Physics Flashcards
Theatre pollution (RCoA new book, past Q)
The introduction into an area of a substance which is harmful to living creatures or the environment.
Matter (solid/liquid/gas) vs. energy (heat, noise, radiation) pollution.
Sources - gas induction, semi closed systems, spillage, leaks, failure to turn off, post extubation exhalation
Reduction - TIVA, regional, ventilation, low flows, scavenging
Scavenging - active/passive
Other reasons for efficient ventilation - laminar flow (400 changes/h, filtered air), temperature control
COSHH
Pulse oximetry (RCoA old book)
660/940/590/805 Isobestic points Beer-Lambert SpO2<80% is by extrapolation only Causes of inaccuracy 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
Diathermy (RCoA old book)
Heat concentrated at probe where resistance highest
Heat produced depends on power
Sine wave (0.5MHz) for cutting, damped/pulsed sine wave for coag (1-1.5MHz) - high frequencies as muscle sensitive to lower ones
Unipolar - indifferent electrode attached to pt (large, good contact, attached to area with good blood flow to dissipate heat otherwise risk of burns)
Bipolar - earth-free, localised current, lower power
Accidental switching on avoided by isolated quiver and audible tone upon use
Prevent pt touching earthed point/equipment - risk of shock
Microshock
Infarction of organ with thin vascular pedicle e.g. testis
Fire risk with flammable agents - skin prep, volatiles
EMI with pacemakers and other devices
Rotameters (RCoA old book)
Constant pressure, variable orifice
Viscosity important at laminar flows
No static, notched bobbin to make it rotate, calibrated for single gas
Altitude
Safety - colour, order, scales, hypoxic guard, visibility of bobbins
Gas laws (RCoA old book)
Boyle’s law - gas in closed spaces e.g. N2O and PTX
Charles’ law - thermometers
(3rd gas law)
Dalton’s law of partial pressures - gas mixtures, altitude
Avogadro’s law
Henry’s law - O2 carriage, hyperbaric chambers
Ayre’s T piece (Mendonca)
Ayre’s T piece with Jackson-Rees modification = Mapleson F. For up to 25-30kg. Other paed alternatives = paeds circle system, Ayre’s T piece, Humphrey ADE.
Benefit of bag: monitor respiration, can apply IPPV or CPAP.
Pros of T piece: compact, valveless, low resistance.
Cons: pollution, high FGF in SV.
SV: need FGF 2-3x MV to prevent rebreathing. IPPV: FGF = MV.
Vol of reservoir should = VT (otherwise air entrainment/rebreathing).
Components of a breathing system: reservoir bag, circuit, APL valve, FM. Ideal system: easy to use, reliable, cheap, low dead space, low resistance, economical, efficient, appropriate for adults/paeds/SV/IPPV.
Mapleson A is best for SV, D for IPPV.
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
EtCO2: tension of CO2 in exhaled gas at end expiration. Represents PACO2 (alveolar CO2). Depends on PACO2 and volume of dead space. PACO2 depends on CO2 production, CO/pulmonary blood flow, and alveolar ventilation.
EtCO2 high/low: actual changes in CO2 production, altered alveolar perfusion or ventilation, technical factors.
High EtCO2: increased CO2 production (fever, sepsis, MH, tourniquet release), increased alveolar perfusion (high CO), reduced alveolar ventilation (hypoventilation, endobronchial), technical factors (exhausted soda lime, leak).
Low EtCO2: reduced CO2 production (hypothermia), reduced alveolar perfusion (low CO/arrest, PE), increased alveolar ventilation (hyperventilation), technical factors (disconnection).
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
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.
Depth of anaesthesia monitoring (Mendonca, past Q)
Clinical
- Movement in the unparalysed pt
- Evan’s PRST system (pressure, rate, sweating, tears) - often confounded by drugs
- Experience
Instrumental
- MAC and EtAA (still the best)
- Isolated forearm, oesophageal contractility, A/V/somatosensory evoked potentials, R-R variation, frontalis EMG, EEG and derivatives e.g. BIS, Narcotrend (latter = automatic analysis and grouped A-F; GA level is E)
Difference between awareness and consciousness?
Consciousness - response to command
Modified Structured Brice Interview detects explicit recall. Implicit recall is more common however.
Incidence of awareness = 1-2/1000. May be a genetic component as higher rates reported in Chinese populations.
PTSD:
- Re‐experiencing the trauma, with flashbacks or nightmares.
- Avoidance of things or situations related to the trauma.
- Hyper‐arousal, which may increase sleep disturbance, irritability and fear.
EEG: alpha, beta, gamma and delta waves. EEG is complex to interpret and is affected by hypoxia, hypercapnoea and hypotension. BIS is reduced by sux and roc (BJA 2015 trial in awake volunteers).
Somatosensory evoked potentials: median or ulnar nerve stimulated, and responses detected at spinal cord and cerebral cortex levels.
