Inhalant Anaesthetics Dr.Kumari - PowerPoint Presentation

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Inhalant Anaesthetics

Dr.Kumari

Anjana

Assistant Professor

Deptt

. of Veterinary Pharmacology & Toxicology

Bihar Veterinary College, Bihar Animal Sciences University, Patna

INHALATIONAL ANAESTHETICS

MAC:

MAC is the minimum alveolar concentration; the

lowest

concentration of the inhalant

anesthetic

in pulmonary alveoli

in producing immobility due to surgical incision in 50% of the individuals

.

It

is the measure of potency of inhalation anaesthetics

.

The

anaesthetic

potency of an inhaled

anaesthetic

is inversely related to MAC.

Potency = 1/MAC; i.e. an agent having low

anaesthetic

potency will have a high MAC value and vice-versa

.

INHALATIONAL ANAESTHETICS contd

…

Classification

of Inhalational

Anaesthetics

:

Gaseous agents

:

Nitrous

oxide and Cyclopropane

.

Volatile liquids

:

Methoxyflurane

, Halothane, Ether, Chloroform,

Enflurane

,

Isoflurane

,

Desflurane

,

Sevoflurane

.

Blood: Gas Partition Coefficient

: The blood/ gas solubility is a

measure of the speed of anaesthetic

induction

, recovery and change of

anaesthetic

levels.

Lower

the blood/ gas partition coefficient, the

more

rapid the

anaesthetic

induction

or rate of change of

anaesthetic

level in response to a stepwise change in

anaesthetic

delivery

.

Oil:

Gas Partition Coefficient:

A measure of fat solubility, determines the potency of an

anaesthetic

and also influences the kinetics of its distribution in the body, the main effect being that high lipid solubility delays recovery from

anaesthesia

.

Volatile Anaesthetics

Parameter

Ether

Halothane

Methoxyflurane

Properties

Characteristic

oduour

and sweetish taste. Oxidizes to peroxides (irritate resp. tract) upon exposure to moisture.

Characteristic sweetish

odour

. Decomposes upon exposure to sunlight (0.10%

thymol

is added as preservative).

Characteristic pungent

odour

. Decomposes upon exposure to sunlight. Butylated

hydroxytoulene

is added as preservative.

MAC (%)

3

Least

potent.

Slow

induction

0.75-1.20

.

Induction 3-5 min

0.23

Slow

induction (10 min)

CNS

All stages are seen

Stage II bypassed

Stage II bypassed

CVS

Induction- release of adrenaline: increase in heart rate & BP. Stage III: Fall in BP and COP (depression of VM centre). Does not sensitize heart to

catecholamines

.

Direct myocardial depression (reducing

intracellutar

Ca

++

). Sensitizes heart to

catcholamines

(arrhythmia)

No change in heart rate or mild tachycardia. Adrenaline can induce cardiac arrhythmia.

Respiration

Initial stimulation followed by progressive depression. Increase bronchial secretion.

Depression with increase in duration of

anaethesia

, may develop acidosis.

Initial stimulation followed by progressive depression with increase in

anaethesia

.

SK. Muscle relaxation

Sig. effect. Dose of

dTC

to be reduced to one-third

Low to satisfactory relaxation.

dTC

can be used if needed.

Adequate relaxation dTC can be used if needed.

Liver

Prolonged anesthesia lowers liver glycogen. Not hepatotoxic

Hepatotoxic like chloroform

No significant effect

Kidney

Long duration: oliguria/anuria due to ADH release

No significant effect

No significant effect

Body Temperature

Hypothermia

Malignant hyperthermia in pig and horse (

persisten

muscle contraction due release of Ca

++

from

sarcoplasmic

reticulum) and hypothermia in others.

Hypothermia

Foetus

& Uterus

No significant effect

Reduce uterine contractions

.

Neutralizes oxytocin. Readily crosses placenta.

Reduce uterine

contractions

Neutralizes

oxytocin

. Readily crosses placenta.

GIT

Nauses

& vomition common during induction or recovery.

No vomition during induction or recovery.

Nauses

& vomition common during induction or recovery

Merits

Safest in small animals with proper premedication

.

Ready control of

anaethesia

. Good analgesia, muscle relaxation. Cheap.

No

costly equipment is needed.

