Choice of a local anaesthetic technique depends upon its feasibility
for a particular procedure and the patient’s willingness and ability to
co-operate, as well the surgeon’s and anaesthetist’s preference. Local
anaesthesia may be the reliable and traditional method for some minor surgical
procedures which do not warrant general anaesthesia. One of the main advantages
is the continuation of pain relief into the postoperative period, by either
drugs with a prolonged duration of action or delivery of further local
anaesthetic increments via a catheter.
However,
local anaesthesia is not infallible, and may be contraindicated by allergy or
local infection. Epidural and intrathecal anaesthesia includes sympathetic
blockade which may result in vasodilatation and systemic hypotension, and may
confer greater intraoperative risk than a carefully managed general anaesthetic.
Complications
may be local, such as infection or haematoma, or systemic if overdosage or
accidental intravascular injection leads to toxic blood levels. The latter may
manifest as depressed conscious level, convulsions and/or cardiac arrest
(particularly bupivacaine), and may be heralded by circum-oral paraesthesia and
light-headedness. Addition of adrenaline to the local anaesthetic solution
increases the risk of cardiac arrhythmia associated with accidental
intravascular
injection. Prilocaine overdosage causes methaemoglobinaemia. Recently
introduced local anaesthetics such as ropivicaine and laevobupivacaine are
claimed to have enhanced safety profiles.
Addition
of adrenaline (commonly 1:200 000—1:125 000 concentration) to the local
anaesthetic solution hastens the onset and prolongs the duration of action, and
permits a
higher dose of drug to be used as it is more slowly absorbed into the
circulation. Adrenaline should not be used in hypertensive patients, or for
patients taking either monoamine oxidase inhibitor or tricyclic antidepressant
drugs, as its cardiovascular effects are potentiated. It should not be used in
end arterial locations, where there is no collateral circulation, such as
fingers and toes, or around the retinal artery.
The
potential risk of life threatening sequelae mandates the availability of
appropriately skilled personnel and resuscitation equipment including oxygen, as
prerequisites if local anaesthesia is practised.
The
following exemplify sensible upper dose limits suitable for a 70 kg adult.
• Lignocaine 200 mg (10 ml of 2 per cent) or lignocaine with adrenaline
(1:200 000) 500 mg. Lignocaine 1 per cent is effective for most sensory blocks
and’ addition of adrenaline enables a greater volume to be used. Thus, up to
50 ml of lignocaine 1 per cent with adrenaline (1:200 000) can be infiltrated
into the tissues.
• Bupivacaine 150 mg (30 ml of 0.5 per cent). Addition of
adrenaline would enhance the safety of this high dose. Bupivacaine is more
cardiotoxic than lignocaine. Bupivacaine 0.25 per cent is effective for
sensory block against moderate stimulus. Bupivacaine must never be injected into
a vein, and is absolutely contraindicated from use for intravenous regional
anaesthesia. (Bier’s block is commonly used for procedures such as reduction
of Colle’s fracture and carpal tunnel decompression.) Bupivacaine is a
long-acting drug lasting for about 6 hours.
• Prilocaine 400 mg (40 ml of 1 per cent). The presence of
blue—brown skin colour indicates methaemoglobin toxicity.
Topical
anaesthesia
Topical anaesthetic agents are used on the skin, the urethral mucosa,
nasal mucosa and the cornea. The agents used are amethocaine, because it is well
absorbed by mucosa, cocaine for its vasoconstrictive properties, lignocaine and
prilocatne. A lignocaine and prilocaine eutectic mixture (‘EMLA’ cream) is
commonly used on the skin of children before venepuncture.
Local
infiltration
This is the method most commonly used by both surgeons and physicians.
It is not necessary to starve the patient preoperatively unless the procedure
carries a high risk of intravascular or intrathecal injection. Infiltration of
local anaesthetic drug may be into or around a wound, ideally with particular
attention to neuroanatomical territories and boundaries. Contraindications are
local infection
Regional
anaesthesia (without general anaesthesia)
Regional anaesthesia involves blockade of major nerve trunks which
innervate the site of surgery. It is usually performed by an anaesthetist with
the necessary skills. However, both ultrathecal (spinal) and epidural
anaesthesia should only be conducted by experienced practitioners using full
aseptic techniques.
