4.2 Central Neuraxial Blocks - Epidural Flashcards
Epidural space
Where
Extends
Epidural space (extradural) lies
between spinal dura mater
and walls of
vertebral canal.
It extends from the
base of the skull to
the sacral hiatus.
Epidural space
contains
It contains fat, lymphatics, areolar tissue, nerves and venous plexus.
There is no free fluid present.
Batson’s venous plexus
Batson’s venous plexus, present in the epidural space, is a valveless system in continuity with the pelvic veins and venous system of the abdominal and thoracic body wall.
This explains the more common
blood vessel puncture in pregnant
patients, as epidural veins
are engorged due to caval compression
(especially in paramedian insertions).
Anterior dura v posterior nerve supply
periosteoum
flavum
The anterior dura is heavily
innervated by nerves,
but the posterior
dura is poorly supplied.
The periosteum is pain-sensitive but the
ligamentum flavum is not.
This allows spinal anaesthetic with little
pain.
Posterior border of epidural space
The ligamentum flavum
forms the posterior
boundary of epidural space.
Epidural in elderly /obese
epidural fat increases
with obesity and decreases with age.
This explains
why epidural local anaesthetic
requirement is lower in the elderly
Spinous processes angulation
Cervical
Thoracic
Lumbar
Spinous processes of cervical, thoracic and lumbar vertebrae have different angulation.
They are relatively straight, posteriorly directed
at cervical, lower thoracic and lumbar levels.
But they are caudally inclined at thoracic levels
Greatest degree of angulation Spinous processes
greatest degree of angulation is at
T3–T7,
making medial approach to
epidural space technically difficult.
Hence a paramedian
approach is easier at this level.
Vertebral levels and landmarks
Most prominent
spine scapula
inferior angle scapula
line connecting iliac crest
line connecting post inf iliac spine
C7: vertebrae prominens
T3: spine of scapula
T7: inferior angle of scapula
L4: line connecting iliac crest
S2: line connecting posterior inferior iliac spine.
Absolute contraindications epidural
Absolute contraindications for epidural blockade include
patient refusal, infection at injection site, allergy to local anaesthetic, raised intracranial pressure and hypovolemia.
Relative contraindications epidural
Relative contraindications include a platelet count < 100 000 per mL, hypertension, sepsis, anatomical abnormality of spine, previous spinal surgery and an uncooperative patient
physiological effects of epidural anaesthesia
Cardiovascular Block below + Above T4:
physiological effects of epidural anaesthesia
Cardiovascular Block below T4:
venodilation and arterial dilation,
decrease in venous return and
systemic vascular resistance
compensatory reflex
vasoconstriction above the blockade
Block above T4:
blocks the cardio-acceleratory fibres,
resulting in bradycardia and hypotension
physiological effects of epidural anaesthesia
Respiratory
Respiratory
Minimal effects on pulmonary function
Vital capacity,
tidal volume,
minute ventilation and
dead space are maintained
Forced expiratory volumes reduced
physiological effects of epidural anaesthesia
Renal
Renal
Urinary retention may occur from
blockade of S2–S4 fibres
Due to renal autoregulation,
renal blood flow is maintained.
physiological effects of epidural anaesthesia
Gastrointestinal
Gastrointestinal
Increase in secretions,
peristalsis and a
contracted gut occur
because of unopposed
parasympathetic action
The visceral perfusion is well maintained
Nausea from increased gastric peristalsis
physiological effects of epidural anaesthesia
Neuroendocrine
Neuroendocrine
Epidural blockade abolishes the
stress response to surgery by
blocking the afferent sensory fibres
Thoracic epidural prevents an
increase in post-operative
nor-epinephrine release,
which causes vasospasm,
thus offering cardioprotective benefits
Which epidural lower v upper abdo is more effective decreasing stress response
Epidural anaesthesia
more effectively decreases stress response
with lower abdominal
and lower-limb surgeries
than upper-abdominal
and thoracic surgeries.
This is because not all the nociceptive
afferent fibres in upperabdominal
and thoracic surgeries may be blocked.