Potent; used in small or large animals; rapid induction (3-5 min) & recovery (10-15 min): nonirritant, ready control of

anaesthesia

(low blood solubility), nonflammable &

nonexplosive

Most potent

.

Can be used in small or large animals.

Excellent

muscle relaxation and analgesia. Nonflammable.

Demerits

Highly flammable. Difficult to use in hot climate. irritant to resp. tract. Delayed induction without proper premedication

Resp.

and

cardiac

depression and poor muscle relaxation and analgesia. Malignant hyperthermia in pig and horse.

Expensive

(requires closed circuit apparatus)

Easy control of

anaesthesia

not possible (high blood solubility).

Recovery

prolonged.

Poor

vaporization.

Needs

close circuit apparatus

Contra-

indications

Aminoglycoside

antibiotics (synergistic curariform effect)

Aminoglycoside antibiotics (synergistic

curariform

effect), catecholamine’s and Ca Channel blockers. CHF

Noradrenalin or epinephrine without premedication

Uses

Not used in human or

vety

, surgery.

Mainly

use in lab animals for surgery or euthanasia

Used in small and large (horse) animals, mainly for maintenance (2-8 ml/45 kg/

hr

) of

anaesthesia

with NO

2

, after induction by ultra-short acting barbiturate.

Used in small or large animal surgery for induction (3% MF+N

2

O(70%)+O

2

(27%) and maintenance (2-3%)

Enflurane

A colorless, pungent, nonflammable volatile liquid, chemically related to methoxyflurane

. The most frequently

used potent

anaesthetic

in human

surgery.

It

is classified as a

convulsive

anaesthetic

(epilepsy like seizures;

disscociative

-cataleptic

anaesthesia

).

Its

MAC for

horse

is 2.12

%.

It

causes

CNS excitation in dogs

causing muscular twitching (face, neck,

limb

and abdomen) if diazepam

preanaethesia

is not given.

In

comparison to halothane this does not sensitize heart to catecholamine’s and has more depressant action on respiration and

better muscle relaxation

.

Isoflurane

Though an isomer of enflurane does not cause CNS excitation.

It is about one and half times more potent

than

enflurane

MAC 1.3

%.

It provides

satisfactory skeletal muscle relaxation

(synergistic neuromuscular blockade with curariform

agents).

Chloroform

It is replaced

by other safer anaesthetics, sometimes used for euthanasia.

Its use is associated

with

risk of death

of the animal during induction, prolonged

anaesthesia

and during

post-

anaesthetic

period.

During

induction majority of deaths

occur due to direct toxic effect on heart.

During

stage I the animal tries to avoid inhaling chloroform

vapours

by temporary

breath-holding

, which is followed by reflex

deep breathing

taking a high concentration of chloroform

vapours

into lungs, from there through pulmonary veins into the heart, causing ventricular fibrillation

and/or

cardiac arrest

.

Cardiac toxicity may be

avoided by proper premedication

(sedatives) and slow administration of chloroform.

Prolonged surgical

anaesthesia

may cause

respiratory failure

due depression of medullary respiratory centre.

exposure to

air and light

chloroform

gets oxidized to

phosgene gas

(a marked lung irritant).

Phosgene formation is prevented by

adding ethyl alcohol @ 1 per cent.

Nitrous Oxide (N2

O; Laughing gas)

It is discovered by

joseph Priestly (1772).

It

is a colorless nonirritating and nonflammable gas.

It

causes

excitement, delirium and amnesia in humans

, hence the name laughing gas

.

Has very low

anaesthetic

potency

(MAC 188 (cat) 255 (dog)%)

and hence must be combined with other inhalation anaesthetics (halothane or

methoxyflurone

).

It has good analgesic, but poor muscle relaxant effects.

To

avoid hypoxia, it is used in combination with oxygen (nitrous oxide 70% oxygen 25%) and other inhalation

anaesthetic

(0.2 – 2

%).

Cyclopropane

Colorless gas with a characteristic

odour.

It

can be used in small animal surgery of short duration after diluting with oxygen (4 times) through a closed circuit system

.

Morphine premedication and

catechloamines

are contraindicated due to marked respiratory depression and cardiac arrhythmic respectively

.

It causes adequate skeletal muscle relaxation. It forms flammable mixture with air

.

Thank You