Anaestheska III Week 5+ Flashcards
What are the properties of an ideal breathing circuit?
Simple and safe Deliver intended fio2 Spont and controlled vent Low FGF requirement Protect from barotrauma Remove waste Easy maintenance and low cost
What is dead space?
Where no gas exchange occurs
There is a mix of inspired and expired gases
How does the APL valve work?
Lightweight disk sits on knife edge seating held on by spring
Tension in spring = valve opening pressure
Dial movement adjusts the tension
What are safety issues with APL?
Malfunction could keep APL open = ^^ dead space
High positive pressure if left closed resulting in barotrauma - safety relief valve at 60cmh20
Exhaled water vapour may cause disk to stick - hydrophobic material
Describe the reservoir bag.
Ellipsoidal, anti-static rubber
0.5-2L size
Accommodates FGF during expiration as reservoir for next inspiration
Can assist ventilation and act as visual aide for spont breathing
Highly compliant so accommodates gas limiting the pressure to 40cmH20
Describe circuit tubing.
Must provide laminar flow: uniform shape and large diameter
Must have optimal compliance
Corrugations a resist kinking and improve flexibility
What is the design of the mapleson A magill system?
Bag in middle
FGF opposite end
APL patient end
How does the Magill system work?
Insp: APL closes, FGF comes in and mixes with exhaled dead space gas
Exp: pt expires forcing APL open, bag is refilling
Pause: exp gas meets FGF in tube and is forced out APL
Pro vs con for magill circuit?
Yes spontaneous
FGF=MV
No controlled
3xMV needed
Large dead space not for
How does the Lack system work?
Insp: APL closes and FGF comes in outer tube
Exp: patient expires into tube so APL opens; FGF fills bag
Pause: expired gas forced through inner tube and out APL by incoming FGF
Pro vs con of lack?
As with Magill
Risk of inner tube malfunction causing high dead space
APL at machine end makes system less cumbersome
What is the design of the Mapleson B system?
Bag one end
FGF, patient and APL at other
Corrugated tube in middle acts as reservoir
How does the Mapleson B work?
Insp: mix of alveolar and FGF inhaled
Exp: expires into tube and bag; APL opens
Pause: expired gas and FGF forces out
Pro vs con for Mapleson B?
No spontaneous FGF 2x MV needed No controlled FGF 2x MV needed Gas waste and pollution
What is the design for the Mapleson C?
(Waters without absorber) Bag one end FGF, patient and APL other Short tube Used in emergencies Works like Mapleson B
What is the design for the Mapleson D-Bain system?
APL and bag one end FGF and patient at other Co-axial (also parallel version) Inner tube FGF Outer tube expired gas
How does the Bain system work?
Insp: FGF flow to patient via inner tube
Exp: expires into reservoir tube which mixes with the still flowing FGF
Pause: FGF washes expired gas out, also filling system with FGF for next inspiration
Pro vs con for Bain system?
No spontaneous FGF 2xMV needed Yes controlled 70ml/kg needed Good for head/neck surgery Low R and dead space Assisted scavenging Inner tube may fail Bag movement not good indicator
How does the Ayres T piece work?
Insp: FGF flow to patient from reservoir tube
Exp: expires into reservoir tube which mixes with FGF still flowing
Pause: FGF washes expires gas out; fills system with FGF for next inspiration
Pro vs con for Ayres T piece?
Yes spontaneous 3x MV needed Yes controlled FGF=MV due to longer exp pause allowing refill time Low R and dead space Valveless - less leaks Pollution and gas waste Tube length long enough to not entrain air
How does the Jackson Rees Modification work?
As with Mapleson E
Pro vs con for Jackson Rees modification?
Yes spontaneous and controlled 2.5-3x MV needed Visual respiratory movements Easy IPPV Tubing length vs entrainment Pollution and gas waste Great
What is the design of the Mapleson A Lack system?
Co-axial version of A designed to assist scavenging
Inspiration outer tube, exp inner
Parallel version also available
FGF, APL and bag machine end
What is soda lime for?
Absorbs carbon dioxide
Low FGF requirement so highly efficient
Low pollution
Conserves, warms and humidifies
What is the physical design of the soda lime canister?
Vertical
Two ports: one delivers insp gas, other receives exp gas
Each port has unidirectional valve
What happens if carbon dioxide rebreathing occurs?
Acidosis results
Acts as an anaesthetic and will cause coma
What are the components of soda lime?
94% calcium hydroxide
Sodium hydroxide
Potassium hydroxide
Water
pH 13.5
Colour change ethyl violet dye <10
Why is silica added to the soda lime?
Granules prone to powder formation which can cause high resistance and stick to valve
The silica helps to harden and prevent powder formation
Why is zeolite added to soda lime?
Helps maintain pH for longer and retain moisture to increase the amount of carbon dioxide absorption
Also helps reduce formation of carbon monoxide and compound A
What are the risks of soda lime?
Dust on valves Dust cause high resistance Corrosive (alkaline) Compound A Carbon monoxide Channeling Leaks
What is the granule size of soda lime?
