Equipment + Measurement

Question 1

Gas

Answer 1

Exists in the gaseous state at room temp. Its liquefaction at room temperature is impossible, since the room temperature is above its critical temperature.

Question 2

Vapour

Answer 2

Gaseous state of a substance below its critical temperature. At room temperature and atmospheric pressure, the substance is a liquid

Question 3

Critical temperature

Answer 3

Temperature above which a substance cannot be liquefied no matter how much pressure is applied

Question 4

Pulse Oximetry

Answer 4

• Probe with 2 LEDs (red 660nm, infrared 940nm) and a photodetector
• A microprocessor analyses the signal (calculates R:IR absorbance ratio = R value)
• 3 phase cycle >200Hz
  => Red on, IR off
  => Red off, IR on
  => Both off (baseline - so interference from ambient light can be detected and subtracted)

Isobestic point = 590nm + 805nm (wavelength at which oxy- and deoxyHb absorb the same proportion of the two wavelengths of light)

Pros
- Safe, non invasive
- Accurate within clinical range (70-100%)
- Fast response time, cont reading
- Changes in audio tone
- Plethysmography trace gives index of peripheral perfusion
- Accuracy not effected by HbF or HbS

Cons
- Susceptible to errors -> inaccurate reading
- Potential for burns if fault

Sources of error
- Low signal to noise ratio (pulsatile component only 1-2% of total signal, hence reading prone to inaccuracies is any further reduction is signal)
- Arrhythmias
- Different Hb (metHb - absorbs red and IR light 1:1 => falsely low readings. COHb => falsely high)
- Electromagnetic interference (e.g diathermy)
- Coloured dyes and nail polish (falsely low)
- Sats <75% (reading extrapolated)
- Lag in reading change (up to 30s)
- Venous pulsations (may be interpreted as arterial -> falsely low)

Question 5

Humidifiers

Answer 5

Passive - HMEs
Active - THRIVE

HME
- Pleats of paper provide large SA which condensation/ vaporisation can occur
- Hygrosopic (water retaining)
- Hydrophobic (water repelling), or combination of both

MOA
- Expire warm, fully saturated air
- Gas passes through HME, loses heat to cooler ambient temps => condensation + secondary hygroscopic + hydrophobic properties
- Condensed water evaporated and returned to patient on next inspiration
- Relative humidity of 60-70% achieved

Pros
- Cheap
- Lightweight
- Portable
- May contain anti-bacterial filter

Disadvantages
- Incr resistance (require 5-10cmH2O PS to compensate)
- Incr apparatus deadspace
- Obstruction with mucous + water accumulation => incr resistance
- Not as effective cf active humidifer
- Efficiency decr with large VT
- Only lasts 24hrs

Question 6

Humidity and intubation

Answer 6

Absolute humidity - mass of water vapour present in a given volume of air (mg/L or g/m3)

Relative humidity - the ratio of the mass of water vapour present in a given volume of air to the mass of water vapour required to saturate that volume at that temp (%)

Intubation bypasses humidification function of nose
- Dehydration of mucosa
- Altered ciliary function
- Inspissation of secretions -> obstruction, atelectasis, V/Q mismatch
- Loss of body heat, esp neonates

Question 7

DINAMAP

Answer 7

Components
- Cuff with tube
- Case with microprocessor, pressure transducer and solenoid valve

Question 8

Arterial Pressure waveform

Answer 8

• Primary/ fundamental frequency = HR (60-120bpm = 1-2Hz)
• Dicrotic notch represents change in pressure due to closure of AV
• Rate of rise of upstroke reflect myocardial contractility. Slow rise requires inotropic support. Max upward slope related to speed of ventricular ejection
• Position of dicrotic notch - PVR. In vasodilated - lower on curve. Vasoconstrictor - high on curve
• Downstroke slope - resistance to outflow. Slow fall seen in vasoconstriction
• SV can be estimated by measuring area from start of upstroke to dicrotic notch, then x HR = CO
• Systolic time = myocardial O2 supply
• Diastolic time = myocardial O2 demand
• MAP = average pressure throughout cardiac cycle

