Balanced anesthesia
Balanced anesthesia is an anesthetic method for surgical patients during their operation, which was proposed by John Lundy in 1926. The purpose of balanced anesthesia is not only to be less dangerous than using only one drug for general anesthesia but also to minimise the potential adverse side effects which may be caused by the anesthetic agents. The concept of balanced anesthesia is that of applying two or more medications or techniques in order to ease pain, relax the muscles, and have autonomous reflexes suppressed in the patient. In other words, it is an anesthesia method to maintain stable vital signs. There are numerous factors that come into play when the anesthetist decides to use this method of anesthesia. These factors include, but are not limited to: patients' major organ functions, general condition and compensatory capacity. By making use of adequate types and appropriate amounts of agents and accurate anesthesia methods, the anesthetist will promote a successful, safe, and efficient surgery.

Principles of pharmacokinetics
The scope of pharmacodynamics is the effects caused on the body by a medicine. However, the distribution of medicine, clearance and the concentration of medicine in tissue, blood or plasma are part of the pharmacokinetic features of the medicine. Using the anesthetic injection medicine, which is a portion of balanced anesthetic techniques, can be made like a single intermittent dose or as a single injection. It should keep a constant rate infusion (CRI) during the time. Both the foreseeable pharmacodynamic effects and foreseeable concentration of plasma can be offered by the CRI of specific medicine. It has similarity on keeping the invariable concentration of end-tidal by using the vaporous precise device, which can provide the volatile anesthetic.

When the administration rate exceeds the clearance rate, a stable-state concentration has been achieved by delivering the medicine as a CRI. In addition, if the medicine has distributed fully at equilibrium in the body, which is called the volume of distribution at a stable state. In case the loading dose was administered after the CRI, the time period of which will keep the concentration at a stable state equals 3 time constants or  5 terminal half-lives of the specific medicine. The bolus dose can full with the volume of the medicine in an efficiently and effectively way in order that the medicine can be cleared and delivered. This also can promote to achieve the stable state in a prompter approach.

Administering a CRI has two important methods: targeting the infusion rate, and making the infusion rate constant.
 Make the infusion rate constant
It is relatively simple to make the infusion rate stay stable and supposed that the stable-state concentration is gained on the basis of proper distribution. Whereas, due to the saturated state of tissues and the same rate of infusion, it has the possibility to exceed the clearance rate and cause a higher concentration in plasma than the expected concentration. Hence, for this modality, it is necessary to adjust the CRI as the time goes by.
 Target the infusion rate
This type of infusion rate is changed based on the specific rate constants. These rate constants control the movements of medicine in the compartments on the basis of saturated state of the tissues, which is decided by the study population in prior pharmacokinetic studies. Because a large amount of knowledge about the specific pharmacokinetic constants  and the target-controlled infusion system which is made up of the computer program and the syringe pump needs to be known, it is extremely difficult to set up the target infusions in clinical conditions. Therefore, making the infusion rate constant are applied more widely than targeting the infusion rate.

Frequently used anesthetic agents
The quantity of a single anesthetic which is used for balanced techniques has similarity with that which is used for standing sedation. However, compare to the doses used for TIVA (total intravenous anesthesia), which is always lower than using the single anesthetics. The doses of anesthetics required differ, and depend on the required duration of anesthesia, the requirements for anesthesia to volatile, expected pain of injection of anesthesia, the experience the anesthetist using various medicines, and other factors.

According to a latest review of using the injection of anesthesia from the American association, for short-duration anesthesia, whose duration is around 20 minutes, the recommended induction method is to use xylazine as a sedative medicine. In addition, diazepam and ketamine are recommended after the xylazine.  For longer duration anesthesia, whose duration is over 30 minutes, the most common anesthetics is the combination of guaifenesin, ketamine, and xylazine or isoflurane.

The pharmacokinetics of the two most common anesthetic agents, xylazine and ketamine, used during the operation are:

Xylazine

Xylazine is the most widely anesthetic agent used for short-duration operations.