4-8 mesh
(4-8 openings per inch)
3-4mm spheres which allow more even gas flow throughout. Also allows longer life, lower dust and reduced resistance
What’s the mechanism of soda lime?
Exhaled gas go to canister where carbon dioxide gets absorbed and heat + water is produced (exothermic)
This warm/humid gas then rejoins FGF
Why does low FGF exhaust like faster?
Most exhaled gases go out the APL but with low flows very little exits the APL so therefore goes through the lime
What are the safety features of soda lime?
Clear canister houses valves and lime Colour change Sphere, specific sized granules Added silica Uneven canister filling can result in channeling
How is compound A formed?
When sevo used with soda lime
Due to alkali metal in lime degrading the sevo
Increased by temp, low FGF and high sevo concentration
How is carbon monoxide formed?
When volatiles containing CHF2 moiety (enf, des, iso) used with dry granules
Production of carboxyhaemoglobin can occur
What is a closed system?
No gas escapes
FGF replaces what is consumed by patient and lime
FGF and pollution both very low
FGF must match perfectly
What is a semi closed system?
Pressure relief valve allows excess gas to escape
High flows can be used
What may happen if unidirectional breathing valves fail?
Low efficiency
Rebreathing
Hypercarbia
What is the unit of measurement for theatre pollution?
Ppm
Particles per million
What are some methods to reduce theatre pollution?
Theatre ventilation with 15-20 air changes per hour Non-recirculated Circle system TIVA RA Scavenging
What are some causes of pollution?
Bad mask fit Paed breathing systems Failure to turn of gases at end and to intubate Uncuffed tube Vaporiser filling Exhaling vapour in recovery Machine leaks
What’s the ideals of scavenging?
Not affect ventilation Not affect dynamics of system Collecting device, system to carry away and method for regulating pressure Checked daily Passive/active
Describe passive scavenging.
Simple, no cost
Collecting system has shroud connects to APL
30mm connector
Receiving system can be used (2 valves protect against high and negative pressures)
Disposal via copper pipes to atm or ventilation system
Driven by patient effort
What is the safety features of passive scavenging?
Valves to protect from high and negative pressure
30mm/19 connection prevent connect to breathing system
Wind at outlet causing +/- pressure
Outlet should have mesh
Protect the tubing to prevent leaks
Need long tubes (^R)
Describe active scavenging.
Collection and transfer similar to passive
Receiving system usually valveless, open ended reservoir between receiving and disposal
Antibacterial filter downstream
Flow indicator between reservoir and disposal
Reservoir has 2 valves
Vacuum created by fan, pump or Venturi system
Cope with 30-130L/min rates
What are safety features of the active scavenging system?
30/19mm connector
Receiving system capable of coping with changes in flow rates
Increased demand (-P) allow ambient air to entrain so maintaining pressure
Opposite occurs with high +P occurs
Reservoir prevents -/+P to patient
Independent pump used
What are the order of components for active scavenging?
From expiratory valve: 30mm connection Collecting system Transfer system Receiving system Vacuum generator
What are the features of an ideal filter?
Filter air and liquid borne from 99.99-99.999% Bidirectional Minimal dead space Minimal resistance Unaffected and not affect agent Work when wet and dry Prevent liquids passage Light, not bulky, not traumatic Humidify Transparent Low cost
What are the basic components of a filter?
2 ports: 15mm and 22mm
Sample port on anaesthetic side
Filtration element in middle
What is an electrostatic filter?
Material exposed to electric field producing felt-like material and high polarity. One type of fibre becomes + and other -
Usually 2 polymer fibres used
Flat layer of material used so low R
Relies on charge to attract particles
99.99% effective
Charge efficiency high when dry but low when wet and increases R
Charge decays with time so limited life
What happens when a hygroscopic layer is added to electrostatic filter?
= HMEF
Pressure drop across element and therefore resistance is higher with water absorption
What is a hydrophobic filter?
Lasts long periods
Rely on natural electrostatic interactions
99.999% efficiency
Pleated paper of inorganic fibres achieve high SA and higher R
Forces between H2O are > forces between H2O and filter therefore it collects on surface without absorbing
What are the characteristics of the ideal filter?
Provide humidification Low resistance Low dead space Microbiological protection Maintain body temp Safe and convenient Economical
What is a HME?
Heat and moisture exchanger
Passive and effective
Retains a portion of expiratory moisture + heat
It then returns it on inspiration
Achieves 60-70%
Warmed to 29-34 degrees
Delivers absolute humidity of 30g/m3 at 30 degrees
How does the HME work?
Exhaled gas passes causing water vapour to condense on cooler HME medium. This is evaporated and returned to patient with next inspiration, humidifying gas.
>temp difference = > transfer
5-20mins before optimal
0.2 nanometer pore size = HMEF
What might affect the HME performance?
Water vapour content and temperature of insp/exp
Flow rates - how much time gas is in contact with medium
Larger medium = better performance
Low thermal conductivity helps maintain temp difference across HME
What are the safety features of a HME?
