Physics Flashcards
osmolarity vs osmolality
Osmolarity = number of osmotically active particles in 1 litre of solution
Osmolality = number of osmotically active particles in 1 kg solvent
Osmolality used as temperature alters volume not mass
tonicity is the osmotic properties of a fluid in relation to a membrane
Measuring osmolarity and osmolality
Osmolarity is calculated (difficult to measure)
glucose + urea + 2 x Na
Osmolality is measured by osmometer - depression of freezing point of water (higher osmolality, lower freezing point)
Osmolar gap = difference between two i.e. unmeasured osmoles (methanol, ethanol, ethylene glycol)
Osmole
unit of measurement describing number of moles of a compound that contribute to osmotic pressure and that would depress freezing point by 1.86 K
Colligative properties
properties of a solution which are affected by osmolarity
- depression of freezing point (1.86K per osmole)
- reduction of vapour pressure (less space for solvent on surface)
- increase boiling point
- increase osmotic pressure
Clinical applications of osmolality
- SIADH - reduced osmolality
- DI - increased osmolality
- TURP syndrome - reduced osmolality
- Hyperosmolar states e.g. HSS
Transducer
Converts one form of energy into another form
Active - generate electric current directly in response to stimulation e.g. piezoelectric
Passive - external power source - change in resistance etc converted to equivalent electric current
Ultrasound
Sound waves at frequency above human hearing > 20KHz
Medical USS 2.5 - 15 MHz
Principles
- Array of piezoelectric crystals
- USS waves generated by piezoelectric effect - electric voltage across piezoelectric crystal makes it vibrate
- Frequency of vibration corresponds to frequency of current
- USS travels through tissue and reflected at tissue interfaces - when there is a change in density e.g. boundaries between tissues
- reflection transduced into display
- strong reflections from solid structures are white
- weaker reflections grey
- absence of reflections e.g. blood black
velocity of sound waves in tissue is 1540m/s
Ultrasound modes
A-mode - transducer scans a line through the body, echoes plotted on screen as function of depth
B- mode - brightness - linear array of transducers produces beam of USS in a plane, reflections viewed as two dimensional image
M-mode - motion - rapid sequence of B mode follow each other in sequence - examination of moving structure
Doppler mode - Doppler effect - increase in frequency of signal when the source of signal is approaching the observer and decrease in frequencyy as sound source moves away.
change in frequency i.e. increase in frequency of RBC moving towards and decrease in frequency of RBC moving away = doppler shift, can be used to calculate velocity
v = Fd x 1540 / 2 x emitted frequency x angle
USS image quality
Not all of the USS beam is reflected
- absorption - tissue absorbs acoustic energy
- reflection - only some of acoustic energy is reflected directly back to the probe
- refraction - deflected acoustic energy at difference angle
- divergence - spread out of energy
Improved by
- amplitude - increasing strength of the use wave increases reflected acoustic energy
- frequency - higher frequency better resolution but worse penetration (linear = 10MHz, curvilinear = 5MHz)
- Gain - screen brightness
How is blood pressure measured
Non-invasively
- Mercury sphigmanometer
- Oscillometery
Invasively
- Arterial line
Non-invasive non automated BP measurement
Sphygmomanometer
- Centre of cuff over brachial artery
- Width 20% greater than diameter of arm
- Aneroid gauge or column of mercury
- Cuff inflated above systolic, released 2-3mmhg/sec
- Auscultation over brachial artery for Korotkoff sounds
1 = blood flow in artery first appears
2 = muffling
3 = rising in volume
4 = fall
5 = absence
Oscillometric
- cuff / aneroid gauge
- cuff deflated below systolic, needle on aneroid gauge oscillates
- pressure at onset of oscillation = systolic
- max oscillations = mean
- decreasing oscillation = diastolic
Von-Recklinghausen oscillotonometer
- two overlapping cuffs on upper arm
- proximal = sphygmomanometer
- distal = measure oscillation
Non-invasive automated BP measurement
DINAMAP - device for indirect non-invasive automated MAP
- similar to von-recklingausen
- single cuff, pressure transducer, microprocessor, display
- pressure transducers measured pressure and oscillations
- onset of oscillations = systolic, max = MAP, diastolic derived
- reliable but need 2 min intervals, arrhythmias unreliable, fail to record if < 50
Penaz
- LED through small finger cuff
- light detected other side
- amount of light absorbed by tissues proportional to volume and therefore BP
Invasive arterial blood pressure measurement
gold standard, beat to beat
