M5: Anesthetics

Question 1

What is Anesthesia

Answer 1

Definition: a state of loss of sensation

Purpose: protect the patient from the pain associated with surgery → to provide a still surgical field

Question 2

Types of Anesthesia

Answer 2

(1) GENERAL ANESTHESIA

Components:
- Hypnosis = loss of consciousness
- Amnesia = loss of memory
- Analgesia = loss of response to pain
- Areflexia = loss of autonomic response
- Relaxation = skeletal muscle relaxation

IDEAL ANAESTHETIC:
- Rapid and smooth induction of anesthesia
- Prompt recovery after administration is stopped
- High therapeutic index
- No (or minimum) adverse effects

MECHANISM OF ACTION:
1. Facilitation of GABA-mediated inhibition at GABAA receptor:
Strengthens the inhibitory function of GABA, increasing CNS depression

2. Antagonism of glutamic acid excitation of NMDA receptor:
   Decreases excitation in the CNS, enhancing CNS depression

(2) CLASSES OF GENERAL ANESTHETICS:
1. INHALED:
- Administration & Distribution: lungs → heart (blood) → brain
-Elimination: lungs → liver

Commonly Used Inhaled Anesthetics: Nitrous Oxide (N2O):
Characterized by:
- Low potency – incomplete anesthesia
- Rapid onset and offset
- Prominent analgesic effect
Adverse Effects:
- Hypotension
- Respiratory depression: Minimal

2. Volatile Anesthetics:
   - Isoflurane
   - Halthane

Characterized by:
Liquid at room temperature → aerosolized for inhalation
Potent drugs – produce:
- Unconsciousness
- Analgesia
- Muscle relaxation
- Bronchodilation

Adverse effects:
- Hypotension
- Respiratory depression

Question 3

Minium Alveolar Concentration (MAC)

Answer 3

1. The concentration of inhaled anesthetic required to prevent 50% of subjects from responding to a painful surgical stimulus with “gross purposeful movements”
2. Determines anesthetic potency:
   inverse relationship (↑ MAC = ↓ potency)

E.g.,
1. Nitrous Oxide (MAC = 104)
2. Isoflurane (MAC = 1.15)
3. Halothane (MAC = 0.74)

Question 4

Intravenous Anesthetics (part of General Anesthesia)

Answer 4

Administration, distribution & redistribution:
- Injected directly into the bloodstream → goes to the brain, muscle, adipose tissue, heart
- Patient awakes after bolus dose due to redistribution not elimination

Commonly Used IV Anesthetics:

1. PROPOFOL:
   - Very rapid offset despite prolonged infusions (rapid hepatic & extra-hepatic metabolism
   - Ideal ambulatory anesthetic
   - Prevents nausea and vomiting
   - Does not cause changes to heart rate

Adverse Effects:
- Hypotension
- Apnea

2. KETAMINE
   - Often used in pediatric anesthesia
   - Used as a recreational street drug
   - Anesthetic of choice for compromised blood flow
   - Provides analgesia
   - Does not usually cause apnea

Adverse Effects:
- Hypertension
- Tachycardia
- Elevated intracranial pressure
- Dysphoria

3. ETOMIDATE

Question 5

Balanced Anesthesia

Answer 5

Combination of different anesthetics:
- Inhaled, iv anesthetics, sedative-hypnotics, opioids, and neuromuscular blocking drugs

Aim:
- Avoid high dose of a single anesthetic
- Minimize adverse effects

Question 6

LOCAL ANESTHETICS
- Mechanism
- Termination of Effect
- Administration and Uses
- Adverse Effects and Toxicity

Answer 6

Loss of sensation confined to a discrete area of the body

DRUG = LIDOCAINE

MECHANISM:
Block voltage-gated sodium channels →
↓ action potential →
Block sensory nerve conduction

EFFECT: Block pain sensation +
1. Autonomic function
2. Temperature sensation
3. Light touch
4. Proprioception
5. Motor function

TERMINATION OF EFFECT:
Systemic absorption → distribution → elimination

ADMINISTRATION & USES:
1. Topical
2. Local Infiltration
3. Nerve block
4. Spinal or epidural

ADVERSE EFFECTS & TOXICITY:
1. Allergy:
- Very rare
- Mostly due to additives’ effect

2. Systemic Toxicity:
   - Due to overdose or accidental iv injection
   - Sedation, light-headedness, visual & auditory disturbances, restlessness, convulsions, and CVS depression