MRI (Mendonca, Krishnachetty, past Q)
- Unpaired protons (mostly in water, but also phosphorus) act as bar magnets which align in a static magnetic field
- An electromagnetic field is superimposed perpendicularly and turned on and off, causing the protons go in and out of alignment (precession) and as they relax they release the radiofrequency energy which they had absorbed - this is then measured
Earth’s magnetic field 0.5-1 Gauss
1 Tesla = 10,000 Gauss
MRI = 1.5-3T
Frog levitation = 16T!!
(Weber = magnetic flux, Tesla = magnetic flux density; so 1T = 1 Wb/m2)
MRI safe <5 Gauss (PPMs malfunction if closer than this)
MRI conditional <50 Gauss (ferromagnetic objects become projectiles if closer than this)
T = relaxation time constant
T1 - early image, water dark
T2 - later image, water light
Problems
- Patient - ICU, paeds, screening required
- MRI - magnet, noise, local heating/burns, cold, EMI (ECG mimics hyperkalaemia)
- Equipment and monitoring - need MRI safe stuff, Faraday cage, wave guide (bronze), long lines, capnography delay, limited access
- Remote location
MR safe equipment will not cause harm but will not necessarily function.
MR conditional is safe within limitations.
(MR compatible = old term)
Gadolinium contrast: enhances T1 and reduces T2 signal. Does not normally cross BBB so can show when this is breached. SEs: nephrotoxicity (nephrogenic systemic fibrosis) in pts with pre-existing renal impairment.
Quenching: when the helium (which cools the coils to maintain superconductivity) rapidly boils off - spontaneous or deliberately to shut down the field. Helium gas will be produced - normally vented but risk of backing up, therefore need O2 sensor in scan room.
Temperature measurement (Mendonca, past Q)
Graph of heat loss inc combined GA/neuraxial
Hyperpyrexia and hyperthermia
Causes of high temp in athlete after exercise
NMS and serotonin syndrome
Measurement:
- Non-electrical (liquid expansion (Hg, alcohol), gas expansion (Bourdon), chemical, bimetallic strip, infrared (tympanic - black box radiation principle))
- Electrical (thermistor, resistance thermometer e.g. platinum wire, thermocouple (Seebeck))
Heat loss in theatre:
Vasodilatation, exposure of body cavity, irrigation/infusion cold fluid, theatre temp.
Radiation 40% - infrared radiation - increase room temp/blankets
Convection 30% - air layer next to pt warms and rises, causing convection current; exacerbated by laminar flow - forced air warmer/fluid warmer
Evaporation 15% - sweat/fluids - latent heat of vaporisation - cover pt
Conduction 10% - avoid contact with metal
Respiration 5% - inspiring dry gases - HME
Blood gas measurement (Mendonca)
pH: pH electrode
- Glass electrode (Ag/AgCl) with bulb of H+-sensitive glass in a buffer soln
- Reference electrode of calomel (Hg/HgCl) or Ag/AgCl in KOH
- Potential diff generated, size proportional to difference in [H+]
CO2: Severinghaus
- H+-sensitive glass electrode
- Ag/AgCl reference electrode
- CO2 + H2O –> H+ + HCO3-
- Number of H+ proportional to PCO2
O2: Clark electrode (polarographic electrode)
- Platinum cathode / Ag/AgCl anode
- KOH solution
- Electrons consumed at cathode and formed at anode
- Voltage applied and current flows, size of which is proportional to PO2
Standard bicarb: plasma conc of bicarb when Hb fully saturated, temp 37 and PCO2 corrected to 5.3kPa.
Base: amount of base/acid required to restore 1L of blood to normal pH at a PCO2 of 5.3kPa and a temperature of 37C.
PCO2, PO2 and pH are measured; others are calculated.
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
TOE: 180 degree view of heart. Good for valves and aorta. Can exclude vegetations. Good for CHD, RWMAs/LV function intraop.
CI: varices, oesophageal tumour, C-spine instability
Comps: perf, bleeding, microshock
Sodalime (Krishnachetty)
Sodalime, baralyme, litholyme, Amsorb, Dragersorb, Medisorb. Named for the lesser ingredient (most of all of them is Ca(OH)2). Litholyme does not produce compound A or CO.
Soda lime: 5% NaOH, 94% Ca(OH)2, 0.1% KOH, silica and indicator dye (ethyl violet). 1kg absorbs 120L CO2. 4-8 mesh - granules pass through a mesh with 4-8 strands per inch. Compromise between resistance to flow and surface area.
Permits low FGF, prevents rebreathing, less waste/pollution, provides heat/humidification.
Soda lime exhaustion: increased RR, BP, HR, arrhythmia, acidosis
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
Waveform: percussion (systolic), tidal (brain compliance), dichrotic (aortic recoil)
Lundberg A - plateau waves, always pathological
Lundberg B - vasospasm
Lundberg C - can be normal
