Induction of Anesthesia

By | 2011-10-30

Injectable agents

Advantages of injectable anesthetics include ease of administration, avoiding problems related to breath-holding and prolonged induction, low cost and good availability. Disadvantages include a recovery often dependent on organ metabolism, difficulty reversing medications in emergency situations, prolonged recovery periods and necrosis of muscle cells at injection sites. Also, due to the renal portal system, drugs injected into the caudal half of a reptile may either be excreted by the kidneys before they take effect, or increase renal damage if the case of nephrotoxic agents. Injectable medications should therefore be administered in the cranial half of the body.

Ketamine

The effects of ketamine are species and dose dependent. Recommended levels range from 22-44mg / kg intramuscularly (IM) for sedation and 55-88 mg / kg IM for surgical anesthesia. Lower doses are required if the drug is combined with a premedicant such as midazolam or medetomidine.

Anaesthesia is induced in 10-30 minutes but may take up to 4 days to wear off, particularly at low environmental temperatures. Therefore, ketamine is mainly used at lower doses to sedate the animal and allow intubation and maintenance of anesthesia using inhalational agents in species such as Chelonia, which may breath-hold.

Disadvantages of ketamine include pain on administration and renal excretion: due to the renal portal system administration of ketamine in the cranial half of the body is recommended.

Propofol

Propofol produces rapid induction of and recovery from anesthesia, and is becoming the induction agent of choice. Its advantages include a short elimination half-life and minimal organ metabolism, making it safer in debilitated reptiles. Disadvantages include the need for intravenous administration, although use of the intraosseous route has been shown to be successful in green iguanas at a dose of 10 mg / kg. Propofol does produce transient apnoea and cardiac depression, often necessitating positive pressure ventilation.

Doses of 10-15 mg / kg of propofol administered via the dorsal coccygeal (tail) vein in Chelonia have successfully induced anesthesia in under 1 minute. Used alone this will provide a period of anesthesia of 20-30 minutes or will allow intubation and maintenance of anesthesia using inhalational agents.

Succinylcholine

This is a neuromuscular blocking agent and produces immobilisation without analgesia. It should only be used to aid the administration of another form of anesthetic, or for transportation and not as a sole method of anesthesia.

It can be used in large Chelonia at doses of 0.5-1 mg / kg intramuscularly (IM) and will allow intubation and conversion to inhalational anesthesia. Crocodilians can be immobilised with 3-5 mg / kg IM, with immobilisation occurring within 4 minutes and recovery in 7-9 hours. Respiration usually continues without assistance at these doses, but is important to have assisted ventilation facilities to hand as paralysis of the muscles of respiration can easily occur.

Inhalational agents

The usual inhalational agents may be used to induce anesthesia, either in an induction chamber or via face masks. Face masks may be either bought, or for snakes, syringe cases may be modified to form elongated masks for induction.