General Anaesthetics

Question 1

Components of GA effect

Answer 1

Loss of arousability in resp to noxious stimuli
Loss of pain sensation in resp to noxious stimuli
Loss of mobility in resp to noxious stimuli
Loss of memory
Attenuation of autonomic responses to noxious stim

Question 2

Classification of GAs

Answer 2

Inhalation Anaesthetics
Volatile Liquids: ethers, halogenated hydrocarbons, halogenated ethers
Gases: Nitrous Oxide

IV Anaesthetics
   Barbiturates
   Propofol
   Etomidate
   Ketamine

Question 3

MAC

Meyer and Overton’s Lipid Theory

Answer 3

Anaesthetic potency directly related to their lipid
solubility
MAC = Minimal Anaesthetic Concentration.
Conc of anaesthetic in alveolar space which causes
anaesthesia in 5% of patients. Typical: 1.3xMAC

Meyer and Overton’s Lipid Theory
GA act by being dissolved in lipid membranes of CNS
neurons
Anaesthesia develops when anaesthetic reaches critical
conc. in neuronal cell membrane

Question 4

Mechanism of Action GAs

Answer 4

Activiation of some ligand gates Cl channels
   GABA-A R (Inhalation except NO, IV except ketamine)
   Glycine R (Inhalation except NO)

Inhibition of some ligand gated Na channels
   Ach R (all inhalation, barbiturates)
   Glu R, NMDA type (NO, Ketamine)

Activation TREK-1 (K channel–> K efflux–> hyperpolar)

–>
Hyperpolarisation of neurons–> inhibiting synaptic transmission

GA DO NOT inhibit axonal conductance; do not affect voltage gated Na channels at anaesthetic concentrations

Question 5

Components of GA effect and corresponding inhibition of synaptic transmission

Answer 5

Thalamus, Hypothalamus, Cortex
–> sedative, loss of consciousness and arousability

Spinal Cord
–> Muscle Relaxation

Thalamus and Spinal Cord
–> Analgesia

Hippocampus
–> Amnesia

Question 6

Signs and Stages of GA

1) Stage of Analgesia

Answer 6

Stage of Analgesia: pain sensation lost, consciousness
kept
Used: Minor operations, traumatised with
subanaesthetic dose

Question 7

Signs and Stages of GA

2) Stage of Delirium

Answer 7

Stage of Delirium: consciousness lost

Presence of motor hyperactivity (increased muscle tone)
jaw set, irregular breathing, vomiting, defecation

Sympathetic Hyperactivity: hypertension, tachy

Hyperactivity due to blockage of small inhibitory
neurons

Doesn’t occur in modern anaesthetics due to rapid induction;
Combo IV and inhalation–> balanced anaesthesia: best of both worlds

Question 8

Signs and Stages of GA

3) Stage of surgical anaesthesia

Answer 8

Stage of surgical anaesthesia

1) Return of regular breathing
2) Intercostal ventilation weakens then ceases, only
diaphragmatic ventilation remains
3)Pupils gradually dilate
4)Loss of various reflexes
Corneal Reflex
Peritoneal Reflex
Pupillary Reflex–> warning sign, sign of hypoxia

Other warning signs
Decrease BP, tachy
Shallow, irregular ventilation, cyanotic skin

Question 9

Signs and Stages of GA

4) Stage of medullary paralysis

Answer 9

Respiration ceases (paralysis of resp center)

Heart stops (paralysis vasomotor center)

Inhalation anaesth. –> narrow th window–> most dangerous drugs in clinical use

What to do?
Stop anaesthesia, provide O2, Cardiopulmonary resuscitation

To prevent stage 4: monitor using EEG