May obstruct from mucus or water Single use Max 24hours use: risk of H2O accumulation = ^R Efficiency low with ^TV 2 way gas flow required to work Place close to patient Increases dead space and R (WOB)
What are the components of a hot water bath?
Disposable reservoir with inlet and outlet for insp gas to pass
Heated sterile water partly filled
Thermostat controlled element with temp sensors in reservoir and near pt
Tubing delivers gas to pt
Water trap along tubing between pt and humidifier (lower than pt)
Electric power
Describe the receiver system for the AGSS?
Main interface between breathing and disposal system.
Must protect from +/-P
Provides capacity to cope with peak flows
Can be open: reservoir tubular open to atm therefore providing air break (needs active disposal)
Closed: reservoir is bag with P release valves
Describe the disposal system for the AGSS?
Passive: driven by respiration, tube from patient to outside, short and wide tube to reduce R
Active: fan/pump draws gas, used with open receiver system
What are the affects of dry and cold gases?
Dehydration Hypothermia Infection Atelectasis/shunt Reduced FRC Reduced compliance Cell damage - dysfunction mucocilliary elevator, damaged cilia Thick mucus
What is a nebuliser?
Produces most of micro droplets of water suspended in a gaseous medium
Amount of water droplets delivered does not depend on temperature
Smallest droplets more stable and get further in airway
Can deliver medicine
How does a gas driven nebuliser work?
Capillary tube with bottom end in water container
Top end close to Venturi constriction
High pressure gas flows through Venturi = -P
H2O drawn up capillary tube and broken into fine spray
Smaller droplets achieved as spray hits anvil/baffle
Mostly 2-4 micrometer produced
How does a spinning disk nebuliser work?
Centrifugal force created by motor driven spinning disk causes micro droplets to be thrown out
What is a laryngoscope?
Perform laryngoscope and aide intubation
Handle - houses battery
Blade connects to handle
Light source: bulb screwed onto blade, electric connection made when open
Light can be in handle or transmitted along fibre optic
How does a laryngoscope work?
Blade advance and lifts epiglottis to view cords
4 Mac sizes
Left blade available
Interchangeable blades on handle: ISO international standard - colour marks same systems (green)
How does a straight blade work?
Advanced over posterior border of epiglottis which is lifted upward/forward in order to view larynx
How does a curved blade work?
Inserted at right angle
Sweeps tongue to left
Tip reaches vallecula which pulls epiglottis up from behind
What is a McCoy blade?
Like Mac but with hinged tip operated by lever at handle
Also available as straight blade
What are the safety features of laryngoscopes?
Can cause trauma
Light source may fail
Large cheat difficult - short handle or angled blade
Needs sterilisation
What is the polio and kessel blade?
At angle to help with DI
Polio 135 degrees
Kessel 110 degrees
What is a fibreoptic scope?
Aide intubation
Evaluate airway
Confirm placement
Perform trachea bronchial toilet
How does a fibreoptic scope work?
Uses light transmitted through fibres
Fibres provide low refraction
Light enters at a specific angle, travels fibre repeatedly striking and reflecting glass at same angle until it comes out the end
Each fibre carries small part of the picture so arrangement crucial throughout scope
Variable cord length and diameter
What are safety features of fibreoptic scopes?
3.0-7.0 diameter tubes useable Easily damaged Needs reprocessing Light failure Blocked channels Only use one channel at a time
What are magill forceps?
Small/large
Used to guide tube, remove foreign body, insert throat pack
What’s a retrograde intubation set?
Assists DI
18G intro needle, guide wire, 14G hollow guide catheter with distal side port, 15mm
Intro inserted into cricoid membrane
Guide wire advanced in retrograde direction to exit oral/nasal
Catheter inserted antegrade into trachea
Tube introduced over catheter
What are the features of an ET tube?
PCV/silicone Radio opaque line Outer/inner diameter Beveled tip Cuff Murphy eye Vocal cord indicator 15mm connection Level markings (cm) Single use marking Pilot cuff One way valve Oral/nasal marking Curvature
What is the bevel for?
Left facing and oval shaped
Improves view of cords when laryngoscope inserted on the right
Shape allows to push through cords, separating them
What is a high pressure, low volume cuff?
Prevents passage of secretions
High pressures on walls of airway
Can cause necrosis
What is a low pressure, high volume cuff?
Pressure applied over larger area so less pressure risk over longer time
Possible wrinkles so less reliable seal
Why should pressure be regularly checked?
May increase with nitrous oxide use and temperatures
Could be a slow leak
Movement from moving patient
Nearby surgery
Why is a nasal tube advantageous?
Can’t bite
More tolerated
Frees up the mouth
Epistaxis risk
What are 8 risks of intubation?
Linking Herniated cuff Occlusion by secretions Bevel against wall Oesophageal intubation Trauma - sore throat Dental damage Failed airway
What is an armoured tube?
Plastic or silicone rubber Embedded spiral of metal or tough nylon Had a thicker wall so OD bigger Wire prevents kinking Strong, flexible Introducer often used Cannot cut to length High risk bronchial intubation 2x markers for vocal cords
What is a RAE tube?
