Equipment and data Flashcards
Pulse oximetry (RCoA old book)
Red 660/ infrared 940 Isobestic points 590/805 Beer-Lambert laws (concentration and thickness respectively) SpO2<70% is inaccurate/extrapolated Causes of inaccuracy Time lag 30-60s Burns possible CO-oximeter - part of modern blood gas analyser; measures carboxyHb and methHb as well as oxyHb; therefore useful for elucidating causes of hypoxia, and demonstrating falsely low/high pulse oximetry readings
Defibrillator (Mendonca)
A step-up transformer converts 240V to 5000V. A rectifier converts AC to DC. Electrical charge is stored in a capacitor before release. An inductor slows the discharge from the capacitor, lengthening the shock duration, and also absorbs some of the energy. Thoracic impedance (about 50 Ohms) is reduced by gel pads, large paddle size, and by delivering shock in expiration. Only 4% of the delivered energy passes through the heart. Critical mass theory = a critical mass of cells needs to be depolarised. Capacitance is determined by surface area of the capacitor plates and thickness of the insulating layer between. Monophasic = single current pulse in one direction. Biphasic = two consecutive pulses in opposite directions. Defib threshold lower in the latter - more efficient, smaller capacitor and battery needed. Energy = 1/2 QV. Q = charge. ICD: energy 0.1-30J. Can perform sync/unsynced shocks and anti-tachycardia pacing. Always also has PPM function for anti-bradycardia backup. External defib over ICD can cause EMI and myocardial burns. Difference between defibrillation and cardioversion = syncing. 90% success but high relapse rate. CI to cardioversion: digoxin toxicity and AF >48h without anticoagulation (need anticoag 3/52 before and 4/52 after). Remember: press sync every time, and need to press and hold shock button. Elective: keep SV, deep sedation or GA, pre-O2 but no airway required; propofol/sevo but not opioid.
Capnography (Mendonca, past Q)
Confirmation of ETT: clinical signs, capnography, fibreoptic bronchoscopy, oesophageal detector. Gold standard = direct visualisation and presence of EtCO2 for 6 consecutive breaths.
Pattern recognition
y axis can be % or kPa
Normal trace - see loose notes
Phase 1 - dead space, no CO2 2 - alveolar gas mixes with dead space 3 - plateau, EtCO2 is right at end 0 - inspiration Alpha and beta angles Can get phase 4 in obesity and pregnancy (upswing after phase 3 - reduced thoracic compliance) Reverse phase 3 - emphysema Camel hump - lateral position Notched phase 2 - leak Dual capnogram - single lung transplant, kyphoscoliosis, endobronchial (sometimes)
Normal EtCO2 = 38mmHg/5%
Flat trace: disconnection, extubation, oesophageal intubation, analyser not in circuit, analyser malfunction/blockage, resp/cardiac arrest, apnoea testing, complete obstruction
High CO2
- Excess production: fever, sepsis, MH, bicarb, tourniquet release, CO2 embolism, overfeeding, high CO/BP
- Reduced clearance: hypoventilation, endobronchial intubation, partial airway obstruction, rebreathing, leak, inadequate FGF, exhausted soda lime
Low CO2
- Reduced production: hypothermia, hypotension, cardiac arrest, PE, apnoea, total airway obstruction, disconnection, leak, condensation in sample line
- Excess clearance: hyperventilation
Sudden rise: correction of obstruction, ROSC
EtCO2 high/low: actual changes in CO2 production, altered alveolar perfusion or ventilation, technical factors.
Volume capnography
- Gives V/Q info
- Can measure dead space
- Gives CO2 elimination info
- Gives mixed expired CO2 conc
IR detects molecules of differing atoms (i.e. not O2).
Mainstream - cuvette with quartz window; beam of IR light passes through stream. Sensor must be heated to 39C to prevent water condensing on the sensor and causing inaccuracy.
Sidestream - sample aspiration 50-150ml/min. More convenient but time delay (transit and rise time) - esp in MRI.
Factors affecting IR capnography
- N2O - collision broadening (false high)
- Water vapour (false high)
- Response time - faster = more accurate. Total of transit time and rise time.
