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Equipment - Coggle Diagram
Equipment
Depth of Anaesthesia monitoring
BIS (bispectral index)
What is it
Correlates with effect site concentrations of anaesthetic drugs
Not an accurate relfection of depth - more to measure drug effect
Reflection of bioelectrical signal for cerebral cortex
Values
1 MAC Halothane 57
1 MAC Sevoflurane 32
Ketamine increases CBF and CMRO2 - so can increase BIS despite increased depth
N2O also increases BIS
Dimensionless number based on
Synch fast:slow ratio
Burst suppression
Beta ratio of EEG
Aim 40-60 (out of 100)
Physiological changes that affect BIS
Hypothermia
Hepatic encephalopathy
Hypoglycaemia
Hypovolaemia
Hypotension
Physiological sleep
Aims to reduce incidence of awareness and titrate anaesthesia
Risk factors of AAGA
Anaesthetic factors
NMBs, difficult intubation, circuit disconnection, equipment failures
Surgical factors
Obstetric, cardiac, trauma emergency
Patient factors
Prev AAGA, haemodynamic instability, extremes of age, hepatic disease, hyperthyroid, chronic EToH/opiate use, high BMI
Isolated forearm technique
Gold standard
Tourniquet forearm above arterial pressure prior to NMB dose
Pt can signal with hand if conscious
Issues
Pts under sedation follow commands but aren't distressed
AAGA can occur without NMB (though rare)
50% of patients who respond won't recall
Most are processed EEG monitors
Record breif epoch and break wave down via fourier transformation
Gives different frequency of waves
Compressed spectral array
Entropy
Uses compressed spectral arrays
Gives two numbers
State entropy
Only lower EEG frequencies (just brain)
Response entropy
Include higher frequencies (might be EMG)
?Includes grimace to pain
Features
Generates dimensionless number out of 100, aim 40-60
Interference with Eschmann tables
Unreliable with Ketamine & N2O
Narcotrend
Uses CSA
Auditory Evoked Potentials
Look for EEG evoked by clicking sounds
Clicks at 7Hz via headphones
Intraop TOE
General
Used in cardiac and non cardiac procedures
Sits it oesophagus or stomach
Recommended
Intraop Mx of valve repair/replacement
Useful to target fluid filling requirements
Uses
Can assess MV repair
Detectic systolic anterior motion & mitral stenosis
Limitations
Limited views of ventricular apex
Difficult to assess AS
AR and MR shouldn't be assessed under GA
Cardiac Output Monitoring
PAC
General
Gold standard for CO monitoring
Highly invasive
Allows various pressure measurements
Features
Antimicrobial non latex polymer
5-7 lumens
Thermistor @ distal end
CVC lumens for pressure monitoring, venous sampling and cold injectate
Distal balloon
+/- FO intrared spectometry for continuous SvO2 monitoring
Distal lumen - pressure transduction & mixed venous sampling
+/- heating coils for self calibrating continuous CO monitoring
110cm CVC with markings every 10cm
Insertion
Catheter inserted into sheath into SVC/RA
Inflate balloon with 1.5ml air
Sheath into central vein (RIJV preferred as feeds into RA)
Smooth insertion and balloon carried with blood RA->TV->RV->PV->PA
Full asepsis
Once tip in place, inflate balloon - gives wedge pressure (let down afterwards!)