Additives to epidurals
Epinephrine
Duration
how
added benefit?
Epinephrine added to local anaesthetic solution (short- and medium prolongs the duration of action both by pharmacodynamic and pharmacokinetic effect.
Pharmacokinetic effect –
slower drug clearance from epidural space,
resulting in decrease in peak plasma
concentration,
mainly by decreasing blood flow in the dura mater.
Pharmacodynamic effect –
acts on α2 receptors,
decreasing pain
transmission.
Additives to epidurals
Epi affect on BP
Epinephrine may increase the incidence of hypotension, because of
its β2 (vasodilatation) effect
Additives to epidurals
Soda Bic
Benefit
How
Adding bicarbonate to local anaesthetic increases the unionised form, hence more drug can penetrate the lipid membrane, thereby increasing the speed of onset.
Additives to epidurals
Soda Bic
Solution for ligno / ropiv
recommended solutions of sodium bicarbonate to be added to local
anaesthetic are 1 mEq/mL for lignocaine and other shorter-acting agents, and
0.1 mEq/mL for bupivacaine and ropivacaine.
Site of epidural injection
What affects density
What affects height
Site of epidural injection is important
(should correlate with the
dermatome of surgical
incision for maximal effect).
Concentration of local anaesthetic mainly affects the density of the
block.
Volume affects the height of the block.
Usually 1–2 mL per segment
is used for epidural blockade.
Epidural height affected by patient weight and height?
Weight and height have no correlation
with the spread of epidural
drug, except in extreme scenarios.
LOR to fluid described by
What sign given
issues if use too much NaCL
The loss or resistance (LOR) to
fluid to identify the epidural space was
first decribed by Dogliotti.
It mainly gives a visual sign of entry into
epidural space and is not dependent on the feel.
If a large volume of
saline is used it may produce inadequate
blockade due to dilution of
local anaesthetic.
How differentiate fluid from catheter csf v nacl
Fluid obtained later after catheter placement can be
differentiated from CSF by urine reagent strip.
LOR air issues
LOR using air might result in false LOR,
as air is compressible.
Use of air may cause venous air embolism,
headache,
pneumocephalus and
patchy block.
Test dose for epidural
The test dose for epidural anaesthesia
includes 3 mL of 1.5%
lidocaine with epinephrine 15 mcg.
How can intravascular placement be detected
exceptions?
An increase in heart rate by
20% is indicative of intravascular
placement,
except in patients on beta blockers,
pregnant patients in
active labour and patients
under general anaesthetic.
How can intravascular placement be detected in those on Beta bloqs
Systolic blood pressure changes of about 20 mmHg are used as an
indicator of intravascular placement in patients on beta blockers.
How can intravascular placement be detected in paeds
Peaked P waves and
T-wave changes on ECG
indicate intravascular
placement in paediatrics.
How an intrathecal placement be detected
Dense motor block within
5 minutes of test dose
indicates intrathecal placement.
When to give test dose to those in labour
In patients in active labour, the test dose must be given after a
contraction.
Elderly patients difference
how does this affect spread
In elderly patients, there is a
reduction in size of the intervertebral foramina,
limiting the spread of
local anaesthetics out
of the epidural space.
Should dose be changed in elderly
why
Additionally,
reduction in the fat content allows
a more cephalad spread.
Hence, the same dose as in adults
will lead to a higher block.
Consequently,
dosing for the elderly should be reduced
Pharmacokinetic considerations of epidural in children
- Higher CNS toxicity:
2.
higher free fraction and higher
potential for toxicity
3.
Higher initial plasma concentrations
- Decreased duration of action
5.
Drug accumulation after a continuous infusion.
- Reduced drug metabolism and clearance: im
Pharmacokinetic considerations of epidural in children
Higher CNS toxicity:
blood–brain barrier is more permeable.
Pharmacokinetic considerations of epidural in children
higher free fraction and higher
potential for toxicity
Lower α1-acid glycoprotein levels:.
