3. Equipment and Physics Flashcards
Who first studied nerve stim
Who first used it to perform block
How were nerves blocked prior to this
Electrical nerve stimulation was first studied
by French physiologist Louis Lapicque in 1909.
It was first used to perform nerve blocks by
Von Perthes in 1912.
Before this, nerves were blocked by
direct instillation of local anaesthetics
(by dissection and exposure of nerve plexus) or paresthesia techniques.
Nerve stim
How does the technique work
The technique of electrical nerve stimulation
is based on the premise
that a current of sufficient amplitude
applied for a sufficient time will
depolarise a nerve.
In the case of nerve blocks,
this means either motor response
or sensory stimulation (since most nerves are
mixed).
Which is more commonly stimulated Motor or sensory
What does the Cathode do
What does the Anode do
However, it was also noted that stimulating
motor fibres was easier
than sensory fibres,
and more importantly,
application of a cathode
depolarised the nerve,
while an anode hyperpolarised the nerve.
What does the Cathode do
What does the Anode do
application of a cathode
depolarised the nerve,
while an anode
hyperpolarised the nerve.
What is the gold standard
At present,
ultrasound guidance is becoming more popular,
but electrical nerve stimulation is
still the commonest method employed.
However, no method of nerve blockade is described as gold standard.
What is Rheobase
Rheobase
is the minimum current of
indefinite duration required
to depolarise a nerve.
How do we calculate the total charge required to depol nerve
The total charge (Q) required to depolarise a nerve is
the product of the current intensity (I) \+ the duration (t) for which it is applied. Q = I × t
What current intensity is required for depolarisation
In turn,
the current intensity
required to produce depolarisation
is given by the following equation
(where Ir is the rheobase
and C is the chronaxie):
I = Ir × (1 + C/t)
t = infinity, we get I = Ir, and so Q = Ir.
what is Chronaxie
Chronaxie is the minimum duration
of current twice the rheobase required
to stimulate a nerve
(as shown in the previous answer).
Chronaxae related how to fibre size
It is inversely proportional to
fibre size and
hence ease of stimulation.
What is the Chronaxiae of Aa fibres
Aα (motor) has a
chronaxie of 0.05–0.1 millisecond,
Aδ (sensory) is 0.15 millisecond
C (unmyelinated sensory) 0.4 millisecond
Hence, stimulating motor nerve
requires shorter pulses than sensory fibres.
Desirable properties of electrical
nerve stimulation are
x 5
- the most important
- Short pulse width:
- Square-wave current
- Cathodal stimulation
- Constant current generator * most important
- Frequency: 2hz
Short pulse width
Refers to
Why is this advantageous
pulse width refers to the time duration
for which the current is applied.
Shorter pulse width has two advantages:
1 Since the motor fibres have a smaller chronaxie, shorter pulse width stimulates them but not the sensory fibres.
This results in motor responses
but not painful paresthesia,
which is undesirable anyway.
2 Shorter pulse width may be superior
to longer in estimating needle to- nerve distance
Square-wave current
Slow rising current allows
for accommodation
(resulting in difficulty in nerve stimulation)
of nerve fibres.
This can be avoided by the
square-wave form of applying
current (abrupt rise and abrupt fall).
Cathodal stimulation
Cathodal stimulation:
it is preferable to stimulate
the nerve with needle as cathode,
since this then depolarises it,
whereas needle as anode
hyperpolarises the nerve
(necessitating application of higher
current for stimulation).
Constant current generator
Constant current generator (not fixed):
a peripheral nerve stimulator (PNS) should deliver the same current despite changing impedance applied.
This is the most important property of the
peripheral nervous system (PNS).
Frequency:
Frequency:
a stimulation frequency of
2 Hz is better than 1 Hz,
since it allows
faster manipulation of needle.
Describe important things during PNS
Negative
Positive
Distance between
Current
During nerve stimulation,
the following things are vital:
- Negative (cathode) to needle.
- Positive (anode) to patient.
- It was considered that the anode
site should be at least 20 cm away
from the needle site to reduce direct muscle stimulation, but this has
been found to be unnecessary.
4. Acceptable current is between 0.2 and 0.5 mA. Above 0.5 mA, the needle may be further away from the nerve, and such injections may not be successful.
Below 0.2 mA, injection may be intraneural.
components of a peripheral nerve stimulator
Microcontroller
Constant current generator
(most important)
Oscillator
Clock reference
LCD display
Controls
components of a peripheral nerve stimulator
Explained
- Microcontroller
Brain of the peripheral nervous system:
processes variable, like
current, pulse width, frequency - Constant current generator
(most important)
Generates the same current despite changing impedance
3 Oscillator
Generates the desired frequency
4.
