Introduction

Anesthesia is a condition of temporary loss of pain. It’s induced by using certain drugs, either for medical purposes such as surgery, skin biopsies, screening, dental work, and diagnostic tests or for animal research [1].

Scientists use anesthesia to prevent animals from feeling any pain and stress during experimental clinical studies of drugs or the development of regenerative medicines – this prevents pain and stress from altering the quality of the produced results.

A class of drugs called anesthetics induces anesthesia. Popular anesthetic agents used in lab animals include ketamine, propofol, isoflurane, and halothane[1]. Usually, after administration, the vital parameters (of the lab animals) are monitored for side effects or health conditions.

Also, the administration of the anesthesia differs based on the type of pain relief needed. For example, they can be delivered to animals either by inhalation, spray, eye drops, injection, topical lotion, or skin patch.

This article reviews the types of anesthesia, anesthetic agents, general components of anesthesia machines, and the side effects of anesthesia on animals.

Different Types of Anesthesia

Based on the different effects each type of anesthesia has on the animals, they are categorized into three:[1]

●     General Anesthesia

In general anesthesia, the whole body is rendered unconscious and immobile. It’s mainly used during more invasive and time-consuming procedures like surgery.[1] General anesthetics are given either as inhaled gas which takes longer to wear off, or administered directly through the bloodstream, which starts working immediately but rapidly wears off.

●     Regional Anesthesia

Regional anesthesia is used to numb a larger part of the body, like everything below the waist or the arm or leg. It’s generally used during minor surgeries[2]. It’s safer and doesn’t have many side effects and complications like general anesthesia and sedation. Examples are epidurals and spinal blocks used during cesarean deliveries.[2].

●     Local Anesthesia

Local anesthesia is used to numb even smaller areas than that numbed by regional anesthesia, such as during dental work or stitching minor wounds.[3] At the time of the procedure, the administered person remains awake. Unlike general anesthesia, the side effects and complications generated due to local anesthesia are rare and minor.

Common Anesthetic Agents

The best anesthetic agents effectively reduce pain with the least side effects and soothe the recovery process.[4] Anesthetics are categorized into the following groups based on their means of administration:

1. Inhaled (or Volatile) Anesthetic Agents

Inhaled anesthetic agents are generally supplied as liquids;[3] thus, they require a vaporizer and carrier gas (like oxygen) to be administered to animals. To determine the right type and dosage of volatile anesthetics for any animal you’re working on, consult with an OAR or IACUC veterinarian.[4]

Furthermore, if the animal is deeply anesthetized, the anesthetic agents are quickly removed through the animal’s lungs and scavenged carefully to minimize its exposure to people nearby.

Commonly used inhaled anesthetic agents are:[5]

    • Halothane: It’s highly volatile and requires a calibrated vaporizer for its administration to animals. Except after deep anesthesia, animals take around 1-3 minutes to recover from this anesthetic. Its side effects include malignant hyperthermia and rare hepatitis.
    • Nitrous oxide: It comes in the form of a gas cylinder and is used in combination with other anesthetic agents, such as isoflurane and propofol. The gas should be carefully used on ruminants, and side effects include minimal cardiovascular and respiratory depression.
    • Isoflurane: It’s highly volatile and is administered to animals using a calibrated vaporizer – this prevents exposure to high concentrations of the gas. It takes 1-3 minutes for recovery, and the drug must be appropriately scavenged after use. Side effects include respiratory depression, hepatitis (low risk), and little hepatic metabolism[5].
    • Sevoflurane: It’s one of the commonly used volatile anesthetic agents. Sevoflurane is often delivered to animals in a mixture of oxygen and nitrous oxide. Though the drug is safe to administer, it has certain potential side effects, such as hepatotoxicity and accelerated Alzheimer’s.

2. Injectable Anesthetic Agents

These are anesthetic drugs administered through a needle or syringe,[5] and once injected, the effects of these drugs are difficult to control. The recovery of animals from this anesthesia depends on the blood-to-tissue transfer of the drug, its metabolism, or both. Examples are:[5]

    • Ketamine: The drug is usually used with another drug, such as xylazine or diazepam. The period of anesthesia depends on the drug’s dosage. However, though the animal is unresponsive to any pain, some reflex-like swallowing and blink reflexes remain active.
    • Sodium pentobarbital: It rapidly begins to work after injection, but it has a shorter duration of action. It provides up to 60 minutes of anesthesia, and at a higher dose, it can cause cardiac arrest.[6]
    • Urethane: It provides a longer period of anesthesia and is generally used in surgeries. However, the drug is carcinogenic, and the animal administered with the drug should not be allowed to recover from anesthesia.[5]

How to Anesthetize Lab Animals - Procedures

The type and age of an animal greatly determine the approach and drug dosage to anesthetize the animal. For example, anesthesia is induced using anesthetic gas in small lab animals.