Pharmacokinetic considerations of epidural in children
Decreased duration of action
due to high cardiac output
increasing the uptake of
local anaesthetic agents from neuraxial spaces.
Pharmacokinetic considerations of epidural in children
Reduced drug metabolism and clearance
immature liver and kidneys.
Due to this, decrease infusion rate after 24 hours.
Regarding paediatric epidurals:
What type of isomer is prefered
why
infusion?
Regarding paediatric epidurals:
The single isomers like
ropivacaine and
levobupivacaine are
preferred.
The reduced toxicity and
less motor blockade offer immense
benefits particularly in infants and neonates.
Prolonged infusion of
ropivacaine (72 hours) has
not shown increased toxicity.
The vasoconstrictive properties of
ropivacaine may delay its systemic
absorption, reducing systemic toxicity.
Regarding paediatric epidurals:
Test doses
symptoms of concern
why late
good way to monitor IV spread?
Test doses:
convulsions, arrhythmias and respiratory or cardiac arrest may be the first signs of toxicity in children, as procedures are mostly
performed under general anaesthetic.
Monitoring ECG changes is a
specific and more reliable method of detecting intravascular spread.
Regarding paediatric epidurals:
Additives
@ what age
why
Additives:
because of concerns regarding spinal cord toxicity and the risk of apnoea, additives are not commonly used below 6 months of age.
Regarding paediatric epidurals:
Anatomy and type of epidural
Anatomy:
because the spinal cord extends
till L3 and subarachnoid space
till S3–S4 in infants,
caudal epidural is preferred over lumbar
epidurals.
Also, due to less fat and fibrous tissue, it is easier to insert catheters to higher levels from lower approaches.
When comparing lumbar epidural and thoracic epidural for abdominal
surgery, the disadvantage of lumbar epidural is
x 3
1 Hypotension and bradycardia is more common
2 Rescue analgesia is required more often
3 Motor block is extensive
Hesitancy with thoracic epidural
how may we compromise with level insertion
Difficulties in needle insertion, uncertain placement of catheters and the potential of neurological problems with thoracic epidural are the reasons for hesitancy in its use for abdominal surgery.
On the other hand,
extension of lumbar block may
be accepted as a compromise.
Extension of lumbar block for abdominal surgery
may not be ideal
for 3 reasons,
But this may not be ideal
for several reasons, as follows
1.
Higher chances of bradycardia, vasodilatation and hypotension (Bezold–Jarisch reflex) with lumbar epidural.
2.
Lumbar blocks are difficult to maintain
(frequent regression seen) and
need frequent rescue doses.
3. Lumbar block cause more hypotension, and reflex vasoconstriction above the block may lead to myocardial ischaemia.
Should we use lumbar epidurals for abdo surgery
Evidence suggests that lumbar epidurals should be avoided in patients
undergoing abdominal or thoracic procedures
Benefit of CSE
x 3
(onset / dose / 1st stage)
Combined spinal epidural (CSE)
offers benefits of both spinal and epidural.
- The onset of surgical anaesthesia is
comparable with a single-shot
spinal and is faster than epidural anaesthesia.
2. CSE allows the use of lower dose of anaesthetic for spinal and later prolongation of block if required with epidural administration of local anaesthetic.
3.
There is significant reduction in the
duration of first-stage labour
in primiparous patients.
Disadvantages of CSE:
few disadvantages of CSE:
1.
Inability to test the epidural
catheter after spinal injection.
2.
Possibility of failed epidural catheter.
3.
The risk of greater spread of
spinal drug after epidural injection
What ways can CSE be performed
CSE can be done with two techniques:
- needle-through-needle technique (NTN)
2.
two separate injections at different levels
What distance is important with CSE
under / over
How far does needle protrude in NTN
The knowledge of
posterior epidural space distance (PED) is very
important with NTN technique
Underestimation of PED
may result in spinal block failure and
overestimation may cause
damage to neural structures.
The spinal needle protrudes 10–15 mm beyond the epidural needle in NTN
technique
Where is PED widest and narrowest
PED is widest in the lumbar region and
narrowest in the cervical region.