Clock reference
Synchronises the current with the frequency
- LCD display
For current amplitude, frequency and the pulse width selected
6Controls
For selecting parameters
appropriate settings of a PNS
for performing a nerve block include
- negative lead to needle
2.
positive lead to patient
3.
a square-wave impulse (to prevent accommodation)
- pulse duration 0.1 millisecond (for stimulating motor nerve fibres preferably)
5.
frequency of 2 Hz (better than 1 Hz)
- an initial current of 1–2 mA
7.
a final current of 0.2–0.5 mA
(> 0.5 mA, the needle may be further
away from the nerve,
and such injections may not be successful;
< 0.2 mA, the injection may be intraneural)
What is the law that governs the
principle of nerve stimulation
The current required is
inversely proportional
to the square of the distance
between the needle and the nerve
Coulombs
Coulombs Law
The inverse-square law (Coulomb’s Law)
dictates that the current required
(I) to stimulate a nerve,
is proportional to the minimal current (i),
and
inversely to the square of the distance (r)
from the nerve (k is a constant)
.
I = k(i/r2)
How may nerve stimulation be altered in elderly, diabetics or those with neurological diseases,
Usually, a motor response between 0.2 and 0.5 mA is sought and considered appropriate. However, in elderly, diabetics or those with neurological diseases,
higher currents may be needed due to slower
nerve velocities and lower motor amplitudes.
What is the Raj test
How is it performed
What does it confirm
Explain the mechanism
The disappearance of the
motor response induced
by a low current (0.5 mA)
following injection of local anaesthetics
or normal saline (conducting solutions),
confirms the proximity of needle to the
nerve and constitutes the Raj test.
This does not result due to the
physical displacement of the nerve
but due to the dissipation of
current density near the nerve.
What is the Tsui test.
How is it performed
What does it confirm
The exaggeration of motor response induced by a low current (0.5 mA) following injection of 5% dextrose (non-conducting solutions),
confirms the needle-to-nerve
proximity as well
and constitutes the Tsui test
Is a sensory response able to elicit a motor response
a lack of motor
response does not rule
out the possibility of sensory nerve
contact by the injection needle
Peripheral Nerve Stimulators
- Optimal Range
Optimal range for a PNS is 0–5 mA.
This is because some patients
may need higher current for stimulation
(diabetics, elderly, neurologic disease).
Newer devices may have higher ranges (0–10 mA) used for epidural stimulation.
Higher ranges (0–80 mA) are used in neuromuscular monitors.
Peripheral Nerve Stimulators
Percutaneous nerve stimulation
Percutaneous nerve stimulation is a
new technique involving the
stimulation of nerves non-invasively.
The current needed for this is higher than invasive stimulation, but offers the identification of insertion points in especially difficult cases (obese).
What range should the PNS be checked in.
Biomedical engineering departments
have measured the accuracy of
PNS in the higher current ranges (> 1 mA) in the past.
It was subsequently argued that since the current used for performing nerve blocks is in the range of 0.2–0.5 mA, it is prudent to check the accuracy in this range. This has been adopted by some manufacturers
Which are more accurate insulated or non
Non-insulated needles were the
first to be used.
Both the tip and the shaft
were conductive,
causing current dispersion
and lower accuracy.
What is an issue with non insuated needles
They also caused local muscle
stimulation through the
shaft of thenneedle.
What are insulated needles coated in
Why are they beneficial
The development of Teflon-coated
insulated needles resulted in
better precision.
This is because only the tip is conductive,
and hence the current is not dispersed.
What type of needle tips are available
Various needle-tip designs are prevalent.
Among the sharp needles,
the tip may have a
long (standard, 15°) bevel
or short (30° or 45°) bevel
Which bevel cuts nerves
Which bevel causes blunt trauma
The long-bevel needles may
cause sharp cuts on nerves,
while the short bevel
leads to blunt nerve damage.
Which type of needle bevel is more frequently associated with injury
Which is more severe
Although nerve injury is
more frequent with long-bevel needles,
it may be more severe if it occurs u
sing a short-bevel needle.
Which bevel is used more frequently these days
Blunt-bevel needles offer more resistance as they pass through tissue planes and thus give a better feel.
Hence they are most commonly
used nowadays
Needle gauge is an important consideration while performing blocks
Superficial injections
Needle gauge is an important consideration while performing blocks.
Superficial injections are
best given using 25/26-G needles.