Below are procedures on how to anesthetize animals based on the type of anesthetic agents used.[5]

1. For Inhalation Anesthesia

  • Put the animal in a chamber and flood the chamber with an anesthetic gas mixed with a carrier gas at a suitable concentration (based on the species and age of the experimenting animal).
  • After anesthetizing the animal, open the chamber in a fume hood.
  • Remove the animal from the chamber and place a nose cone or other anesthetic apparatus to prolong the anesthesia duration.[5]
A lab animal in an anesthesia chamber

Figure: An image of an animal being anesthetized in a chamber with volatile anesthetics.[5]

Sometimes, masks are put on animals’ noses and mouths to breathe in the anesthetic. However, it’s challenging to apply to large animals, and some animals might hold their breath during anesthesia procedures.

2. For Injectable Anesthesia

Injectable anesthesia can be given intraperitoneally, intravenously, intramuscularly, or subcutaneously. For example, for regional anesthesia, the drugs are injected into the subarachnoid or epidural spaces.

Below are some approaches used to deliver injectable anesthetics to animals:[5]

  • Aquatic animals are generally anesthetized by adding anesthetics to water and allowing the animals to swim until they are entirely anesthetized. E.g., tricaine methanesulfonate is used to anesthetize frogs and fish.
  • To anesthetize amphibians and reptiles, hypothermia is induced using certain anesthetics or analgesics. A neural tissue temperature below 9°C produces unconsciousness by blocking transmissions to the brain and central nervous system.[5]

There are also some modern anesthesia machines available to anesthetize animals. They mix the anesthetics and other gasses and keep the animal in anesthesia during the whole procedure[7].

Anesthesia Machine: Multimodal Output

Figure: The image of a Multi-output Small Animal Anesthesia Machine.

Although the machine has hundreds of parts, some critical components mentioned here have a significant role in facilitating the procedure[7].

  • Oxygen flowmeter: It keeps a consistent oxygen flow at a set point.
  • Oxygen source: Pressurized air tanks and airlines help the animals breathe.
  • Patient breathing circuit: It’s a combination of hose and face mask fitted over animals’ noses and mouths to deliver the anesthetics.
  • Vaporizer: It transforms the liquid form of the anesthetics to vapor to help the animals inhale it.
  • Scavenging system: It helps to prevent the anesthetic contamination in the room while removing excess gasses from the machine
  • Heads-up display: It’s a monitor showing the consistency and flow of the gasses.

Factors to Consider When Selecting a Method of Anesthesia

Several factors impact the effect of anesthetic drugs or induced anesthesia in animals. Some of these factors are mentioned below:[5]

    • Species: Different species respond differently to the same dose of anesthetics. Thus, the familiarity with one drug in one species should not be assumed to be suitable for other species because different doses need to be prepared for different animal species, especially for injectable anesthetics. For some other non-mammalian and bird species, the doses of inhalation anesthetics should be considered.
    • Age: Most risks of using anesthetics are observed either in young animals or old ones. The younger ones might take a longer recovery time, and for the old ones, any pathological conditions in the respiratory system may cause increased complications. The inhalation anesthetics are generally used on these groups to get more control over their responses[6].
    • Weight: Fat animals might have more difficulty when given anesthetics than thinner ones. For example, the fat animals might go under hypoventilation (animals unable to breathe) and possess lower circulation and distribution of anesthetic drugs.
    • Health: If animals are having any health complications, such as lung, kidney, or liver disorder, or have gone into surgeries, they might develop more severe complications after their exposure to anesthetic agents.
    • Other factors: Some non-anesthetic drugs can affect the response of anesthetic agents. For example, pentobarbital anesthesia might be extended by the use of Chloramphenicol. Similarly, some antibiotics might attenuate the actions of muscle relaxants.[6]

Side Effects of Anesthesia

Anesthetic drugs also have side effects like other drugs, which should be considered before anesthetizing any lab animal. Below is a list of effects associated with general anesthetics:[5]

    • Respiratory Depression: Anesthetics cause a decrease in muscles’ contractility and loss of muscle tone. This leads to a decrease in respiratory rate and short breaths.
    • Cardiovascular Depression: Direct influence of anesthetics on the heart leads to a fall in blood pressure and a reduction in the heart’s contractility and cardiac output.
    • Loss of Temperature Control: The mechanisms regulating steady body temperatures, such as the temperature-regulating centers in the brain and shivering-like processes are inhibited by certain anesthetic agents. Especially in small animals, the condition leads to hypothermia[5].

Conclusion

Anesthesia is a general procedure used in labs to make animals unconscious and minimize their pain while performing critical experiments on them. A variety of anesthetic agents are available to induce anesthesia and based on their effect, they are categorized into general, regional, and local anesthesia.

Also, based on the mode of delivery, the anesthetic agents are classified into inhalation anesthetics and injectable anesthetics. However, before administering anesthetics to animals, it’s essential to consult a veterinary expert for suitable doses. The doses and types of anesthetics given to animals are based on their species, age, and health condition.

The delivery of anesthetics to animals can be more controlled by using modern anesthesia machines. These machines provide more control over the flow and consistency of gasses and help you to track the records.

If you’re looking for a suitable anesthesia machine to perform your research work, check out our advanced multi-output anesthesia machine!

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