intra-arterial cannula - short, stiff
column of fluid pressurised to 300mmHg with 4ml.hr flush
transducer - mechanical energy (movement of diaphragm) to electric
electric signal amplified and processed
- saline column in contact with diaphragm and 4 strain gauges
- tension of strain gauges alter as diaphragm moves
- change in tension changes electrical resistance, measured by wheatsonte bridge
Complications of arterial cannulation
Early
- haematoma
ischaema (vasospasm)
Late
- ischaemia (thrombosis
- infection
Any time
- exsanguiantion
- intra-arterial injection
Damping and resonance
resonance - every system has resonant frequency - system will ossilate if left alone. if frequency coincides with frequency of arterial waveform - increased amplitude and distortion
Damping - inherent tendency of system to resist oscillations
- optimal 0.67
- critical 1.0
- over damped - waveform stops quickly due to compliant tubing, air bubble > 1
- underdamped - resonance causes the trace to oscillate and overshoot
Information from arterial waveform
- slope of upstroke - myocardial contractility
- downslope - SVR. steep downstroke with low dicrotic notch indicates low SVR. high dicrotic notch implies high SVR
- respiratory swing in IPPV - hypovolaemia
Pulse contour analysis
SV proportional to AUC systolic portion of curve
CO = SV x HR
SVV - min SV divided by max. > 15% suggests fluid responsive
Cardiac output measurement
Clinical
Non-invasive - TT electrical impendence
Minimally invasive - ODM PICCO
Invasive - PAC
PAFC
Fick principle - uptake of substance by organ is equal to amount entering and leaving
PAC indirectly measures CO by thermodilution
- cold saline 10ml proximal lumen
- change in temperature measured by thermistor in tip
- rate of change in temperature reflective of CO and calculated by Stewart-hamilton equation (AUC log change in temp over time)
Pulse contour analysis
Arterial pressure waveform morphology
- slope up - contractility
- AUC up to dicrotic notch - SV
- downstroke / position dicrotic notch - SVR
Computer algorithms to calculate CO
- SVV - change in CO over respiratory cycle
- calibrated - PICCO - CVC / specialised art line measure transpulmonary thermodilution. LIDCO uses lithium. Stewart-Hamilton eq
Uncalibrated use height weight nomograms e.g. lid rapid
Oesophageal doppler
USS probe at 45 degrees to aorta
USS beam reflected at different frequency when red blood cells are in motion
- measure blood velocity multiplied by CSA aorta
- velocity plotted against time
waveform
- contractility - peak velocity
- preload- flow time
- SV - stroke distance area under the velocity / time curve
Thoracic electrical bioimpedence
Resistance to alternating current flowing through the body
electrodes neck / chest. resistance to current flowing from outer to inner electrodes measured.
Bioimpedence is related to water content of the thorax
nomograms to estimate volume of electrically participating tissue
impedance changes throughout the cardiac cycle as the volume of blood in the thorax changes - microprocessor analyses and estimates SV
Sensitive to movement, diathermy, arrhythmias
Electrical current
Rate of flow of electrons past a point in a conductor
Measured in Amperes (SI unit)
- 1 coulomb of charge passing a point in one second.
Voltage and current
Ohms law V = I x R
V = Voltage I = current R = resistance
Current directly proportional to voltage, inversely proportional to resistance
Types of current
Direct current
- electrons flow in single direction
- current constant and doesn’t change with time
Alternating
- electrons alternate flow with time
- current alternates between positive and negative values
- UK 50Hz (50 x per second) 240 V
Electrical injury
Damage to skin or internal organs from electrical circuit. Extent of injury
- current (i.e. amount of electricity)
- pathway - which tissues injured
- duration
- type of current - AC worse than DC
Microshock = usually safe current transmitted directly to the heart via conductor e.g. CVC causes arrhythmias
Electrical safety - general measures
regular maintenance
patients not in contact with earthed objects
correct humidity
antistatic shoes and flooring
Electrical safety - equipment classification
Class 1 - any conducting part connected to earth by an earth wire. Faulty live supply - current flows to east and causes safety fuse to melt
Class 2 - conducting part double insulated
Class 3 - voltages no higher than safety extra low voltage < 25V AC - unlikely to cause macro shock
Degree of protection
Type B - can be class 1, 2 or 3, maximum leakage current is < 100mcgA
Type BF 0 uses floating circuit - equipment circuit separated from mains
Type CF - highest protection max leakage < 10mcgA - suitable for cardiac
Diathermy
Heating effect of high frequency AC electrical current to cut and coagulate.
