The use of adjuncts to inhalants for better anesthetic management (Proceedings)

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 example.

     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.

Inhalant agents

Isoflurane and sevoflurane are the inhalant agents available in veterinary practice. Both will cause cardiopulmonary depression in a dose-dependent manner. Critically ill patients seem to be more sensitive to the depressant effects of the inhalant agent. The inhalant agent will cause unconsciousness and some degree of muscle relaxation. With the use of these adjuncts, the vaporizer setting will be lower than the settings we use in typical general anesthesia using only inhalant agent. Using sevoflurane as the inhalant agent, it may actually reduce the total cost of anesthesia for the practitioners.

Adjuncts to inhalant agent

Opioids

The opioids that are used in balanced anesthesia include fentanyl, morphine, methadone, hydromorphone, and remifentanil. These opioids are classified as full agonists. Full agonists are more efficacious analgesic compared with the agonist-antagonists and partial agonist. The full agonists are known to reduce the minimum alveolar concentration (MAC) of the inhalant agents.

An opioid should be given as a premedicant. During maintenance, the opioid can be given as a CRI or intermittent IV boluses. Fentanyl and remifentanil are preferably given as a CRI because of their short duration of action. Morphine and hydromorphone can be given either as a CRI or boluses. The dosages are given in Table 1.

Table 1. Dosages of opioids that can be used as adjuncts

When giving other injectable agents like ketamine and/or lidocaine during maintenance, the author does not reduce the dose of the opioid. When giving multiple drugs, it should be the vaporizer setting that should be adjusted when trying to balance the anesthetic technique.

Ketamine

Ketamine is an NMDA antagonist that can be administered during anesthesia as part of balanced anesthesia. It is considered as an adjunct analgesic to prevent "wind-up". However, it has been shown in dogs that it also decreases the MAC. In most cases, ketamine is given with an opioid and lidocaine as a CRI during anesthesia. Its use with the other injectable agents just mentioned is very useful in patients undergoing surgical procedures associated with severe pain. The dose of ketamine during surgery is 10 ug/kg/min (0.6 mg/kg/hour). If ketamine is not used as induction agent, the bolus of 1.0 mg/kg should be given before starting the CRI. Its administration can continue until the postoperative period. The dose during the postoperative period is 3-5 ug/kg/min (0.18-0.3 mg/kg/hour).

IV lidocaine

Lidocaine is a local anesthetic that can be administered as a CRI as part of balanced anesthesia. It has been shown that IV lidocaine can reduce the MAC of isoflurane. It can be used by itself during anesthetic maintenance in neurological cases (for neurophatic pain), abdominal procedures, and surgical procedures with mild to moderate pain. If the pain involved is severe, it is prudent to give an opioid and ketamine with lidocaine. The bolus dose for lidocaine in dogs is 1.0-2.0 mg/kg and the CRI rate is 50-80 ug/kg/min. We avoid the use of lidocaine in cats because of reported significant cardiovascular depression associated with the use of IV lidocaine in cats.

Local anesthetic techniques

There are many local anesthetic techniques that can be performed to balance the anesthetic technique. These are performed before surgery. Epidural administration of local anesthetic and morphine can be used in hind leg procedures and abdominal surgeries. A brachial plexus block can be used for front leg procedures. There are specific nerve blocks that can desensitize certain areas. For example, maxillary nerve block can be performed in maxillectomies. Retrobulbar block will be useful for enucleation. Practitioners should utilize these blocks most especially in procedures associated with severe pain. When using bupivacaine, it is important to think about the toxic dose when performing these techniques. We try to limit our total dose to 2.0 mg/kg.

Neuromuscular blockers

Atracurium is the most common neuromuscular blocker we use in practice. It can be given at 0.1-0.25 mg/kg IV just before the start of the surgical procedure. Neuromuscular blockade can be maintained with repeated administration of atracurium or by CRI. For a CRI, the dose is 0.5 mg/kg/hour. The main advantage of atracurium is its unique degradation in the body. Patients with hepatic and renal diseases can be given atracurium because it is degraded by the Hofmann reaction which does not require function of the liver and the kidney.

It will be useful to have a nerve stimulator to monitor the degree of neuromuscular blockade. It is important to remember that the presence of breathing does not rule out some degree of neuromuscular blockade. Even if the animal is breathing, some neuromuscular blockade will weaken the diaphragm resulting in hypoventilation and possibly hypoxemia.

Edrophonium, an anticholinesterase, is the reversal agent we use because it has less muscarinic effect than neostigmine. The patient should show some signs of recovery from neuromuscular blockade before edrophonium is administered. The dose is 0.5 mg/kg given IV at a slow rate.

Specific cases

Here are some cases where adjuncts are used during general anesthesia. Application of these techniques should be considered in surgical procedures associated with severe pain and in critically ill patients.

Intraocular procedures (e.g. Phacofragmentation)

Premedication

     • Acepromazine – 0.02-0.05 mg/kg IM (depending upon the age; older dogs, lower dose)

     • Buprenorphine – 0.01-0.02 mg/kg IM

Induction

     • Propofol (4.0 mg/kg given to effect)

Maintenance

     • Isoflurane or sevoflurane

     • Lidocaine – Bolus 1-2 mg/kg IV; CRI 50 ug/kg/min

     • Atracurium – 0.1-0.25 mg/kg; Maintenance: half of the original dose or CRI at 0.5 mg/kg/hour

     • Retrobulbar block using lidocaine and bupivacaine instead of using the neuromuscular blocker. Retrobulbar block produces a central eye and analgesia.

Pelvic fractures

Premedication

     • Dexmedetodine – 5.0-10.0 ug/kg IM (use in stable patients and those without a cardiac problem)

     • Morphine 0.5-1.0 mg/kg IM or Hydromorphone 0.1 mg/kg IM

Induction

     • Diazepam and ketamine or

     • Propofol

     • Epidural morphine and bupivacaine – if the lumbo-sacral junction can be felt easily

Maintenance

     • Isoflurane or sevoflurane

     • Morphine - 0.1-0.3 mg/kg/hour

     • Lidocaine – 50 ug/kg/min

     • Ketamine – 10 ug/kg/min

Septic patient

Premedication

     • Hydromorphone – 0.05 mg/kg or other opioid of choice

Induction

     • Diazepam and ketamine

     • or

     • Neuroleptanalgesic combination – diazepam 0.2 mg/kg IV and hydromorphone 0.2 mg/kg IV; if this is the technique chosen, premedication with hydromorphone does not have to be done

     • or

     • Diazepam 0.2 mg/kg IV and Fentanyl 10 ug/kg IV

Maintenance

     • Isoflurane or sevoflurane (lower concentration)

     • Fentanyl CRI 0.5-2.0 ug/kg/min

     • Lidocaine CRI 3 mg/kg/hour after a bolus of 1.0-2.0 mg/kg IV

Summary

Using adjuncts during inhalant anesthesia is a technique that can be utilized in critically ill patients, healthy patients responding poorly to inhalant and patients in severe pain. It also can be used in surgical procedures with particular requirements like a central eyeball or excellent muscle relaxation. The goals of this technique are to minimize the profound cardiopulmonary depressant effects of inhalant agent and to maintain a stable anesthetic depth. The choice of drugs or techniques to balance anesthesia depend on the procedure involved as well as the degree of pain.