Pharmacokinetics

Pharmacokinetics of xylazine may be influenced by anesthesia since after an intravenous therapy about 1.1 mg/kg, the half-life of xylazine will increase to 118 minutes and the clearance will decrease to 6 mL/kg/min. Based on a recent study, if injecting the morphine, which is 0.1 or 0.2 mg/kg, in the vein at the same time can extend the terminal half-life to about 150 minutes and the clearance will not be influenced.

Ketamine

Ketamine is the most widely anesthetic agent used for longer duration operations.

Pharmacokinetics

After an intravenous therapy, which is about 2.2 mg/kg, mixed with the 1.1 mg/kg xylazine the half-life of xylazine is approximately 66 minutes and the clearance is around 31 mL/kg/min when patients are halothane-anesthetized. If only managing the xylazine and ketamine, the terminal half-life will be 42 minutes and its clearance will be 27 mL/kg/min. When the CRI of ketamine is kept stable for an hour at 2.4 mg/kg/h, the terminal half-life will be 46 minutes and the clearance will be 32 mL/kg/min.

Advantages of balanced anesthesia
Compared with general anesthesia, balanced anesthesia has various advantages. In some cases, applying balanced anesthesia is considerably cheaper than the usual anesthesia. Secondly, it can reduce the death rate. Furthermore, it offers more stable operating conditions for veterinarians.

Balanced anesthesia can not only decrease the risk to patients from the operation, but also increase patient's safety and comfort. The 3 main advantages of balanced anesthesia are making patients calm, minimizing the pain patients suffer, and decreasing the adverse effects using the anesthetic agents.

Balanced anesthesia can make patients calm by using drugs such as: medetomidine, diazepam or midazolam, and acepromazine. It is considerably important to make patients calm before the surgery because it can decrease the difficulties to work out the issues and decrease the patients' stress. Stress may result in unpredictable consequences, including tachypnoea, hypertension and tachycardia. These may be harmful to the anesthetized patients. In addition, anxiety and stress may cause the nociceptive pain. Therefore, using balanced anesthesia can keep patients clam, which can lead the operation's success.

Another advantage of using balanced anesthesia is that it can decrease the chance of adverse effects. All medicines may have adverse effect on patients; some serious adverse effects of anesthesia may be caused by inhalant anesthetics, although in general these medicines are highly safe and useful. Using the correct amount of balanced anesthetic agents, the adverse effects can be reduced by some extent.

Balanced anesthesia can also minimize the pain patients suffer. Pain may delay wound healing, decrease appetite, and even result in death. Using the proper amount of analgesics can reduce the amount of inhalant anesthetics required and help patients reduce the pain.

Choosing to use balanced anesthesia requires an evaluation of the operation and patient. Choosing the suitable type and amount of anesthetic agents is the best method for an operation.

Technique in cats and dogs

The technique of balanced anesthetic has been applied widely with cats and dogs.

It is not necessary to use general anesthesia in veterinary surgery for cats and dogs. The most common method of general anesthesia for cats and dogs is inhalant anesthetic agents because they are both easy to manage and the depth of anesthesia can be predictable. In addition, the depth of anesthesia can be changed and recovery in time if some unexpected situation happened suddenly. Although inhaled anesthetics will cause an unconscious state in which cats and dogs will not recall or perceive pain, the depth of anesthesia may not prevent the variety of reflex reactions to harmful stimuli which may happen during the operation. In order to prevent these reflex reactions, it may be required to increase the concentration of inhalant anesthetic agents, which may result in serious cardiovascular and respiratory depression for young patients. High amounts of inhalant anesthetic agents will make the patients who have serious systemic disease be depressive and even increase the morbidity and mortality. With the balanced anesthetic technique, the low concentration of inhalant anesthetic agents and other medicines used during the operation can alter the perception of painful stimuli. In other words, using balanced anesthetic techniques for cats and dogs can decrease the morbidity and mortality effectively. Therefore, in this situation, using balanced anesthetic techniques in cats and dogs is less risky for operation than using the general anesthesia. According to a report from a teaching hospital, the rate of complications resulting in death in cats and dogs using the balanced anesthesia are relatively low, at 1/9 and 1/233 respectively.

References

Anesthesia