Introduction to anaesthesia
(The loss of sensation)
Classical reasons:
▪ Humane aspects;
- Prevention of pain.
- Humane handling to prevent animal injury.
▪ Technical efficiency;
- Ease of surgery.
- Personnel protection.
Objectives of anaesthesia:
1- provide relief from pain.
2- Control and immobilization of the patient.
3- Ensure patient safety and survival.
Types of anaesthesia:
- General anaesthesia; unconsciousness.
- Local anaesthesia; blocking nerves endings.
- Analgesia.
Preanaesthetic examination:
(Minimize risk of anaesthetic morbidity and mortality)
▪ History.
▪ Respiratory system evaluation:
- Respiratory pattern / effort.
- Pulmonary diseases; Pneumonia.
- Cough; a moist cough when is suppressed by anaesthesia, the airway secretion will
obstruct the lung.
▪ Cardiopulmonary evaluation:
-Auscultation of heart sounds; heart rate and rhythm; pulse rate and quality.
- Cardiovascular failure.
- Hypovolaemia and dehydration.
- Dysrhythmia.
- Anaemia; Hypoproteinaemia → Hypoalbuminaemia → Drug unbound →
Increase peak activity.
Hepatic system:
-
Metabolism of many drugs.
-
Hepatitis, Jaundice; lead to hypoalbuminaemia.
Renal system:
-Regulate the body's fluid volume, mineral composition and acidity.
- Dehydration, haemorrhage → Arterial hypotension → Renal ischaemia →
Uraemia → Death.
What would contraindicate general anaesthesia?
- Severe respiratory diseases.
- Congestive heart failure.
- Ventricular dysrhythmia.
- Severe electrolyte or acid base disturbances.
- Acute anaemia.
- Full stomach.
Preparation of the patient:
Preoperative starvation; An empty stomach reduces pressure on the lung, and
reduces the chance of vomiting.
-Time of starvation depends on species; about 12 hours for single stomached animals,
more (24-48) for ruminants.
-Water should be freely available.
-Small mammals and birds are not usually starved.
Pain and the central nervous system:
-CNS comprises; brain and spinal cord.
-Peripheral nervous system consists of; cranial and spinal nerves, and consists of
bundle of thousands of nerve fiber.
- The nerves connect the receptors with the CNS and CNS with the muscles and organs.
- Nerve fibers act as a wire transmitting messages as electrical pulses. (in one direction
only)
Synapse; the region where two or more neurons meets and where impulses are passed
from one to another.
Neurotransmitter; are chemicals that move electrical impulses across a synaptic gap to
the next neuron. It is stored in vesicles on synaptic membrane.
Neurotransmitter are ;
- Excitatory " glutamate" or inhibitory "GABA",
- Circulating hormones " dopamine, adrenaline, noradrenaline".
- Acetylcholine.
Principles of sedation
Definitions:
▪ Tranquillizers; drugs that reduce fear and apprehension. Example is the
acepromazine. Overdose of these drugs does not increase the sedative effect but may cause
CNS stimulation and muscle twitching.
▪ Sedatives; drugs that relieve anxiety and make the animal sleepy. Drugs in this group
include the alpha 2-adrenoceptor agonists (e.g. Xylazine).
▪ Hypnotics; drugs that induce sleep. e.g. thiopentone.
Reasons for using sedatives:
-
To control normal animals for minor procedures.
-
To reduce fear and resistance to anaesthesia.
-
Help to ensure smooth induction, and to reduce the dose of the anaesthetic.
General rules for using the sedative agents:
- Sensory input " noise, etc" should be minimized, as no animal will sedate
effectively when is significantly disturbed " high adrenaline level".
- Choose the correct drug and route, and consider the species differences,
age and the condition of animal "disease, pregnancy".
- Sedatives are NOT anaesthetics. Never trust the effect and still handle with
care "horse kick, dog bite".
ALPHA2 ADRENOCEPTOR AGONISTS
"Most important sedatives in Vet. Med."
Agents include: Xylazine and Detomidine.
Mechanism of action:
Alpha-2 receptors are found in the CNS, and PNS.
