Pre-Anesthetics

There are several pre-anesthetic drugs, or drug combinations to choose from. The table below illustrates several of those possibilities. Balanced pre-anesthetic protocols include drugs from different classes, used with the specific purpose of producing mild sedation, decreasing anxiety, providing analgesia, increasing muscle relaxation, decreasing saliva and respiratory secretions and to suppress vomiting and regurgitation. Additional reasons to use pre-anesthetic drugs include the reduction of unwanted drug effects, the ability to decrease doses of induction and maintenance drugs, the provision of a smooth anesthesia induction and recovery, and to contribute to post-operative analgesia. Moderate dosing, using combinations of different classes of drugs, is preferable to the administration of a large dose of a single drug. Premedication combinations may include anticholinergics (if appropriate), benzodiazepines, opioid analgesics, phenothiazine tranquilizers, alpha-2 agonists, or dissociative drugs.

Premedication protocols: combine drugs from each category

Single Injections of Sedative or Analgesic Drugs

Administer a single injection of an analgesic drug at the painful points of the procedure (see table below).

Maropitant (Cerenia®)

Maropitant is a neurokinin (NK1) receptor antagonist that blocks the action of substance P in the central nervous system. Maropitant has been shown to decrease sevoflurane MAC in dogs undergoing ovariohysterectomy by 24% after IV injection of 1 mg/kg followed by 0.03 mg/kg/h CRI. Another study in dogs showed sevoflurane MAC was reduced by 30% after 5 mg/kg IV followed by 0.15 mg/kg/h CRI. In cats undergoing ovariohysterectomy, a single injection of 1 mg/kg IV reduced sevoflurane MAC by 16%. The same study showed no further decrease in MAC after a single injection of 5 mg/kg IV. A maropitant CRI was not administered in the feline study.

Midazolam CRI

Midazolam, a benzodiazepine, may be administered as a CRI to produce muscle relaxation or mild sedation. It is usually administered in combination with fentanyl or any of the opioid CRIs listed below.

Combined Drug Analgesic CRIs

A continuous infusion of analgesic drug has several advantages over intermittent boluses. This method of administration avoids "peak and valley" plasma drug concentrations, potentially slow onset of analgesia, and allows a continuous reduction in vaporizer dial setting.

Drugs used as CRIs during anesthesia to reduce MAC

Epidural Analgesia

The area over the desired site of needle placement is shaved and surgically scrubbed. Strict aseptic technique must be followed for epidural injections or catheter placement. In dogs and cats, the most common site for epidural injection is at the lumbo-sacral space (L7–S1). Tuohy needles accentuate the pop through the ligamentum flavum more than spinal needles, indicating entrance into the epidural space. The loss of resistance technique using a saline filled syringe, or "hanging drop" technique may aid in identifying placement into the epidural space. If the intrathecal space is entered inadvertently, the dose of analgesic is decreased by 50% and the local anesthetic dose is decreased by 90%.

Do not exceed epidural volume of 6 ml total in any dog.

Single Injection Epidural Protocols: Dogs

1.  Preservative free morphine 0.05 to 0.2 mg/kg diluted in 0.9% saline to give a final total volume of 0.1 to 0.2 ml/kg

2.  Preservative free morphine 0.05 to 0.2 mg/kg plus an equal volume of 0.5% bupivacaine

3.  Methadone 0.1 mg/kg diluted in 0.9% saline to give a total volume of 0.1 to 0.2 ml/kg

4.  Preservative free morphine 0.1 mg/kg combined with dexmedetomidine 0.0005 mg/kg

5.  Oxymorphone 0.05 to 0.1 mg/kg with or without a local anesthetic

6.  Hydromorphone 0.03 to 0.1 mg/kg with or without a local anesthetic

7.  Fentanyl 5 to 30 mcg/kg with or without a local anesthetic

8.  Buprenorphine 0.005 to 0.03 mg/kg with or without a local anesthetic

Single Injection Protocols: Cats

1.  Preservative free morphine 0.1 mg/kg diluted in 0.9% saline to give a final total volume of 0.1 ml/kg

Epidural Catheter

Epidural catheters are placed most commonly for chronic or severely painful procedures. Examples include chronic perineal hernias, cranial abdominal tumors (pancreatic), extensive degloving injury of the hind limbs, multiple rear limb fractures, etc. They may also be placed for thoracic surgery and used in combination with a local or regional block: intercostal blocks, wound soaker catheter. For cranially placed catheters, bupivacaine or other local anesthetics are not included.