- Atmospheric pressure - increases density of IR-absorbing molecules
Last ICS review (2016) recommended use of capnography in cardiac arrest and CPAP (as disconnection/occlusion alarm). Existing recommendations (2011) are for intubation/tracheostomy, continuous use in all intubated pts, and on transfers.
Capnography/capnometry/colourimetric devices, Severinghaus electrode, infrared, mass spec/Raman spec/photoacoustic spec.
Collision broadening
Mass spec
Separates substances according to MW. Sample aspirated into vacuum chamber where an electronic beam ionises and fragments the sample. Ions are then accelerated by an electric field, and deflected by a magnetic field according to their MW. Lighter ions are deflected more. Detector plates measure molecules. Rapid response time and accurate but bulky and expensive.
Raman
Uses ‘Raman scattering’ for CO2. Sample is aspirated into a chamber where molecules are scattered by an argon laser beam. A measurable change of wavelength of light occurs.
Photo-acoustic
Uses pulsatile infra-red radiation. An acoustic signal is generated and measured by microphone.
BP measurement (Mendonca, past Q)
Non-invasive:
- Discontinuous (sphyg (Hg or aneuroid), DINAMAP or von Recklinghausen oscillotonometer)
- Continuous (Penaz/Finapress, Doppler)
Von R has 2 cuffs, DINAMAP 1 cuff. At SBP, oscillations start. At MAP, oscillations are maximal. Diastolic is calculated. Bladder needs to cover 80% of arm circumference. Width should be 20% greater than arm diameter. Midline of bladder to be over brachial artery. Too small a cuff over-reads, too large under-reads. Obese pts have conical arms - difficult. NIBP inaccurate at extremes of BP (over-read at low, under-read at high). Arrhythmias cause inaccuracy, as can movement/shivering. Can cause nerve damage and petechial haemorrhage.
Finapress = short for FINger Arterial PRESSure. Infra-red photo-plethysmograph detects finger blood volume. Finger cuff pressure constantly adjusts to keep volume constant (null deflection). The applied pressure correlates with arterial pressure and is displayed as waveform.
Invasive: arterial line
Arterial cannula, tubing, transducer, pressurised flush system (300mmHg), cable and monitor.
Cannula: Teflon (lower thrombosis risk), short, wide and stiff so less effect on natural frequency and damping.
Fluid-filled tubing transmits arterial pressure to the transducer. Filled with saline +/- heparin (anything more viscous would cause over-damping). Flow = 3-4ml/h to reduce clot formation. Tubing is non-compliant and should be bubble-free. Length max = 122cm and number of 3-way taps minimised (they have narrower lumen and reduce natural frequency so cause damping).
Info from arterial trace: HR (+ regularity of rhythm), BP, indication of preload (respiratory swing), contractility (upslope), afterload (downslope and position of dichrotic notch) and stroke volume (AUC up to dichrotic notch). CO can then be calculated.
Zero calibration to RA eliminates the effect of atmospheric pressure on the measurement.
Damping: tendency to resist oscillation. Caused by bubbles, clots, kinking, 3-way taps, narrow/long/compliant tubing. Damping coefficient of 1 = critical damping. DC of 0 = oscillation indefinitely. Optimal damping = 0.67 - best compromise between speed of response and accuracy (oscillation/overshoot). Over-damping under-reads SBP and over-reads DBP (values get closer). Under-damping over-reads SBP and under-reads DBP (values diverge). MAP stays same. Testing for optimal damping: square wave test/fast flush. 300mmHg is applied, causing a square waveform then oscillation. In optimal damping, 2-3 oscillations occur before values settle. Over-damped = no oscillations. Under-damped >3-4 oscillations.
Comps of art lines: haemorrhage, haematoma, thrombosis, ischaemia, dissection, nerve damage, infection, aneurysm, accidental drug administration, AV fistula.
