Module 10 - Analgesia

Local Anesthetics

Local anesthetics provide pain relief by blocking pain stimuli from reaching the central nervous system (brain and spinal cord).  They differ from the previously discussed opioids and NSAIDs in that they abolish pain rather than diminish it and make it more tolerable.  However, they are limited in their usefulness by the need to reach the activated pain receptors or the nerves leading from these receptors to the brain or spinal cord.  An advantage of local anesthetics is their local activity with few systemic effects.  This allows for local pain relief without distorting other physiological systems that could interfere with the experiment.

The primary use of local anesthetics in analgesia therapy is in the relief of pain in the skin.  Wound edges infiltrated with local anesthetics provide relief for up to about six hours after surgery, the time over which the acute pain of surgical wounds is at its greatest and when there is the greatest need for pain relief.  Local anesthetics may also be infiltrated around nerves if the opportunity arises.  Intercostal nerves, for example, may be blocked to relieve pain following a thoracotomy and to permit better respiration.

Summary of Actions of Local Anesthetics

Duration of Effects

Local anesthetics last for up to six hours, depending on the agent.  The effects will be prolonged if the preparation included epinephrine.  Topically applied local anesthetics may last for less than one hour.

Routes of Administration

Local anesthetics may be given by several different routes depending on the objectives.  Local infiltration may be used to block receptors in the skin and underlying tissues, or to block nerves coursing through the area.  In the latter case, anesthesia (and paralysis) is provided at a site remote from the injection site.  Topical application on mucous membranes or subcuticular structures will effectively block receptors in these areas.  For example, local anesthetic sprays are used to block receptors on the vocal cords to prevent spasm during endotracheal intubation.  When given into the spinal epidural or subarachnoid spaces, all activity in the nerves will be blocked, so that there will be a loss of sensation and a loss of motor function.

Local anesthetics are poorly absorbed through intact skin.  One preparation that is absorbed through skin is a mixture of prilocaine and lidocaine, however absorption is relatively slow and anesthesia is limited to about five millimeters under the skin.  This is enough to anesthetise the cutaneous receptors.  The preparation requires a minimum of 30 minutes to penetrate the skin and will require longer to achieve maximum penetration. This is effective when applied to the surface of the rabbit's ear over the artery or vein and wrapping with a occlusive bandage for 30 minutes before injecting or taking a blood sample.

Local Anesthetics in Combination with Other Drugs

Frequently epinephrine is added to local anesthetics to cause local vasoconstriction and to slow the uptake of the local anaesthetic and increase the effective duration.  Although the amounts of epinephrine are small, they may be sufficient to cause an elevation of blood pressure and heart rate.  This effect is well recognized by dentists who use this type of preparation.

Adverse Reactions to Local Anesthetics

Overdoses of local anesthetics may cause convulsions in people and animals.  In general, the more potent the anaesthetic, the lower the toxic dose.  Procain, prilocaine, lidocaine have low relative anesthetic potencies and high toxic doses while tetracaine and bupivacaine have a high relative anesthetic potency and low toxic doses.  Toxic doses vary between animal species and range from 1-5 mg/kg to 15-25 mg/kg.  These figures usually refer to local anesthetics injected intravenously, however, toxic levels may be achieved following administration by other routes. 

Special care must be taken when giving local anesthetics to small laboratory animals, as it is very easy to exceed the toxic dose.  Injectable local anesthetics come in solutions of 0.5-5.0%.  Even the lowest concentration has 5mg/ml and for the least toxic local anesthetic, 0.1 ml could be toxic for a 20g mouse.  Inadvertent intravenous injection of local anesthetic may cause a decrease in cardiac function with a fall in blood pressure.

Examples of Local Anesthetics

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Lidocaine and Prilocaine (EMLA)