Anaesthesia – Work, Classification, Uses

Definition

An “Anaesthetic” is a drug used to induce temporary loss of sensation or awareness.

They may be divided into two broad classes :-

1. General Anaesthetics= which result in a reversible loss of consciousness.
2. Local Anaesthetics= which cause a reversible loss of sensation for a limited region of the body without necessarily affecting consciousness.

Anaesthetics are distinct from analgesics, which block only sensation of painful stimuli.

LOCAL ANAESTHESIA

These are leaves of Coca plant i.e. Erythroxylum novogranatense.

Leaves of the coca plant used as naturally occurring local anesthetic.

WORK :-

Local anaesthetic agents prevent transmission of nerve impulses without causing unconsciousness.

They act by reversibly binding to fast sodium channels from within nerve fibers, thereby preventing sodium from entering the fibres, stabilising the cell membrane and preventing action potential propagation.

Each of the local anaesthetics have the suffix “-caine” in their names.

Local anaesthetics can be either ester-or amide-based.

1. Ester local anaesthetics such as procaine, amethocaine, cocaine, benzocaine, tetracaine are generally unstable in solution and fast-acting, are rapidly metabolised in the blood plasma and liver and more commonly induce allergic reactions.
2. Amide local anaesthetics such as lidocaine, prilocaine, bupivacaine, levobupivacaine, ropivacaine, mepivacaine, dibucaine and etidocaine are generally heat-stable.

LOCAL ACTIONS

Local anaesthetics produce anaesthesia by inhibiting excitation of nerve endings or by blocking conduction in peripheral nerves.

This is achieved by anesthetics reversibly binding to and inactivating sodium channels.

SYSTEMIC ACTIONS

• On central nervous system

The initial CNS symptoms are blurred vision, dizziness, tongue paresthesia (tingling sensation) and numbness.

Excitatory signs such as nervousness, restlessness, and muscle twitching are the result of blockade of inhibitory pathways.

The early symptoms are slurred speech, drowsiness, unconsciousness, and then respiratory arrest.

• On cardiovascular system

Local anesthetics have directs effects on the heart and peripheral blood vessels. They block the fast sodium channels in the fast conducting tissue of Purkinje fibers and ventricles resulting in a decrease rate of depolarization.

• On Peripheral vascular effects

Low doses of local anesthetics may cause vasoconstriction, where as, moderate or high doses result in vasodilation.

Cocaine is the only local anaesthetic that causes vasoconstriction at all dose.

CLASSIFICATION OF DRUGS

1. Injectable anaesthetic

• Low potency; short duration Procaine, Chloroprocaine

• Intermediate potency and duration Lignocaine, Prilocaine

• High potency, long duration Tetracaine (Amethocaine), Bupivacaine, Ropivacaine, Dibucaine (Cinchocaine)

2. Surface anaesthetic

Soluble– Cocaine, Lidocaine, Tetracaine, Benoxinate

Insoluble – Benzocaine, Butyl Aminobenzoate (Butamben) ,Oxethazaine

PHARMACOKINETICS

Because LA (Local Anaesthesia) act in their site of administration, pharmacokinetic characteristics are not important determinants of their efficacy, but markedly influence their systemic effects and toxicity.

Soluble surface anesthetic (lidocaine, tetracaine) are rapidly absorbed from mucous membranes and abraded areas, but absorption from skin is minimal.

Procaine does not significantly pancreatic mucous membranes. Rate of absorption depends on the blood flow to the area of application or injection .

liver Amide -linked LA (lidocaine, etc) are degraded only in the liver microsomes by dealkylation and hydrolysis.

Metabolism of lidocaine is hepatic blood-flow dependent The maximal safe dose of LAs is lower in patients with hepatic disease and in the elderly who have decreased liver function.