The drug’s mechanism of action is mainly through its agonist activity at presynaptic
adrenergic alpha-2 receptors, that decreases noradrenaline release from adrenergic nerve
terminals in CNS and PNS.
Pharmacokinetic:
These drugs are metabolized by the liver.
Clinical uses:
-Excellent sedatives, provide moderate to heavy sedation.
- Provide analgesia, and muscle relaxation.
- Several combinations are in widespread use. (Synergistic with some anaesthetics agents
like ketamine)
Side effects:
-
Bradycardia (decreased heart rate).
-
Respiratory depression (rate and depth).
-
Hypoxaemia and pulmonary oedema in ruminants.
-
Induce vomiting in small animals (dogs and cats).
-
Stimulation of pregnant uterus, which may lead to abortion. (data sheet gives
contraindication in late stage of pregnancy.
General considerations:
-
There is a marked species differences in dosage with xylazine (xylazine dosage for
cattle is one tenth of horse and dog dose).
-
Detomidine is very potent in horses (approximately 10 times than xylazine, but very
low effects in camel).
-
Detomidine is less ecbolic effect than xylazine.
Phenothiazines derivatives
▪ Acepromazine (ACP)
Mechanism of action: Dopamine antagonism in CNS
Pharmacokinetic: Metabolized by liver.
Clinical uses:
-
Calming effects, and sometimes sedation.
-
Anti emetic effect in dogs and cats.
-
Maximal sedative effects are obtained with the recommended doses. (excessive dose
does not increase the degree of sedation)
Side effects:
-
Priapism in male horses.
GENERAL ANAESTHESIA
Definition:
It is a state o f unconsciousness produced by reversible drug that produces depression of
the CNS, in which the patient neither feels nor remember painful stimuli.
Induction and maintenance of anaesthesia:
General anaesthesia can be induced by injectable agents (IV/IM) or inhalation (gaseous)
agents. Onset of anaesthesia is faster with IV injection than with inhalation.
The duration of action of IV induction agents is generally 5 to 10 minutes, after which time
spontaneous recovery of consciousness will occur. In order to prolong anaesthesia for the
required duration (usually the duration of surgery), anaesthesia must be maintained.
Usually this is acheived by the total inravenous anaesthesia (TIVA), or by the inhalation
system.
Mode of action:
- Unknown specific receptor sites.
-No method of antagonism.
Stages of anaesthesia:
Stage One: voluntary excitement might be observed.
Stage Two: animal appeared to be unconscious, but involuntary excitement could occur.
Stage Three: Surgical anaesthesia.
Stage Four: Anaesthetic overdose.
Injectable anaesthetics
Barbiturates: (Thiopentone)
- Widely used as effective iv induction agent in all species.
- Good hypnotics, but very poor analgesics.
- Very slow metabolism by the liver. (It is cumulative, and can't be used for TIVA)
Propofol:
- Produces an excellent quality of anaesthesia.
- Very rapidly metabolized by liver. (Non-cumulative, and TIVA can be used)
- Disadvantages: - Very respiratory depressant.
- Very expensive.
Dissociative anaesthesia: (Ketamine)
Definition: It is a type of anaesthesia that characterized by complete analgesia combined
with only superficial sleep.
Ketamine:
-
Used extensively and effectively in most domesticated species, specially when inhalant
agents are unavailable.
-
Metabolized by liver, and marked cumulating does not occur.
Anaesthetic properties:
-
Side effects of ketamine may become clear after incremental doses, and those are bad
quality anaesthesia, poor muscle relaxation, movement, excitation, tremors and
convulsions.
-
It usually used in combination with alpha 2 agonists (xylazine) to improve the quality
of anaesthesia and to reduce the side effects of ketamine.
-
Difficult to assess the depth of anaesthesia. The anaesthetic depth is only indicated by
response to painful stimuli. Eye may be open, with the presence of palpebral reflex.
-
Stimulant action in sympathetic nerves (tachycardia, and increase cardiac output),
but also there is a direct cardiovascular depressant action.
Inhalational anaesthetics
- Inhalation anaesthetics are divided into two categories, gaseous and volatile
agents.
