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)
