GENERAL ANAESTHETICS

By Dr .Elza Joy Munjely,JR II,Govt. Medical College ,Kottayam.

CARDINAL FEATURES

Immobility

Amnesia

Attenuation of autonomic responses

Analgesia

unconsciousness

Balanced anaesthesia-

• All these modalities achieved

• Using combination of anaesthetic drugs

• So that the dose of individual drugs can be reduced along with their side effects

HISTORY

Humphrey Davy

• 1800• First person to make

N2O • Noted –

Euphoria,analgesia,LOC

HISTORY

Horace Wells

• American dentist• 1844• First used N2O to

patients for dental extraction

HISTORY

William Morton

• 1846• American Dentist• Ether- tooth extraction

HISTORYJohn Collins Warren

• Chief surgeon of Massachusetts

• 16 th October 1846-ETHER DAY =WORLD ANAESTHESIA DAY

• Operation theatre-ETHER DOME

HISTORY

James Simpson

• Professor of midwifery at Edinburgh university

• 1847• Chloroform• To relieve pain of

childbirth

HISTORY

John Snow

• Calculated dosages for the use of ether and chloroform as surgical anaesthetics

• Designed the apparatus to administer ether & mask to administer chloroform

• 1853• administered chloroform

to Queen Victoria during her 8th delivery

STAGES OF ANAESTHESIA

STAGE 3-Surgical anaesthesiaFrom regular respiration-loss of spontaneous respirationDivided into 4 planes

STAGE 4-Medullary paralysisLoss of spontaneous respiration – failure of circulation & death

MEASUREMENT OF ANAESTHETICPOTENCY

MAC- MINIMAL ALVEOLAR CONCENTRATION

The alveolar partial pressure of a gas at which

50% of humans do not respond to a surgical

incision

Two important characteristics of Inhalational anaesthetics which govern the anaesthesia are :

2. Solubility in the blood

(blood : gas partition

coefficient)

OIL-GAS PARTITION COEFFICIENTS

• It is a measure of lipid solubility of the anaesthetic

• Measure of anaesthetic potency

• solubility of general anaesthetics in lipid is the potency

BLOOD-GAS PARTITION COEFFICIENT

• Ratio conc. in blood conc. in gas

• Lower the Blood-gas partition coefficient faster the induction & faster the recovery.

Anaesthetic Blood/Gas Oil/gas

Nitrous oxide 0.47 1.4

Halothane

2.4 224

Isoflurane

1.4 97

Sevoflurane

.65 42

Desflurane

.42 18.7

MECHANISM OF ACTION

EFFECT OF ION CHANNELS

Potentiation of GABA at GABAA

receptors- almost all anaesthetics (except

cyclopropane,ketamine,Xenon,N2O)

EFFECT OF ION CHANNELS

Activation of Two – pore Domain potassium channels-

can be directly activated by low conc. of volatile & gaseous anaesthetics, thus reducing memb. excitability

EFFECT OF ION CHANNELS

Inhibition of excitatory NMDA receptors-

• Competitive antagonist for glycine - Xenon

• Noncompetitive antagonist of glutamate-Ketamine

• NMDA channel blocker-N2O

EFFECT OF ION CHANNELS

• Other ion channels-ligand-gated channels including glycine,nicotinic & 5HTreceptors as well as at cyclic nucleotide –gated K+ channels

• Inhibition of presynaptic Na channels inhibition of NT release at excitatory synapses

CLASSIFICATION

INHALATIONAL

GAS• Nitrous oxide• Xenon

VOLATILE LIQUIDS• Ether• Halothane• Isoflurane• Desflurane• Sevoflurane

INTRAVENOUS

Fast acting drugs• Thiopentone sod.• Methohexitone sod.• Propofol• Etomidate

Slower acting drugsBENZODIAZEPINES• Diazepam• Lorazepam• MidazolamDISSOCIATIVE ANAESTHESIA• KetamineOPIOID ANALGESIA• Fentanyl• Alfentanil• Sufentanil• Remifentanil

