Info

nerve block (peripheral)

3% + felypressm (dental use)

up co 40 ml up to 20 ml up to 20 m

1. Time to peak effect is about 5 min, except bupivacaine (see text).

2. Maximum doses of local anaesthetic plus vasoconstrictor are toxic in absence of the vasoconstrictor and so substantially less should be used. All doses are approximate only; larger amounts may be safe, but deaths have occurred with smaller amounts, so that the minimum dose that will suffice should be used.

3. Maximum dose of adrenaline (epinephrine) is 500 micrograms (see below).

4. Concentrations of solutions and dose of drug: errors of calculation occur with sometimes fatal results.We provide these figures becuse experience of conducting examinations with medical students has taught us that they frequently lack the facility of calculating the dose of a drug in a given volume of known concentration.

1% means one gram in 100 ml = 1000 mg in 100 ml = 10 mg per ml: 2% = 20 mg per ml, and so on.

It is traditional to express adrenaline (epinephrine) concentrations as I in 200 000, or I in 80 000, or I in 1000.

I in 1000 means 1000 mg (1.0 g) in 1000 ml = I mg per ml.

I in 200 000 means 1000 mg (1.0 g) in 200 000 ml = 5 micrograms per ml.

Thus the maximum dose of adrenaline (epinephrine), 500 micrograms (see above), is contained in 100 ml of I in 200 000 solution.

Notes:

1. Time to peak effect is about 5 min, except bupivacaine (see text).

2. Maximum doses of local anaesthetic plus vasoconstrictor are toxic in absence of the vasoconstrictor and so substantially less should be used. All doses are approximate only; larger amounts may be safe, but deaths have occurred with smaller amounts, so that the minimum dose that will suffice should be used.

3. Maximum dose of adrenaline (epinephrine) is 500 micrograms (see below).

4. Concentrations of solutions and dose of drug: errors of calculation occur with sometimes fatal results.We provide these figures becuse experience of conducting examinations with medical students has taught us that they frequently lack the facility of calculating the dose of a drug in a given volume of known concentration.

1% means one gram in 100 ml = 1000 mg in 100 ml = 10 mg per ml: 2% = 20 mg per ml, and so on.

It is traditional to express adrenaline (epinephrine) concentrations as I in 200 000, or I in 80 000, or I in 1000.

I in 1000 means 1000 mg (1.0 g) in 1000 ml = I mg per ml.

I in 200 000 means 1000 mg (1.0 g) in 200 000 ml = 5 micrograms per ml.

Thus the maximum dose of adrenaline (epinephrine), 500 micrograms (see above), is contained in 100 ml of I in 200 000 solution.

sory blockade can be achieved without causing motor weakness. The rate of onset of ropivacaine is similar to bupivacaine, but its absolute potency and duration of effect is slightly less. The indications for ropivacaine are similar to those of bupivacaine.

Ester

Cocaine (alkaloid) is used medicinally solely as a surface anaesthetic (for abuse toxicity, see p. 192) usually as a 4% solution, because adverse effects are both common and dangerous when it is injected. Even as a surface anaesthetic sufficient absorption may take place to cause serious adverse effects and cases continue to be reported; only specialists should use it and the dose must be checked and restricted. Cocaine prevents the uptake of catecholamines [adrenaline (epinephrine), noradrenaline (norepinephrine)] into sympathetic nerve endings, thus increasing their concentration at receptor sites, so that cocaine has a built-in vasoconstrictor action, which is why it retains a (declining) place as a no longer available as a preservative-free solution and most clinicians now use lidocaine instead. Crystals of prilocaine and lidocaine base, when mixed, dissolve in one another to form a eutectic emulsion that penetrates skin and is used for dermal anaesthesia (EMLA, see p. 358), e.g., before venepuncture in children.

Bupivacaine is long-acting (t'/2 3 h) (see Table 18.1) and is used for peripheral nerve blocks, and epidural and spinal anaesthesia. Whilst onset of effect is comparable to lidocaine, peak effect occurs later (30 min).

Levobupivacaine is the S-enantiomer of racemic bupivacaine. The relative therapeutic ratio (levo-bupivacaine:racemic bupivacaine) for CNS toxicity is 1.03, indicating that levobupivacaine is marginally less toxic.

Ropivacaine may provide better separation of motor and sensory nerve blockade; effective sen surface anaesthetic for surgery involving mucous membranes, e.g. nose. Other local anaesthetics do not have this action, indeed most are vasodilators and added adrenaline (epinephrine) is not so efficient.

Obstetric analgesia and anaesthesia

Although this soon ceased to be considered immoral on religious grounds, it has been a technically controversial topic since 1853 when it was announced that Queen Victoria had inhaled chloroform during the birth of her eighth child. The Lancet recorded 'intense astonishment ... throughout the profession' at this use of chloroform, 'an agent which has unquestionably caused instantaneous death in a considerable number of cases'. But the Queen (perhaps ignorant of these risks) took a different view, writing in her private journal of 'that blessed chloroform' and adding that 'the effect was soothing, quieting and delightful beyond measure'.

The ideal drug must relieve labour pain without making the patient confused or uncooperative. It must not interfere with uterine activity nor must it influence the fetus, e.g. to cause respiratory depression by a direct action, by prolonging labour or by reducing uterine blood supply. It should also be suitable for use by a midwife without supervision.

Pethidine is widely used. There is little difference between the effects of equipotent doses of morphine and pethidine with regard to analgesia, respiratory depression, and nausea and vomiting (but it may delay labour less). All opioids have the potential to cause respiratory depression of the newborn but this can be reversed with naloxone if necessary. The popular choice of pethidine for analgesia during labour in the UK is not because of any clear pharmacological advantage, but because it remains the only opioid licensed for use by midwives.

Nitrous oxide and oxygen (50% of each: Entonox) may be administered for each contraction from a machine the patient works herself or supervised by a midwife (about 10 good breaths are needed for maximal analgesia).

Epidural local anaesthesia provides the most effective pain relief, but the technique should only be undertaken after adequate training. In the UK, only anaesthetists insert epidural anaesthetics.

Spinal anaesthesia is now used more commonly then epidural anaesthesia for Caesarean section. The vast majority of Caesarean sections are now undertaken with regional rather than general anaesthesia.

General anaesthesia during labour presents special problems. Gastric regurgitation and aspiration are a particular risk (see p. 347). The safety of the fetus must be considered; all anaesthetics and analgesics in general use cross the placenta in varying amounts and, apart from respiratory depression, produce no important effects except that high doses interfere with uterine retraction and may be followed by uterine haemorrhage. Neuromuscular blocking agents can be used safely.

Anaesthesia in patients already taking medication

Anaesthetists are in an unenviable position. They are expected to provide safe service to patients in any condition, taking any drugs. Sometimes there is opportunity to modify drug therapy before surgery but often there is not. Anaesthetists require a particularly detailed drug history from the patient.

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