Ring, Adair, Elwyn Preformed shape Bend where tube emerges so that connection at chin/forehead High risk bronchial intubation Cuffed RAE = 1 Murphy eye Uncuffed = 2 Murphy eye
What is a laser tube?
Withstand carbon dioxide or potassium-titanyl-phosphate laser
Reduce fire and damage risk
Beams are reflected and de focused to reduce strike of healthy tissue
Flexible stainless steel body or wrap of laser proof foil
Double cuff for extra protection
Cuff may be filled with meth blue or saline
What’s an evoked potential tube?
Used if risk of nerve damage Bipolar stainless steel contact electrodes are embedded in tube above cuff where they touch cords This is connected to a nerve stimulator Earth connects to patient Provide visual and audible warnings NIM = neural integrity monitor
What is a micro laryngeal tube?
Better excess to larynx
Small diameter; same length
Sufficient length for nasal too
Ivory PCV to reduce trauma
What are the components of a tracheostomy tube?
Introducer
Wings on proximal part allow fixation
Adjustable flange to fit variable thickness of tissue
Cuff/uncuff
15mm
Square tip to reduce obstruction risk against tracheal wall
Suction lumen above cuff
Some have inner cannula: secretions can collect/dry out (obstruction) on inner which can then be replaced instead of whole thing
Neonate to adult size
Some have one-way flap valve and window for speech
What are the benefits of a tracheostomy?
More comfort Less sedation needed Better access for oral hygiene Allow oral nutrition Aides bronchial suction Less dead space Less resistance Less glottis trauma
What are the safety features of a tracheostomy?
Haemorrhage Mis placement, occlusion Pneumothorax Blocked by secretion Infection Over inflation Granulomata Tracheal dilatation Scar
What is a fenestrated tube?
Window in curve channels air to cords allowing speech
After deflation patient can breathe around cuff, through window and through tube - less R and weaning
May be fenestrated inner cannula
What is a larnygectomy tube?
Cuffed tube inserted through tracheostomy to facilitate IPPV during neck surgery
Offer better access by allowing breathing system to be connected away from field
Replaced with tracheostomy at end
What is a speaking valve?
One way speaking valve can be fitted to tube (if cuffed must be down)
Mounts on top
What’s a tracheostomy button?
Once tracheostomy removed, button inserted into stoma to keep patent and allow suction still
What’s a percutaneous tracheostomy tube?
Insert between 1+2 or 2+3 ring
Withdraw any tube to tip just below cords
Insert intro needle and seldinger guide wire through to trachea
Use F/O throughout, ensure midline puncture, free of tube, posterior wall not damaged and assess position
Forceps/dilatory dilate tissue over wire
Trache inserted over wire then remove
Can be done at bedside
What is a double lumen tube?
Selectively deflate one lung while vent other
2 separate colour codes lumen: tracheal and bronchial
Each have bevel and cuff with colour coded pilot balloon, labelled
2 curves: standard anterior to fit trachea and other for L/R
Y connector - each lumen to limb
What is a left DLT?
Inflates left lung and deflates right lung
ALWAYS USE
Left lobe to carina is 5cm
What is a right DLT?
Inflates right lung and deflates left lung
Risk of upper lobe obstruction
Eye in bronchial cuff to facilitate vent of upper lobe
Right lobe to carina is 2.5cm
How is position of DLT checked?
Use F/O
Auscultation
Inflate tracheal until no leak - both lungs vent
Clamp tracheal
Inflate bronchial until no leak (3ml) - one lung vent
Check other lung vent by clamping bronchial and opening tracheal - one lung vent
What is an endobronchial blocker?
Alternative to DLT
Blocker catheter with distal cuff, pilot balloon and guide loop
Multi port adaptor - 15mm
Blocker advanced over F/O using loop and tracked into main bronchus
Inflate cuff when in place
Vent maintained in other lung via tube
What is an oropharyngeal airway?
Anatomically shaped device fits into oropharynx to maintain airway patency
Anaesthetic reduces tone resulting in obstruction by tongue or soft palate
000-6
What are the components of an oropharyngeal airway?
Curved body Inner channel Flat anteroposteriorly Curved laterally Flange to prevent travel Bite portion is straight and prevents occlusion Colour code
How does oropharyngeal airway work?
Keeps airway patent by preventing tongue and epiglottis from falling back
Size = distance from incisor to mandible angle or corner of mouth to tragus
Adult: Insert upside down partially then rotate 180 degrees - prevent tongue being pushed back
Paed: insert right way up and depress tongue if needed
What’s a Bergman airway?
Assists with F/O
Guides F/O
Maintains airway and protect scope
Side opening allows removal of F/O
What are the risks of an oropharyngeal airway?
Trauma to tissue
Broken teeth
Could trigger gag
Can make worse if incorrect fit
What is a nasopharyngeal airway?
Through nose to nasopharynx Distal end sits just above epiglottis Curved body Left facing bevel Proximal flange Softer plastic
What’s the mechanism of a nasopharyngeal airway?
Alternate to oropharyngeal of mouth won’t open or not tolerated
Need lubrication
Left bevel eases insertion through right nostril
What are the safety features of nasopharyngeal airways?
