P18: General Inhalation Anaesthetics

What is general anaesthesia?

a state of unconsciousness induced by pharmacological means that results in controlled and reversible CNS depression, where vital functions are preserved

Why does general anaesthesia occur?

reversible paralysis of CNS ganglion cells located in cerebral cortex and spinal cord

When is general anaesthesia induced?

before surgery, painful or prolonged procedures, or for diagnostic purposes

What can irreversible anaesthesia and depression lead to?

euthanasia

What are the purposes of general anaesthesia induction?

analgesia, amnesia, immobility, unconsciousness, skeletal muscle relaxation

How can general anaesthesia be induced?

single drug or combination (hypnotics, sedatives, paralytics, analgesics)

What types of drugs are used as premedication for general anaesthesia?

neuroleptics, sedatives

What is the difference between general anaesthetics and local anaesthetics?

general: given systematically and exert their main effects on the CNS
local: only work locally to application

What are the two classed of general anaesthetics?

inhalation anaesthetics (liquid, gaseous)
injectable anaesthetics

What are the advantages of general inhalation anaesthetics?

possibility of complete control of depth of anaesthesia;
possibility of lengthening or shortening of anaesthesia;
rapid awakening without excitation

What are the disadvantages of general inhalation anaesthetics?

need for special equipment;
need for erudite worker

What are the reflex responses used to determine the depth of anaesthesia?

pedal (reflex);
palpebral (reflex);
laryngeal/swallowing (reflex);
corneal (reflex);
pupillary (reflex);
skin and muscle relaxation

Describe the pedal reflex

obtained by adding firm pressure to the interdigital skin in dogs and cats, squeezing claws together in cattle and swine, and firm pressure on the pastern of horses;
reflex disappears in early stage 3, plane 1 and indicates deep anaesthesia

Describe the palpebral reflex

simulated by tapping the skin in the inner corner of the eye or by running a finger along the eyelashes;
reflex disappears in plane 2 of stage 3 in small animals

Describe the laryngeal/swallowing reflex

stimulated by attempting to pass an endotracheal tube;
disappears in plane 2 of stage 3

Describe the corneal reflex

obtained by gentle palpation of the lateral aspect of the cornea causing the closure of the eyelids;
disappears in plane 2 of stage 3
(not always reliable in dog, particularly if eyeball is rotated)

Describe the pupillary responses

pupil dilated in early excitement stage 2;
becomes progressively constricted in stage 3;
dilates again with entry into stage 4
(heavily influenced by pre-medication, species variations exist)

Describe the skin reflex and muscle relaxation

(jaw tone)
decreases from maximum in stage 2 to stage 3;
muscles relaxed and skin reflex is lost in plane 1 of stage 3

What are the stages of anaesthesia?

stage 1: induction or pre-anaesthetic
stage 2: excitement
stage 3: surgical or deep general anaesthesia
stage 4: overdose or paralytic

What are the characteristics of stage 1 of anaesthesia?

period between initial administration of the induction agents and loss of consciousness;
defecation, urination, mydriasis;
patient progresses from analgesia without amnesia to analgesia with amnesia;
loss of pain sensation but remains conscious;
all reflexes present

What are the characteristics of stage 2 of anaesthesia?

period following loss of consciousness and marked by excited and delirious activity;
respirations and heart rate may become irregular and accelerated;
may be uncontrolled movements and sounds, vomiting, breath holding, pupillary dilation, mydriasis, nystagmus

What is used to keep excitement stage of anaesthesia as short as possible?

rapidly acting drugs

What are the characteristics of stage 3 of anaesthesia?

unconscious with analgesia;
ready for surgery;
skeletal muscles relax, vomiting stops, respiratory depression occurs;
pulse, blood pressure and colour of mucous membranes do not change and are controlled,
eye movements slow and stop

How many planes are there in stage 3 of anaesthesia?

4

Describe plane 1 of anaesthesia

miosis, nystagmus, eyes become fixed centrally (cats and dogs),
skin and pedal reflexes lost

Which plane of anaesthesia is described as "light"?

plane 1

Describe plane 2 of anaesthesia

loss of palpebral and laryngeal reflexes;
respiration is regular

Which plane of anaesthesia is described as "surgical"?

plane 2

Describe plane 3 of anaesthesia

respiration is depressed;
cardiac contractility and blood pressure is reduced;
corneal reflex is lost

Which plane of anaesthesia is described as "deep"?

plane 3

Describe plane of anaesthesia

overdose;
paralysis of the intercostal muscles and shallow abdominal respiration;
pupils very dilated;
pupillary reflex is lost

What are the characteristics of stage 4 of anaesthesia?

too much medication relative to the amount of surgical stimulation and patient has severe brain stem or medullary depression;
stop of respiration and potential cardiovascular collapse;
vital functions cease;
lethal without cardiovascular and respiratory support;
recovery is possible if inhalation anaesthetics are withdrawn and artificial respiration instituted;
reversal agents can be used with parenteral anaesthetic agent overdose