- Gas→Circuit→Inspired→Alveolar→Artery
↔ Brain
- Comparison of inhalant and injectable anesthetics:
Injectable Technique
Inhalant technique
Injectable technique
Expensive Equipment
Cheap (needles, syringes)
Airway control and high O2
Not necessarily
Better control of anesthetic depth
Once given, suffer the consequences
Ease of elimination (ventilation)
through metabolism & Excretion
Pollution
No
Potency:
MAC (Minimum Alveolar Concentration);
It is the percentage of an anaesthetic which prevents movements to a standard stimulus in
50% of the subjects.
Agents in veterinary use:
Nitrous oxide: N2O
-
Low potency, and can’t be used alone.
-
Used as a carrier gas because of its excellent analgesic properties.
Halothane:
-
Widely used in veterinary anaesthesia to produce general anaesthesia.
-
Very potent and effective agent.
-
Approximately 20% of the dose is metabolized by liver.
-
Very rare occurrence of hepatitis "especially in goats".
-
Halothane has been replaced more recently by isoflurane and sevoflurane.
Isoflurane, Sevoflurane:
-
Newer agents that in several properties are superior to halothane.
-
Only 1 or 2% are metabolized by liver.
-
Speed induction and recovery.
Agents used in the past:
Ether:
-
Very old and save anaesthetic.
-
It is inflammable agent. (may cause explosion)
Chloroform:
-
Very powerful anaesthetic, which is no longer used because of its toxicity.
Administration of inhalation agents:
The administration requires:
-
Source of oxygen.
-
A vaporizer or source of anaesthetic gas.
-
A breathing system: The purpose is to carry the oxygen and the anaesthetic gases to
the patient and to remove the CO2.
Types of breathing systems:
- Non-rebreathing system:
The expired gases are vented outside and cannot be rebreathed. e.g. T-piece, and
Lack systems.
- Rebreathing system:
The expired gases are passed through a CO2 absorber to remove the CO2.
The Rebreathing system (Circle system) contains inspiratory and expiratory tube
with unidirectional valves, CO2 absorber and rebreathing bag.
Local anaesthesia
Definition:
- Local anesthesia is defined as the loss of sensation in a limited area of the body.
- Local anaesthetics are the drugs that act directly on peripheral nerves to block the
conduction of nerve impulses.
Cocaine was the first local anaesthetic discovered, but has no role in modern anaesthetic
practice.
Lignocaine: (Lidocaine)
The most widely used for local analgesia, has been used safely and effectively for every
possible type of local anaesthetic procedure. Rapid onset of action extended up to 2 hours.
Mechanism of action: Block the sodium channels.
Local anaesthetic techniques:
▪ Surface analgesia:
By agents, which cause freezing, e.g. ice, ethylchloride.
Or by agents like Lignocaine applied topically in the form of cream or spray.
▪ Intrasynovial analgesia:
Useful for both diagnosis of lameness, and for general pain relief.
▪ Infiltration analgesia:
It is applied by injection of local anaesthetic subcutaneously at the site of operation. Most
minor surgeries can be done by using this method.
Disadvantages; - Irritation, infection, and distortion of the wound.
- Swelling, and some delay in post-operative healing.
▪ Field analgesia:
Wall of analgesia are made around the surgical site.
Forms of field analgesia;
-
Inverted L block; to desensitize the flank area of ruminants. e.g. laporatomy and
caesarean operations.
-
Ring block; injecting local analgesia in a circular manner around the site of
operation. e.g. teat, feet, digit.
▪ Regional analgesia:
It is made by blocking the nerves that innervating the site of operation.
e.g. Cornual block for dehorning, Mandibular nerve block for dental surgeries.
Fig. Diagrams of needle placement sites for cornual nerve block in a cow and in a goat.
▪ Spinal analgesia:
It is the injection of local anaesthetic around the spinal nerves.
Types of spinal analgesia:
- Subarachnoid injection: (spinal analgesia)
The needle penetrates the dura mater.
- Epidural analgesia:
Injection of local analgesia above the dura mater.
Site of injection:
Caudal block; "posterior block"
Is made between the coccygeal intervertebral spaces.
Indications; surgeries around rectum, vagina, and tail.
Anterior block;
The injection is made further cranially, usually at lumbo-sacral space.