Overdamped - coefficient >1
Underdamped <1
Critical 1
Optimal 0.64
Fibreoptic bronchoscope (Mendonca, past Q)
Body, insertion cord, light source, working channel, eyepiece, dioptre ring
Body houses a lever that manipulates tip
Working channel can accommodate suction, epidural catheter or O2
Cord 55-60cm; light transmission bundle, working channel and control wires to tip
Light carried to end of scope in fibres
Light from object reflected onto lens
Light focused onto image transmission bundle which carries image to eyepiece
Optical fibres are 6-10microns in diameter, arranged in bundles
Each fibre has a glass centre (core) where light travels, an outer optical material surrounding it (cladding) and a plastic coating (buffer coating) which protects the fibre from damage and moisture.
Light hitting the cladding is either absorbed (very little), transmitted/refracted or reflected. At a critical angle of incidence, all light is completely reflected (total internal reflection). This is repeated until the light emerges at the other end.
Fibreoptic relay system: transmitter, optical fibre, optical regenerator and optical receiver (photodiode).
Ultrasound (Mendonca, past Q)
2002 NICE - US guidance of CVCs
US > 20kHz
Piezo-electric crystal in probe generates US waves
Waves reflected at tissue interfaces, converted into electrical signal
Tissues can be hyper/hypo/anechogenic (latter = blood, air)
High frequency = better resolution but poorer depth penetration, and vice versa
Modes: A (single transducer), B (linear array), M (motion, for valves etc), 2D (commonest), Doppler.
Doppler: transcranial, echo, cardiac output, fetal
BART - blue away, red towards probe
US uses: lines, blocks, drains/taps, echo, FAST, CO, MCA flow, VTE diagnosis
PTX on US - loss of lung sliding, loss of comet tails (B lines (short)) but A lines (long) still present, lung point (junction between sliding and non-sliding parts) (NB lung bullae can appear similar)
Spine sign on US - indicative of basal consolidation as normally can’t see the spine through aerated lung
Pleural effusion estimated volume: max depth in mm x 20ml
FAST can detect 200ml with 90% sensitivity.
ICP monitoring (Krishnachetty, past Q)
Raised ICP: HA worse in am, N/V, altered mental status, papilloedema/visual dist.
M-K curve steeper with blood and CSF as non-compressible so decompensate faster.
Cushing: HTN, bradycardia, Cheyne-Stokes respiration
Managing raised ICP: approaches to reduce blood, brain and CSF
Measurement
Indirect - clinical features, imaging evidence, papilloedema
Direct - EVD and ICP bolts (extraparenchymal - extradural, subdural, subarachnoid; intraparenchymal). Bolts work by wire strain gauge or fibreoptic pressure transducer.
EVD ‘kept at 15cm’ means CSF column is allowed to reach 15cm above the zero point before it starts to drain. Can be recalibrated at the bedside and can treat raised ICP. Extra/intraparenchymal devices are subject to drift. All devices carry risk of CNS infection. If invasive ICP monitor CI, can use transcranial Doppler (Lindegaard ratio - MCA to carotid flow ratio. >3 = vasospasm).
Waveform: percussion (systolic), tidal (brain compliance), dichrotic (aortic recoil)
Lundberg A - plateau waves, always pathological
Lundberg B - absence indicates compromised autoregulation
Lundberg C - can be normal
RRT (Krishnachetty, past Q)
Indications: pH, K+, fluid, urea, sepsis, drug toxicity, severe hyperthermia
Types: HD, PD; intermittent/continuous, VV/AV
Modes:
Haemofiltration - water moves across by hydrostatic pressure/ultrafiltration and other molecules follow by convection (solute/solvent drag); removes larger molecules and water; no dialysate fluid
Haemodialysis - diffusion; countercurrent dialysate; removes small solutes down conc gradt, does not remove water
Haemodiafiltration - combined mode
SLED - slow low efficiency HD. hybrid, faster than CRRT but slower than HD
SCUF - slow continuous ultrafiltration - slow water removal.
Haemoperfusion - no semi-permeable membrane, blood passed over activated charcoal or ion exchange resin. Good for removing some drugs but also removes glucose and platelets.
PLEX - removes substances up to 500kDa. Replaced with HAS/FFP.
Plasmapheresis - centrifugation rather than membrane.
Choosing mode: aim of therapy (size of particles to be removed), CV stability (CRRT>HD), resources/staffing.
