**6. Systemic and toxic effects of local anesthetics**

Accidental intravenous injection of local anesthetics is the most common cause of adverse reaction associated with local anesthetic administration. In severe cases it can cause cardiac arrest.

When the plasma concentration of local anesthetics is excessive, sufficient cardiac sodium channels become blocked so that conduction and automaticity become adversely depressed. For example, excessive plasma concentration of lidocaine may slow conduction of cardiac impulses through the heart, manifesting as increased PR interval and widened QRS complex on the ECG.

Local Anesthesia for Husbandry Procedures and Experimental Purposes in Farm Animals 239

Problems occur through infection (never inject local analgesic through infected tissues), irritation, distortion of the wound, swelling and some delay in post-operative healing.

A variation of infiltration anesthesia designed to minimize these effects is field anesthesia. Here, walls of anesthesia are made by infiltrating the tissues around (rather than at) the

Advantages include absence of distortion of the anatomical features in the line of incision; muscle relaxation and no interference to healing. An example of a field anesthesia technique which is widely used in cattle is the Inverted L or 7 block for anesthesia of the abdominal

Ring blocks whereby the tissue all around a distal organ is infiltrated with local anesthetic, is another form of field anesthesia: examples of where this is used is on the teats of cattle (epinephrine must not be used here, as vasoconstriction could lead to ischemic necrosis and

Spinal anesthesia is the injection of local anesthetic around the spinal cord. When local anesthetics such as lidocaine or bupivacaine are used, all the segmental nerves (sensory and motor) which pass through the anesthetic are paralyzed, although when opioids are used

Epidural (or extradural) anesthesia refers to depositing of local anesthetics into the extradural space. The needle enters the spinal canal, but does not penetrate the meninges.

True spinal anesthesia refers to the subarachnoid access in which the needle penetrates the

For caudal and epidural anesthesia the injection site used is between coccygeal C1 and C2. For a 500 kg bovine 5-10 ml 2% lidocaine will give caudal anesthesia without causing hind

Onset of paralysis of the tail should occur in 1-2 minutes. The block will last 1-2 hours.

By the time 100-150 ml 2% lidocaine is injected, the block will be sufficiently anterior to allow surgery of the hindlimbs, mammary tissue, flanks and abdominal wall. However, the bovine will be recumbent. Injection of local anesthetics can be carried out at the lumbosacral

There is a danger of accidental subarachnoid injection. Segmental epidural anesthesia, where the anesthetic is injected into the epidural space at the region required can be used for analgesia of any 'segment' with less overall side effects. It is more difficult to perform; penetration of the meninges is likely, but in skilled hands it is a very useful technique.

Spinal anesthesia is divided into two types: epidural and true spinal.

The anesthetic is therefore limited to the canal outside the dura mater.

dura mater, and the analgesic is injected into the cerebrospinal fluid.

Larger doses will produce increasingly anterior effects.

junction in order to produce an anterior block with less anesthetic.

surgical site.

**7.4 Spinal anesthesia** 

only sensory block occurs.

**7.4.1 Epidural anesthesia in cattle** 

limb ataxia or paralysis.

sloughing of tissue) or around the limb of cattle.

fossa.

Effects of local anesthetics on calcium and potassium ion channels and local anesthetic induced inhibition of cyclic adenosine monophosphate (cAMP) production may also contribute to cardiac toxicity.

Bupivacaine is more cardiotoxic than lidocaine. It is always important to draw back on syringe to check to be not in vein before injecting local anesthetics.

General overdose depends on blood levels, therefore is influenced by total dose and speed of uptake from the tissues.

As a very rough guide, the toxic dose of lidocaine would be 8 mg/kg and 4 mg/kg of bupivacaine.

In very small animals such as small mammals (goat kids) and birds this amount can be easily exceeded using solutions of standard concentration, so it must be diluted carefully and used with caution.

Signs of overdose are initial sedation, followed with increasing dosage by twitching, convulsions, coma and death.

Reports implicate prilocaine, benzocaine, lidocaine and procaine as causative agents to produce methemoglobinemia in some animals.
