Review of Pharmacology - 9E (2015)

Review of Pharmacology
Contd...
Anaesthesia
LA causing
methemoglobinemia
B – Benzocaine
P – Prilocaine (Max)
L – Lignocaine
• Chlorprocaine is the shortest acting local anaesthetic and is contra-indicated
in spinal anaesthesia (It may cause paraplegia due to the presence of sodium
metabisulphite as preservative, which is neurotoxic).
• All LAs if absorbed in systemic circulation can cause CNS toxicity that manifests
as excitation followed by depression. Initial excitation is due to inhibition of
inhibitory neurons. Thus LAs may lead to seizures followed by coma at high
doses.
• Dibucaine is the most potent, longest acting and most toxic LA whereas
chlorprocaine is the shortest acting LA.
• Lignocaine is the most commonly used LA and is the drug of choice for ventricular
tachycardia. It can precipitate malignant hyperthermia due to release of calcium.
Dose is limited to 7 mg/kg with adrenaline or 4 mg/kg without adrenaline.
• Bupivacaine is the best drug for regional block but it is also the most cardiotoxic
LA. Due to cardiotoxic effect, it should not be used for Bier’s block. It is more
potent and longer acting than lignocaine. Addition of adrenaline does not
significantly increase the duration of action of this drug. For spinal anaesthesia,
0.5% solution is made hyperbaric with 8.25% dextrose in water. Its maximum
dose is 2mg/kg. Most common ECG changes in bupivacaine toxicity are slow
idioventricular rhythm with broad QRS complex. Bretylium is the drug of choice
for bupivacaine induced ventricular tachycardia.
• Ropivacaine is less cardiotoxic congener of bupivacaine.
• Prilocaine produces a metabolite “O-toluidine” which is an oxidizing agent. Latter
can oxidize hemoglobin to methemoglobin that can cause methemoglobinemia.
It is the most suitable LA for Bier’s block.
• Oxethazaine (mucaine) can be used to provide symptomatic relief in gastritis (it
remains unionized in the acidic pH of stomach).
Uses Of Local Anaesthetics
These agents can be used for following types of anaesthesia.
398
A mixture of 2.5% prilocaine and
2.5% lignocaine in 1:1 ratio can
anaesthetize even unbroken
skin. Combination of these two
agents lowers the melting point
of individual drugs and helps to
form a semi-solid ointment. This
mixture is known as Eutectic
mixture.
1. Surface Anaesthesia
It is the topical application of LA to mucous membranes and abraded skin. Only superficial
area is anaesthetized. Lignocaine is the commonly used agent for topical anaesthesia of
mucous membranes of nose, ear, eye, mouth and pharynx. It is also used during proctoscopy,
catheterization and per rectal examinations. Lignocaine is ineffective on intact skin. However
a mixture of 2.5% prilocaine and 2.5% lignocaine in 1:1 ratio can anaesthetize even unbroken
skin. Combination of these two agents lowers the melting point of individual drugs and
helps to form a semi-solid ointment. This mixture is known as Eutectic mixture. Oxethazaine
(mucaine) can be used to provide symptomatic relief in gastritis (it remains unionized in the
acidic pH of stomach).
2. Infiltration Anaesthesia
LA is infiltrated s.c. in the area of operation site for blocking the sensory nerve endings. It is
used in minor surgeries like incisions, excisions, suturing, hydrocele etc. Adrenaline can be
added to the LA to prolong its duration of action and to prevent systemic side effects.
3. Nerve Blocks
LA is injected around the nerve trunks supplying a particular area to anaesthetize all the
nerves coming to or leaving that area. It includes blocks of head and neck (stellate ganglion,
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