Docstoc

General Anaesthetics

Document Sample
General Anaesthetics Powered By Docstoc
					Introduction to Anaesthesia

"Anaesthesia" - Greek word absence or loss of sensation.

5/2/2008

2

Objectives
 History
 Definition  Types of anaesthesia

 General anaesthesia/drugs
 Phases of GA  Regional anaesthesia  Local anaesthesia
5/2/2008 3

Early history
 Ancient/Medieval period

- Opium - Alcohol - Cannabis

5/2/2008

4

             
5/2/2008

History
1776 - Priestey - Ssuggested use of N2O 1845- Horace Wells - N2O 1846- William Morton - Ether 1847- Simpson – Chloroform 1878- ETT 1884- Cocaine 1895-98- Spinal analgesia /anaesthesia 1921- Epidurals 1934- Thiopentone, cyclopropane 1942- Curare 1946- Lignocaine 1951- Suxamethonium 1952- IPPV 1956- Halothane
5

Ideal anesthetic
 potency, efficacy, stability & safety - not a single drug  Take advantage of desirable effects-

e.g. speed of induction-skeletal muscle relaxation
 To minimize their undesirable effects

e.g. respiratory depression, hepatotoxicity

5/2/2008

6

Definition
‘Loss of sensation’
 It allows unbearable medical procedures to be performed

while the patient is relaxed & asleep.

5/2/2008

7

General
Putting to sleep & keeping asleep for surgery or other medical procedures
5/2/2008

Regional
Numbing an area of the body

Local
Numbing a small part of the body

8

Objectives of G A
● ● CNS depressants Reversible loss of pain sensation and consciousness
Permits painful surgical procedures to be performed without patient being aware of it , or react reflexly


1. 2. 3.

Goals of GA
Analgesia Loss of consciousness, amnesia Loss of reflexes to minimize laryngospasm, arrhythmias, salivation, vomiting, postop abdominal distension Skeletal muscle relaxation

4.

5/2/2008

9

Properties of an ideal GA
 Patient -

fast pleasant induction, odourless, tasteless, no side effects  Surgeon relaxation, analgesia, no increased bleeding; long duration possible  Anaesthetist - controllable, potent, no long term effects, easy mixing  Manufacturer- stable, easy to make, pure
All: non-explosive; non-inflammable

5/2/2008

10

Mechanism of Action
 Ion channels - ultimate site of action of general anaesthetics  GA act on CNS-by modifying electrical activity of neurons at molecular level by modifying the function of ion channels  This occurs by anaesthetic molecules - binding directly to ion channels or - by disrupting function of molecules that maintain ion channels  Produce a progressive depression of CNS

5/2/2008

11

Mechanism of Action
 Inhibit release of pre-synaptic excitatory neurotransmitters  Alters postsynaptic responsiveness to the released neurotransmitter  Increases activity of inhibitory ion channels in postsynaptic

receptors
 Enhancing inhibitory neurotransmission within CNS

 Action of GA agent depends upon their selectivity of ion channel

5/2/2008

12

Pharmacokinetics
FICKs LAW:
CNS

Diffusion proportional to: • 1/distance area • diffusion coefficient • diffusion gradient • Solubility in blood (blood/gas part.coef) • Pulmonary blood flow

tidal volume 500 ml

DS 150ml
lungs 6 litres

BODY

INDUCTION and MAINTENANCE concs.
5/2/2008 13

How Do General Anesthetics Work
Franks and Lieb on Mihic et al. (1997) Nature, 385-389 (1997)

Mihic et al.5 found that single amino-acid substitutions at two positions remove the potentiating effects of volatile anaesthetics and ethanol on GABAA and glycine receptors. a, GABAA and glycine receptors bind the neurotransmitters that are released at inhibitory chemical synapses, and open to allow chloride ions to diffuse across the postsynaptic membrane. b, The main effect of volatile anaesthetics is to prolong channel opening and, hence, to increase postsynaptic inhibition. c, The receptor channels consist of pentamers of closely related subunits, and the structure of a single subunit is shown d. Authors suggest that two critical amino acids form a binding site for general anesthetics and ethanol.
5/2/2008 14

Triad of General anaesthesia

Hypnosis

Analgesia
5/2/2008

Muscle relaxation
15

Balanced aneasthesia
Using two /more aneasthetics together with other ancillary medications
1. 2.







anticholinergic muscle relaxant anxiolytics to provide smooth induction, sufficient depth,duration, adequate muscle relaxation, decreased tracheobronchial secretions and minimal danger to vital body systems
5/2/2008 16