Question 9

CO measuring - non-invasive

Answer 9

Calibrated
- PiCCO - uses CVL and thermistor tipped art line, CO estimated by pressure waveform analysis. Calibrated using trans pulmonary thermodilution method
- LiDCO - standard art line, calibrated by lithium dye injected into peripheral (or central vein), fall in conc measured by lithium electrode sampling art line. Need recal Q8hrly.
=> avoids error of pulm heat dissipation
=> cannot be used in pt on lithium
=> freq calibration introduces error
=> muscle relaxants may cross react with lithium
=> underestimated CO when afterload is low
=> cardiac shunts introduce error in both LiDCO and PiCCO

Non-calibrated
- FloTrac/ Vigileo - pressure sensor attached to art line, waveform analysis
- LiDCO rapid - same algorithm as LiDCO but uses nomogram for calibration

Sources of error
- Under or over damped art line
- Used of IABP
- Arrhythmias
- Aortic regurgitation
- SV variation

Question 10

CO measurement - invasive

Answer 10

Pulmonary artery catheter
- Inserted into IJ or subclavian vein, proximal lumen connected to transducer with continuous pressure monitoring and inserted into RA/ RV, then balloon inflated and floats until it is in the PA/ branch
- RA 3-8mmHg, RV 25/0, PA 25/10, PACWP 4-12

Indicator dilution methods
- Uses Fick principle - based on law of conservation of mass. Uptake or excretion of a substance is equal to the difference between the amount of substance entering and the amount leaving the organ

Thermodilution technique
- Requires PAC with thermistor
- Heat lost from blood = heat gained by injectate
- 10-15ml of normal saline at room temp with rapid injection
- Change in PA temp measured and temp-time graph plotted
- CO calculated using mod Stewart-Hamilton equation (large AUC = small CO)
- Gold standard
- Errors - speed of injection too slow (do average of 3), low CO (injectate may increase preload briefly and thus CO, overestimates CO), inaccurate temp recording of injectate, false assumption (thermal equilibrium established by the time mixture reaches thermistor, injected warmed by blood only, flow unidirectional, cold injectate does not depress CO)

Dye dilution technique
- Known amount of indicator dye injected into PAC and conc continuously samples at peripheral art line
- Indocyanin green (short T1/2 and low toxicity), Li also used
- Change in conc over time graph
- Calculated from AUC
- Error introduced from recirculation of the indicator dye which causes a second peak on the graph

USS oesophageal doppler
- Minimally invasive
- Small USS transducer on tip of probed inserted via mouth or nose to oesophagus to T5-6 (descending aorta adjacent and parallel to oesophagus)
- USS beam directed at 45 degrees to direction of aortic blood flow
- Doppler equation -> velocity of flow in aorta calculated, + CO, SV, volaemic status, SVR and myocardial function
- Pros - minimally invasive, nil CVL or art line. No calibration. Continual CO measurement
- Cons - movement of probe may lead to poor trace, frequent probs repositioning required. Estimates may be inaccurate

Question 11

Critical temp of O2 and N2O

Answer 11

O2 = -115C (at room temp = gas)
N2O = 36.5C (at room temp = gas + liquid)

Question 12

TOF

Answer 12

• 4x 0.1ms stimuli 0.5s apart (2Hz)
• TOF ratio = ratio of the 4th twitch height to the 1st twitch height
  => TOF >0.9 = adequate level of reversal for extubation
• Depolarising blockade - reduced the amplitude of all 4 contractions with NO fade
• Non-depolarising blockade - causes fade, reduction of twitch height with each subsequent stimuli
• TOF fade is difficult to evaluate manually or visually when the ratio is >0.4
• TOF count correlates with receptor occupancy
• 2 twitches present (~80% receptors occupied) = anatgonisable block

Question 13

Tetanic stimulation

Answer 13

• 50Hz stimulation for 5s -> sustained muscle contraction
• Non-depolarising blockade
  => Tetanic stimulation fades with time
  =>Blockade of pre-synaptic AchR by NDMR -> no positive feedback -> decr production of Ach vesicles -> decr mobilisation of Ach at time of peak activity
• Effect may last up to 6mins -> underestimation of neuromuscular block
• Painful in awake

Question 14

Post-tetanic potentiation and count

Answer 14

Post-tetanic potentiation - incr response to a given stimulus following tetanic stimulation. Due to incr synthesis and mobilisation of ACh. Additional ACh is able to compete with NDMRs for receptors.

