Maintenance of anesthesia using an inhalant agent has been associated with cardiopulmonary depression. Some small animal patients
are more sensitive to the cardiopulmonary depressant effect of inhalant agents. Providing adjuncts to inhalants indicates
the addition of drug(s) and/or technique(s) before and during anesthetic maintenance to minimize the detrimental effects of
the inhalant to the patient. A term that is coined for this technique is balanced anesthesia. The idea behind this technique
is to take advantage of the desired effects of each drug in small amounts. By doing so, the goal is to minimize the side effects
associated with each drug.
General anesthesia is composed of three major components: unconsciousness (narcosis), muscle relaxation, and analgesia. Choosing
drugs that have these primary effects is considered balanced anesthesia. Using an inhalant agent for unconsciousness, an opioid
for analgesia, and a neuromuscular blocker for muscle relaxation is a good example of "balanced anesthesia." In veterinary
practice, a neuromuscular blocker is rarely used and the real definition of balanced anesthesia cannot be applied. Presently,
balanced anesthesia in veterinary practice implies the concurrent use of an inhalant agent, injectable agent(s), specific
local anesthetic technique, and the administration of drugs epidurally.
Advantages of using adjuncts to inhalant
1. Using adjunctive methods minimizes the cardiopulmonary depressant effects of inhalant agents. This advantage appears
to be more applicable to critically ill patients which appear to be more sensitive to cardiovascular and respiratory depression
caused by inhalant agents. By using this technique, the anesthetic requirement for inhalant agent will be lowered.
2. Maintenance of stable anesthetic depth is relatively easier when using adjuncts during inhalant anesthesia. This technique
is meant to prevent a "roller coaster" type of anesthetic maintenance.
3. By incorporating an analgesic agent during anesthetic maintenance with inhalant agent, analgesia will be provided
up to the postoperative period. The inhalant agents we use today (isoflurane and sevoflurane) do not provide analgesia. Without
any injectable analgesic, animals will wake up in pain. This can result in more problems with controlling the acute pain as
well as the possibility of chronic pain syndromes.
4. Injectable anesthetic agents have been shown to cause less stress response in patients compared with patients receiving
inhalant agents. Since an inhalant agent is administered with injectable agent(s) in balanced anesthesia, the stress of anesthesia
and surgery will be decreased.
5. By using balanced anesthesia, the conditions for some surgical procedures can be optimized. An intraocular procedure
that requires a central eye can be achieved safely with a combination of inhalant agent and a neuromuscular blocker. Orthopedic
procedures that require excellent muscle relaxation can be performed with a combination of an inhalant agent, a neuromuscular
blocker, and an analgesic.
Disadvantages of using adjuncts to inhalant
6. When adjuncts are used during general anesthesia, polypharmacy is being employed. When anesthetic complications occur,
it is more difficult to ascertain the cause of the adverse reaction. An anaphylactoid reaction during anesthesia is a good
7. Extra equipment (ventilator) may be utilized if a neuromuscular blocker is part of balancing the anesthetic technique.
8. There is a greater chance of drug dose miscalculation since more drugs will be used. Most of the drugs will be given
as a constant rate infusion (CRI) which can create confusion during calculation which may potentially, lead to overdosing.
9. When neuromuscular blocker is used as an adjunct to balance the anesthetic technique, ensuring an adequate plane of
anesthesia may pose a challenge to the person watching the patient during anesthetic maintenance. This can become a problem
in critically ill patients that are hemodynamically unstable during anesthesia. The tendency is to decrease the vaporizer
setting to optimize the arterial blood pressure. In doing so the patient may wake up but since the patient has a neuromuscular
blocker, it will not move on the surgical table.