It
is in any case required that a doctor other than the operator is present to
monitor continuously and resuscitate the patient if necessary. If regional
anaesthesia fails, general anaesthesia may be necessary. Compensation for an
inadequate regional block by heavy sedation carries great dangers including
airway obstruction and pulmonary aspiration of gastric contents. These may
easily go unrecognised by a single-handed operator. All patients should be
starved preoperatively and monitored. In emergency surgery, regional
anaesthesia carries the advantage of preservation of the protective laryngeal
reflexes, particularly in emergency obstetric anaesthesia, for which epidural or
spinal regional anaesthesia is commonly the method of choice. The reduction in
blood pressure with spinal and epidural anaesthesia can be advantageous in
reducing intraoperative blood loss, but only if the surgeon strives to achieve
haemostasis prior to wound closure and restoration of normal blood pressure.
When
sedation has been used for surgery under regional anaesthesia, respiratory
obstruction may occur postoperatively when the surgical stimulus has ceased.
Oxygen saturation measurement by pulse oximetry is required monitoring during
regional anaesthesia.
Regional
anaesthesia had a very clear advantage over general anaesthesia when general
anaesthetic agents carried high morbidity and mortality rates. In contemporary
practice this advantage is less pronounced or even reversed. However, regional
anaesthesia may be advantageous for patients who have debilitating respiratory
disease. In cardiovascular disease, general anaesthesia with support of the
circulation and pulmonary ventilation is often more advantageous than risking
hypotension and tachyarrhythmias exacerbating ischaemic heart disease and
resultant angina, which may occur with regional anaesthesia. Regional
anaesthesia does provide excellent analgesia into the postoperative period,
reducing the need for centrally acting analgesic agents.
The
most clear indications for spinal and epidural anaesthesia are in obstetric
practice to spare the mother from the risk of pulmonary aspiration because of
the full stomach usually present in labour, and also to spare the newborn from
the depressant action of the general anaesthetic and analgesic drugs.
General
and regional anaesthesia combined
Combining the two methods of anaesthesia in well-balanced measure
enables a patient to receive a lighter general anaesthetic and to have the
advantage of good postoperative analgesia. At its simplest, the infiltration of
an abdominal wound with local anaesthetic agent will facilitate comfortable
breathing in the recovery room.
Regional
local anaesthetic techniques
Spinal, plexus and major nerve local anaesthetic blockade may be
employed alone or in combination with sedation or general anaesthesia. It is
most commonly used for limb, abdominal and thoracic surgery, and obstetric
analgesia and surgery.
It
is imperative that a second medical practitioner, and not the surgical operator,
is responsible for supervision and monitoring of the patient during the
procedure.
Preoperative
patient preparation for elective regional anaesthesia includes that required for
general anaesthesia, with explanation of the local anaesthetic procedure. In
emergency, it is safer to use regional anaesthesia on an unstarved patient
rather than general anaesthesia, for the risk of aspiration of gastric contents
is much reduced although not absent. Some forms of regional anaesthesia with
long acting drugs, such as epidural bupivacaine anaesthesia, result in prolonged
motor block and may be unsuitable if the patient is expected to be an ambulant
day case.
The
recently introduced subcutaneous low-molecular-weight heparins (LMWH) for
prophylaxis for deep venous thrombosis are longer acting than heparin, and
appear to have increased the risk of intraspinal haematoma. Epidural and spinal
injections (and catheter insertion or removal) should only be performed at least
12 hours after a LMWH dose, and the next LMWH dose delayed for at least 2 hours.
The LMWH doses must therefore be timed appropriately. As with many perioperative
management issues, optimal care depends upon close liaison between anaesthetist
and surgeon.
Electrocardiogram,
pulse oximetry and blood pressure measurements should be performed during
regional anaesthesia. Oxygen by face mask should be given to frail or sedated
patients during surgery.
Common
local anaesthetic techniques
In awake patients the nerve blocks must provide comprehensive numbness
throughout the surgical field. The following field blocks are commonly used.
• Brachial plexus block for surgery on the arm or hand.
• Field block for inguinal hernia repair. The iliohypogastric and
ilioinguinal nerves are blocked immediately inferomedial to the anterior
superior iliac spine. The genitofemoral nerve is infiltrated at the
midinguinal point and at the pubic tubercle. If a large volume of local
anaesthetic is used, the peritoneal sac can be anaesthetised before the
incision, but care must be taken to avoid drug toxicity.
