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 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.
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 and clotting disorder. Not only will local infiltration spread the infection, but local anaesthetic drugs are ineffective in conditions of acidity as produced by infection. Local infiltration in the presence of a clotting disorder may result in haemorrhage, or may produce haematoma, potentially fatal in the airway, as in dentistry.
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.
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 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.
No comments yet.