2. Sacrum Flashcards
Benefits
similar to that provided by a low lumbar epidural. In
contrast to other neuraxial blocks, it requires no equipment other than a needle, syringe
and/or intravenous cannula, and is simple to perform. This is a core area of anatomy
applied to anaesthetic practice.
Relations
It is a large triangular-shaped bone that articulates superiorly with the fifth lumbar
vertebra, inferiorly with the coccyx and laterally with the ilia.
The dorsal roof comprises the fused laminae of the five sacral vertebrae and is convex
dorsally (the curve is variable between sexes and races).
In the midline there is a median crest, which represents the sacral spinous processes.
Lateral to this is the intermediate sacral crest with a row of four tubercles, which
represent the articular processes.
Process
The S5 processes are remnants only and form the
cornua, which are the main landmarks for identifying the sacral hiatus
At S5 this failure of development of the spinous processes and laminae results in a
hiatus in the roof of the canal. It is this sacral hiatus which allows access to the
extradural space. It is covered by the sacrococcygeal membrane..
Sacral contents
In addition to the dura superiorly, the canal contains areolar connective tissue, fat,
the sacral nerves, lymphatics, the filum terminale (which is an extension of the pia
mater originating from the conus medullaris at the end of the spinal cord and which
extends to the coccyx) and a rich venous plexus
Sacral Extradural (Caudal) Anaesthesia
Access to the canal is via the sacral hiatus at the level of the fifth sacral vertebra through
the sacrococcygeal membrane. In up to 7% of subjects, fusion has taken place and so
access is impossible. (Some authorities believe this to be an overestimate.)
Drug doses:
Adults, a typical dose would be levobupivacaine 0.5% 20 ml
Armitage in 1979: 0.5 ml kg1 of (levo)bupivacaine 0.25% for sacral block
(circumcision, hypospadias, anal procedures),
1.0 ml kg1 for low thoracic block (for inguinal herniotomy)
and 1.25 ml kg1 for higher thoracic block up to T8 (for orchidopexy).
Additives
The addition of clonidine 2.0 μg kg1 will double the duration of
effective analgesia, while ketamine 0.5 mg kg1 (preservative free) has been shown to
increase it by four times.
Given the well-recognized association between ketamine
and accelerated neuronal apoptosis in young animals, the potential risks would
seem to far outweigh the benefits of any increased duration of analgesia
swoosh’ test is similar in principle, except that auscultation is performed
as the local anaesthetic itself is being injected.
Differences between adults and children
: the dura mater usually ends at the level of S2 in adults
(although it can descend to within about 5 cm of the hiatus in some subjects).
At birth the dura is as low as S4, but by around 2 years of age it ascends to adult levels.
The sacral hiatus is easier to locate in children because it is not overlain
by the sacral fat pad that later develops in adults.
The sacral extradural space in children offers lower resistance to longitudinal spread
than the adult. Epidural fat in children has a loose and wide-meshed texture, whereas
in adults it becomes more densely packed and fibrous. There is less fibrous connective
tissue in the sacral epidural space than in adults, and this combination of factors
means that local anaesthetic spread is greater.
Complications
intrathecal injection are more likely in children less than
2 years of age. Otherwise the incidence both of intrathecal and intravascular injection
does not differ from that seen in adults. If too large an aspirating pressure is applied
to a syringe used to check that the needle is not intravascular, this may collapse the
vessel and give a false negative. For that reason, the syringe should be removed from
the needle or cannula, which should be allowed to drain freely
Sympathetic effects: children up to and beyond the age of 6 years show cardiovascular
stability in the face of blocks that would cause sympathetic blockade and
hypotension in adults. This is probably due to delay in the maturation of the
autonomic nervous system
Complication
Complications: these include failure;
intravascular injection (false negative aspiration
may occur in 10% or more of cases, as negative pressure collapses the vein);
intraosseous injection in young children;
bowel perforation;
dural and subdural puncture (which is characterized by an extensive, patchy block of slow onset).
In obstetric practice the fetal head is vulnerable to an inaccurately placed needle.
There are also the potential complications associated with the particular drugs injected
(local anaesthetics, opiates, clonidine, ketamine