PRECAUTIONS

1. Before injecting the LA, aspirate lightly to avoid intravascular injection
2. Inject the LA slowly and take care not to exceed the maximum safe dose, especially in children
3. Toxicity after topical application or regional injection is influenced by the relative rules of absorption and metabolism. Those rapidly absorbed but slowly metabolized are more toxic.

ADVERSE EFFECTS

(1) CNS effects – light headedness, dizziness auditory and visual disturbances, mental problems, disorientation, shivering, , involuntary movements, finally convulsions and respiratory arrest. This can be prevented and treated by diazepam .

(2) Cardiovascular toxicity of LAs is manifested as bradycardia, hypotension, cardiac arrhythmias and vascular collapse.

(3) Vasoconstrictor (adrenaline) containing LA arrhythmic purposes should be avoided for patients with ischaemic heart disease, cardiac arrhythmia, thyrotoxicosis, uncontrolled hypertension.

USES AND TECHNIQUES OF LOCAL ANAESTHESIA

1. Surface anaesthesia

It is produced by topical application of a surface anaesthetic to mucous membranes and abraded skin.

Only the superficial layer is anaesthetised and there is no loss of motor function.

Onset and duration depends on the site, the drug, its concentration and form, eg, lidocaine (10%) spray in the throat acts in 2-5 min and produces anaesthesia for 30-45 min.

Addition of Adr does not affect duration of topical anaesthesia.

phenylephrine can cause mucosal vasoconstriction and prolong topical anaesthesia.

2. Infiltration anaesthesia

Use under the skin in the area of operation; blocks sensory nerve endings.

It’s action is almost immediate and duration is shorter than that after nerve block. eg; lidocaine 30-60 min, bupivacaine 90-180 min

Infiltration is used for minor operations, e.g. incisions, excision, hydrocele.

3. Conduction block

The LA is injected around nerve trunks so that the area distal to injection is anaesthetised and paralysed.

Choice of the LA and its concentration is mainly indicated by the required duration of action.

4. Spinal anaesthesia

The LA is injected in the subarachnoid space between L2-3 or L3-4 is below the lower end of spinal cord.

The primary site of action is the nerve roots in the cauda equina rather than the spinal cord.

Lower abdomen and hind limbs are anaesthetised and paralysed.

The level of anaesthesia depends on the volume and speed of injection, specific gravity of drug solution and posture of the patient.

Drug– Lidocaine, Bupivacaine ,Tetracaine .

Spinal anaesthesia is used for operations on the lower limbs, pelvis, lower abdomen.

LA for spinal anaesthesia primarily depends on the nature and duration of the operative procedure.

NOTE – Women during late pregnancy require less drug for spinal anaesthesia, because inferior vena cava compression leads to engorgement of the vertebral system and a decrease in the capacity of subarachnoid space.

Complications of spinal anaesthesia;

• Respiratory system
• Intercostal muscles may be paralysed, but diaphragm (supplied by phrenic nerve) maintains breathing.
• Hypotension
• heart contributes more to the fall in BP .
• Paralysis of skeletal muscles of lower limb.
• Low heart rate
• Headache
• Cauda equina syndrome (resulting in prolonged loss of control over bladder and bowel sphincters)
• Septic meningitis
• Nausea and vomiting after abdominal operations
• Epidural anaesthesia; The spinal dural space is filled with semi liquid fat through which nerve roots travel.

5. Epidural anaesthesia

The spinal dural space is filled with semi liquid fat through which nerve roots travel.

6. Intravenous regional anaesthesia (Intravascular Infiltration anaesthesia)

IVRA, Bier’s block anesthesia is an anesthetic technique on the body’s extremities where a local anesthetic is injected intravenously and isolated from circulation in a target area.

General Anaesthesia

General anaesthetics (GAs) are drugs which produce reversible loss of all sensation and consciousness.

Cardinal features of General Anaesthesia:-

• Loss of all sensation,especially pain

• Sleep etc.

TO KNOW MORE ABOUT GENERAL ANAESTHESIA, CLICK HERE.