Monopolar - AC 200KHz - 6MHz. energy between active electrode (instrument) and neutral plate. very high current density at instrument with heating effect. current density low at plate - large area of contact. cutting / coagulation depending waveform
Bipolar - much lower AC frequency. Energy between two points of surgical forceps. current passed through tissue held by forceps. no neutral plate.
Dangers of diathermy
- Burns - accidental discharge, poor application of neutral plate
- Fire - flammable material may be ignited
- pacemarkers - monocular may inhibit or damage (bipolar safest)
ICP waveform
Trifid
- respiratory sinusoidal waveform
- arterial pulsatile 3 peaks
- P1 = arterial pulse. P2 = brain compliance. P3 = dicrotic notch (aortic valve closing)
Raised ICP
- Amplitude of all 3 rises
- decreased P1 suggests decreased cerebral perfusion
- increased P2 suggests reduced cerebral compliance
Electrical charge
physical property that causes it to experience a force in an electromagnetic field
- SI unit is coloumb
- 1 coulomb - quantity of electric charge which passes a point when 1 ampere current flows for 1 second
Capacitance
measure of ability to hold electric charge
Farad SI unit
A capacitor has 1 Farad of capcitance if potential difference of 1 volt is applied across its plates when they hold 1 coulomb of charge
capacitor - device able to store energy in form of electric charge - two metal plates separated by insulator. storage increased by greater SA and greater distance between two plates. Energy = 1/2 x Q x V
Defibrillator
Electrical equipment that delivers dose of electrical current to the heart
Two electrical circuits
- charging circuit - DC battery or AC mains with rectifier to convert to DC. Capacitor 5000 V applied across the plate stores electrons
- Discharging circuit - inductor - coil of wire that prolongs the duration over which the electrical discharge takes place. patient, switch (between charging and discharging circuits.
Without inductor - rapid discharge of electrons, doesn’t defibrillate and causes burns
Monophasic - single discharge of current travels in one direction. Biphasic - two consecutive pulses, travels in one direction then back. lower energy required.
Transthoracic impedence
impedance presented by patient during cardio version. influences by
- paddle size
electrode coupling
paddle position
- obesity
CO2 absorption options
- Soda lime (sodium hydroxide)
- Baralyme (calcium hydroxide, barium hydroxide)
- Litholyme
Sodium hydroxide – > Compound A and Carbon Monoxide with halogenated volatiles
Soda Lime
94% calcium hydroxide, 5% sodium hydroxide
Colour dye pH < 10
pH 13.5. 1kg - 120L CO2
Exhaled gases reach canister, CO2 absorbed, heat and water produced which rejoin FGF
Overall reaction
Ca(OH)2 + CO2 –> CaCO3 + H20 + heat
4-8 mesh in size. uniform and smooth flow. high surface area but not too much resistance
Allows low flow, less waste, avoids rebreathing, humidification
Harmful products sodalime
Compound A (nephrotoxic)
- fluromethylether produced from sevoflurane
- high servo / low FGF
Carbon Monoxide
- produced when system left for time
- higher concentration of volatile
- type of volatile (des)
Pros and cons of circle system
Pros
- Low flow
- economy of anaesthetic consumption
- reduced pollution
- warming and humidification
Cons
- Slow changes in inspired anaesthetic concentration
Components of circle system
- inspiratory limb with one way valve
- expiratory limb with one way valve
- vaporiser in circuit / out of circuit
- soda lime
- bag with APL valve
- fresh gas supply
How do you measure ICP
Clinically - symptoms, papilloedema
Non-invasively - CT imaging
Invasively - EVD, ICP bolt
ICP waveform
Normal - P1 / P2 / P3
P1 = systolic upstroke, P3 = diastolic downstroke P2 = dicrotic notch (brain compliance). P1 > P2 > P3
Lundberg A = plateau waves, steep increase in ICP lasting 5-10 mins. always pathological
Lundberg B = oscillations of ICP 0.5-2waves/min associated with unstable ICP
Lundberg C = oscillations with frequency 4-8 waves / min, relationship with cardiac and respiratory cycles
Neurophysiological monitoring
MEPs - trancranial stimulation of motor cortex, producing muscle contraction. strong correlation to spinal cord ischaemia. monitors descending anterior and lateral corticospinal tracts
SSEPs - electrical impulses delivered to peripheral nerve which are detected in sensory cortex. transmission via dorsal columns.