Don’t use if on blood thinners, deformities of nose or sepsis
Don’t force - false passage
Too big may cause necrosis
What are the advantages of a supraglottic airway?
Don't need direct vision Quick Low CVS disturbance Low coughing Conduit for intubation - aperture aligns with glottic opening Useful in CICO 2nd generation has gastric port
What are the components of a laryngeal mask?
Transparent tube Wide diameter (low R) 15mm Elliptical cuff Forms seal around posterior larynx Inflates via pilot balloon with valve Bars to prevent epiglottis obstruction
What are the components of a proseal?
Lumen adjacent to airway traverses floor of mask to tip to drain secretion/air
Contained in bite block
3D elliptical cuff inflation with 2nd cuff behind bowl to improve seal (rear boot or dorsal cuff)
Seal up to 35cmH2O
What is a reinforced LMA?
Stainless steel wire spiral in wall
Flexible, kink resistant
How do you check an LMA?
Inflate and check for hernia Check lumen patent Tube bends to 180 degree without occlusion Inspect for faults, damage, wear Inspect when removed for blood
What is an I-gel?
Extraglottic airway Anatomical fit without cuff Has second drain tube Large lumen 15mm Non-inflatable gel like cuff with ridge at superior anterior edge to prevent epiglottis occlusion Wise oval cross sectional body to prevent rotation and bite Gel material moulds into place
What is a seldinger technique?
Obtain access to lumen
Lumen penetrated by needle, wire passed through, needle removed, then catheter passed over wire
Risk haemorrhage, perforation organ, infection, equipment loss inside
What is a face mask?
Anatomic fit Range of sizes Air filled cuff supports body - helps snug fit 22mm Transparent Flavoured Size proportional to dead space Valve to change cuff volume
What is a catheter mount?
Flexible link between system and airway Variable length Corrugated tubing Concertina design 15mm and 22mm ends May have inbuilt sample port or humidifier
What is ECG?
Electrocardiogram
Measure electrical activity of heart with potentials of 0.5-2 MV at skin
Shows HR, ischemia, arrhythmia, conduction
No assessment of output or function
Bipolar leads measure change between 2 electrodes
What are the components of ECG?
Skin electrodes:
Silver/silver chloride electrode
Held in cup separate from skin by foam pad and conduction gel
Colour coded cable to machine
Variable length and no. Electrodes
Must be all one length in set to reduce electromagnetic interference
How does ECG work?
Clean skin, abrade, good gel contact, all same type Less impedance on bony prominence High/low pass clean signal Signal boosted by amplifier 3-lead most common: 2 active 1 earth
What is the CM5 arrangement?
R- manubrium sternum
L- 5th intercostal space L anterior axillary
N- L shoulder
Detects ST changes very well!
Lead II ideal for arrhythmias!
What are safety features of ECG?
Incorrect placement
Muscular interference
Diathermy current travel here if pad not on correctly
High/low ventricular rate alarms and ST segment monitor
Raised ST can show early ischemia
Interference from mains, diathermy
What is Einthoven’s triangle?
Arrangement of 3-lead Lead II follows direction of depolarisation so is best Dot 1 (R) always - Dot 3 (L) always + Dot 2 varies depending on lead set Unused dot is earth Heart current travels from - to +
What is a 5-lead ECG?
Utilises V1 which gives view of heart directly below it
More comprehensive picture, better arrhythmia analysis, better ischemic monitor, more balance representation of right and left heart
V1 is right stern all border, 4th space
What creates an upright or upside down ECG trace?
Depolarisation toward + dot is upright
If travelling away it creates inverted wave
What is DINAMAP?
Device for indirect non-invasive automatic mean arterial pressure
What are the components of NIBP?
Microprocessor controls sequence and pneumatic pump
Pneumatic pump causes inflation
Solenoid valve causes deflation
Pressure transducer detects oscillations
How does NIBP work?
Microprocessor control sequence of inflation via pneumatic pump
Pump inflates cuff to higher P than previous systolic
Solenoid valve deflates incrementally
Return of flow causes oscillations sensed by transducer and interpreted by microprocessor
What are safety features of NIBP?
Cuff must be correct size:
Cover 2/3 of arm, middle of bladder over brachial artery, bladder width 40% of mid circumference of limb
Can do venous stasis
Must inflate/deflate quick (3mmHg/s)
Can be affected by arrhythmia
External pressure on cuff can affect
Frequent measurement can cause ulna nerve palsy and petechial haemorrhage of skin
What is pulse oximetry?
Measure of how much haemoglobin in the artery is carrying oxygen %
What is beers law?
Amount of light absorbed is proportional to concentration of light absorbing substance - haemoglobin
The more haem = more absorbed
What is lambert’s law?
Amount of light absorbed is proportional to the length of the light pathway - size of artery
Wider artery has same haem per unit area but more overall area so therefore more light is absorbed
What are the LEDs used and there wavelengths and how does they relate to pulse oximetry?