G A classification
Inhalation GA
 Volatile liquids- Diethyl ether, Ethyl chloride

Halothane, Enflurane ,Isoflurane  Gases- Nitrous oxide Intravenous GA- Nonvolatile
 Ultrashort acting barbiturates- Thiopental

sodium, Methohexitol  Nonbarbiturates- BZD -Midazolam  Ketamine  Propofol
5/2/2008 17

Hypnosis
Death

Coma Hypnosis sedation

Amnesia Awake
5/2/2008 18

Hypnotic drugs-intravenous
 Gold standard- thiopentone (Ultra short

acting barbiturates)  Methohexitol  Propofol  Etomidate  Benzodiazepines-Midazolam  Ketamine
5/2/2008 19

Intravenous agents
 Thiopentone, methohexitone-fast, pleasant induction, last

25 min; depress respiration  Propofol- 60 sec onset, pain at site of administration, 75% day cases  Etomidate- some spontaneous movements  Ketamine- hallucinations (droperidol), children, NMDA receptors  Midazolam- benzodiazepine, water soluble, anxiolytic, sedative, muscle relaxant, anticonvulsant, amnesia
5/2/2008 20

Inhalational anaesthetics
Volatile liquids  Diethyl ether  Ethyl chloride  Halothane  Enflurane  Isoflurane  Sevoflurane  Desflurane Gases  Nitrous oxide
5/2/2008 21

Inhalation agents
 Nitrous oxide:sweet smelling colourless gas, low

potency (80%), excellent analgesic, low blood gas partition coefficient (fast action), bone marrow depressed in long term use  Halothane: liquid, potent (0.75%), higher blood gas partition coefficient, respiration and vascular muscle depressed, biotransformed (15%)  Isoflurane: less potent (1.3%), less biotransformed, expensive

5/2/2008

22

Analgesia
 Good analgesia = good anaesthesia
 Hypnotic sparing effect  Opiates

 Local anaesthetics
 NSAIDS  Paracetamol

5/2/2008

23

Analgesia-Opiates
 Gold standard – morphine
 Derivatives- diamorphine, codeine  Synthetic agents

- Pethidine
- Fentanyl/Alfentanil-short acting - Remifentanil-ultra short acting

5/2/2008

24

Narcolept analgesia
Fentanyl + Droperidol

-

severe respiratory depression, responsive to commands but not to pain, used where patient’s co-operation is required. Newer agents pregnanolone, desflurane??

5/2/2008

25

Analgesia-NSAIDS
 Gold standard – aspirin
 Ibuprofen  Diclofenac

 Cox-2 inhibitors

5/2/2008

26

Muscle relaxation
 Aids intubation
 Helps surgeon/surgery  Surgery of long duration

 Reduces maintenance dose of anaesthetics agents

5/2/2008

27

Muscle relaxants
Two types  Depolarising-short acting eg;suxmethonium
 Non-depolarising- medium/long acting - Tracurium - Vecuronium - Rocuronium
5/2/2008 28

Skeletal Muscle relaxants

1.Pancuronium 2.Rocuronium 3.Atracurium 4.Succinylcholine
5/2/2008 29

Prerequisites
Oxygen

Suction
Tilting trolley

5/2/2008

 Resuscitation drugs  Monitoring  Anaesthetist  Skilled assistance  Drugs and machine

30

Preanesthetic Evaluation

 To determine which drugs (dosages), additional invasive monitors

and/or analgesic therapies required by the patient  Patient's age, wt, medical history, current medications, previous anesthetics, & other factors relevant for administering anesthesia noted

5/2/2008

31

Preanesthetic medication
 Defined as administration of drugs before the

anaesthetic is given

AIMS
- Relief of anxiety - Relief in salivary and mucous secretion - Inhibition of undesirable side-effects. (Bradycardia, Muscle spasms)

5/2/2008

32

Pre-anaesthetic medication


Improve comfort, reduce pain, apprehension & anxiety  Promote pleasant, smooth induction, recovery & amnesia  Reduce side effects of GA
1.

2.

Narcotic analgesics- tranquillisers (benzodiazepines + neuroleptics) Anticholinergics; + special drugs for particular GA or clinical conditions.
5/2/2008 33

Adjunctive Anaesthetic Medication
 Facilitates the induction and maintenance of anaesthesia
 Minimizes the adverse effects of anaesthesia

5/2/2008

34

Phases of general anaesthesia
 Induction
 Maintenance  Recovery

5/2/2008

35

Induction
 Intravenous- majority
 Inhalational- children, needle phobics  Monitoring

 Preoxygenation
 Hypnotic/analgesic and or relaxant  Mask/LMA/ET tube

5/2/2008

36

Intubation is the placement of a tube into an external or internal orifice of the body. In tracheal intubation, an endotracheal tube is passed through the nose or mouth, through the larynx, and into the trachea.