Stagnara wake up test - wake patient up and get them too ollow commands
Pollution in theatre
- gas induction
- spillage
- leaks
- open circuits
- failure to turn off vaporiser
Concerns - spontaneous abortion
- malignancy
- infertility
- mortality
- global warming
N20 negative effects
- bone marrow toxicity (megaloblastic anaemia)
- neurotoxicity (subacute degeneration of the spinal cord)
- teratogenicity
- spontaneous miscarriage
- substance absue
Reducing anaesthetic gas pollution
- ventilation (15x per hour)
- closed circle circuits
- soda lime
- avoid leaks
- avoid gas induction
- avoid volatile
- low flow
- monitoring pollution
- scavenging
COSHH recommendations for volatiles
8 hour time weighted average
halothane 10ppm
enflurane 50ppm
isoflurane 50ppm
N20 100ppm
Passive scavenging
- patient dependent
- no active positive or negative pressure
- 30mm connector
- gases vented to atmosphere by patients spontaneously ventilation or ventilator
- Cardiff aldasorber - charchol absorbing volatiles
Active scavenging
- collecting system - collect expired gases from breathing system via 30mm connector
- transfer system - wide bore tubing
- receiving system - reservoir. valved guard against positive or negative pressures
- disposal system - vacuum, vented outside
disadvantages of active
excessive positive pressure - barotraua
excessive negative pressure - rebreathing
laminar flow
ultra clean environment
prosthetic implants
400 air changes per hour - air passes through high efficiency filters
Epidural filter
disc shape
hydrophilic supported membrane
Filter size 0.22microns
Filters viruses, bacteria, foreign bodies
Physiological Humidifcation
inspired gas is warmed as it passes over vascular nasal epithelium
- humidified by water evaporation from respiratory surface
- 100% relative humidity just distal to carina
- at 37 degrees 44mg/L
Gases delivered from pipelines are dry to prevent condensation and corrosion
Humidifcation
Passive
- HME
- circle breathing system (soda lime produces heat and water)
- cold water bath - inspired gas bubbled through cold sterile water. full saturation at room temperature, patient further heats
Active
- hot water bath - heating element added to cold water bath to increase absolute humidity. disks are condensation
- gas driven nebuliser - aerosolises water various droplet sizes - venturi effect
- ultrasonic nebuliser - electrical to mechanical energy, fine mist generated from water covered membrane
HME
HMEF
- hygroscopic membrane, pleated to reduce dead space
- 0.2 microns pore size
- 60-70% relative humidity (30g/m3 at 30degrees)
- 15/22mm connectors. sampling tube connection
- warm humidified exhaled gas condenses on filter medium, next inspiratory breath humidified and returns to patient
- 100ml dead space
- Affected by temperature / water vapour content / medium size / flow rates
- cons - resistance to flow, location near patient, obstruction with mucous
Ideal humidifer
- adequate humidification
- low resistance to flow
- microbiological protection
- warms
- economical
Other filters
Filter needles
- prevent particulate and organism contamination
- 0.2 microns
Fluid filter
- 15 microns to prevent particulate
Blood filters
- 170-250 microns - whole blood
- 20-50 microns 70-80% leucodepletion
- electrostatic - 100% leucodepletion
types of RRT
Intermittent
- IHD ( 300-400ml.min flow rates. tap water / reverse osmosis)
- Peritoneal dialysis
Continious
- CVVHF
- CVVHD
- CVVHDF
Haemofiltration
- extracorporeal circuit
- blood filtered through semipermeable membrane driven by hydrostatic pressure
- ultra filtrate discarded and replaced with suitable solution
- Particles < 50kDa filtered by convection (dragged through)
Haemodialysis
- extracorporeal circuit
- blood passes in countercurrent system next dialysate separated by semipermeable membrane
- solutes move across membrane along concentration gradient obeying ficks law
Haemodiafiltration = combination of two
Similarities between CVVHF and CVVHD
Both achieve
- water removal
- solute removal
- correction of electrolytes and acid-base
Set up
- extracorporeal
- central vei
- roller pumps
- filter
Peritoneal dialysis
same principle as haemodialysis
peritoneum acts as membrane
SLED
sustained low efficiency dialysis
intermittent
daily longer sessions
slower solute removal and fluid than IHD but faster than CRRTT
Type of RRT depends on
- stability (IHD more CVS derangement)