PTC
- Single twitch at 1Hz are started 3s after tetanic stimulation for 10s
- Number of twitches is inversely related to the level of block
- Useful when profound neuromuscular blockade is required
- 1st twitch of TOF returns with PTC of 9
- PTC should not be repeated within 6mins, as result inaccurate

Question 15

Double burst stimulation

Answer 15

• 2x short burst 50Hz tetanic stimulation 750ms apart
• each short burst consists of 3x0.2ms stimuli separated by 20ms
• Fade is easier to detect manually
  => Bigger response
  => Two twitches are easier to assess than four
• Same requirements as TOF
  => Ratio >0.9 required for adequate reversal
• can detect fade 60% difference cf 40 in TOF

Question 16

BIS

Answer 16

BIS algorithm
- Records the frontal EEG
- Analyses the waveform using various techniques (time domain, frequency domain, bispectral analysis)
- Computes scores for several variables

Time domain - power vs time
- Looks at burst suppression ratio (burst suppression time/ epoch time )
- Epoch time- duration of the EEG sample analysed (63s)

Frequency domain analysis - power vs frequency
- Fourier analysis - complex signal broken down into sines waves/ frequency/ amplitude
- Principle that deepening anaesthesia higher frequencies and power in lower frequencies increases
- Calculates the spectral edge frequency (SEF95)
- With deepening anaesthesia → ↓SEF95

Bispectral analysis - examines relationship between the phase of pairs of frequencies (biocoherence)
- Biocoherence - measure of degree of phase coupling between signals (0 random, 1 invariable phase relationship)
- Deepening GA -> incr biocoherence in delta-theta range

Question 17

Entropy

Answer 17

• Combination of time and frequency domain analysis
• ‘Regularity’ or the amount of disorder of the EEG signal used
• Fourier transformation of raw frontal EEF into its component sine waves and then power spectrum obtained

State entropy (SE) index - -predominantly to EEG activity

Response entropy (RE) index - EEG and includes EMG activity from frontalis muscle

Question 18

Laser

Answer 18

Light amplification by stimulated emission of radiation

• Monochromatic - same wavelength and frequency
• Coherence - photons in phase
• Collimated - photons move parallel to each other, little divergence of beam, negligible loss of power with distance

Components:
1. Energy source
2. Lasing medium (solid, liquid, gas)
3. Optical resonator (cavity with reflective mirrors and a route out for the last beam to leave the cavity)

Mechanism
- Energy source passed through easing medium, housed in a resonator made of mirrors
- Stimulated absorption - as easing medium excited, electrons enter a higher energy level (unstable excited state). Losing medium gives the laser its name and dictates wavelength emitted
- Spontaneous emission - as electrons fall back to their resting state spontaneously, they release a photon in a random direction. Energy contained by photon is proportional to its frequency
- Stimulated emission - occurs when an electron in excited state is hit by photon. Decays back to its resting state releasing another photon that is parallel to, in phase with, and of same wavelength as the photon it was struck by. Stimulated emissions results in amplification of light release - some photons move parallels to the axis of the chamber, these photons bounce back and forth between the mirrors hitting more excited electrons, causes chain reaction (amplification)
- Laser output - beam is focused through lens. Absorption of laser light results in localised heating and vaporisation

Hazards
- Eyes, skin burns, fires, smoke inhalation

Question 19

Viscoelastic/ TEG

Answer 19

R (reaction time)
- 4-8mins
- Start of sampling process to initial fibrin formation (2mm)
- Incr - anticoagulants, deficiency of coagulation factors/ inhibition (e.g heparin)
- Decr - hyper coagulable condition

