Review of Pharmacology - 9E (2015)

CHAPTER
9
Anaesthesia
Local Anaesthetics
These drugs act by blocking the conduction of nerve impulse along the axon. Small diameter
and myelinated fibres are blocked first whereas unmyelinated and thick fibres are blocked
at last. Thus, the order of blockade of fibres is B, C, Aδ and then Aα, β and γ. Autonomic
fibres are blocked first, then sensory (cold temperature sensation is lost first followed by
heat, pain and proprioception) and finally motor are blocked at last. Order of recovery is in the
reverse order.
Local anaesthetics (LAs) can be classified into amide and ester types. Amides are
usually long acting and chances of allergic reactions are less whereas esters are short acting
(due to metabolism by esterases present in the plasma). Ester LAs can cause allergic reactions
and also antagonize the action of sulfonamides due to degradation of PABA.
Amide class
* Lignocaine * Cocaine
* Prilocaine * Procaine
Ester class
* Bupivacaine * Chlorprocaine (Shortest acting)
* Dibucaine (Longest acting) * Tetracaine (Amethocaine)
* Mepivacaine * Benzocaine
* Etidocaine
* Ropivacaine
Mechanism of Action
All LAs are weak bases. These drugs act by penetrating the axonal membrane (in unionized
form) and blocking the voltage gated sodium channels from within (in ionized form).
Sodium bicarbonate speeds the onset of action of LAs by increasing the unionized form
(weak bases are unionized in the alkaline medium) that can penetrate the axonal membrane.
Vasoconstrictors like adrenaline can prolong the duration of action and decrease the
systemic toxicity. Alternative vasoconstrictor felypressin (synthetic vasopressin) can also be
used with LAs in order to avoid cardiovascular complications due to adrenaline.
Important Points
• Small diameter axons are more susceptible to block than large diameter fibres.
• Myelinated fibres are more sensitive than non-myelinated.
• Sequence of block is type B > type C > type A.
• In functional terms: Autonomic > sensory > motor.
• Among sensory fibres sequence of block is pain > temperature (cold before heat) >
touch > deep pressure > proprioception.
• All LAs are vasodilators except cocaine (act as sympathomimetic due to inhibition
of nor-adrenaline reuptake) which is a vasoconstrictor. Therefore all LAs decrease
BP except cocaine (increases).
• Cocaine should NEVER be given by intravenous route or with adrenaline.
• Cocaine is the only ester which is not metabolized by pseudocholinesterase. It is
metabolized in the liver.
• Procaine is the local anaesthetic of choice in malignant hyperthermia.
Adrenaline is added to LA to
make them long acting whereas
sodium bicarbonate makes them
fast acting.
Lignocaine with adrenaline
SHOULD NOT be used for:
• Areas with end arteries
e.g. for ring block of fingers,
toes, penis, pinna (absolute
contraindication)
• When an inhalational agent
especially halothane which
sensitizes myocardium to
adrenaline is used.
• Myocardial ischemic
patients
• Hyperthyroid patient
• Severe hypertensives
• Intravenous regional
anaesthesia (Bier’s block)
Contd...
https://kat.cr/user/Blink99/