Veterinary inhalant anaesthetics

1. Inhalant Anaesthetics
Dr.Kumari Anjana
Assistant Professor
Deptt. of Veterinary Pharmacology & Toxicology
Bihar Veterinary College, Bihar Animal Sciences University, Patna

2. 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.

3. INHALATIONAL ANAESTHETICS contd…
Classification of Inhalational Anaesthetics:
Gaseous agents:
Nitrous oxide and Cyclopropane.
Volatile liquids:
Methoxyflurane, Halothane, Ether, Chloroform,
Enflurane, Isoflurane, Desflurane, Sevoflurane.

4. 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.

5. 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.

6. 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.

7. 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

8. 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.

9. 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 NO2,
after induction by ultra-
short acting barbiturate.
Used in small or large
animal surgery for
induction (3%
MF+N2O(70%)+O2(27%)
and maintenance (2-3%)

10. 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.

11. 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).

12. 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.

13. Nitrous Oxide (N2O; 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%).

14. 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.