Needle gauge is an important consideration while performing blocks
Single shot injections
The 21/22-G needles are best for single-shot injections,
Needle gauge is an important consideration while performing blocks
Continuous catheter injections
Catheter size
18/19-G Tuohy-tip needles
are best suited for
continuous catheter techniques.
In such cases, 20-G
catheters are used.
Needle length is an
consideration when doing nerve blocks.
Shorter needle may not be sufficient, while longer needles may have potential
for tissue damage if introduced further than needed
•
25 mm
what block
Interscalene
50 mm
what block
Cervical plexus
Supraclavicular
Axillary
Femoral
Popliteal (posterior)
100 mm
Infraclavicular
Popliteal (lateral)
Paravertebral
Lumbar plexus
Sciatic (posterior)
150 mm
Sciatic (anterior)
An in-line pressure-monitoring
provides
An in-line pressure-monitoring
device measures pressure while
injecting local anaesthetic.
It provides an objective
assessment of pressures
rather than subjective feel.
The latter can vary between
individuals and devices.
I
Intraneural injection pressures
Perineural
Intraneural injection pressures are
>20 psi while
those made perineurally
have lower pressures of 5–20 psi.
Therefore, the device guides
placement of the tip according to injection pressures.
The continuous catheter technique involves
2 elements and sizes
The continuous catheter technique involves
the use of larger needles
(Tuohy-, Sprotte- or facet-tipped 18/19 G)
and fine stimulating catheters (20 G).
Once a nerve is stimulated using the needle, the
catheter is threaded through the needle.
In the case of non-stimulating catheters,
how is this done
In the case of non-stimulating catheters,
the perineural space is dilated
by injecting saline and threading
the catheter 3–5 cm beyond the
needle tip.
Benefit of Catheters
How were they limited
Although the use of catheters
helped to prolong postoperative
pain relief,
they were limited by secondary block failures
(primary block refers to the block
following initial injection,
while secondary block is one following continuous infusion).
How was secondary failure improved
This was improved by stimulating catheters.
They are threaded
along the nerve,
and their position confirmed using
electrostimulation in real time.
This is followed by initial bolus
and continuous infusion,
both through the catheter.
This has reduced secondary failures.
‘catheter over needle’
and
‘catheter through needle’
Which is more frequently used
which has a problem with leakage
why
Systems with ‘catheter over needle’ and ‘catheter through needle’ are available.
Though the latter are more prevalent, their use is often plagued by leakage through the injection site (because the hole made by the needle is larger than the catheter size).
Tunnelling the catheter
reduces the chances of dislodgement and helps maintain the
catheter for a longer time.
Ultrasound waves are
Beyond what
which is
Clinical U/S are what freq
(> 20 KHz) are waves
beyond the audible frequency
range of audible sound
(20–20 000 Hz).
Clinically used ultrasound is in the 1–20-MHz frequency range.
Describe how an US Probe works
Ultrasound waves are generated by
applying an electric field
to
piezoelectric crystals
to produce a
series of pressure waves.
The pressure waves are transmitted from the probe head and reflected back dependent upon the tissue type.
The returning pressure waves are
detected and generate an electric current
that is converted
into a two-dimensional image.
This interpretation assumes the speed of
sound in soft tissues to be 1540 m/second.
The speed of sound in soft tissues
The speed of sound in soft tissues to be 1540 m/second.
The various modes of ultrasound in use are:
A Mode
B Mode
M Mode
Doppler mode
A-mode:
A-mode:
the simplest type of ultrasound.
A single transducer scans
a line through the body with
the echoes plotted on screen as a
function of depth.
B-mode:
B-mode:
the commonest mode.
A linear array of transducers
simultaneously scans a
plane through the body
that can be viewed as a
two-dimensional image on screen.
M-mode:
M-mode:
M stands for ‘motion’.
Ultrasound pulses are emitted
in quick succession,
recording a video in ultrasound.
This can be used to determine
the velocity of specific organ structures
such as
cardiac valves and jets
Doppler mode:
Doppler mode:
This mode makes use of the Doppler effect
(change in frequency of a wave for an observer moving relative to the source of the wave)
in measuring and visualising blood flow.
What type of waves are US waves
What are their properties
Ultrasound waves are sound waves
Wavelength (λ)
Amplitude (A)
Frequency (f)
Period (τ)
Velocity (c):
Wavelength (λ):
Distance between two consecutive corresponding
points of the same phase.
Amplitude (A)
maximum height of the wave.
Frequency (f)
number of complete cycles per second.