Red - 660 nm
Infrared - 940 nm
Oxyhaemoglobin absorbs more IR than red
Deoxyhaemoglobin absorbs more red than IR
They absorb a different amount of light at different wavelengths
Pulse ox works out sats by comparing how much red/IR is absorbed as a ratio
How does the pulse ox distinguish arterial results from surrounding tissue?
Surrounding tissue will also absorb some light
Arteries are pulsatile so any changing absorbable just be from an artery as path length changes with each pulse (lambert)
Unchanging absorption must be from surrounding tissue
What is the plethysmographic waveform?
Graphical form of pulsatile change in absorbance
Shows signal quality
How does the pulse ox account for ambient light?
Detector reads red, IR and ambient light
Pulse ox rapidly switches LEDs on/off in specific sequence (100/s)
Red on: detect red and ambient
Red off, IR on: detect IR and ambient
Both off: detect ambient only which can then be subtracted
What are some problems with pulse oximetry?
Movement Incorrect placement Too much ambient light Electromagnetic interference - diathermy Poor perfusion affects pulsation Nail polish, IV dyes and some drugs interfere with wavelength Carbon monoxide joins Hb and detected as oxyHb Below 70% not accurate Change site 2 hourly - pressure sore Venous pulsation eg valsalva
What are the generals steps to a pulse ox measurement?
Detector detects light from LED
Ratio of red to IR calculated
Pulsatile vs non-pulsatile
Flicking on/off to remove ambient
What is a capnograph?
A device that records and displays waveform and value of end of expiration
What is a capnogram?
Graphical plot of carbon dioxide partial pressure or percentage vs time
What is a capnometer?
A device that shows numerical concentration without a waveform
How does end tidal carbon dioxide analysis work?
CO2 absorbs IR
Amount absorbed is related to amount of CO2 in sample chamber
Remaining IR goes to photodetector which produces heat
Heat is measured by a temp sensor and the temp is inversely proportional to amount of CO2 in sample chamber
Light also passes reference chamber of room air to compare
RR measured from rise/fall
Describe a side stream analyser?
1.2mm ID tube samples gas at a constant rate of 150-200ml/min
Tube connects to adaptor near pt and delivers to sample chamber
Teflon so impermeable to CO2 and unreactive to agents
Moisture trap and exhaust allows humid gas to be vented or returned
Tube needs to be close as possible
Time delays
What is a main stream analyser?
Sample chamber within patient gas stream High dead space Heated to 41 degrees to prevent condensation No time delay Can't measure other gases Only with intubated patients No gas removed from system
Compare side stream and main stream analyser.
Side: time delay, moisture trap, sample other gases, removes gas - return, leak risk, need calibration, use on non-intubated
Main: no time delay, heat chamber, doesn’t remove gas, dead space, can’t measure others, can’t use on non-intubated, no calibration
What are cardiogenic oscillations?
If sample line aspirates gas due to prolonged pause or expiration
Ripples appear on the trace during alveolar plateau in sync with the heart beat
Smoothed by high lung volume and peep
How is oxygen analysed?
Measure of FiO2
Galvanic, polarographic and paramagnetic method
How does the paramagnetic analyser work?
O2 attracted to magnetic field
The O2 is attracted and agitated leading to a change in pressure in both chambers
The pressure difference is proportional to pp difference between the chambers
What are the benefits of the paramagnetic analyser?
Very accurate Most common Most rapid and can be breath-by-breath Continuous High and low alarms Displays pp and % (Pauling) Needs water trap - affected by vapour
What are the components of nitrous and agent analyser?
Sample tube Sample chamber IR light source Optical filters Photodetector
What is the mechanism of the nitrous and agent analyser?
Sample enters chamber and exposed to IR
Photodetector measures the light reaching it across correct wavelength
Absorption of IR proportional to concentration
Electrical signal analysed and process
Optical filters used to select correct wavelength
How are shapes used to identify agents in the analyser?
Can measure up to 3 agents
5 sensors produce spectral shape which represents spectral signal of agent in sample
This is compared with spectral shapes stored in memory to ID
Amplitude of shape is inversely proportional to the amount of agent
Optical filter used to filter desirable wavelengths
How do piezoelectric crystals work in analysis?
Can measure agent concentration
Lipophilic coated crystal undergoes changes in natural resonant frequency when exposed to lipid soluble agents
Change in frequency proportional to pp of agent
Lacks agent specificity
Sensitive to vapour
What is mass spectrometry?
Breath-to-breath
Charge particles of sample with beam and then separate components into a spectrum according to their mass to charge ratio
Done in high vacuum avoids interference
Abundance of ions at certain mass:charge ratios is determined and relates to composition of mixture
Magnet used to separate ion beam into spectrum
(Sample hit with high energy electrons and exposed to MF to sort ions)
High expense
What systems can measure oxygen?
Paramagnetic Polarographic Galvanic Mass spec Raman spec
What systems can measure CO2?
IR
Mass spec
Raman spec
What systems can measure agent concentration?
IR
Mass spec
Raman spec
Piezoelectric
What is a pneumotachograph?
