position
Areas of the CNS effected by general anaesthesia
Cerebral cortex
Thalamus
Pontine reticular activating formation
spinal cord
Methods of scavenging
"removal of waste anaesthetic gases" Active Passive Cardiff Aldasorber - Canister with activated charcoal connected to APL valve via tubing
Cardiff aldasorber adv vs disadv
adv
- small, portable
- no set up cost
Dis
- doesnt absorb N2O
- have to weight the charcoal to see if used up
- requires replacing every 12 hours
- heating canister releases volatiles again
Volatile max concentrations
N2O 100 PPM
Volatiles 50 PPM
Halothane 10 PPM
Averaged 8 hour concentration
Note other countries have different levels
What are the different types of ventilators
Can be classified by:
- PPV vs NPV
PPV can be subclassified:
- Volume cycled
- Time cycled
- Pressure cycled
Can also be classified by MoA
- minute volume dividers
- bag squeezers
- Intermittent blowers
HFOV
Pendulft principle
HFOV
Pendulft principle
Ideal portable ventilator
Light easy to move Resistant to damage Adequate gas supply & low gas consumption Adequate battery Easy to use
Multiple ventilatory mechanisms
Adult and children
Oxylog 3000
Hamilton T1
Laser types
Nd YAG
- crystal used as lasing medium
- 1064 nm (near infrared)
- endoscopic surgery - cutting
Argon
- gas lasing medium
- 400-700 nm (near blue)
- good penetration through clear tissues
- used in eye surgery and derm (birthmarks)
CO2
- gas lasing medium
- 10.6 micrometer (infrared region)
- highest power laser avaliable
- poor penetration
- superficial use
How are lasers classified
Class 1 - power doesnt exceed maximum permissible exposure for eye
Class 2 - power up to 1mW and visible laser beam only. Eye protected by blink reflex
Class 3a - up to 5mW, visible spectrum only but laser beam must be expanded, eyes protected by blink
Class 3b - power up to 0.5w and any wavelength. hazardous so eye protection worn
Class 4 - power >0.5w and any wavelength. hazardous and capable of igniting flammable materials. eye protection essential