Direct column of blood to LA - gives LA filling pressure
Complications
Pulmonary infarction if balloon is left inflated
Pulmonary artery rupture - mortality 30%
Balloon rupture + air embolism
Thromboembolism
Knotting of catheter in heart
Endocarditis
Ventricular arrhythmias
PV and TV regurgitation
Thermodilution
AUC is used to calculate via Steward Hamilton equation
Average of 3 readings
Cools blood - time for it to arrive @ thermistor - temp change is inversely proportional to CO
Bolus cold fluid into proximal port - 10ml of 10C saline
Disadvantages
? doesnt change patient outcomes
Inaccurate if shunts present
High complication rate (4 in 1000)
Invasive
Indications
Guide pressor vs inotropes
W/ PAFC - measure R heart and PA pressures
Guide fluid therapy
Differentiate between cardiac and non cardiac causes for pulmonary oedema (PAFC)
Optimise O2 tissue delivery
'Non invasive' CO monitors
Oesophageal Doppler
General
Cross sectional area calculated from normogram via age/sex/height/weight
RBC velocity time x CSA = blood flow
Measures movement of RBCs moving away from it in descending aorta
Gives CO measurement of descending aorta only
Features
Connects to monitor with pt parameters set in
Probe inserted/retracted/rotated to achieve best possible trace
Oral or nasal insertion (not awake pts)
90cm long, 6mm diameter, insulated flexible probe with 4-5MHz transducer mounter @ 45 degrees @ tip
Measurement
Calculated variables (SVR & SVRI)
FTc (corrected flow time) - porportion of cardiac cycle where there is forward flow (<0.33s means fluid responsive)
CO/SV/HR
Peak velocity is broad surrogate for LV contractility
Advantages
Probes can be left in place for 24h
Relatively good concordance with PAC (esp trends)
Easy to use
Less invasive than PAC
Disadvantages
Inaccurate if aortic pathology
Contraindicated in some oesophageal issues
Operator dependent signal
Estimate of CSA may be inaccurate
Intubated patients
CSA is dynamic - pressors/clamps etc
Assumed 70% of blood flow - may not be correct
Diathermy interferencs
Pulse Contour Analysis
General
Most use thermodilution calibration
Analyse pulse pressure waveform
Non invasive but require Aline & CVC
PiCCO
General
Waveform analysis and AUC of systolic portion
Specific proximal Aline (brachial/femoral)
Calibrated via thermodilution - 15ml saline @ 8 degrees into CVC
Aline has thermistor @ tip - measures drop after pul vessels and left side of heart
Greater thermal loss to tissue than PAC so temp curve is delayed and shallower
Regular calibration allows continuous CO monitoring
Advantages
Continuous monitoring
Lots of variables that can be derived
Less invasive than PAC
Can be used in kids > 2kg
Good correlation with PAC
Disadvantages
Requires accurate Aline transduction as uses diachrotic notch
Inaccurate with shunt
Regular recalibration
CVC and proximal Aline needed
Flotrac
Standard Aline
No calibration required
Uses normogram
20s of enhanced data collection initially and derives values from there
LiDCO
General
Calibration system withdraws blood @ 4.5ml/min
PVC membrane allows Li ions to permeate and generate current
2-4umol/kg Li ions injected into peripheral and CVC
AUC via Stewart Hamilton -> SV derived via algorithm
Waveform analysis by lithium dilution
Advantages
Good correlation with PAC
Continuous once calibrated
No CVC needed
Less susceptible to dampening
Any Aline can be used
Disadvantages
NMB interfere with lithium analysis
Li contrainidcated in pregnancy or pts <40kg
Cant be used if pts on lithium
Regular calibration
Rapide
No calibration - works via normogram of pt details
Non continuous
Good for trends
Only licensed for surgery
PPV
13% = fluid responsive
<10% non fluid responsive
Gives value for Aline swing
No good if arrhythmia
Partial Gas Rebreathing
Works via Fick's principle
Uptake of substance by organ = blood flow x AV conc difference
CO ~ blood to lung, AV conc difference can be CO2 (ETCO2 as surrogate)
Short period of rebreathing wont increase total body CO2, so amount CO2 rises will relate to CO
Advantages
Non invasive (pt already intubated)
Disadvantages
Requires stable ventilation paramenters
IPPV
Require intubation
Inaccurate if lung disease causing increased AA gradient
Thoracic Bioimpedence
Small high frequency current passed through chest during respiration
Measures impedence change due to blood volume in chest & measures pulsatile component
Truely non invasive
Disadvantages
Muscle activity causes interference
Leakage currents from other devices
High signal-noise ratio
Inaccurate in critically ill
Finapres
Cuff around finger with LED - volume of finger increases as it fills with blood
Pressure in cuff increases to keep light absorption constant
Small, portable and easy to use
Quite well validated
Diathermy
Unipolar
Power 400W (J/s)
Isolating capacitor is placed between pt plate and the earth
Capacitor has low impedence to diathermy high freq current but high impeance to 50Hz current - protecting pt from electrical shock
Cutting and indifferent electrode
Frequency 500kHz-1MHz
Bipolar
Power output 40-140W (J/s)
Circuit is not earthed
Freq 0.5-1MHz
Waveform types
Coagulation mode - pulsed square wave
Cutting mode - continuous square wave