Drug Protocols for Epidural Catheters: Dogs

1.  Preservative free morphine at 0.1 mg/kg for intermittent injections every 4 h

2.  Preservative free morphine 0.1 mg/kg/24 h and lidocaine 0.2 mg/kg/h

3.  Fentanyl loading dose 0.5 to 1 mcg/kg then 0.5 to 1 mcg/kg/h with bupivacaine 0.25 to 0.5 mg/kg/h

4.  Morphine loading dose 0.03 to 0.05 mg/kg then 0.003 to 0.008 mg/kg/h

Local Anesthetics

The local anesthetics are tertiary amines that inhibit the generation and conduction of nerve impulses by blocking voltage-gated sodium channels. Myelinated and smaller nerve fibers seem to be more susceptible to blockade compared to large nerve fibers. Autonomic and pain fibers are blocked before motor and other sensory fibers. Systemic absorption is determined primarily by dose and site of administration. Uptake from mucosal surfaces is rapid and almost complete. With SQ administration, uptake is much slower. The principle metabolic pathway of local anesthetic is hydrolysis by plasma cholinesterase with urinary excretion of metabolites. Plasma cholinesterase is produced in the liver; therefore, hepatic function is somewhat important for local anesthetic metabolism. To increase nerve contact and spread of local anesthetic in tissues, volumes can be increased using physiologic saline or other compatible fluids for dilution.

Local anesthetic drugs. All doses in mg/kg

Extending Duration of Analgesia

Buprenorphine 0.003 mg/kg added to local anesthetic blocks of lidocaine and bupivacaine doubles the duration of analgesia. The addition of morphine 0.075 mg/kg or hydromorphone 0.015 mg/kg to bupivacaine local anesthetic blocks doubles the duration of analgesia.

Local Block Techniques

Splash Block

The application of local anesthetic directly to the wound surface. Topically administered local anesthetic does have some benefit. Total doses should not exceed those recommended above. To increase available volume of solutions, local anesthetic can be diluted in 0.9% NaCl to increasing surface area contact.

Local Infiltration

Local anesthetic is injected into the tissue surrounding the surgical area. This technique can be effective when anatomy for specific conduction blocks may be odd (i.e., brachycephalic dogs and cats).

Conduction Nerve Blocks for Dentistry

There are several texts with excellent diagrams that demonstrate the landmarks for conduction nerve blocks.

Maxillary Nerve Block

Maxillary nerve block desensitizes the maxilla, upper teeth, nose, and upper lip.

Infraorbital Nerve Block

The infraorbital nerve block desensitizes the upper lip, nose, roof of the nasal cavity, and related skin ventral to the infraorbital foramen.

Inferior Alveolar Nerve Block

The inferior alveolar branch of the mandibular nerve is blocked desensitizing the cheek teeth, canine, incisors, skin and mucosal of the chin and lower lip.

Mental Nerve Block

The mental branch of the mandibular nerve is blocked desensitizing the lower lip.

Local Anesthetic Blocks for Feet and Toes

Conduction nerve blocks are an excellent way to manage the acute pain experienced by patients undergoing procedures of the feet or toes. Several hours of comfort can be provided while additional pain control methods for longer term analgesia are commenced.

Intravenous Regional Anesthesia (IVRA)

Commonly termed a Bier Block, intravenous regional anesthesia can be used for controlling intra-operative pain of the distal extremities. The effects of IVRA are short lived. Once the tourniquet is removed, sensation returns within 10 to 15 minutes and analgesia persists for only about 30 minutes post-op. Strict adherence to local anesthetic dosing is necessary because a relatively large amount of local anesthetic enters the general circulation after the tourniquet is removed. An Esmarch bandage is applied to desanguinate the extremity and the cuff tourniquet is applied proximally to the bandage. The bandage is removed and local anesthetic solution is injected IV distal to the tourniquet. The tourniquet cuff pressure should be measured (using a sphygmomanometer) and inflated to apply 100 mm Hg pressure over the measured systolic blood pressure in that limb. The paw is desensitized within 10 minutes. Maximum time for cuff inflation is 90 minutes.

Ring Block

Local anesthetic is injected circumferentially around the limb at the middle of the radius/ulna or tibia/fibula.

Conduction Block for the Foot

The radial, median, and ulnar nerves are blocked to desensitize the distal foot and toes.

Adverse Effects of Local Anesthetics

Local anesthetics have a relatively high margin of safety when used correctly. Inadvertent overdose and accidental intravenous administration are the two most common causes of toxicity. Doses need to be calculated carefully and the appropriate size syringes should be used for drawing up drug. The hub of a one ml syringe may hold up to an additional 0.1 cc. In a small dog or cat, administration of this additional amount may be enough to lead to toxicity. For drawing small volumes that will be subsequently diluted, a U100 insulin syringe, which has no hub, is the most accurate and safest method of measuring drug.

Systemic reactions to local anesthetics involves the CNS and cardiovascular systems. CNS toxicity occurs at lower doses that cardiac toxicity. Muscle twitching and seizure are usually the first signs of toxicity. If large doses are given, seizures are followed by unconsciousness, coma, respiratory arrest, and death. Cardiovascular signs of toxic overdose are peripheral vasodilation, hypotension, sinus bradycardia, sinus arrest, ventricular arrhythmias and cardiovascular collapse.

Other reactions to local anesthetics include direct tissue damage, especially from solutions containing vasoconstrictors used in extremities with limited collateral circulation (i.e., paws), methemoglobinemia, allergic reaction, and anaphylaxis.

Acute systemic toxicity produced by inadvertent intravenous administration can be treated successfully by supporting cardiovascular function with administration of IV fluids, providing IPPV, oxygen supplementation, and controlling seizures.