Anesthetic Agents & Anesthesia

• Anesthesia is the loss of sensation by giving anesthetic agents.
• Agents which cause the loss of sensation are called as anesthetic agents.

These are of two types;

Local

• These cause loss of sensation but animal is conscious.

General

• These cause loss of sensation due to unconsciousness.

Objectives of Anesthesia

• Sensory block to have analgesia. In this, sensory stimuli from the periphery are blocked.
• Mental block to have hypnosis or Amnesia (animal know nothing, what is happening around). Activity of CNS is blocked.
• Motor block to have muscle relaxation
• Reflex block to have hyporeflexia
• In case of local anesthetics, only one purpose is not the case of interest which is the mental block.

Stages of Anesthesia

• Based on ether anesthesia, there are following stages of anesthesia;
• Excitement stage (voluntary movements occur)
• Delirium stage (involuntary movements occur)
• Surgical Stage
• Medullary paralysis

There are 3 stages of anesthesia;

• Induction
• Maintenance
• Recovery

Ideal Properties of Anesthetic Agent

• It should cause rapid & smooth induction and recovery.
• It should be easy to maintain anesthesia (light or deep).
• It should be non-toxic and safe to internal organs.
• It should be non-irritant.
• It should be non-inflammable and non-explosive.
• Myocardial sensitization to epinephrine should be least.
• It should be compatible with other drugs.
• It should be easily available, cheap and easy to administer.

Pre-anesthetic Agents

These are the drugs given before anesthesia.

Why to give?

• To reduce anxiety of patient
• To reduce the amount of anesthetic agent
• To reduce the adverse effects of anesthetic agent
• To reduce the toxicity problems
• To reduce secretions especially Bronchial & Salivary
• To increase analgesia

Examples

• Opiods (Morphine, Mepridine) which induce analgesia
• Tranquilizers (Xylazine, Acepromazine) which reduce anxiety
• Anticholinergic drugs (Atropine sulphate, scopolamine) which reduce secretions
• Muscle relaxants (Gallamine, Pancuronium, d-tubocurarine, succinylcholine)

Medication during Anesthesia

• To reduce Blood pressure
• To induce muscle relaxation
• To stop bleeding

Post-Anesthetic Medication

• Analgesics
• Anti-inflammatory
• Anti-microbial
• Cholinergic drugs to relieve retention of urine E.g. Bethanicol

Introduction to Pharmacology of CNS

CNS consists of two main organs;

• Brain
• Spinal Cord
• This is the information superhighway of the body. It carries information up to the brain and instructions back down.

Brain

It consists of following parts;

Cerebellum

• Mostly deals with movement. It regulates and coordinates movement, posture and balance. Also involved in learning movement

Cerebrum

• The cerebrum or cortex is the largest part of the human brain, associated with higher brain function such as thought and action.

Midbrain

• Midbrain/ Mesencephalon- the rostral part of the brain stem, which includes the tectum and tegmentum
• It is involved in functions such as vision, hearing, eyemovement, and body movement.

Types of Drugs in CNS

• In CNS, synapses are the main junctions at the neuronal terminals.
• Drugs are based on the type of neurotransmitters which are of 2 types;
• Excitatory
• Inhibitory

Stimulants

• Drugs stimulating the excitatory neurotransmitters are called stimulants. E.g. Ach, Dopamine, epinephrine and nor-epinephrine.

Depressants

• Drugs stimulating inhibitory neurotransmitters are called depressants. E.g. GABA (gamma amino butyric acid)(Brain), Glycine (spinal cord).
• Depressants are mainly used in human practice.

Stages of Stimulation

Stages of Depression

Terminology

Sedation

There is slight depression but animal is awake.

Hypnosis

Animal is greatly depressed and seems to be asleep but can be awakened.

Tranquilizers

Slight Depression but animal behaves abnormally to external stimuli.

Narcosis

There is greater depression and animal is asleep and can be awakened but will again sleep. There is good analgesia.

Anesthesia

Animal is under greater depression and asleep but cannot be awakened. There is hyporeflexia.

Dissociative Anesthetics

Animal is superficially anesthetized and there is myotonia (movement in muscles).

There is analgesia but reflexes are present.

Basal Anesthetics

This anesthesia is maintained by other agents.

Adrenergic Blocking Agents...Pharmacology

• Adrenergic blocking agents are drugs that selectively inhibit specific receptor sites from sympathetic stimulation.
• Blocking agents may interact with specific alpha and beta receptors. The release of nor-epinephrine from storage sites may be blocked.

Types of Adrenergic Blocking Agents

Adrenergic blocking agents are of two types;

• α-Blockers
• β-Blockers

α-Blockers

• Phenoxybenzamine is the prototype drug which blocks α-1 and α-2 receptors.
• Phentolamine, Tolezoline…… blocks α-1 and α-2 receptors
• Przosine, Doxazosine, Terazosine….. Blocks α-1 receptors
• Atipamazole…. blocks α-2 receptors

β-Blockers

• Propranalol is the prototype drug which blocks both types of beta receptors.
• Timolol, Ndolol, Pindolol also block the both types of beta receptors.
• Atinilol, Esmalol blocks beta-1 receptors.
• Butamoxine blocks beta-2 receptors.
• Labetalol, Carvedalol block both alpha & beta receptors.

Therapeutic Uses

α-Blockers

• Phenoxybenzamine blocks α-1 and α-2 receptors by making covalent bond, It is irreversible inhibition.
• Other alpha blockers are reversible inhibitors.
• For hypertension, these drugs are used.
• Orthostatic Hypotension…In standing condition, hypotension occurs due to these drugs. So, initial dose should be given before bed.
• Pheochromocytoma….Tumor of chromafin cells in adrenal glands, so, adrenaline quantity increased and Blood pressure remains constantly high. To diagnose it, phenoxybenze is used which will lower the blood pressure.
• To remove the toxicity of α-2 agonists, alpha-2 antagonists are used.

β-Blockers

• These are used as anti-hypertensive drugs. E.g. propranalol
• Timolol is used to treat glaucoma by decreasing the intraocular pressure.