What shape is the dural sac
how does this affect paramedian NTN
The dural sac is triangular in shape,
and thus paramedian approach
increases the risk of failed
spinal block with NTN technique.
Describe NTN tech
Benefit
Disadvantage
NTN:
This involves use of two separate
needles for epidural and spinal,
but use of same intervertebral space.
This is associated with single
skin puncture and hence
less discomfort to the patient.
However, as spinal block is given
prior to epidural catheterisation, confirmation
of epidural placement of catheter is not possible.
Describe STN tech
dsiadvantage
Separate spinal and epidural block (SNT)
at two different
intervertebral spaces.
This is slightly more uncomfortable to the
patient because of two skin punctures.
It also takes more time than
the NTN technique
CSE Test dose
what does it prove / not prove
The test dose
(3 mL of 1.5% lignocaine and epinephrine 1:200 000) is
used to rule out intravascular placement
or intrathecal placement.
If the catheter is intravascular,
there would be tachycardia due to
epinephrine, whereas a dense block with test dose will confirm intrathecal placement.
The test dose does not confirm proper
epidural placement.
How might epidural cathter be confirmed
Nerve stimulation may be used as a technique for confirmation of
epidural placement of catheter
Confirmation of epidural space
What strength confirms epidural
what strength indicates outside
what about subarachnoid
how sensitive / specific is this test
Tsui test can be used for confirmation of epidural placement of catheter
Epidural placement of catheter is
confirmed if a motor response
is obtained with a current strength
of 1–10 mA
motor response with > 10 mA indicates that the catheter is
outside the epidural space,
whereas a motor response with < 1 mA indicates
subarachnoid location of catheter
Its sensitivity and specificity is 100%.
Failure of spinal block with CSE is mainly because
needle length
1.
spinal needle too short –
not able to reach dura
2.
spinal needle too long –
deviation from the midline
3. with NTN technique as spinal needle enters via the epidural needle, it is not stabilised by the surrounding ligaments.
Therefore, the hand must be very
stable during injection of drug.
Slight movement of hand
may result in inappropriate drug delivery.
Issues with NTN technique
1. possibility of depositing metallic debris in the intrathecal space with NTN technique due to friction between the needles.
- There may be subarachnoid flux of
epidurally administered local anaesthetic with CSE,
leading to higher block than the same dose administered solely epidurally.
3. Similarly intrathecally administered local anaesthetic may lead to higher block with CSE than single-shot spinal, as epidurally administered air or saline decreases the lumbar CSF volume.
CSE in labour vs spinal
Foetal brady
PDPH
Neuro injury
Metal deposition
- There is increased risk of foetal bradycardia with CSE as compared
- The incidence of PDPH is rare
- There is a higher risk of neurological
injury with CSE when compared
with a single-shot spinal. - There is no increased risk of metal deposition with NTN combined
spinal epidural
There is increased risk of foetal bradycardia with CSE as compared
with single-shot spinal.
The rapid onset of analgesia with CSE causes a
fall in catecholamine levels,
leading to foetal bradycardia. However,
this effect is transient, lasting only few minutes.
The incidence of PDPH with CSE
with CSE because of the use of
small gauge
atraumatic spinal needles.
Epidural volume extension’ (EVE)
Epidural volume extension’ (EVE) via a combined spinal–epidural (CSE) technique is the enhancement of a small-dose intrathecal block by epidural saline boluses.
Epidural volume extension’ (EVE)
Advantages
The advantages are as follows.
1.
Theoretically, it may help
reduce the total intrathecal local anaesthetic
dose required.
However, a recent study did not find the technique
dose-sparing.
2.
It allows a faster motor recovery.
3
It does not alter the pain scores (VAS), peak sensory block height, time
for sensory regression (to T10)
and lowest systolic blood pressures.
sacral vertebral foramina form a triangular canal
called the sacral canal,
which is a continuation of the
lumbar spinal canal.
