General anaesthesia Flashcards
Characterize general anaesthesia
State of unconsciousness induced by pharmacological means that results in controlled and reversible CNS depression.
Reversible paralysis of CNS ganglion cells located in the cerebral cortex and spinal cord.
Purposes of anaesthesia induction:
Analgesia - loss of pain response and sensation
Amnesia - loss of memory during procedure
Immobility - loss of motor reflexes
Unconsciousness - senselessness
Skeletal muscle relaxation to position the patient
Name two classes of general anaesthetics
Inhalation anaesthetics (liquid + gaseous)
Injectible anaesthetics
Advantages and disadvantages of inhalation anaesthesia
Advantages:
* Possible to have complete control on the depth of anaesthesia
* Possible to lengthen or shorten the anaesthesia
* Rapid awakening without excitation
Disadvantages:
* Need for special equipment
* Need for an erudite worker
Methods for administration of general inhalation anaesthetics:
- Open: substance is dripped into gauze mask. Act slowly, used for small animals
- Semi-closed: substance in collected in a special bag, and re-used for effectively re-action
- Closed: endotracheal tube, low consumption of anaesthetics, 2 modifications
Resorption of inhalation anaesthetic depends on:
The difference between partial pressures of an anaesthetic in alveoli and in inhaled air. The higher partial pressure in inhaled air than alveoli, the more rapid resorption.
Name factors influencing inhalation anaesthesia:
- Solubility of anaesthetic: low solubility in blood - diffuses from the lungs to the arterial blood more quickly
- Concentration of the anaesthetic in inspired air
- Intensity of pulmonary ventilation
- Blood flow through lungs
- Concentration of gradient of anaesthetic between arterial and mixed venous blood
Which reflex responses are used to determine the depth of anaesthesia?
- Pedal - digital withdrawal reflex - deep anaesthesia
- Palpebral reflex - light to medium
- Laryngeal reflex/swallowing: light
- Corneal reflex: phase III
- Pupillary responses: dilated –> constricted
- Skin reflex and muscle relaxion
Name the stages of anaesthesia
Stage 1: Induction or preanaesthetic phase
Stage 2: Excitement stage
Stage 3: Surgical anaesthesia, deep general anaesthesia
Stage 4: Overdose or paralytic phase
Characterize stage 1:
- Between initial administration of the induction and loss of consciousness
- Defecation, urination and mydriasis may occur
- Analgesia without amnesia to analgesia with amnesia
- Loss of pain sensation, but remain conscious
- Reflexes are present
Characterize stage 2
Excitement stage
* Following loss of consciousness marked by excited + delirious activity
* Respiration and heart rate are irregular and accelerated, shallow breathing
* Uncontrolled movements, sounds, vomiting, breath holding and pupillary dilation
* Pulse and respiration increase
* Mydriasis, nystagmus, nausea and emesis, regurgitation
* Can be avoided by rapidly acting drugs
Characterize stage 3
Surgical anaesthesia
* Unconscious and analgesia, ready for surgery
* Pulse, bp and colour of mucosa do not change
* Eye movements stop
4 planes:
I. Mydriasis, nystagmus, fixing, loss of skin and pedal reflexes
II. Loss of palpebral and laryngeal reflexes
III. Respiration more depressed, cardiac contractibility and bp released, corneal reflex lost
IV. Overdose
Characterize stage 4
- Severe brain stem or medullary depression
- Stop of respiration and potential cardiovascular collapse
- Vital functions cease to function
- Lethal without support
- Reversal agents or withdrawal of anaesthesia can be used if artificial respiration is induced before profound cardiovascular depression occurs
How do we distinguish eyeball position?
Using additional monitoring of reflexes and vital functions
Name liquid inhalation anaesthetics
- Ether
- Halothane
- Isoflurane
- Enflurane
- Desflurane
- Sevoflurane
Liquid inhalation anaesthetic - ether
Good analgesia and muscle relaxation, low respiratory depression and blood pressure.
Induces strong excitatory reactions (need premedications).
Contraindicated in respiratory diseases as it irritates the respiratory tract.
Undesirable effects: slow awakening, nausea, vomiting, explosiveness, meat from animals smells and must not be consumed.
Liquid inhalation anaesthetic - halothane
Rapid induction and recovery, potency and nonflammability.
Minimal side effects.
CNS depressant, weak analgesic effect.
Progressive depressant of the respiratory system.
Metabolites of halothane are hepatotoxic.
Some pig breeds are predisposed to increased sensitivity to halothane - stress response –> PSE and swine malignant hyperthermia syndrome.
Liquid inhalation anaesthetic - isoflurane
Structural isomer with enflurane.
Most common for dogs, cats, horses and birds.
Lack epileptogenic property (enflurane has).
May precipitate myocardial ischaemia in patients with coronary disease.
Irritant to respiratory tract.
Liquid inhalation anaesthetic - enflurane
Halogenated anaesthetic similar to halothane.
Less metabolism than halothane (less risky).
Faster induction and recovery than halothane (less accumulation in fat).
Risk of epilepsy-like seizures.
Liquid inhalation anaesthetic - desflurane
Similar to isoflurane, but faster onset and recovery.
Respiratory irritant, can cause coughing and laryngospasm.
Day-case surgery.
Liquid inhalation anaesthetic - sevoflurane
Similar to desflurane, doesn’t irritate respiratory tract.
General anaesthetic for dogs.
Induce and maintain general anaesthesia.
Fast and smooth onset.
Induction - 7%, maintenanse 3-4%.
Side effects: low bp, abnormally fast breathing, muscle stiffness, irritation, temporary inability to breathe, small muscle twitch and vomiting.
Name gas inhalation anaesthetics:
- Nitrous oxide (laughing gas)
* with oxygen, colorless, non-irritant, nonflammable, light anaesthesia, low blood sulibility, limited cardiovascular + respiratory system depression, minimal toxicity
* combination with halothane, premed. with atropine and acepromazine - Carbon dioxide
* analgesic and anaesthetic effect, with oxygen in 30-40% conc., without excitation - Metakain
* fish and reptiles
* protection period of 21 days
