Local anaesthesia

1. LOCAL ANAESTHESIA

2. Local Anesthetics
 Local anesthesia is any technique to induce the
absence of sensation in a specific part of the body,
generally for the aim of inducing local analgesia,
that is, local insensitivity to pain, although
other local senses may be affected as well.
 Reversible inhibition of sensory nerve impulse
conduction
 Prevent transmission of information to the CNS
 No loss of consciousness

3. Clinical Usefulness
Depends on:
 Inherent Anesthetic potency
 Rate of onset
 Duration of effect
Which in turn is dependent on:
 Physiochemical properties
 Inherent vasodilator activity

4. Physiochemical Properties
1. Lipid Solubility
2. Protein Binding - duration
3. pKA – determines onset of anesthesia

5. Molecular
weight
pKa
Protein
binding
Maximum
dose (mg)
Lidocaine 234 7.9 65 %
300
500 w/ Epi
Racemic
bupivacaine
288 8.01 95 %
175
225 w/ Epi
Ropivacaine 328.89 8.07 94 % 150
L-bupivacaine 324.9 8.09 > 97 % 150-375
Pharmacokinetics of Common
Local Anesthetics

6. Vasodilator Properties
 All except cocaine exhibit biphasic effect
on vascular smooth muscle
 Extreme low concentration provides
vasoconstriction
 Concentration for regional anesthesia
provides vasodilatation

7. NON PHARMACOLOGIC FACTORS
INFLUENCING ACTIVITY
1. Dosage
2. Addition of vasoconstrictor
3. Site of injection
4. Additives
5. Mixture of LA

8. 1. Dosage
• Primary qualities of regional anesthesia
(onset, duration, depth) are related to the
mass of the drug  volume x
concentration
2. Site on Injection
• Due to the particular anatomy of the area
of the injection, variation in the rate of
absorption & amount of drug used
• In Spinal anesthesia, lack of nerve
sheath around spinal cord are responsible
for the rapid onset

9. 3. Additives
a. CO2
b. NaHCO3 – increase pH near pKA more ionized 
faster entry faster onset
c. KCl
d. Dextran
4. Mixture of Local Anesthesia
 basis is for the mixture of local anesthesia to
compensate for the short duration of action
and the long latency of others

10. Clinically Useful Local Anesthesia
1. AMINO-ESTERS
 Ester link between aromatic and main portion of
the molecule
2. AMINO-AMIDES
 Amide linkages

11. Local Anesthetics Classification
CH2 N
CH2
CH2
CH3
CH3
CH2 CH2 N
CH2
CH2
CH3
CH3
CH3
CH3
CH3
CH3
NH C
O
1) Amides
Lidocaine, Bupivacaine,
Ropivacaine, Levobupivacaine
Procaine, Chloroprocaine,
Cocaine, Tetracaine
C
O
C
2) Esters

12. Mechanism of Action
 Prevent generation and conduction of
nerve impulses by decreasing or
preventing the large transient increase in
permeability of excitable membranes to
Na+

13. Indications for local anesthesia
 Most frequent use: regional anesthesia.
 Analgesic espescially post operative pain.
 Lidocaine (Xylocaine) also reduces blood pressure response to direct
laryngoscopic tracheal intubation, an effect probably secondary to
generalized cardiovascular depression.
 Treatment of intractable cough.

14. Local Anesthetics
Mechanism of Action
blocks voltage-sensitive Na+ ion channels
in neuronal membrane
prevent Na+ influx (Depolarization)
prevent transmission
of nerve impulses

15. a. Low anesthetic potency & short duration of
action  procaine & chloroprocaine
b. Intermediate anesthetic potency & duration
of action  lidocaine, mepivacaine &
prilocaine
c. High anesthetic potency and prolonged
duration of action  tetracaine,
bupivacaine : racemic and
levobupivacaine, ropivacaine,
ethidocaine

16. Essential Precautions:
 Secure intravenous access before injection
of any dose that may cause toxic effects .
 Always have adequate resuscitation
equipment and drugs available before
starting to inject.

17. Advantages of local anaesthesia
 Non inflammable.
 Excellent muscle relaxant effect.
 During local anesthesia the patient remains conscious.
 It requires less skilled nursing care as compared to
other anesthesia like general anesthesia.
 Maintains his own airway.

18.  Less pulmonary complication.s
 Aspiration of gastric contents unlikely.
 Less nausea and vomiting.
 Contracted bowel so helpful in abdominal and pelvic
surgery.
 Postoperative analgesia.
 There is reduction surgical stress.
 Earlier discharge for outpatients.