- resources / experience
- size of particles to be removed (smaller molecules - dialysis, larger - filtration)
RRT disadvantages
- Cost
- Nursing workload
- Complications of line insertion
- Fluid shifts
- altered drug metabolism
- low PB removed easily
- Higher Vd have lower RRT clearance
- ABx - teic, linezolid no adjusstment
- mero / tan are filtered - increase dose
- gent and vanc highly filtered - levels and increase dose
Terms in RRT
Dose = volume of plasma ‘purified’ per unit time. typically 25-30ml/kg/hr. Effluent flow rate (dialysate + ultra filtrate)
Filtration fraction = fraction of plasma removed from blood 20-25%
Sieving coefficient = ratio of concentration of solute in ultra filtrate vs plasma. 1 = fully filtered 0 = no filtration. determined by size, polarity, pore size, protein binding
Humidification importance
without humidification, respiratory tract dries out, keratinise, reduced ciliary clearance, inflammation, mucus plugging
- passive : HME
- Active : hot water baths, nebuliser, cascade
Humidity in theatre
Too low relative humifity
- static electricity
- increased evaporative fluid loss from patient
To high relative humidity
- microbial growth
- unpleasant
Optimium is 40-60% at 20 degrees
Humidification expressed as
Absolute humidity - mass of water vapour per unit volume g/m3
relative - ratio of mass of water to mass of water if fully saturated expressed as percentage
The amount of water vapour a given volume of air can contain increases with increasing temperature. If air is 100% saturated and temperature decreases then condensation occurs. temp at which condensation occurs is the dew point
Measuring humidity
Hair hygrometer
- relative humidity
- hair increases in length proportional to increasing humidity (water is absorbed), moves pointer on scale
wet/dry bulb hygrometer
- two glass thermometers one in air one in water
- thermometer in water reads lower temp due to cooling effect from evaporation. the rate of evaporation is dependent on ambient humidity (lower humidity, more evaporation, lower temperature)
- difference in temperature between two measured and calculate relative humidity
Renaults hygrometer
- silver tubing in ether
- air blown through ether, evaporation and cooling
- condensation on shiny outside
- temperature at condensation is due point (temp when air fully satirated)
relative humidity = svp at dew point / sip ambient temp
Absolute humidity measured by mass spectrometry - molecules ionised then deflected by magnet based on mass:charge ratio - quantify amount of water.
heat definition
state of energy an object has in relation to kinetic energy of atoms or molecules
transfer down temperature gradientt
temperature deifntiion
measure of average kinetic energy of the molecules within a substance
- substances with higher temperature have higher average kinetic energy
- lower temperature lower average kinetic energy
- determines whether heat can be transferred
- SI = Kelvin 1/273.16th triple point of water
(triple point is the temp and pressure when substance can exist in solid, liquid, gas at equilibrium)
- celcius - 0k = -273.15C
Temperature measurement classiication
Non-electrical
Electrical
Infrared
Non-electrical methods of temp measurement
Liquid thermometer
- volume of liquid increases as temp rises
- calibrated against fixed points
- mercury or alcohol
- simple, cheap, reusable
- slow, easily broken
Bourdon gauge
- usually measured pressure
- attach to liquid-containing tube
- inc temp inc vol liquid
- small inc in pressure on hollow spiral tube of bourdon gauge which unwinds and moves pointer
Bimetallic strip
- copper / steel
- different coefficients of expansion
- fixed at ends
- increase in temp –> expansion at different rates –> bending of strip –> move pointer
- cheap, simple, slow
Liquid-crystal
- colour change in response to change in temp (thermochromism)
- cheap, limited range
Electrical methods of temperature measurement
Platinum resistance
- resistance increases with temperature
- circuit, battery, platinum wire, Wheatstone bridge
- accurate, expensive
Thermistor
- semi-conductive material, metal oxide
- resistance decreases with temperature
- Wheatstone bridge
- small, robust, cheap, but drift
Thermocouple
- Seebeck effect - junction of 2 different metals potential difference develops in proportion to temperature
- iron / Constantan
- second cold junction reference