Period (τ):
time taken for one complete wave cycle to occur.
Velocity (c):
Calculted how
speed at which sound waves
pass through a medium. It
may be calculated using the equation: c = λ × f
What is relationship between wavelength and frequency
why
wavelength and frequency
bear an inverse relationship.
Since the velocity within a medium is constant,
How does frequency affect image quality
high
Higher-frequency beams
experience more attenuation
(directly proportional)
and hence have lesser penetration.
They are used for performing superficial blocks (interscalene and supraclavicular).
How does frequency affect image quality
Low
Low-frequency beams have
better penetration and allow
visualisation of deep structures
(infraclavicular and sciatic nerve blocks).
Linear probes generate
Curved generate
Linear transducers generate high-frequency ultrasound,
while curved array
probes low-frequency ultrasound.
Linear array
freq ax res attenuation depth image best
Linear array
High frequency (6–13 MHz) Greatest axial resolution More attenuation Limited depth of penetration Rectangular images Best for superficial structures (e.g. brachial plexus)
Curved array
freq ax res attenuation depth image best
Curved array
Low frequency (2–5 MHz) Decreased axial resolution Less attenuation Deeper penetration Sector-shaped image Best for large or deep structures (e.g. sciatic nerve)
Phased array
freq ax res attenuation depth image best
Phased array
Consists of many small ultrasonic elements High-resolution beam Characteristic image is sector-shaped Used for echocardiography
J-shaped
J-shaped (hockey-stick footprint)
probes are linear array probes
which are small in size
hence ideally suited for paediatric usage.
Axial resolution
Axial resolution is the ability
of the system to display small structures
along the axis of the beam
as separate from each other.
It is
directly proportional to the
frequency of the beam.
Lateral resolution
is the ability of the system to
display small structures
side by side (same depth)
as separate from each other.
Attenuation is
What does it cause
The sum total of reflection, refraction, scattering and absorption
It leads to
loss of clarity of the image.
How can attenuation be corrected
It can be corrected
by time-gain compensation
(also called depth-gain compensation).
Attenuation is directly proportional
to the frequency.
So higher frequency
ultrasound beams
undergo higher attenuation
and
allow the best visualisation of superficial structures.
What is an artefact
What are the diff types artefact
x4
An artefact
is an image,
or part of it,
that does not correspond
to the anatomy of the
structure being examined.
Shadowing
Post-cystic enhancement
Reverberation
Anisotropy
Shadowing
Shadowing:
when the ultrasound beam
cannot pass through a structure
(e.g. bone),
the beam is reflected back
and the tissues immediately
behind the structure appear dark.
Post-cystic enhancement
Post-cystic enhancement:
when the ultrasound beam passes
through a fluid-filled structure
such as the
urinary bladder,
cysts or blood
vessels,
very little is reflected,
and therefore the tissues behind the
fluid appear bright.
Reverberation
Reverberation:
occurs when ultrasound is repeatedly reflected
between two highly reflective surfaces.
Anisotropy
Anisotropy:
is the property of
tendons, nerves and muscles
to vary in their ultrasound
appearance depending on the
angle of insonation of
the incident ultrasound beam.
what is echogenicity
On returning to the transducer,
the amplitude of an
echo is represented
by the degree of brightness
(i.e. echogenicity)
of a dot on the display.
Each tissue displays a different echogenicity,
allowing identification of structures.
Anechoic:
Anechoic
veins/arteries offer no reflection and appear black.
Hypoechoic
Hypoechoic:
muscle and central nerve plexus
offer weak reflections
and appear dark.
Hyperechoic
Hyperechoic:
bone and peripheral nerves
offer strong reflections and
appear bright.
Factors determining needle visualisation under ultrasound
improves technique size of needle angle of insertion depth echogenicity
Technique deteriorates improves
Technique Out of plane In plane
Size of needle Smaller (22 G) Larger (17-G Tuohy)
Angle of insertion Steep Shallow
Depth of insertion Deep Shallow
Echogenicity Non-echogenic Echogenic (cornerstone reflectors)
Cornerstone reflectors
how
introduced by some companies
(Pajunk),
these are intended to improve
needle visibility under ultrasound,
even at steep angles.
They do so by reflecting all ultrasound
waves without
losses.
real-time spatial compound imaging
In real-time spatial compound imaging,
a transducer array is used
to rapidly acquire several
overlapping scans of an object
from different angles.
These scans are averaged to
form a compound image that
shows improved image quality
because of reduction of artefacts.
technologies involve either one of the following:
Needle-guidance systems:
Sonic GPS (Ultrasonics) and eTrAX needle
systems (CiVCO).
