1

General anaesthesia

Henning Andreas Haga

Associate professor

Norwegian School of Veterinary Science

Anaesthesia

• A reversible drug induced condition of unconsciousness.

• Goals:– Unconsciousness

– Muscle relaxation

– Analgesia

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Anaesthetics

• Injections

• Gas

Side effects

• Respiratory depression

• Cardiovascular depression

• Risk of reduced oxygen tension (hypoxia)

• Balance between moving animals and respiratory arrest.

3

Injection

• Subcutaneously (under skin)

• Intramuscullary (into muscle)

• Intraperitoneally (into abdominal cavity)

• Intravenously (into vein)

4

• Subcutaneous, intramuscular or intraperitoneal injection.

• Simple, but less control

Injection

Injection

• Subcutaneous, intramuscular or intraperitoneal injection

• Simple but less control

Time

Eff

ect

5

Intravenous anaesthesia

• Rapid effect

• Easy to administer more

• Intravenous access

TidPla

smak

onse

ntra

sjon

Intravenous induction

• Rapid

• Good control

• Can be given to effect

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Intravenous anaesthesia

0

0

Tid

Pla

smak

ons

entras

jon

(ng/m

l)

Infusjonskons

0

0

Tid Plasm

akonse

ntras

jon (ng/m

l)

Boluskons

Plasmakons

Infusjonskons

Intravenous anaesthesia

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Species difference

• 10 mg xylazine is a suitable dose to sedate a cow (500 kg) and a cat (5 kg)

• Diclofenac, NSAID kills vulture through residues in carcasses.

• Ketamine doses: – Horse 2.5 mg/kg– Pig 15 mg/kg– Rat 70 mg/kg

• Diazepam (Valium©), excitation in some animals

Anaesthetic gas

Anaesthetic gas

Gas anaesthesia

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Why gas?

Anaesthetic machine

• You need to convert fluid into gas

• You need to remove CO2

• You need to administer oxygen

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Gas induction

• Possible to use only one anaesthetic

• Rapid recovery• No drug need to be

matbolised• Stressful• Sevoflurane better for

induction than isoflurane

Maintainance of gas anaesthesia

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Maintainance of gas anaesthesia• Transition from

injection to gas, need to consider for how long the injection has effect

Tid

Gass

Injeksjon

MAC

• Minimum alveolar concentration

• The alveolar gas concentration inhibiting movement as a response to a standardised nociceptive stimulus in 50 % of animals.

• MAC differs between species, but the order between the gases within a species is similiar.

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MAC

MAC

1,89 %Mean

1,65 %11

1,85 %10

1,85 %9

2,35 %8

1,75 %7

2,05 %6

2,25 %5

1,85 %4

1,65 %2

1,65 %1

Individual isoflurane MACPig number

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Perception →←Modulation

Transduction →

Transmission →

General anaesthesia

↓↓↓↓

Analgesia

Blodtrykk, mmHg

55

60

65

70

75

0 5 10 15 20

Tid, minutter

Gje

nnom

snitt

stry

kk, m

mH

g

Bedøvelse Kastrasjon

Kontroll

Testikkel

Streng

Theta power

59

60

61

62

63

0 5 10 15 20

Tid (minutter)

The

ta p

ower

(dB

)

Streng

Kontroll

Testikkel

Injeksjon Kastrasjon

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Aims of anaesthesia

• Unconsciousness

• Muscle relaxation

• Analgesia

With one drug?

Balanced anaesthesia

• In contrast to one single anaesthetics doing all

• Combination of drugs to minimise side effects and taylor drugs to the need of the patient

• Drug combination depends upon:– Species

– Nociception to be expected

– Duration of procedure

– Equipment and expertise avaiable

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Maintainance

• Gas for unconsciousness.

• Analgesics should optimally take care of nociception.

Monitoring of anaesthesia

• Manuel:– Respiratory

movements

– Pulse rate and quality

– Mucous membrane colour

• Electronic equipment– Pulsoximetry

– Multi gas analyser

– Elektrocardiography

– Blood pressure

– etc....

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Temperature

Recovery from anaesthesia

• Take care of the needs of the animal– Oxygen

– Temperature

– Fluids

• Analgesia!– Do not let an animal

wake up to pain.

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Take home message

• Pain and nociception induces malign physiological changes

• Combination of analgesics may be wise• Anaesthetised does not feel pain, but they respons

to nociceptive input• Anaesthesia is a balance between respiratory and

cardiovascular compromise and moving animal.• Perioperative pain management important for

postoperative pain.