- cheap, rapid, stable, accurate. required amplification
infrared temp measurement
all objects emit electromagnetic radiation, wavelength characteristic and dependent on temp
at 37 degrees, radiation at 9340nm
tympanic thermometer - measures emitted radiation, focused onto detector, converted to electrical signal
NICE temperature guideline
- pre-op measure and document. warm if < 36
- walk to theatre if able
- intra-op - measure and document in anaesthetic room and every 30mins
- induction only when above 36
- warm fluid
- force air warmer if > 30min
- measure and document every 15mins in recovery
- do not discharge until > 36
consequences of hypothermia
CVS
- bradycardia, vasoconstriction, arrhythmias, J waves
Neuro
- reduced CBF, CMO2, neuroprotection
- reduced level of consciousness at 33, coma 30
Resp
- shivering increased VO2
- increased PVR
Renal
- reduced renal blood flow due to reduced CO
Haem
- decreased platelet, clotting enzyme function
- left HbO2 curve
metabolic
- reduced BMR
Heat loss in theatre
- induction - vasodilation and heat loss
radiation 40% - heat loss from exposed body parts to neighbouring objects
- treat - increase room temp, cover patient
convection 30% - air layer next to body warms and rises, producing current
- treat - forced air warmer
evaporation 20% - sweat and skin
- treat - cover patient
respiration 10% - drying gases
- treat - humidifying gases
Stroke volume variation
SVmax - SVmin / SVmean
10% + suggests fluid responsiveness
Properties of ideal cardiac output monitor
- Cheap
- accurate
- Easy to insert / non-invasive
- Minimal risks to insertion
- Provide continuous data
- No need to recalibrate / not subject to drift
- operator independent
Oesophageal doppler
flow = velocity x area
area = nomogram
velocity = doppler equation
Derives
- PV = contractility
- CO / CI
- Stroke distance - area under systolic waveform (SV from SD and nomogram)
- Flow time corrected - preload 330-360ms
Sterility of equipment
Critical Equipment
- penetrates mucous membranes to enter sterile body area e.g. surgical instruments, needles, endotracheal tubes
Semi-critical
- contacts mucous membranes, doesn’t enter sterile area e.g. laryngoscope blade
Non-critical
- contacts skin only e.g. NIBP, pulse ox
Making equipment sterile..
Decontamination
- removal, inactivation or destruction of pathogens on surface
- can comprise of cleaning, disinfection or sterilisation
Cleaning is first stage of decontamination
- all that is required for non-critical equipment
- removal of foreign material from object
- detergent or enzymatic soak
- must disassemble
- manual - washing with detergent
- mechanical - waster, ultrasonic cleaner
Disinfection
- destroys most bacteria and all viruses
- does not remove bacterial spores or some mycobacteria
- Thermal = boiling 70-100degrees e.g. pasteurisation = heating to 70-80degrees for 30mins
- chemical = heat sensitive equipment e.g. endoscopes. glutaraldehyde, hydrogen peroxide
Sterilisation - complete destruction / killing of all microorganisms including spores
- Steam (autoclave) - reliable and cheap e.g. 121 degrees 1 bar 20 mins
- Dry heat - mewl
- Ethylene Oxide - 50-60 degrees, expensive, toxic
- Radiation - gamma rays
Peripheral nerve monitoring
Clinical
Qualtitative - nerve stimulator
Quantitative - Mechanomyography, acceleromyography, electromyography
Characteristics of peripheral nerve stimulator
Portable
Battery powered
Easy to use
Able to deliver different impulses
- single twitch
- ToF 2Hz
- Supramaximal –> maximal = all muscle fibres supplied by nerve are stimulated. supramaximal is 20% greater
- Tetanic 50Hz
Train of Four
Series of stimulations at 2Hz (4 over 2 seconds) of approx 70mA voltage (supra maximal)
applied to superficial nerve e.g ulnar, facial
Comapre T1 to T0 (controle) and T1:T4
Can calculate extent of block
- Loss of T4 = 75%
- Loss of T3 + T4 = 80%
- Loss of T2 + T3 + T4 = 90%
- Loss of all = 100%
Depolarising - all reduced, no fade (repeated dose lead to phase 2 (similar to non-depolarising)
Non-depolarising - fade
Double burst stimulation
- 2 tetanic 50Hz separated by 750ms
- used when profound block
Infrared spectroscopy and CO2 measurement
- Molecules containing 2 or more different atoms absorb IR
- Different molecules absorb IR of different wavelengths
- Gases in anaesthesia absorb IR at characteristic wavelengths - CO2 at 4.28um