Describe how eyeball position can be used to test depth of anaesthesia

surgical anaesthesia (halothane, isoflurane): eyeball is rotated medially and ventrally;
early and late anaesthesia: eyeball is centrally placed

Why should eyeball position alone not be used for determination of depth of anaesthesia?

position varies depending on anaesthetic agent and species of animal

What are the methods of administration of general inhalation anaesthetics?

open;
semi-closed;
closed

Describe the open method of administration of general inhalation anaesthetics

substance is dripped onto gauze in a mask;
anaesthetic acts slowly;
manifestations of excitation are more intense;
applicable to small animals

Describe the semi-closed method of administration of general inhalation anaesthetics

substance is collected in a special bag where is is reused effectively for re-action

Describe the closed method of administration of general inhalation anaesthetics

use of anaesthesia apparatus and insertion of endotracheal tube;
depending on instrumentation, it has 2 modifications: 1-way and 2-way
advantage: low consumption of anaesthetic

What are the principles of using inhalation anaesthetics?

resorption depends on the difference between the partial pressures of anaesthetic in alveoli and inhaled air;
higher partial pressure in inhaled air in comparison to alveoli, the more rapid the resorption;
the lower partial pressure in inhaled air in comparison to alveoli, the more rapid the loss of anaesthetic from the body

What are the factors affecting resorption of inhalation anaesthesia?

solubility of anaesthetics;
concentration of anaesthetic;
intensity of pulmonary ventilation;
blood flow through the lungs;
concentration gradient of anaesthetic between arterial and mixed venous blood

How does the solubility of anaesthetics affect the resorption?

anaesthetic with low solubility in blood diffused from the lungs into arterial blood more quickly and there is only a relatively small number of molecules for achieving the increase in partial pressure of arterial tension

How does intensity of pulmonary ventilation affect the resorption of anaesthetics?

respiratory diseases decrease the resorption

What are examples of liquid inhalation anaesthetics?

ether, halothane, isoflurane, enflurane, desflurane, sevoflurane

What are examples of gas inhalation anaesthetics?

nitrous oxide, carbon dioxide, metakain

What are the characteristics of ether?

colourless, volatile liquid with typical odour, with a sweet, burning taste;
very explosive;
induces anaesthesia with good analgesia and muscle relaxation with low respiratory depression and blood pressure;
induces strong excitatory reactions (premedication needed);
irritates respiratory tract;
slow awakening, nausea, vomiting

What are the contraindications of ether?

patients with respiratory diseases;
animals for consumption

What are the characteristics of halothane?

clear, volatile liquid;
rapid induction and recovery, potency, and noninflammability, with minimal side effects;
weak analgesic effects;
metabolites are hepatotoxic

What is halothane used for in pigs?

selection of stress-resistant pigs;
pigs with increases sensitivity to halothane are more prone to porcine stress syndrome resulting in pale, soft, exudative meat and malignant hyperthermia;
malignant hyperthermia: elevated body temperature during anaesthesia, tonic and tetanus cramps with stretching of fore and hind limbs

Which inhalant anaesthetics are structural isomers?

isoflurane, enflurane

What are the characteristics of isoflurane?

volatile liquid;
most commonly used for dogs, cats, horses and birds
lacks epileptogenic property;
faster induction and recovery than halothane (less accumulation in fat)
little hepatic metabolism (less toxicity)
may precipitate myocardial ischaemia in patients with coronary disease;
irritant to respiratory tract

What are the characteristics of enflurane

volatile liquid;
faster induction and recovery than halothane (less accumulation in fat)
little hepatic metabolism (less toxicity);
risk of epilepsy-like seizures

What are the characteristics of desflurane?

volatile liquid;
faster onset and recovery compared to isoflurane;
respiratory irritant (cause coughing and laryngospasm);
useful for day-case surgery

What are the characteristics of sevoflurane?

similar to desflurane with lack of respiratory irritation;
for dogs;
induce and maintain general anaesthesia;
fast and smooth onset;
induction: 7%
maintenance: 3-4%
side effects: low blood pressure, abnormally fast breathing, muscle stiffness, irritation, temporary inability to breathe, small muscle twitching and vomiting

What are the characteristics of nitrous oxide?

gaseous anaesthetic with medicinal oxygen 70:30 or 50:50;
colourless, non-irritant, sweet smelling, non-flammable;
light anaesthesia;
low blood solubility, limited cardiovascular and respiratory system depression and minimal toxicity;
used in combination with halothane;
premedication with atropine and acepromazine

What are the characteristics of carbon dioxide?

analgesic and anaesthetic effects, with oxygen administered 30-40% for induction of anaesthesia without excitation

What is metakain used for as a general anaesthetic?

fish and reptiles