Sacral hiatus
Sacral hiatus is formed by
failure of the laminae of S5 to meet,
thus exposing its dorsal surface.
Sacral cornuae are formed
are formed by inferior articular process of
S5 of each side.
How to identify the sacral hiatus
Practically, the sacral hiatus is identified by drawing an equilateral triangle, the base of which is formed by the posterior superior iliac spine.
Regarding the technique for caudal block
short version
lateral decubitus
22-G intravenous canula
short-bevel
Strict asepsis
angle of 45° plane of the sacrum
horizontally and advanced by 2–3 mm
technique for caudal block:
detailed
technique for caudal block:
Best position for children is lateral decubitus with hips and knees
flexed at 90°. Adults are best positioned prone for this block.
A 22-G intravenous canula or a needle with a stellate is best used
to avoid tissue coring into a hollow needle.
The needle should be short-bevel to appreciate penetration of
sacrococcygeal membrane.
Strict asepsis is stressed because of frequent soiling from the anal
area.
The needle is first placed at an angle of 45° to the plane of the
sacrum, and placed at the sacral hiatus to pierce the sacrococcygeal
ligament. Then it is placed horizontally and advanced by 2–3 mm.
Doses of local anaesthetic for caudal block
(as described by Armitage)
Sacro-lumbar
Sacro-lumbar
- 25% bupivacaine
- 5 mL/kg
Doses of local anaesthetic for caudal block
(as described by Armitage)
Upper abdominal
Upper abdominal
0.25% bupivacaine
1 mL/kg
Doses of local anaesthetic for caudal block
(as described by Armitage)
Mid-thoracic
Mid-thoracic
- 25% bupivacaine
- 25 mL/kg
Caudal block vs Lumbar
Vasodilation
Caudal block results in less peripheral vasodilatation than lumbar epidural, as sympathetic outflow from the spinal cord ends at L2.
Caudal block
autonomic effects
Caudal block results in blockage of S2–S4 contribution to parasympathetic system, affecting the bladder and the bowel distal to the colonic splenic flexure.
Caudal block Dose v lumbar
The sacral canal varies
widely anatomically,
requiring almost double the
dose of local anaesthetic as
compared with lumbar epidural in
order to achieve the same level of blockade
methods of confirmation of caudal placement of the Tuohy needle
Whoosh test:
Inject 2–3 mL of air in the caudal epidural space and auscultation over the thoracolumbar area produces the characteristic sound. It may cause patchy block and air embolism
methods of confirmation of caudal placement of the Tuohy needle
Swoosh test
Swoosh test: in paediatric patients, local anaesthetic or saline is used
instead of air
methods of confirmation of caudal placement of the Tuohy needle in epidural space
nerve stimulator
response
Correct needle placement in the epidural space
can be identified with a
nerve stimulator
(current 1–10 mA) and
anal sphincter contraction
as the end response
methods of confirmation of caudal placement of the Tuohy needle in epidural space
Is ultrasound useful
Ultrasound may be useful in children
< 6 months old, as after this age
there is ossification of vertebral
bodies preventing good visualisation
Uses of caudal epidural block:
x 3
Uses of caudal epidural block:
1
Caudal block is used for surgical procedures both above and below
the diaphragm in paediatrics.
2
It may be used for labour analgesia, gynaecological, lower-limb and anal surgeries.
3
It is mainly used for chronic pain management in adults
Caudal contrast + Block
‘Christmas tree’ appearance is
normally seen on injection
of contrast in the caudal space.
It is due to spread of the dye
in the caudal canal and along the nerve roots
as they exit the vertebral column.
Epidural adhesions are
diagnosed with its characteristic absence.
In order place risk of last
epidural vs brachial plexus v caudal
The risk of local anaesthetic toxicity is as follows:
caudal >
brachial plexus block >
lumbar or thoracic epidural
ECG change after IV Local anaesthetic
what other changes
T-wave changes on the
ECG are the earliest changes
in paediatric patients following
intravascular placement of local anaesthetic following caudal block. These are then followed by heart rate and blood pressure changes