5/2/2008

37

Stages of anaesthesia
 Alcohol  General Anaesthesia

1.Dizzy, delightful 2.Drunk, disorderly 3.Dead drunk 4.Dangerously deep

1.Amnesia, analgesia 2.Uninhibited response to stimuli 3.Surgical anaesthesia 4.Vital centre depression

5/2/2008

38

Stages of anaesthesia

 Lasts until consciousness is lost

 Analgesia persists post operatively

( Ether is Lipid soluble & is released slowly from tissue stores.)

5/2/2008

39

 Begins with loss of consciousness
 Ends with the beginning of rhythmic respiration.  Most difficult to manage as patient may move, vocalise,

salivate & vomit.
 Increased muscle tone, hypertension , tachycardia may occur

5/2/2008

40

Stage 3 – Surgical Anaesthesia

5/2/2008

41

• Begins

with reappearance of regular respiration.

• Loss of automatic eyelid closure • Loss of reflex closure of eyelids on conjunctival stimulation

5/2/2008

42

• Begins

with cessation of eye movements.

• Ends with beginning of respiratory muscles paralysis except diaphragm

5/2/2008

43

• Begins

- increased abdominal excursions.

• Ends with paralysis of all respiratory muscles , except the diaphragm.

5/2/2008

44

• Begins

with cessation of inter costal muscles paralysis.

• Ends with Diaphragmatic paralysis.

5/2/2008

45

 Begins with complete respiratory paralysis.
 Medullary centers are completely paralyzed.  EEG waves - smaller & eventually disappears.

 Ends with failure of circulation.
 Heart and circulation fails.

5/2/2008

46

5/2/2008

47

Maintenance
 Intravenous or inhalational
 Oxygen –40%-100%  Nitrous oxide

 Muscle relaxant
 Analgesia

5/2/2008

48

Recovery
 Turn off agent
 Reverse relaxation  Cough reflex

 Extubate when awake
 Recovery position  Monitor until discharge

5/2/2008

49



During recovery from deep anesthecia patients reflexes returns in reverse order from that they were lost

 GA are used in two main ways1. To produce complete anesthesia - patient is kept in deep anesthesia for prolonged periods for major surgery ( in phase 2 or 3 of stage III ,when muscle relaxation is optimal) 2. To produce basal ( light) anesthesia- for minor and diagnostic procedures
5/2/2008 50

Post operative Medications
 Anti-Emetics. (To relieve Nausea and vomiting)
 Cholinergics (To relieve Abdominal distention and

Urinary retention)
 Analgesics (To relieve pain)  Laxatives (To relieve Constipation)
5/2/2008 51

Monitoring
Continuous Electrocardiography (ECG)

 Continuous Pulse Oximetry (SpO2)
 Blood Pressure Monitoring

Agent concentration measurement
Low oxygen alarm

Circuit disconnect alarm
Carbon dioxide measurement (capnography)
5/2/2008 52

Advantages
 No absolute contraindications
 Quick to establish  Never fails to work

5/2/2008

53

Disadvantages
 Poly pharmacy
 Effects on various systems  Allergic reactions

 Recovery profile
 Awareness

5/2/2008

54

Regional anaesthesia
 Spinal/epidural
- surgery below umbilicus - Provides analgesia/muscle relaxation

 Plexus blocks eg brachial plexus
 Intravenous- Bier’s block

5/2/2008

55

5/2/2008

56

Regional anaesthesia

Analgesia
5/2/2008

Muscle relaxation
57

Local anaesthetics
 Lignocaine- quick/short acting
 Bupivacaine/levobupicvacaine- slow

and long action  Ropivacaine- as above  Amethocaine- topical  Prilocaine- intravenous

5/2/2008

58

Advantages
 Effective alternative to GA
 Avoids polypharmacy  Allergic reactions

 Extended analgesia
 Patient can remain awake  Early drink/feed

5/2/2008

59

Disadvantages
 Limited scope
 Higher failure rate  Time constraints

 Anticoagulants/Bleeding diathesis
 Risk of neural injury

5/2/2008

60

5/2/2008

61


				
DOCUMENT INFO
Shared By:
Stats:
views:755
posted:10/31/2009
language:English
pages:61