Local Anaesthetic Techniques
FEATURES OF LOCAL ANAESTHESIA
Avoidance of the adverse effects of general anaesthesia. These may range from relatively minor postoperative nausea and vomiting, sore throat or myalgia to major issues such as respiratory impairment, awareness, airway complications or aspiration pneumonitis. In addition, the management of many patients with significant medical co-morbidity such as diabetes, obesity, or chronic pulmonary disease, can be improved or simplified. In elderly patients, acute perioperative cognitive impairment may be limited by reducing or avoiding psychoactive drugs and maintaining contact with their surroundings.
Postoperative analgesia. Local anaesthetic techniques can be used to provide effective prolonged postoperative analgesia whilst avoiding the systemic effects of other analgesic drugs, especially opioids. This can be provided using long-acting agents or by utilizing continuous catheter techniques, either neuraxial or peripheral. Some patients may be distressed by the accompanying numbness and motor block, but adequate preoperative explanation should minimize this concern. In addition, it is important that both nursing staff and patient are aware of the risk of tissue damage to any blocked area whether from direct trauma or indirect pressure from poor positioning or prolonged immobility. Simple techniques such as supporting the arm in a sling after brachial plexus block may help prevent injury and encourage earlier mobilization.
Preservation of consciousness during surgery. The ability to assess neurological status continuously may be an advantage in patients with a head injury, diabetes or those undergoing carotid endarterectomy. Patient positioning may be safer, more comfortable and damage to pressure areas or joints avoided if the patient is awake. Airway and neck manipulation can be avoided; this may be especially important in a patient with severe rheumatoid arthritis or an unstable cervical spine. The awake patient undergoing caesarean section under regional anaesthesia is able to protect her own airway and experience the birth of the child.
Sympathetic blockade and attenuation of the stress response to surgery.
Improved gastrointestinal motility and reduced nausea and vomiting. This can allow earlier feeding and more rapid mobilization and discharge.
COMPLICATIONS OF LOCAL ANAESTHESIA
It encourages careful, meticulous practice
It provides the anaesthetist with valuable information on block onset and efficacy
It alerts the anaesthetist to early complications such as inadvertent intravenous injection or intraneural injection.
Local Anaesthetic Toxicity
Clinical Features and Treatment
The clinical features and treatment of LA toxicity are described in Chapter 4.
Prevention
The following precautions are useful to minimize the risk of LA toxicity:
GENERAL MANAGEMENT
Selection of Technique
Local anaesthetic drugs may be administered by:
Resuscitation Equipment
an anaesthetic breathing system through which oxygen may be administered under pressure via a face mask or tracheal tube
a laryngoscope with two sizes of blade, a range of tracheal tubes and an introducer
a table which may be rapidly tilted head-down
intravenous cannulae and fluids
thiopental or propofol to control convulsions
drugs to treat bradycardia or hypotension, especially atropine, ephedrine and metaraminol or phenylephrine
lipid emulsion 20% for treating serious systemic toxicity (see Ch 4).
Regional Block Equipment
Needles
The use of very fine spinal needles (26G) has significantly reduced the incidence of post-spinal headache as has the use of pencil-point 25G Whitacre and 24G Sprotte needles (Fig. 24.1A). The 27G Whitacre needles appear to be associated with the lowest incidence of post-spinal headache but confident and successful use of these needles requires greater expertise than is needed for the use of larger needles. For peripheral blocks, short-bevelled needles allow greater tactile appreciation of fascial planes and appear to reduce the likelihood of nerve damage. A variety of insulated needles are available for plexus and peripheral nerve blockade using a nerve stimulator (Fig. 24.1B). Ultrasound needle visibility may be improved by using echogenic needles which have ‘corner stone’ reflectors positioned at the distal end of the cannula shaft (Fig. 24.11C).
FIGURE 24.1 (A) Left to right: Quincke, Whitacre, Sprotte and Spinocath needles. (B) Left to right: standard-bevelled, short-bevelled, insulated short-bevelled and insulated Tuohy needles.
Immobile Needle Technique
For plexus and major nerve blocks, local anaesthetic drug is drawn into labelled syringes and connected to the block needle by a short length of tubing (Fig. 24.2). This allows the anaesthetist to hold the needle steady while aspiration tests are performed and syringes changed. The system must be primed to prevent air embolism and also to avoid image artefact when using ultrasound-guidance.
Catheters
Continuous administration of local anaesthetic drugs has been made possible by the development of high-quality catheters, which are introduced through a needle (or occasionally over a needle; Fig 24.1A) and may be left in position for hours or even days. Careful fixation is essential to maintain the position of the catheter in the postoperative period. Catheters, in particular spinal (subarachnoid) catheters, should be labelled clearly to prevent accidental overdosage.
Nerve Stimulators
Few anaesthetists now aim to deliberately elicit paraesthesiae when performing a major nerve block; many still use the nerve stimulator (Fig. 24.2) but an increasing number now use ultrasound-guidance. It is important to explain to the patient the sensation elicited by nerve stimulation. It causes little discomfort unless the contracting muscle crosses a fracture site, when duration of stimulation should be kept to the absolute minimum necessary to confirm needle position. The incidence of paraesthesia with short-bevelled insulated needles is very low because of their ability to stimulate without direct neural contact. They are also more likely to displace nerves rather than penetrate them.
CENTRAL NERVE BLOCKS
Physiological Effects of Subarachnoid Block
Prevention of Hypotension
Bradycardia may occur because of:
neurogenic factors, particularly in awake patients, i.e. vasovagal syndrome
paradoxical Bezold-Jarisch reflex; decreased venous return and heightened sympathetic tone leads to forceful contraction of a near empty left ventricle, with consequent parasympathetically mediated arterial vasodilatation and bradycardia
Indications for Subarachnoid Block
Types of Surgery
Urology: Subarachnoid block is commonly employed for urological procedures such as transurethral prostatectomy, but it should be remembered that a block to T10 is required for surgery involving bladder distension. Perineal and penile operations may also be carried out using a low ‘saddle block’, peripheral blockade or caudal anaesthesia.