Pharmacology
Anaesthesia
Which of the following local anaesthetics is typically used for a Bier's block:
Answer:
Prilocaine hydrochloride is a local anaesthetic of low toxicity which is similar to lidocaine hydrochloride but is more extensively metabolised and is less toxic in equipotent doses.. It is most frequently used for intravenous regional anaesthesia (i.e. Bier's block).Local Anaesthetics
Pharmacology / Anaesthesia
Last Updated: 4th April 2019
Local anaesthetic drugs act by causing a reversible block to conduction along nerve fibres. The drugs penetrate the nerve in a non-ionised lipophilic form, but once inside the axon, some ionised molecules are formed and these block the voltage-gated Na+ channels, preventing the sodium current and generation of action potentials.
They vary widely in their uses, potency, toxicity, duration of action, stability, solubility in water, and ability to penetrate mucous membranes.
Lidocaine
Lidocaine is the most widely used local anaesthetic. It acts rapidly and when administered with adrenaline the block lasts about 90 minutes. Lidocaine is effectively absorbed from mucous membranes and can therefore be used as a surface anaesthetic in concentrations up to 10%. Except for surface anaesthesia and dental anaesthesia, solutions should not usually exceed 1% in strength. Lidocaine also has class 1b antiarrhythmic properties (blocks inactivated voltage-gated Na+ channels) and may be given intravenously in the treatment of ventricular arrhythmias.
Prilocaine
Prilocaine hydrochloride is a local anaesthetic of low toxicity which is similar to lidocaine hydrochloride but is more extensively metabolised and is less toxic in equipotent doses.. It is most frequently used for intravenous regional anaesthesia (i.e. Bier's block). Bier's block anaesthesia is an intravenous regional anaesthesia technique in which an extremity (usually the arm) is anaesthetised by injecting a local anaesthetic solution (typically prilocaine) into a vein after the limb has been exsanguinated and a tourniquet placed on it. The tourniquet prevents a potentially toxic dose of local anaesthetic from leaving the extremity and blood from entering it, giving the patient an anaesthetised extremity and the surgeon a bloodless field.
EMLA
EMLA cream, an effective topical local anaesthetic, is a 50/50 mixture of 2.5% prilocaine and 2.5% lidocaine.
Bupivacaine
Bupivacaine has a longer duration of action than the other local anaesthetics, up to 8 hours when used for nerve blocks. It has a slow onset, taking up to 30 minutes for full effect. It is often used in lumbar epidural blockade and is particularly suitable for continuous epidural analgesia in labour, or for postoperative pain relief. It is the principal drug used for spinal anaesthesia.
Vasoconstrictors
Local anaesthetics cause dilatation of blood vessels. The addition of a vasoconstrictor such as adrenaline/epinephrine to the local anaesthetic preparation diminishes local blood flow, slowing the rate of absorption and thereby prolonging the anaesthetic effect. Great care should be taken to avoid inadvertent intravenous administration of a preparation containing adrenaline/epinephrine, and it is not advisable to give adrenaline/epinephrine with a local anaesthetic injection in digits or appendages because of the risk of ischaemic necrosis.
Adrenaline/epinephrine must be used in a low concentration when administered with a local anaesthetic. Care must also be taken to calculate a safe maximum dose of local anaesthetic when using combination products. In patients with severe hypertension or unstable cardiac rhythm, the use of adrenaline/epinephrine with a local anaesthetic may be hazardous; for these patients an anaesthetic without adrenaline/epinephrine should be used.
Toxicity
Toxic effects after administration of local anaesthetics are a result of excessively high plasma concentrations; severe toxicity usually results from inadvertent intravascular injection or too rapid injection. Following most regional anaesthetic procedures, maximum arterial plasma concentration of anaesthetic develops within about 10 to 25 minutes, so careful surveillance for toxic effects is necessary during the first 30 minutes after injection.
Local anaesthetics depress other excitable tissues producing:
- Central nervous system effects (with increasing toxic doses)
- Sedation, lightheadedness, anxiety and restlessness
- Twitching, tremor and visual disturbance
- Convulsions and coma, with respiratory depression resulting from medullary depression
- Cardiovascular effects
- Vasodilation (partly by direct action on blood vessels and partly by blocking their sympathetic nerve supply) with hypotension
- Myocardial depression with bradycardia
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
