iThoughts

Central Idea

EEG

50-200 microV
delta

0-4 Hz
children, sleep, otherwise abnormal

theta (θ)

4-8 Hz
drowsiness

alpha

8-13 Hz
closed eyes, relaxed, increased cortical activity

beta

13 Hz and above
frontal, alert, active, anxious

Nerve stimulators

for regional

up to 150V
20 - 80mA (for skin electrodes)

for NMB assessment

types

single twitch

0.2ms at 0.1-1Hz
compare to pre-NMB
reduced magnitude after 75% blocked

double burst

2 bursts of 3 stimuli of 0.2ms at 50Hz separated by 750ms
better for fade

TOF

4 stimuli at 2Hz
loss of T4: 75%, T3: 80%, T2: 90%, T1: 100%
T4:T1 ratio

70% = adequate reversal

depolarising: no fade

tetany

50-100Hz for 5 seconds
no fade = no block
depolarising: reduced but sustained contraction
post-tetanic facilitation (liberation of presynaptic Ach)

post-tetanic count

position

onset and offset faster in central muscles (good blood supply)

diaphragm
larynx
orbicularis oculi

slower in peripheral nerves

adductor pollicis
common peroneal

foot dorsiflexion

pharynx and larynx have fast onset (like central muscles), but slow offset (like peripheral muscles)

measurement

visual and tactile
MMG
EMG
accelerometry

15-40mA

pressure

types

gauge

measured pressure independent of atmosphere
oxygen cylinder = 137 bar

absolute

gauge + atmospheric
oxygen cylinder = 138 bar

measurement

atmospheric

barometer

mercury
U shaped glass tube, one end closed, near vacuum in the space. Atmospheric pressure pushes on the open end of tube and forces the mercury around the U

difference between things (gauge pressure)

manometer

mercury or water
both ends open (with semi-permeable membranes to reduce risk of spillage)
e.g. old fashioned blood pressure measurement

aneroid

no liquid
high pressure

Bourdon

e.g. cylinder pressures

low pressure

bellows

e.g. airway pressures

temperature

a measure of the average kinetic energy of the atoms and molecules that make up a substance
units

kelvin

1K = 1/273.15 of the thermodynamic temperature of the triple point of water

centigrade
farenheit

to get C = (F-32 / 1.8)

measurement

non-electric

expansion

liquid

mercury

freezes at -39 C
boils at 350 C

alcohol

freezes at -114 C
boils at 78.5 C

dial

aneroid gauge
bimetallic strips

liquid crystal
Dial

bimetallic strip
Bourdon gauge

filled with volatile liquid which expands as temperature rises

electric

infrared

pyroelectric effect: ceramic crystals polarise and produce a voltage when infrared light is reflected back onto them

resistance thermometer

resistance of metal increases linearly with temperature

thermistor

resistance of metal oxide decreases exponentially with temperature (but can be virtually linear in clinical practice)
small and robust
pulmonary artery catheters

thermocouple

Seebeck effect
voltage between two dissimilar metals increases with temperature
most commonly copper and constantin

in the body

oesophageal

accurate if lower 1/3

rectal

0.5-1 C higher than core

PAC

best, but invasive

skin

innaccurate in shock

bladder

high flow needed for accuracy

tymphanic

good correlation with hypothalamic temp, innaccurate w. ear wax

patient heat loss

Royal College Exam Room
Radiation

up to 50%
emission of energy in the form of waves or particles

Convection

up to 30%
transfer by the movement of liquid or gas

Evaporation

up to 20%
conversion from liquid to gas, causing heat loss through latent heat of vapourisation

Respiration

up to 10%
convection and evaporation

Conduction

up to 5%
direct transfer between adjacent molecules

reduction

NICE guidance

forced air warming if high risk or surgery >30mins
measure temp every 30 mins
fluids >500mL should be warmed

methods

theatre

22-24 C
50% humidity
no draft

patient

warm pre-op
covered
forced air warming

reduces convection AND radiation

anaesthetic

HMEF
warmed fluids
low flow anaesthesia