Measure gas flow and calculates flow rate, TV and MV
Component: tube with fixed resistance from a bundle of parallel tubes, 2 P transducers either side
Sense a change of P across resistance
(Gas flow through is laminar)
Change in P proportional to flow rate
TV summated over minute - MV
Bi directional
Water vapour at resistor affects accuracy - hearing tubes help
What factors affect pneumotachograph readings?
Location Gas composition Gas temperature Humidity Dead space
What is a peripheral nerve stimulator for?
Monitor transmission across neuromuscular junction
Establishes depth, reversal and type of block
What is EMG and NMT?
Electromyography
- electrical activity
Neuromuscular transmitter
- transmission across junction
What nerves can be stimulated?
Ulna
Common peroneal- at fibula neck
Posterior tibial - at ankle
Facial
What is the tetanic stimulation mode?
A tetanus of 50-100Hz is used to detect residual block
Fade will be apparent
What is TOF mode?
Assess degree of block Ratio of 4th to 1st twitch 4x 2Hz twitches over 2s Fade noticed first, then lose 4th then 3rd etc twitch 2 twitches to reverse >3 absent for ideal abdo surgery
What is the double burst function?
More accurate assessment of residual block
2x short 50Hz tetanus
Each twitch comprises of 2-3 square wave impulses
What safety considerations should be thought of for nerve stimulators?
Awake can hurt
Hand muscles small in comparison to diaphragm therefore result at hand doesn’t reflect true depth of diaphragm block
What is BIS?
Bispectral index
Monitor electrical activity and sedation of the brain
Assess the risk of awareness
Allows titration of hypnotics based on individual needs
Measures hypnotic component but less sensitive to analgesic components
What are the components of BIS?
Display: BIS value, facial EMG, eeg suppression, signal quality index
Forehead sensor with 4 electrodes and a smart chip
Small tines in electrode part outer skin layers and hydrogel to make contact
Reduces impedance and optimise
Flexible design fits most
How does BIS work?
BIS is a statistic analysis, empirical method that quantifies the level of synchronisation of underlying frequencies
Value derived mathematically using info from EEG, frequency and BIS info
Produces linear scale 1-100
What are the values for BIS?
100 - awake 0 - electrical silence 65 - 85 sedation 40 - 60 GA 60 return of conscious state
What are some safety features of BIS?
Inaccurate at low temperatures
Ketamine is dissociative so cannot monitor
Insufficient data for use with neuro disease
Interference with diathermy and EMG
What is entropy?
Measure anaesthetic depth by amount of disorder of eeg signal
Forehead sensors
Calculates the frequency of voltages for each time sample and converts into normalised spectrum
SE: 0-91; RE: 0-100
What is state and response entropy?
State: calculated from low frequency range corresponding to EEG
Response: calculated from high frequencies and EMG from frontal muscle
What are the components of invasive arterial monitoring?
In-dwelling Teflon cannula
Has parallel walls to reduce effect of flow to distal limb
Column of NaCl (hep) at 300mmHg in flushing device
Connects to transducer which connects to amp and oscilloscope
Strain gauge variable resistor used
Thin membrane diaphragm is interface between fluid and transducer
Low compliance tubing so doesn’t move with each pulse
How does invasive arterial monitoring work?
Transducer changes R in response to change in P
NaCl column goes up/down with arterial pulse causing movement of diaphragm
Transducer connects to Wheatstone bridge
Changes in R and I are converted and displayed as systolic, diastolic, MAP
3-4ml/Hr flush cannula to prevent back flow and clotting
How does a Wheatstone bride in the arterial transducer work?
Electrical circuit for comparison of resistors
Galvanometer, 4 resistors in 2 parallel branches
2 known R, 1 Variable and 1 Unknown
The ratio of the 2 branches are compared and a current will flow through the galvanometer if there is an imbalance and therefore a reading
Draw the waves for an optimal, under and over damped pressure wave and for a square wave test.
See notes
What are some safety features of arterial monitors?
Transducer should be at RA level as reference point; every 10cm out gives a 7.5mmHg error
Haematoma, infection, nerve damage, thrombosis, ischemia, bleeding
More peripheral arteries are narrower and less compliant therefore increased amplitude
Narrow/bifurcate arteries impede flow so backward reflect of P wave
Drug injection = occlusion, gangrene
Re-zeroing to reduce baseline drift
Why is zeroing an arterial line important?
Eliminates atmospheric pressure effects
Ensure monitor indicates zero in the absence of applied pressure therefore eliminating drift
How is a CVC placed?
Jugular, subclavian or basilic vein
Sterile with landmark or ultrasound guidance
Seldinger technique
J-shaped soft tip wire inserted through into needle. Tip lessens trauma. Needle removed. Small incision in skin. Dilate. Railroad catheter over wire then remove wire. Aspirate and flush each port. Secure and check with X-ray
What is a PICC line?
Inserted via ACF
Prolonged drug therapy which may be vein irritant
Several months
What is a Hickman line?
Inserted via subclavian vein with proximal end tunnelled under the skin. Cuffed to hold in place and reduce infection
Long term therapy
Months - years
What are some safety features of a CVC?