19.  Suitable for patients who recently ingested food or fluids.
 Local anesthesia is useful for ambulatory patients
having minor
 procedures.
 Ideal for procedures in which it is desirable to have the
patient awake and cooperative.
 Less bleeding.
 Expenses are less.

20. Disadvantages of local anaesthesia
 There are individual variations in response to local
anesthetic drugs.
 Rapid absorption of the drug into the bloodstream can
cause severe, potentially fatal reactions.
 Apprehension may be increased by the patient's ability
to see and hear. Some patients prefer to be unconscious
and unaware.

21.  Direct damage of nerve.
 Post-dural headache from CSF leak.
 Hypotension and bradycardia through blockade of the
sympathetic nervous system.
 Not suitable for extremes of ages.
 Multiple needle bricks may be needed.

22. TOXICITY
Local Anesthetic Toxicity
A. Systemic – CNS, CVS
B. Local – neural & skeletal muscle irritation
C. Specific – addiction, allergy,
methemoglobinemia

23. LOCAL ANESTHETIC TOXICITY
 Depend on blood level of local anesthetic
delivered to the brain and heart
 Appropriate dose and technique rarely
causes adverse reaction
 Toxic levels – usually due to
intravascular injection or excess dose in
extravascular administration

24. Systemic Toxicity
1. CNS Toxicity
• Related to intrinsic anesthetic potency
• LOW DOSE – excitatory
 Mechanism: selective blockade of inhibitory Pathway in
the cerebral cortex  allows facilitatory neurons to
function unopposed
• LARGE DOSES – CNS depression
 Mechanism: inhibition of both facilitatory & inhibitory
Pathway
 Sign: convulsion ceases, respiratory depression, arrest

25. Subjective CNS Symptoms
 light headedness
 Dizziness
 Visual & Auditory disturbances – difficulty in
focusing & tinnitus
 disorientation
 drowsiness

26. Objective CNS Signs
(at low dose)
 shivering
Muscular twitching
Tremors – muscles of face and distal parts of
the extremities
 convulsions – tonic, clonic

27. Factors that Affect CNS Toxicity
 Potency
 Rate of Injection
 Rate at which a particular blood level is
attained

28. Effects of Increase pCO2 on CNS
Toxicity
 pCO2 level is inversely related to
convulsive threshold
 Enhances cerebral blood flow so more
local anesthetic is delivered to the brain
 Decrease plasma protein binding of local
anesthetics, more local anesthetic
available to the brain

29. 2. CVS Toxicity – Cardiac, Vascular
a. Cardiac Effect
• Dose dependent negative inotropic
action – depends on potency of LA
• Inhibit Na conductance in fast channels
• High concentration of lidocaine,
procaine & tetracaine can block slow
calcium channels
• Increase LV EDP, direct pulmonary
vasoconstriction effect

30. 1) INDIRECT EFFECTS
2) DIRECT EFFECTS
a) block sympathetic  innervation
b) other CNS-mediated mechanisms
a) Block  Na+ channels
conduction delay & QRS prolongation
b) Block K+ & Ca++ channels
Cardiotoxicity

31. CVS Toxicity
b. Peripheral Vascular Effects (BIPHASIC)
LOW DOSE – stimulates myogenic contraction and
augments basal tone leading to higher pressure
HIGH DOSE – inhibits myogenic activity vasodilation
COCAINE – initial effect is vsaodilatation followed by
vasoconstriction at low and high doses
Mechanism of Action – inhibits re-uptake of NE by tissue
binding site, therefore no re-uptake leading to increase
free NE, potentiating the effects of adrenergic
stimulation.(hypertension and ventricular ectopia)

32. Local Toxicity
 The more potent longer acting local
anesthetics (e.g. Bupivacaine,
ethidocaine): > degree of localized
skeletal muscle damage than less potent
shorter acting agents like Lidocaine and
Prilocaine
 Reversible
 No clinical signs of local irritation

33. Specific Toxicity
1. Methemoglobinemia – PRILOCAINE
– Mechanism: degraded in the liver to
ortotoluidine which causes oxidation of Hb
 requires 600mg Prilocaine to produce
clinical level of Methemoglobenemia
Reversed with Methylene blue
2. Allergy
– AMINO ESTERS  derivatives of para-
amino-benzoic-acid (allergenic)