Indications; Pelvic and abdominal surgeries, castration, amputation of udder, and
obstetric surgeries.
Disadavntages of Epidural analgesia:
-
Infection.
-
Irritation causing spinal damage.
-
Hindlimb motor paralysis. "problems in large animals"
-
Fracture of pelvic limb.
Anaesthetic emergencies
▪ Human errors
-
It is a major finding that a high percent of anaesthetics deaths in animals were due to
improper observation at the time of their death, and so animals should be under
continuous observation
-
Miscalculation or drug overdose.
▪ Inhalation of vomit
-
The risk of regurgitation and then inhalation of vomit is greatest following
administration of xylazine and ketamine. (most observed in ruminants)
-
Vomit or regurgitation may result in:
- Airway obstruction → death.
- Post-operative pneumonia (usually fatal).
Prevention:
- Adequate starvation.
- If it happens, Clear vomit from mouth and airway.
- Extend the head on the neck and pull the tongue out of the mouth.
- Perform endotracheal intubation. And if this is not an available option, consider
tracheostomy.
- Speed up the recovery.
- Antibiotics are required to prevent pneumonia.
▪ Bradycardia
Causes:
-
Deep anaesthesia.
-
High dose of alpha2 agonists (e.g. Xylazine).
-
Vagal stimulation (surgeries around the eye, ear, or larynx)
Treatment:
-
Identify the cause and address it.
-
Intravenous injection of atropine.
-
If no response, Intravenous injection of adrenaline should be given.
▪ Cardiac Arrest
Common Causes
-
Deficiency of oxygen is the ultimate cause of all cardiac arrests.
-
Respiratory failure.
-
Acid-base and electrolyte disturbances.
-
Drug overdose.
Signs of cardiac arrest
• Absence of a pulse or a palpable or audible heart beat.
• Apnea.
• Loss of consciousness.
• Loss of corneal ocular reflexes.
• Eyes are fixed, wide open.
Action: Cardiopulmonary Resuscitation (CPR)
• Establish a secure airway as quickly as possible.
• Clear the airway of any obstructions (vomits, excessive mucus, tongue, foreign objects).
• Perform endotracheal intubation.
• If this is not an available option, consider tracheostomy.
• Cardiac and external chest compression.
• Intravenous injection of adrenaline. (Repeat it after 3 to 5 minutes if needed)
• Administer sodium bicarbonate when a metabolic acidosis has developed or is strongly
suspected.
Table: Animals anaesthesia; recommended doses at the KFU Vet. Teaching
Hospital.
Animals Sedation
-Acepromazine 0.05-0.1 mg/kg.
Anaesthesia
Sheep &
goat
-Xylazine 0.02-0.2 mg/kg.
- Xylazine 0.1 mg/kg.
+
- Ketamine 0.5 mg/kg.
Horse
-Acepromazine 0.05-0.1 mg/kg.
- Xylazine 0.5-1.0 mg/kg.
- Detomidine 0.02 mg/kg.
- Xylazine 0.5-1.0 mg/kg. or Detomidine 0.02 mg/kg.
+
- Ketamine 2.0-2.2 mg/kg
Cattle
-Acepromazine 0.05-0.1 mg/kg.
-Xylazine 0.02-0.2 mg/kg.
- Xylazine 0.02-0.2 mg/kg.
+
- Ketamine 2.0 mg/kg
Camel
Xylazine 0.2 mg/kg. i.v.
0.2-0.4 mg/kg. i.m.
Xylazine 0.2 mg/kg. i.v.
+
- Ketamine 0.7-1.0 mg/kg. i.v.
-Acepromazine 0.05-0.2 mg/kg.
-Xylazine 1-3 mg/kg. i.m.
0.5-1 i.v.
-Xylazine 1.0 mg/kg.
+
- Ketamine 5-10 mg/kg.
-Acepromazine 0.03-0.1 mg/kg.
-Xylazine 1-3 mg/kg. i.m.
-Xylazine 1.0 mg/kg. i.m.
+
- Ketamine 5-25 mg/kg. i.m.
Dog
Cat
How to calculate drugs dosages:
Dose rate (mg/kg) * Animal weight (kg)
Solution strength (mg/ml)
= Volume of solution (ml)