Cyclopropane,trichloroethylene,methoxyflurane & enflurane are no longer used

INHALATIONAL ANAESTHETICS

PHARMACOKINETICS

DEPTH OF ANAESTHESIA DEPENDS ON

POTENCY OF THE AGENT

PARTIAL PRESSURE IN THE BRAIN

INDUCTION & RECOVERY DEPENDS ON

RATE OF CHANGE OF PARTIAL PRESSURE IN

THE BRAIN

ALVEOLI BRAINBLOOD

FACTORS AFFECTING THE PP OF ANAESTHETIC ATTAINED IN THE BRAIN

1. PP of anaesthetic in inspired gas2. Pulmonary ventilation3. Alveolar exchange4. Solubility in blood5. Solubility of anaesthetic in tissues6. Cerebral blood flow

ELIMINATION

• Same factors which govern induction also govern recovery.

• Most GA eliminated unchanged• Halothane >20% metabolised in liver

SECOND GAS EFFECT

• Occurs when another inhalational anaesthetic is used with N2O

• Rapid uptake of N2O produces a vaccum in the alveoli

• Second gas also undergoes rapid uptake along with N2O

DIFFUSION HYPOXIA

• Reverse of second gas effect occurs when N2O is discontinued after prolonged anaesthesia

• N2O rapidly diffuses out the alveoli & dilutes the alveolar air.

PP of O2 reduced in alveoli

Diffusion Hypoxia

Anaesthetic MAC

Oil:Gas Partition Coeff.

Blood:Gas Partition Coeff.

Induction

Muscle Relaxation Remarks

Ether 1.9 65 12.1 Slow V.Good Irritating,inflammable & explosivePotent,Good analgesia,pungentSafe ininexperienced hands-no need for special equipment

Halothane 0.75 224 2.3 Interm Fair Nonirritant,Potent,preferred for asthmaticsMalignant hyperthermia,hepatotoxic

Isoflurane 1.2 99 1.4 Interm Good Safe in MI, Preferred in neurosurgery

Desflurane 6 19 0.42 Fast Good Out patient surgery,irritant

Sevoflurane 2 50 0.68 Fast Good Pleasant,can be used in paediatric patients

N2O 105 1.4 0.47 fast Poor Least potentGood analgesic,breathing & respiration better maintainedExpand pneumothorax

I.V ANAESTHETICS

INDUCING AGENTS

• Drugs on IV injection produce LOC in one arm-brain circulation time, 11 sec

• Thiopentone sod.• Methohexitone sod.• Propofol• Etomidate

DRUG INDUCTION

MAJOR UNWANTED EFFECTS

REMARKS

PROPOFOL Fast CVS & R.S depression

Propofol infusion syndrome

Used for total IV anaesthesia along with fentanylPreferred for OP surgeriesDOC for sedating intubated pts. In ICU,preferred in asthmatics

THIOPENTAL Fasthangover

CVS & R.S Depression, can precipitate AIPNecrosis on extravasation

Other uses – to control convulsions,Narcoanalysis

ETOMIDATE Fast Excitatory effectsDuring induction& recoveryAdrenocorticalsuppression

Aneurysm surgeries & cardiac disease

KETAMINE Slow Psychomimetic effectsPostop nausea, vomiting SalivationRaised ICT

Dissociative anaesthesiaMuscle tone ↑HR,CO,BP,ICT ↑Preferredfor head & neck surgeries,hypovolaemic pts,Asthmatics

MIDAZOLAM slow

Preferred for endoscopiesfracture settingsangiographies,ECT

FENTANYL Anaesthetic

awareness with dreadful recall

Opioid analgesicTo supplement balanced anaesthesiaNerolept analgesia –along with Droperidol

CONSCIOUS SEDATION

• Monitored state of altered consciousness that can be employed to carryout diagnostic/short therapeutic/dental procedures in apprehensive subjects or medically compromised patients– Diazepam– Propofol– N2O– Fentanyl

THANK YOU