Too far in - arrhythmia Blocked, failure Pneumothorax Infection, bleeding, nerve damage Air embolism Haematoma Tracheal puncture
What are ways to reduce CVC infection rate?
Sterile method Subclavian site Ultrasound guide to reduce number of attempts Minimal number of ports Remove ASAP AB coated lines Sterile, clear dressings
What are the components of a PA catheter?
5-8G Upto 5 ports Distal lumen is PA Proximal lumen in RA Other proximals can infuse drugs Another lumen houses wires for thermistor - 3.7cm from tip Lumen to inflate balloon up to 1.5ml air Can be inflated to float with blood or when wanted PCWP Lumen to connect to P transducer Some can measure mixed venous O2 sats
What is the mechanism of the PA catheter?
Flush lines, test balloon
Connect to P transducer
Partially inflate balloon to get to RA
Watch waveform!
Fully inflate balloon to get tip to pulmonary artery branch where it will wedge. This measurement reflects Left atrial filling pressure
Deflate balloon so catheter floats back to PA. Keep deflated until PCWP wanted
How does thermodilution work?
10ml of cold injectate administered upstream. Thermistor in PA measures change in temperature of blood downstream and a temperature time curve is created. This can then calculate CO. Volume of injectate must be accurate and quick. Mean of 3 readings used.
Low blood = high temperature change
High blood = low temperature change
What are the safety concerns of a PA catheter?
Arrhythmia VT Heart block Valve damage Perforate PA Don't advance >10cm without a change in waveform Thrombosis, infection, pulmonary infarction Whip artifacts
What is the LiDCOrapid?
CO monitor uses arterial waveform analysis to generate cardiac output
Assess fluid and inotrope affects
Algorithms used
Can assess SV and its response to fluid challenges
How does a thermistor work?
A temperature dependant semi conductor and a Wheatstone bridge
A change in temp = change in R
Small and cheap
How does an IR tympanic temperature probe work?
Small probe inserted into external auditory meatus
Detected by a series of thermocouples (thermopile)
Detector receives IR from tympanic membrane
IR signal converted to electrical signal
Must be accurately timed
False low; wax
How does a thermocouple work?
2 strips of different metals in contact from both ends and galvanometer
One junction for measure and one for reference
Metals expand and contract to different degrees with changing temp which produces a voltage compared with reference
Generate a voltage which is temperature dependant
Accurate to 0.1
What are the measurable sites for temperature?
Rectal Oesophageal (core) Tympanic (core) Bladder (accurate if output normal) Skin Axillary
What is the oesophageal Doppler?
Quick estimate of CO
Probe in Oesophagus
Close to aorta; minimal interference
Adequate sedation needed
How does an oesophageal Doppler work?
Principle: high frequency when approach and low when travelling away
Ultrasound waves encounter moving RBC. Frequency determines flow
Probe inserted in mouth with bevel facing up at back of throat
Probe rotated and slowly pulled back
Position: 5-6 thoracic vertebrae adjacent to aorta
What is the mechanism of action of a hot water bath?
Water heated 45-60 degrees
Dry/cold gases enter container where some pass close to surface of water = max saturation
Container has ^ SA for vaporisation which ensures gas is fully saturated
Tubing has poor thermal insulation causing a drop in temp
By ^ temp > body temp it’s possible to deliver it at 37 degrees, fully saturated
Temp measured at pt end and feedback to control water temp
What are the safety features of hot water bath?
Need electricity Potential shock/burn 2nd thermostat if first malfunctions Water trap should be lower than pt to prevent flooding of airway by condensed water Water may be colonised Microbe risk drops with higher temp but burns risk then increases Large, expensive Leak risk
Define relative humidity.
Ratio of the amount of water vapour in the air at a specific temp compared to the maximum it could hold
Define absolute humidity.
Measure of the amount of water vapour in the air. Measured as partial pressure
g H2O vapour/ cm3
What is the minimum humidification outcome for an HME?
30 degrees
30gm-3
What are some comments regarding the position of the filter in the system?
Should be higher than lung level and in a vertical position to reduce chance of a complete block. A block on pt side stops exp but allows insp.
Should be after lime to prevent drying and therefore not encourage compound A/monoxide production
What are the effects from theatre pollution?
Abortion Low fertility Haematological malignancy Renal/liver dysfunction Low mental performance
Describe the collection system for AGSS?
Gather from APL or from ventilator exhaust valve
30/19mm connection
This mustn’t cause R to expiration
What are safety features of the unidirectional valves?
Clear housing
Hydrophobic
Sit on knife edge to avoid sticking
How does colorimetric CO2 work?
It contains paper which changes colour in the presence of CO2 based on pH
What is the conversion between kPa and water?
1 kPa = 10 cmH20
What is a port-a-Cath?
SC port attached to a central catheter
Surgically placed
Prolonged intermittent therapy
Months - years
What is a CVC?
Resuscitation or pressure monitoring
Up to 14 days
What are the indications for central lines?
Chemo Long term AB TPN Haemodialysis Repeated transfusions or samples Peripheral irritant
What are the correct DLT sizes?
41/39 male
37 female
