☻Anesthesiology

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ANESTHESIA

Satisfactory anesthesia is very important for both humanitarian and technical efficacy. Humanity ensures gentle handling of the animal with minimal restraint to minimize possible injury to the animal. Technical efficacy is not restricted to facilitation of the procedure to be carried out on the animal, it must also take into account the protection of personnel from biting, scratching, kicking, accidental self injection by sedative or addictive drugs or possible harmful effects of breathing low concentrations of inhalation anesthetics.

The veterinary anesthetist deals with variety of animal species that exhibit variation in size, temperament, and anatomical and physiological development. Response to anesthesia not only varies according to species and breed, but it also varies among individuals of the same breed. Fear and aggressive reaction of animals to casting (prior to anesthesia) and struggling to escape breathing irritant gases, increase the difficulty of anesthetic administration and brain activity that in turn affect the amount of required anesthetic. Accordingly, sedatives and tranquilizers are used as pre-anesthetic agents to reduce brain activity, fear and struggling to reduce anesthetic dose and to help smooth induction and recovery of anesthesia. 

Terminology: -

1-Anesthesia: -

It is the art and the science related to production of insensibility.

2-General anesthesia: -

It is a state of unconsciousness as a result of controlled reversible intoxication of the central nervous system, and characterized by loss of sensitivity to external stimuli and motor response to such stimuli.

3-Anesthetic agent: -

It is the substance that produces controllable loss of consciousness and absence of motor response to noxious stimuli.

4-Analgesia: -

It is the art and the science related to abolish awareness of pain.

5-Local analgesic: -

It is a substance that when applied to the nerve endings or nerve fibers temporarily prevents the conduction of impulses by the nerve.

6-Local analgesia: -

It is the loss of sensation in a limited area of the body by blocking the nerve endings in this area.

7-Regional analgesia: -

It is the loss of sensation in a limited area of the body by blocking the major nerve(s) to this area.

8-Narcotic agent: -

It is the substance that produces a state of deep sleep (loss of consciousness) that may or may not accompanied by analgesia. Accordingly all anesthetic agents are narcotic but many narcotics are not anesthetics.

9-Hypnotic agent: -

It is a narcotic agent that produces a sleep like state due to moderate depression of the central nervous system, from which the animal can easily be awakened by wide variety of stimuli.

10-Sedative: -

It is a narcotic agent that causes mild depression of the central nervous system. The animal is awake, but less alert, and unaware of its surroundings. It can be used to calm a nervous, excited, or vicious animal.

11-Ataractic or tranquilizer: -

It is a drug with a predominant action in relieving anxiety without producing drowsiness.

AIMS OF ANESTHESIA: -

1-Humanity point of view like prevention of pain during surgical interference.

2-Creation of a safe state under which the surgeon and assistants can work.

EXAMINATION OF THE ANIMAL: -

The general condition of the animal should be evaluated and recorded prior to anesthesia and surgery, including history, temperature, pulse rate, and respiration rate etc….

PREPARATION OF ANIMAL FOR ANESTHESIA: -

1-Fastening 24 hours prior to operation

2-Fluid therapy according to animal state

3-Preanesthetic medication according to the nature of anesthesia, surgery, and animal species

TYPES OF ANESTHESIA

I-Substances have selective transient paralytic action on sensory nerves

I-1-Local analgesia

I-1-A-Surface application       

I-1-A-i-Topical application         

I-1-A-ii-Intra-synovial analgesia

I-1-B- Infiltration analgesia      

I-1-B-i-Intra or sub-dermal infiltration

I-1-B-ii-Linear infiltration analgesia               

I-1-B-iii-Field block analgesia           

I-1-B-iii-1-Cup shape

I-1-B-iii-2-Inverted-L block 

I-1-B-iii-3-Ring block

I-1-C- Intravenous regional analgesia

I-1-D- Local analgesia for fractures

I-2-Regional analgesia                       

I-2-A-Perineural nerve block

I-2-A-i-Peripheral nerve block (head and limbs)

I-2-A-ii-Paravertebral nerve block (trunk)

I-2-B-Spinal analgesia

I-2-B-i-Epidural analgesia

I-2-B-i-1- Caudal epidural

I-2-B-i-2- Lumbar epidural

I-2-B-ii-Intrathecal analgesia or sub arachnoid

II-Sedation, narcosis, and pre-anesthetic medication

II-1-In combination with local or regional analgesia

II-2-In adjunction to general anesthesia

III-Substances have depressant paralytic action on the CNS producing progressive loss of consciousness and voluntary motor function (general anesthesia)

III-1-By inhalation (volatile anesthesia)

III-2-By intravenous injection

III-3-By combination of the mentioned types with or without premeditation

General Considerations in Selecting Anesthetic Method

1-Nature and magnitude of the operation: -

Local infiltration is sufficient for simple interferences like incision of superficial abscess or neoplasm, or castration in immature animals. However some simple surgical operations can’t be performed by local infiltration as a result of severe fibrosis, temperament of the animal, or severity and intensity of surgical procedure.

2-Site of operation: -

Presence of some critical structures in vicinity of site of operation may render local infiltration insufficient as the movement of the animal may endanger his life, and the example is the surgery for retropharyngeal abscess.

3-Duration of operation: -

The duration of operation affects the choice of anesthetic method, especially when adopting general anesthesia. Short-duration, simple dental operations, can be performed by using ultra-short acting barbiturates, while longer interferences can be performed by using longer-acting barbiturates with local analgesia, or inhalation anesthesia.

Pre-anesthetic medication should be considered when the operation is a major operation with long duration and it is required that the animal remain quite for several hours after surgery. Pre-anesthetic medications not only reduce the amount of anesthetic agent and increase duration of anesthesia, but also control undesirable effect of some anesthetics like salivation.

4-Species and breed of animal: -

Not only size and temperament of the animal affect the choice of anesthetic method, but also the anatomy and physiology of some species affect that choice. Generally the larger size animals have greater difficulties and dangers in induction and maintenance of general anesthesia. The safe satisfactory methods for general anesthesia in pets may be unsuitable for large animals, especially for heavy vigorous one, as the upset of locomotor coordination and prolonged recumbency may entail risks.

A-The Horse: -

The animal should be adequately restrained to ensure safety of the veterinarian and the animal himself. Casting methods of conscious animal and frightening expose him to injury, accordingly, many muscle relaxant drugs can be used to induce casting without endangering the animal.

B-Ruminants: -

Generally they are unsuitable candidate for inhalation anesthesia unless endotracheal tube is used, but under field condition, light general anesthesia by intravenous injection has satisfactory results. However the simpler regional analgesic techniques in this species and side effects of general anesthesia make regional anesthesia more popular in these species.

C-The Dog: -

General anesthesia has a high degree of perfection in this species that make this method so popular for veterinarians not only for surgery but also for examination procedures in the animal.

D-The Cat: -

Cat is a difficult subject to be anesthetized quietly and safely as restraint provokes violent struggling. Accordingly cat should be handled quietly with minimal restraint then general anesthesia can be induced by slow intravenous barbit

References: Hall, C.W. and Clarke, K.W. (1983), Veterinary anesthesia, 8th edition; Hall, L.W. (1978), Wrights veterinaray anesthesia and analgesia, 7th edition