Anesthesia Breathing Circuit



Anesthesia is defined as a temporary state of insensibility or a loss of consciousness. Insensibility can be limited to a region usually by injection of drugs that interfere with the local or regional activity of the nervous tissues (local/regional anesthesia). However, in laboratory practices of anesthetization of animals, general anesthesia is usually preferred. As opposed to local or regional anesthesia, general anesthesia results in loss of consciousness, analgesia, suppression of reflex activity, and muscle relaxation. These features of the latter allow researchers and investigators to undertake surgical procedures with precision, without causing any distress to the animals. Often general anesthesia is induced by inhalation or injectable agents or using a combination of these methods.


Anesthesia administration methods and selection of the agent are usually based on the requirements of the procedures or investigation, species, age, and background of the animal being used. Though the use of inhalation agents is a popular method of anesthesia induction, animals can be anesthetized by intravenous, intramuscular, or subcutaneous injections and oral or rectal liquid anesthesia. Although a single anesthesia agent can achieve all the features of general anesthesia, usually a combination of agents is used to create the overall effect. The primary advantage of using a mixture of agents is that undesirable side effects of anesthetic agents can often be minimized.


Laboratories often utilize anesthesia machines to administer and maintain anesthesia. The system comprises vaporizers, ventilators, anesthetic breathing circuits, waste gas scavenging systems, and pressure gauges. The anesthesia is delivered to the animals from the machine via a face mask, a nasal tube, or an endotracheal tube. The system uses compressed gases that are monitored for their flow and pressure rate using flow meters. These gases make their way through the flow meter to a vaporizer where they are transformed into vapors. The vapors then pass on through the breathing system and finally to the subject. Apart from the delivery system, several other instruments are used to ensure adequate airway management and monitoring of the subject throughout the anesthetization process and anesthesia.


Monitoring processes are crucial to ensure the success of the anesthesia procedure. During anesthetization, monitoring of the animal allows for detection of ineffective anesthesia and prevention of injury. The monitoring process should be continued post-anesthesia to ensure proper recovery and to discover any issues that may interfere with the investigatory requirements. Appropriate recovery care is also an essential part of the process. This approach improves the quality of the research while also ensuring animal safety and welfare.


Performing appropriate and effective anesthesia procedures is critical for successful experimental design. Anesthesia has profound effects on the animal’s consciousness and sensation. It enables the experimenters to perform surgical and experimental procedures accurately and with minimal distress to the animal. Achieving adequate anesthesia can be a complex process. Hence, it is important that the anesthetic agents are carefully selected, and appropriate measures are adopted to minimize the unwanted side effects of anesthesia and surgery.


Efforts to induce a general state of anesthesia can be traced back to as early as 4000 BCE. Prior to the development of modern anesthesia, surgical treatments were often avoided by people due to a lack of pain management. Scientific discoveries of the late 18th and the early 19th centuries eventually led to the development of modern anesthetic techniques. By 20th century tracheal intubation and other airway management techniques had become quite common due to the greatly improved safety and efficacy of general anesthesia.

The use of anesthesia in animals was delayed largely due to the misconception that anesthesia induction in animals was unnecessary and painful. However, this ideology was overcome, and veterinary anesthesia became an integral part of animal-based research and investigation procedures. In the 1800s, the potential anesthetic properties of nitrous oxide were suggested by Sir Humphrey Davy which was eventually demonstrated twenty-four years later by H. H. Hickman who mixed nitrous oxide with carbon dioxide to alleviate pain in dogs during surgery. Other drugs and compounds to induce anesthesia were also evaluated in animals. This included diethyl ether, which was extensively used by C. T. Jackson, and chloroform used by Flourens in 1847. In the United States, Dadd (Dadd, 1854) became one of the first advocates of humane treatment of animals and promoted the use of veterinary anesthesia in veterinary surgery. By the end of the 19th century, different routes for anesthetic administrations such as intravenous anesthesia, rectal anesthesia, and intraperitoneal injection were discovered and minimally evaluated. Though the 19th century saw a promising advancement in local analgesic techniques, it wasn’t until well into the 20th century that general anesthesia and humane surgery were adopted into veterinary practices. It can be said that the true beginning of modern era veterinary anesthesia was initiated by the establishment of anesthesia specialty colleges within North America and Europe in the last three decades of the 20th century


Up until the late 19th century, anesthesia delivery did not require a specialized anesthesia machine. However, the introduction of pressurized oxygen and nitrous oxide cylinders required new systems for mounting and delivery. In the year 1917, Henry Edmund Gaskin Boyle designed a continuous-flow anesthetic machine based on the American Gwathmey apparatus of 1912. Since its initial design, Boyle’s machine has seen many improvements and additions to enhance its functions of oxygen delivery, accurate mixing of anesthetic gases and vapors, patient ventilation, and minimization of anesthesia-related risks to patients and staff.


The continuous advancement and investigations in veterinary anesthesiology have revolutionized biomedical research. The expansive research in the field has led to better treatment of laboratory animals and the creation of pain-assessment scales and therapeutic guidelines.


  • Anesthesia is defined as a temporary state of insensibility or loss of consciousness.
  • The requirements of the research procedures, species, age, and background of the animal being used determine the method and agent used to induce anesthesia.
  • In clinical practices, general anesthesia is preferred over local anesthesia since it results in loss of consciousness, analgesia, suppression of reflex activity, and muscle relaxation.
  • Anesthesia can be induced using inhalant or injectable agents.
  • An anesthesia machine comprises a gas compressor, a regulator, a flow meter, a vaporizer, an anesthetic breathing system, and a waste gas scavenging system.
  • Good anesthesia practices involve appropriate anesthesia management and monitoring techniques, and recovery care.
  • Neuromuscular blocking agents only produce paralysis. Thus efforts to monitor the depth of anesthesia are critical for the humane treatment of the animals.
  • Anesthetization of pregnant animals can affect their fetuses. Anesthetics can result in serious acute and/or long-term effects on the fetus. The residual effects of the drugs in cesarean operation delivered fetus can cause sedation and depression in respiratory and cardiovascular systems.
  • Anesthetization of neonatal animals should take into consideration their increased vulnerability to hypothermia, usually low reserves of energy, possible poor pulmonary and circulatory function, and reduced capacity to detoxify a wide range of anesthetic agents (certain species).

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