CLASSIFICATION OF CLASSIFICATION OF GENERAL ANAESTHETICSGENERAL ANAESTHETICS

1.INHALATIONAL

Gases: N2O,Cyclopropane,Xenon

Liquids: Ether, Halothane,

Enflurane, Desflurane,

Isoflurane, Sevoflurane,

Methoxyflurane

2.INTRAVENOUS

Inducing Agents: Thiopentone sodium,

Methohexitone sodium,

Propofol, Etomidate

Dissociative Anaesthesia:Ketamine

Neuroleptanalgesia: Fentanyl+Droperidol

(Analgesic)(Neuroleptic)

BZDs: Diazepam,Lorazepam,Midazolam

Pharmacokinetics• Rapidly diffuse across the alveoli

• Alveoli blood brain

• Depth of anaesthesia-potency & pp

• Induction & Recovery-rate of change of pp

Minimum Alveolar Concentration

• Conc of the inhalational GA that renders 50% of the subjects immobile when exposed to a strong noxious stimulus Halothane 0.75%

Ether 1.9%

Enflurane 1.68%

Isoflurane 1.2%

Desflurane 6%

Sevoflurane 2%

Nitrous oxide 105%

• 0.3 MAC→mild analgesia

• 0.5 MAC→amnesia

• 1.0 MAC→50% patients immobile even after stimulation

• 1.3 MAC→sympathetically mediated response blunted

• 2.0 MAC→potentially lethal

• MAC α 1/Potency

Minimum Alveolar Concentration

limitations1. Leaves 50% subjects

2. At 1.3MAC awareness & recall may still exist

3. Large no. of patients receive muscle relaxants

4. Other indicators of awareness-highly suggestive when present but not definitive when absent

5. A patient who moves with incision is not necessarily awake &one who does not move is not necessarily unconscious

Factors affecting pp of anaesthetic in brain

• PP of anaesthetic in inspired air• Pulmonary ventilation rate• Alveolar exchange• Solubility of anaesthetic in blood• Solubility of anaesthetic in tissues• Cerebral blood flow

1.PP of the anaesthetic in 1.PP of the anaesthetic in inspired airinspired air

PP of the anaesthetic in inspired air

• Increase in inspired anaesthetic conc increases the rate of induction of anaesthesia by increasing the rate of transfer into blood according to Fick’s Law

• Used for mod soluble-halothane- 3-4% →1-2%

Fick’s Law of Diffusion

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•

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Flux=diff in conc x A x Permeability

• Thickness of the path

2.PULMONARY VENTILATION

lung animation.gif

2.Pulmonary Ventilation Rate

• The rate of rise of anaesthetic gas conc in the arterial blood is directly dependent on both rate & depth of ventilation

• Effects- solubility

• 4x ↑ In VR 2x T of halothane bt only 15% ↑ in T of nitrous oxide

3.ALVEOLAR EXCHANGE

ALVEOLAR EXCHANGE

• GAs diffuse freely across alveoli

• Ventilation Perfusion mismatch delays the attainment of equilibrium between blood and alveoli

4.SOLUBILITY IN BLOOD

SOLUBILITY IN BLOOD

• One of the most important factor

• Blood:Gas Partition co efficient –index of solubility

• When an anaesthetic with low solubility diffuses from alveoli into arterial blood, relatively few molecules are required to raise its partial pressure and therefore its arterial tension rises rapidly

5.SOLUBILITY IN TISSUES5.SOLUBILITY IN TISSUES

SOLUBILITY IN TISSUES

• Relative solubility of the anaesthetic in blood and tissue determines its conc in the tissue at equilibrium

• expressed as tissue : blood pc

• =ly soluble in lean tissue & blood. More soluble in fat

• Conc ↑ in white than in grey matter

6.CEREBRAL BLOOD FLOW6.CEREBRAL BLOOD FLOW

CEREBRAL BLOOD FLOW

• Brain is highly perfused

• GAs are quickly delivered

• CO2 inhalation

Second gas effect

• When certain gases like nitrous oxide are administered in high conc, the other anaesthetic gases are also pulled in and their alveolar tension rises more rapidly

• Eg: halothane when given with N2O, delivered at same rate

Concentration effect

• When an anaesthetic is administered in high conc, its alveolar tension rises more rapidly than when the same gas is inhaled in lower conc.

eElimination• gradients reversed

• Through lungs- unchanged,

• Metabolism-halothane>20% in liver

• Lipid soluble anaesthetic-delayed recovery