Local
anaesthetic with 1:200 000 adrenaline prolongs the duration of action and
reduces toxicity by producing vasoconstriction. The line of the skin incision
should be infiltrated with the mixture.
• Regional block of the ankle. This can be used for surgery on
the toes and minor surgery of the foot.
Intravenous
regional anaesthesia
The arm to be operated on is exsanguinated by elevation and/or
compression, and then isolated from the general circulation by the application
of a tourniquet inflated to a pressure well in excess of the systolic arterial
pressure. The venous system is then filled with local anaesthetic agent,
injected via a previously placed indwelling venous cannula. The drug diffuses
from the bloodstream into the nerves to produce an effective block. The arm is
more suitable for this procedure (Bier’s block) than the leg because the large
volume of drug required for the latter can easily lead to toxicity. The
tourniquet must only be deflated after adequate time has elapsed (at least 20
minutes) to allow for the residual venous drug load to fall to a safe level,
before it is washed back into the general circulation. Cardiac arrest or
convulsions may well occur if the tourniquet is accidentally released before the
drug is fixed; this was particularly noted with bupivacaine, which has been
banned from use in this procedure after reports both of a number of deaths and
of directly toxic effects on the heart. Prilocaine 0.5 per cent up to 50
ml is recommended as the safest agent to use. As above, a separate medical
practitioner should supervise the block and monitor the patient, while the
surgeon operates.
lntrathecal
anaesthesia
Spinal anaesthesia in the awake patient is useful for some forms of
surgery in the pelvis or lower limbs. Hyperbaric solutions of bupivacaine are
injected as a ‘single shot’ into the cerebrospinal fluid, to produce rapidly
an intense blockade, usually within 5 minutes. Autonomic sympathetic
blockade results in hypotension, necessitating prior intravenous fluid loading
and titration of vasoconstrictor drugs. If the hyperbaric solution is allowed to
ascend too high, severe hypotension and ventilatory failure occur. This factor
limits the use of spinal anaesthesia to surgery below the segmental level of Tb.
Postoperative
headache, due to cerebrospinal fluid leakage through the dural perforation, is
nowadays much less common as a result of modern needles (very fine with a
round or pencil point tip and side aperture) designed to split rather than cut
the dural fibres.
Spinal
anaesthesia is much used for Caesarean section, prostatectomy and lower limb
surgery. Intrathecal opioid drugs are used to produce postoperative analgesia
but there is a significant risk of respiratory depression.
Epidural
anaesthesia
Epidural anaesthesia is slower in onset than intrathecal anaesthesia,
but has the advantage of multiple dosing and hence
prolonged use, as an indwelling catheter may be threaded into the
epidural space. Hence, epidural anaesthesia can provide good pain relief
extending into the postoperative period. Urinary retention is common,
necessitating catheterisation of the bladder. Epidural anaesthesia also
includes sympathetic blockade, but it is of slower onset, as is the resulting
hypotension, which may be easier to control and can be used to advantage for the
surgery, in reduction of blood loss. If a weak solution of bupivacaine or the
newer ropivicaine is chosen, epidural anaesthesia can be used to produce a
predominantly sensory block for analgesia after upper abdominal or thoracic
surgery. The contemporary trend is to combine weak solutions of local
anaesthetic with opioid agents such as the lipid-soluble diamorphine or fentanyl,
the latter producing analgesia by their action on the opioid receptors in the
spinal cord. However, the potential complication of epidural opioid analgesia is
delayed respiratory arrest from rostral spread and central depression, as late
as 24 hours after the last dose. Hence, regular monitoring of conscious level
and respiratory rate, and facility to immediately reverse the opioid with
intravenous naloxone or to resuscitate, are essential prerequisites.
Epidural
anaesthesia (with bupivacaine or ropivicaine) remains the standard method of
anaesthesia during labour and interventional delivery. In contrast to local
anaesthetic agents, epidural opioid agents alone do not produce hypotension,
so they are preferable for patients who are mobile. There is a current trend
towards their use in labour for this reason, but alone they would not produce
adequate analgesia for surgical intervention.
Caudal
epidural anaesthesia is produced by injection of local anaesthetic agent through
the sacrococcygeal membrane. Its main uses are to supplement general
anaesthesia and for very effective postoperative pain relief. This analgesic
technique is much used in paediatric surgery.