- Measuring the fraction of IR radiation of 4.28um absorbed by a gas mixture, allows CO2 partial pressure to be measured
IR analyser
- one side hot wire emits IR radiation
- IR radiation passes through a filter allowing only specific wavelengths (or multiple)
- IR radiation passes through gas chamber (sapphire), opposite side of chamber lens focuses IR onto a photodetector
- Reference chamber alongside - CO2 free air - removes fluctuations
Side stream vs main stream
Side stream
- gas sample drawn from exp limb close to patient and carried to analyser via tubing
- moisture trap to prevent water interference
- little addition weight, no additional dead space, can measure gases other than CO2
- Disadvantage is delay of 2-3seconds
Main stream
- gas chamber with sapphire windows incorporated into breathing system
- IR radiation passed through gases directly in the breathing system
- advantage is instant anaylsis
- Disadvantage - must be heated, dead space, only measures CO2
Capnogram waveform
Phase 1 - flat bottom line - inspiration and early expiration of dead space gas. CO2 = zero
Phase 2 - expired mixed anatomical dead space and alveolar gas - rapid CO2 rise
Phase 3 - alveolar plateau - end point is ETCO2
Phase 4 - inspiration, CO2 drops to zero
Severinghaus electrode
Modified pH electrode - measures PCO2 in blood
Two electrodes
Glass - silver / silver chloride electrode, gas permeable membrane. detects change in pH via H+ sensitive glass membrane coated in NaHCO3 (buffer)
Silver / silver chloride reference electrode
2-3 min response, temperature maintaiend at 37 and recalibration
Fibreoptic endoscpes
Total internal reflection
refraction - change in direction of beam of light occurring at boundary of two materials with different refractive indices
Bundles of 15,000 glass fibres, each one coating in glass of different refractive index, allowing light to be transmitted along the bundle
Components. Body:
- eyepiece / focussing +/- screen
- light source
- control lever to move tip
- suction port
Tube:
- fibreoptic light bundle, transmits light to distal tip
- image bundle - image –> eye piece
- suction channel
- wires to move tip
Cleaning immediately - enzymatic detergent
disinfection - liquid per acetic acid
Laser uses
Light amplification by stimulated emission of radiation
- ENT - vocal cord resection
- Ophthalmology - retinal surgery
- Urology - lithotripsy
- Dermatology - skin lesions
Properties of laser light
- monochromatic
- high energy, low cross sectional area
- collimated, parallel beam
Principles of laser
- energy applied to an atom causes atom to move from ground state to excited state
- electrons move to higher energy orbits
- electrons spontaneously fall back to resting orbit - releasing energy in form of photon ‘spontaneous emission’
- photons produced similar
- when photons released from atoms in excited state trigger photons to be released from neighbouring atoms in excited state - the released photons are identical. stimualted emission
- population inversion occurs when > 50% atoms are in excited state
components of laser
- energy source
- lasing medium - atoms to be energised
- mirror to reflect and amplify light
- small hole to allow light to leave
types of laser
- CO2 - 10600nm (far IR) - absorbed by all tissues. cellular water boils and cells burst - precise surgical cutting and coagulation - ENT
- Nd-YAG - 1060nm (near IR) - photocoaulgatuon. can be passed down endoscopes
- Argon - 500nm (visible) - dermatological
Laser safety
Minimise risk to patient
- eye wear
- avoid flammable cleaning solutions
- lowest O2 possible
- minimal possible time
- laser safe tubes - non-flammable, reflective, double cuff filled with saline
- damp swabs to protect tissues
Minimis risk to staff
- safety officer
- eye wear
- signs
- locks
Airway fire management
- stop surgery, critical incident
- flood field with saline
- remove breathing circuit and ETT, maintain ventilation with BVM in air
- once extinguished, re-intubate and inspect airway with fibrescope
ICU
Acute lung injury - dexamethasone
Circle circuit
- CO2 absorber - soda lime
- insp and exp limbs with unidirectional valve
- vaporiser
- reservoir bag / apl valve with switch to ventilator
- tubing and FGF
Ads - low flow anaesthesia
- variety of ages
- preserves heat and moisture
disads - complex
- not portable
- low flows - slow change in FGF
- toxic build up e.g. compound a, carbon monoxide with iso