K
- 1-4mins
- End of R (2mm) to 20mm
- Rate of clot formation
- Incr - anticoagulation, clotting factor deficiency
- Decr - incr fibrinogen, incr plt function

A-angle
- 47-74o
- Slope of trace from end of R
- Rate of clot formation
- Incr - anticoagulation, clotting factor deficiency
- Decr - incr fibrinogen, incr plt function

MA (max amplitude)
- 55-73mm
- Marks transition between coagulation and fibrinolysis
- Represents clot strength
- Decr - decr plt function, decr fibrin

LY30
- 0-8%
- Amplitude of trace 30mins after MA
- Represents % lysis 30mins after MA
- Incr - high fibrinolytic activity

Question 20

Paramagnetic oxygen analyser

Answer 20

Components:
- Two chambers separated by a sensitive pressure transducer. Sample gas and reference gas
- Electromagnet rapidly switched on and oof creating a changing magnetic field

MoA
- O2 attracted to magnetic field due to 2 electrons in unpaired orbit. Most other gases used in anaesthesia repelled (diamagnetism)
- Magnetic field causes O2 molecules to be attracted and agitated -> leads to changes in pressure on both sides of transducer.
- Pressure difference across transducer is proportional to the O2 partial pressure difference between the sample and reference gas
- Transducer converts this pressure force to an electrical signal this is displaces as O2 partial pressure or converted to a reading in volume percentage

Question 21

HME

Answer 21

Can achieve relative humidity 60-70%

• Hydrophobic e.g aluminium (less efficient)
• Hygroscopic (water retaining), e.g paper or foam impregnated with calcium chloride or lithium chloride (better efficiency)
• Combine hygroscopic-hydrophobic (best efficiency)

Hydrophobic
- warm humidified exhaled gas pass through HME, causing water vapour to condense on cooler HME medium. Condensed water is evaporated and returned to pt with next inspiration.

Hygroscopic
- Low thermal medium preserved moisture by a chemical reaction with the salt

Other notes
- Greater temp different between each side -> greater potential for heat and moisture to be transferred
- Requires 5-20mins before it reaches optimal ability to humidify dry gases
- Some designs have pore size 0.2um - can filter out bacteria, viruses + particles (HMEF)
- Can incr apparatus DS

Performance of HME effected by
- Water vapour content and temp of inspired and exhaled gases
- Insp and exp flow rates affecting the time the gas is in contact with HME medium, hence heat and moisture exchange
- Volume and efficiency of HME medium - larger the medium, the greater the performance. Low thermal conductivity, i.e poor heat conduction, helps to maintain a greater temp diff across the HME incr potential performance

Problems
- Incr resistance du to water accumulation. Obstruction with mucous -> further incr
- Use for max 24hrs + single pt
- Humidifying efficiency decr when large Vt
- 2 way gas flow required
- Need to place in breathing system close to pt

Question 22

Ideal filter

Answer 22

• Efficient
• Min dead space (esp for paeds)
• Min resistance
• Not affected by anaesthetic agents
• Effective when wet or dry
• User friendly
• Disposable
• Provide some humidification
• Transparent
• Cost effective

Question 23

Filter/ HEPA

Answer 23

Filtration element - electrostatic or pleated hydrophobic

MoA
- Direct interception - large particles (=1um), e.g dust and large bacteria, physically prevented from passing through pores
- Inertial impaction - smaller particles (0.5-1um) collide with filter medium because of their inertia. Tend to continue in straight lines, carried by their own momentum. Particles are held by van der Waals forces
- Diffusional interception - very small particles (<0.5um), such as viruses captured, because they undergo Brownian motion (random movement). Movement increases their apparent diameter so more likely to be captured by filter
- Electrostatic attraction- incr charge on particle or fibres incr filtration efficiency
- Gravitational settling - affects large particles (>5um), rate of settling depends on balance between the effect of gravity and buoyancy of particle.