Pore size 5nm. MW up to 50kDa. SA 0.5-2 m2.
Complications: line-related, activation of clotting cascade/inflammatory response, anticoagulation-related, RRT-related (hypotension, disequilibrium, hypothermia, haemolysis, thrombocytopenia), exsanguination if line left open. Altered drug PK.
Dose: 25-35 ml/kg/h.
Sepsis: can remove TNF, IL-1/6/8, complement, plt activating factor on CVVH with high flow rate. Controversial. No ev improved outcome.
PD not used in ICU because: inefficient, not able to remove fluid, sepsis risk, need surgically placed catheter, causes respiratory embarrassment. However used in PICU because vascular access is challenging and peritoneal surface area is relatively larger than in adults.
Why pts dislike PD: have to be fluid restricted, body image (large abdo/catheter), causes constipation which then causes poor drainage, peritonitis/catheter infections, hernias, discomfort.
MARS/liver replacement - blood passed over a mesh of animal hepatocytes.
Cardiac output monitoring (Krishnachetty, past Q)
CVP inaccuracy: pt (PEEP, TR), equipment (transducer height, damping/resonance, bag pressure)
Non-invasive e.g. NICOM, transthoracic electrical bioimpendance
Minimally invasive e.g. pulse contour methods, FloTrac
Invasive e.g. oesophageal Doppler, TOE, PAC
Ideal properties: accurate, reproducible, non-invasive, quick, simple, cost-effective, operator-independent, continuous measurement, safe, minimal drift fast response time.
Fick principle and partial CO2 rebreathing - easy and non-invasive but need to be on mandatory ventilation and is inaccurate in any condition with V/Q mismatch or barrier to CO2 diffusion, so limited usefulness in critical illness.
ECGs (past Q)
Filters
CM5
Standard leads 1/2/3
Unipolar V1-6
Augmented unipolar aVR, aVL, aVF
QTc Max 440ms men Max 460ms women >500 risk of TdP <350 is short
Decontamination
Cleaning, disinfection and sterilisation (in order of strength of decontamination and how critical the relevant equipment is - non-critical, semi-critical, critical)
Cleaning: physical removal of organic matter with water and detergent
Sterilisation: steam or chemical (ethylene oxide, glutaraldehyde)
Prions (infectious proteins): single use equipment (takes 10 decontamination cycles to reduce to safe levels). Equipment can be quarantined pending diagnosis.
Pacemakers and ICDs
Pacemaker nomenclature (pacing/sensing/response/rate modulation/anti-tachycardia) Rate modulation - via inbuilt accelerometer
Problems: pt, PPM and surgical factors
Diathermy - EMI and local heating
Fixed cardiac output/pacing dependent
Key Qs: indication, mode, are they pacemaker dependent
PPM with magnet - asynchronous pacing (non-sensing)
ICD nomenclature: shocked chamber/anti-tachycardia pacing/method of sensing/paced chamber.
POCT (past Q)
TEG (and differences from ROTEM) Blood glucose ABG PEFR ACT
Scoring systems (past Q)
Ideal: Scores calculated on the basis of easily / routinely recordable variables
Well calibrated and validated
A high level of discrimination
Applicable to all patient populations
Can be used in different countries, health systems or patient cohorts
The ability to predict mortality, functional status or quality of life after discharge
Also considers co-morbidities, organisational aspects, provides a common language for discussion/to evaluate critical care practice, allows ability to compare groups in clinical trials
Uses, features and applications:
Outcome/risk/mortality/prognosis/LoS prediction for individuals and groups
Treatment decisions and treatment location/tertiary referral criteria
Stratification of patients for clinical trials
Assess/compare ICU/hospital performance over time or against other units
Resource allocation
Description of case mix/workload
Features:
Measurement of physiological variables
Some measure interventions
Derived from logistic regression from large demographic data sets
Scoring system types
- Generic (outcome prediction, workload, severity of illness)
- Organ/disease specific
Assessment of scoring systems
- Discrimination - how effective it is at discriminating survivors/non-survivors (area under receiver operator curve, 1 being perfect; >0.7 considered acceptable)
- Calibration - correlation between predicted and actual outcomes
EEG
- Alpha 8-15Hz (awake with eyes closed) - occipital cortex. May represent hypoxia if generalised and unchanged by stimulation.
