Module 11 - Anesthesia

Types of Anaesthetic Agents

Anaesthetic agents should produce a loss of sensation with a minimum of side effects and they should have a calming effect on the animal during the recovery phase. While there is not a requirement for a loss of consciousness during anaesthesia, that is the case with general anaesthetics. Local anaesthetics for example will produce quite localized or even regionalized anaesthesia without any loss of consciousness. As well, it is advantageous for an anaesthetic agent to provide some level of analgesia during the recovery phase.

There are three broad groups of anaesthetic agents namely volatile anaesthetics like isoflurane and halothane, injectable anaesthetics like ketamine, propofol and barbiturates, and local anaesthetics like lidocaine, procaine and bupivacaine. For general anaesthesia inhalant anaesthetics are highly preferred as the anaesthesia is much easier to control and the agent quickly cleared from the body.

Volatile Anaesthetic Agents

The common effects of anaesthetic agents described above apply to the volatile anaesthetics. These drugs are usually supplied as liquids and require a vaporizer and a carrier gas such as oxygen to deliver them to the patient. Altering the concentration of the anaesthetic agent in the inspired gases easily controls the depth of anaesthesia. In the event that the animal becomes too deeply anaesthetised, the anaesthetic agent is quickly removed from the animal through the lungs. It is important to scavenge waste anaesthetic gases to minimize exposure of people to these agents.

Isoflurane

• Highly volatile and must be administered using a calibrated vaporizer to prevent exposure to high concentrations of gas
  
• Must be scavenged to avoid occupational exposure
  
• Respiratory depression greater than with halothane and may necessitate external ventilation
  
• Little hepatic metabolism and a lessened risk of hepatitis
  
• Rapid recovery (1-3 minutes)

Halothane

• Highly volatile and must be administered using a calibrated vaporizer to prevent exposure to high concentrations of gas
  
• Must be scavenged to avoid occupational exposure
  
• May cause cardiac arrhythmias
  
• May cause hepatitis in humans but rare in other species
  
• Will cause malignant hyperthermia in genetically susceptible pigs
  
• Rapid recovery (1-3 minutes) except from very long and deep anaesthesia

Nitrous Oxide

• Comes as a gas in cylinders
  
• Low anaesthetic potency and cannot produce anaesthesia in animals by itself
  
• Causes minimal cardiovascular and respiratory depression
  
• May be used to reduce the concentration of other anaesthetic gases although this effect is less than that seen in humans
  
• Use with caution in ruminants

Injectable Anaesthetics

The general effects of anaesthetics apply to the injectable anaesthetics, with some exceptions. Ketamine, for example, does not cause significant cardiovascular depression at the usual anaesthetic doses. Injectable anaesthetics are easily administered requiring little more than a needle and syringe, but once they have been injected it is very difficult to control their effects. There are no specific antidotes for many of these drugs and recovery from anaesthesia depends on redistribution of the drug from the blood to the tissues or its metabolism or a combination of both processes.

There are many injectable anaesthetic drugs in use, ketamine, propofol, pentobarbital, methohexital, thiopental. The following notes on a few injectable anaesthetics highlight some important features or exceptions from expected effects. Full details on the activities of the drugs in particular species should be obtained from the veterinarian.

Ketamine

• Poor analgesia in most laboratory species and should not be used alone
  
• Increased muscle tone
  
• Many reflexes remain although animal is unresponsive to pain (e.g., swallowing and blink reflexes)
  
• Usually used in combination with another drug (e.g., xylazine, diazepam)
  
• Duration of anaesthesia depends on dose.

Sodium Pentobarbital

• Narrow safety margin
  
• Poor analgesia until animal is completely unconscious
  
• Excitation during the recovery phase
  
• Gives up to 60 minutes of anaesthesia
  
• Controlled drug status

Urethane

• Provides long periods of surgical anaesthesia with little respiratory depression
  
• Urethane is carcinogenic
  
• Animals should not be allowed to recover from urethane anaesthesia

Local Anaesthetic Agents

Local anaesthetics are dealt with in more detail in the Analgesia module. They are used particularly for pain relief following surgical procedures in small animals. They are also employed for regional anaesthesia in larger animals (e.g., sheep and cattle for procedures such as dehorning, castrations and caesarean sections). Frequently they are supplemented with tranquilizing drugs to help provide restraint.