Newer imaging modalities: Multibeam (Sonosite), Cross XBeam (GE)
and Flexi Focus (BK Medical).
How does U/S Aid RA
Can detect what
demonstrate
reduces risk of
Ultrasound provides
real-time visualisation of the nerve
and
surrounding structures during regional anaesthesia.
It allows detection of anatomical variations
and
demonstrates spread of local anaesthetic
during injection.
Intraneural or intravascular
injection can be detected by ultrasound.
How does it improve block
What does the evidence not demonstrate
Evidence shows ultrasound-guided peripheral nerve blocks can be more successful than peripheral nerve stimulator techniques and have a faster onset time.
However, evidence to clearly demonstrate that ultrasound-guided
regional anaesthesia is safer (in terms of neural injury) than using a
peripheral nerve stimulator is still not available.
In the short-axis view
In the short-axis view
tubular structures such as
nerves and blood vessels
appear as though they have been
sliced across their diameter,
like discs of salami.
In the long-axis view,
In the long-axis view, tubular structures are sliced longitudinally
along the length of the tube.
The needle approach is described
In plane
The needle approach is described as
in-plane if the needle remains parallel to
the ultrasound beam,
allowing visualisation of the tip and shaft.
The needle approach is described
Out plane
In the out-of-plane approach,
the needle is inserted more
perpendicular to the ultrasound beam
and can only be visualised as a
dot when the needle crosses the beam.
newer applications of electrical nerve stimulation include the following.
Percutaneous electrode guidance:
Sequential electrical nerve stimulation (SENS):
Epidural stimulation (Tsui test)
Percutaneous electrode guidance
involves percutaneous stimulation
of peripheral nerves to identify a nerve
before skin puncture. This
reduces the number of unsuccessful painful insertion and helps identify
the best insertion point.
Sequential electrical nerve stimulation (SENS):
delivers current at
3 Hz with sequential pulses
of 0.1, 0.3 and 1 msecond to improve
motor response.
Epidural stimulation (Tsui test)
Epidural stimulation (Tsui test):
the placement of wired epidural catheters threaded into epidural space can be confirmed by electrical
stimulation between 1 and 10 mA. The motor responses elicited
direct toward the level of the tip of the catheter. Stimulation at
currents < 1 mA indicate intrathecal placement.
Non-electrical methods to confirm the placement of epidural
catheters
Non-electrical methods to confirm the placement of epidural
catheters include
epidural pressure waveform guidance and
electrocardiographic guidance
Multistimulation technique
Multistimulation technique involves seeking specific component bundle
motor responses separately and then blocking the component nerves
individually. For example, for the axillary block, the radial, ulnar, median
and musculocutaneous responses are sought individually
Multistimulation technique
Advantages
Higher success rate Lower total volume of local anaesthetic needed Shortening of onset times of nerve blocks Lower potential of local-anaesthetic toxicity because of lower doses
Multistimulation technique
Disadvantages
Increased patient discomfort because of multiple needle
redirections
Time for the entire procedure is increased
Theoretically, the risk of nerve damage may be higher
because of repeated needle passage
the incidence of nerve injury with multistimulation technique has been
found to be similar to that of conventional techniques.
Multistimulation technique useful for
Multistimulation technique has been found to be useful for
interscalene, axillary,
infraclavicular, mid-humeral, femoral and sciatic
nerve blocks.
Multistimulation too risky for
However, frequent redirections in the supraclavicular area may
increase the risk of arterial puncture and pneumothorax, and are not
advised.
As the current needed to stimulate nerve
calculation
what current is best used initially
Then what improves specificity
As the current needed to stimulate a nerve
is inversely proportional
to the square of the
distance from the needle,
high currents help in
finding the nerve initially.
Hence they increase the
likelihood of finding the nerve.
Subsequently, lowering the current
as the needle approaches the nerve
helps to improve the specificity of the response.
The resistance encountered by a
stimulating electrode depends
directly on
inversely on
The resistance encountered by a
stimulating electrode depends
directly on the tissue resistance
and
inversely on the conductive
area of the electrode.
How can nerve stimulation electrode be improved
Stimulating a nerve using a microtip electrode (i.e. despite high resistance) improves the specificity of the stimulation.
What is water lipid ratio of nerves
How does this affect stimulation
Water–lipid ratio of tissues
is proportional to conductance,
and
since nerves have a high water–lipid ratio,
they have higher conductance than
skin, muscle, fat or bone;
hence they are stimulated
in preference upon application of a current.