- Beta 15-30Hz (alert) - fronto-temporal region. Primary frequency in drug-induced coma.
- Delta 1-3Hz (non-REM sleep). High voltage delta waves indicate metabolic encephalopathy.
- Theta 4-8Hz (REM sleep). Dominant waveform in children; decrease with age.
Pacing box setup
- RHYTHM/RATE. Reduce rate to 40 to see whether a meaningful intrinsic rhythm exists.
- OUTPUT. Put the rate back up until every beat is paced. Reduce the output gradually to determine the capture threshold. Make a note of it and then double it.
- SENSITIVITY. Put rate back down below intrinsic rate. Put sensitivity up to max and gradually reduce until indicator on pacing box recognises every intrinsic beat. Make a note of it and then halve it (i.e. double the sensitivity).
Pulmonary artery catheter
1-1.5ml air
110cm long
5-8Fr
West zone 3
Pressure transducer on distal lumen
Prox port 30cm from tip - when in position, can transduce this port for CVP
Another port 26cm from tip
Thermal filament 10cm long
Thermistor is 4cm from tip
Est. pressures: RA 5/0, RV 25/5, PA 25/15, PAOP 6-12
PA waveform may have dichrotic notch from PV closure
Pulmonary circulation usually reached 40-55cm from IJ insertion point
PAOP correlates with LA pressure and is measured with balloon inflated. PCWP is closer to capillary pressure (and indicates LVEDP) and is measured with balloon deflated. The terms tend to be used interchangeably although they are not technically identical. Measure at end expiration.
In conditions of low compliance (eg ARDS), less PEEP is transmitted and the PAOP is less affected.
Factors increasing PASP: anything that raises PVR, hypoxaemia, chronic lung disease, PE, ARDS, PHTN, sepsis, L to R shunts.
Factors increasing PADP: all the above plus hypervolaemia, L heart dysfunction, tamponade, constrictive pericarditis.
PAC should not be positioned beyond hilum on CXR and should have no loops.
Sats: SVC 70%, RA/RV/PA 75%, wedge 98%
PAOP does not equal LVEDP in MR or PE
TR makes PAC and CVP readings unreliable
High PAOP: LVF, mitral/aortic valve disease, HOCM, hypervolaemia, L to R shunt, tamponade, RCM, excessive PEEP/iPEEP
Low PAOP: hypovolaemia, large PE
PACMAN trial - PAC no mortality or LoS benefit, and 10% complication rate!
Risks: all CVC risks, knotting/kinking/looping, damage to PA/valves/myocardium, risk of tamponade, pulmonary infarction, arrhythmias, balloon rupture and air embolism, thromboembolism, endocarditis. PA rupture = 30% mortality.
PAC indications
Differentiating cardiogenic and non-cardiogenic pulmonary oedema
Guiding inotropic support in PHTN/HF
Haemodynamic assessment when multiple conditions exist e.g. sepsis + renal/cardiac failure
CO measurements
Intermittent - cold fluid bolus via proximal port; mean of 3 measurements often used; measure at end-expiration
Semi-continuous - heating coil + thermistor; reading every minute. SvO2 can also be measured semi-continuously from a distal fibreoptic probe (works like jugular venous bulb probe).
Info from a PAC
Measured: CVP, RAP, RVP, PAP, PAO/WP, CO, SvO2, ScvO2, HR, core temperature
Derived: CI, SV, SVI, SVR, SVRI, PVR, PVRI, CaO2, CvO2, DO2, VO2
Inaccuracy: incorrect temp/volume of cold fluid bolus, intra-cardiac shunts, measurements not taken at end-expiration.
Stewart-Hamilton equation
Q = I / integral Ci dt
Q = cardiac output
I = indicator amount in moles
Ci dt = integral of indicator conc over time (AUC)
Or:
CO = k(core temp - indicator temp) x vol indicator
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Change in blood temp