Electrostatic
- Element subjected to electric field producing felt-like material with high polarity. One type of fibre becomes positively charged and the other negatively. Usually two polymer fibres used.
- Filtration efficiency 99.99%
- Incr efficiency when dry, deteriorates rapid when wet + incr resistance to flow
- Electrical charge decays with time
- Can add hygroscopic layer to provide humidification

Pleated hydrophobic
- Small pore size
Rely on naturally occurring electrostatic interactions to remove particles
- 99.999% efficiency
- Can repel water even under high pressures
- Longer life span

Question 24

Charcoal/ vapour filters

Answer 24

• Activated charcoal
• Mechanical and chemical filtration
• Remove traces of volatiles from anaesthetic machines
• Used in preparation for vapour free anaesthetic, e.g MH patients
• Place on both insp and exp limb

Question 25

Soda lime

Answer 25

Ca(OH2) = 80%
NaOH + KOH = 5%
Water = 15%
Silica (hardness)
pH indicator (ethyl violet - changes from white -> violet as becomes more acidic from CO2)

CO2 + Ca(OH)2 -> CaCO3 + H2O

• Slow reaction hence intermediary steps to incr speed
• Overall there is heat and water production- incr heat and moisture in circuit

Breakdown products
- Compound A
- CO

Question 26

Compound A

Answer 26

Sevo interacts with CO2 absorbent to produce toxic degradation production (nephrotoxic in rats)

Incr Compound A
- low flow closed circuit
- high sevo conc
- CO2 absorbent with greater strong base (e.g Baralyme >sodalime)
- high absorbent temp
- dry absorbent

Question 27

Temperature monitoring

Answer 27

Electrical
- Thermistor
- Thermocouple
- Infrared
- Liquid crystal displays

Non-electrical
- Mercury
- Bimetallic strip
- Bourdon gauge

Question 28

Diathermy

Answer 28

• High frequency AC current to cause localised heating of tissue.

High frequency has negligible risk of arrhythmia
- At low frequencies VF likely (50-60Hz)

Relies on principle of current density
- High current density at tip of instrument

Cutting
- Continuous, undampened sine wave
- Low voltage (250V), high current (200mA), high frequency

Coagulation
- Short bursts of dampened sine wave
- High voltage (9000V), low current, low frequency
- Less heat produces - current intermittent, total current lower, current dispersed over great area

Monopolar
- Dispersive electrode type - diathermy pad on patient (return plate) - circuit passes from probe, through patient to pad and then back to machine
- Large SA of pad reduces current density and prevents burns at the contact area. Now have split pad design - If pad begins to peel off -> decr current -> alarm and stops functioning

Bipolar
- Electrical current passes between two prongs of diathermy forces
- Only tissue between forceps exposed to current - better safety

Hazard
- Burns to patient
- Fire hazard
- Electromagnetic interference
- Diathermy smoke
- VF
- Metastatic seeding
- Muscle stimulation
- Electrocution

Question 29

Scavenging

Answer 29

Collection and removal of waster anaesthetic gases from the anaesthetising location.
- Prevents exposure of stage to agents + build up of O2 rich environment -> fires

Classification
- Active (high pressure, low flow centralised vacuum) vs passive (positive pressure in circuit to drive flow)
- Open (entrainment of room air) vs closed (positive +/- negative pressure relief valves)
- Passive systems always closed, active may be either

Components
- Collection assembly - input from APL and ventilator
- Transfer tubing - corrugated, resists kinking or occlusion, has diameter specific connections
- Scavenging interface (or receiving system) - flow control, reservoir (allows for the temporary accumulation of waste gases), valve, system volume. The vacuum rate should exceed the average rate of waste gas flow and hence room air will be entrained via the relief ports
- Disposal tubing + assembly – conduit material, vacuum source, venting

Hazards
- Build up of excessive positive pressure
- Build up of excessive negative pressure
- Release of Green house gases

Safety
- Pressure relief valves
- Connection size at transfer tubing ensure not mistakingly connect to breathing circuit