Analgesia for Oral Disease

Analgesia

The concept of pre-emptive analgesia describes the administration of analgesics preoperatively to reduce the severity of postoperative pain. It is important to distinguish between pre-emptive analgesia and alleviation of postoperative pain. In other words, pre-emptive analgesia may block sensitization, but it does not eliminate postoperative pain. Therefore additional measures are still required to ensure a comfortable recovery. The optimum form of pain therapy is constant pre-emptive analgesia, continuously preventing the establishment of sensitization.

The administration of opioids or local anesthetic drugs block central sensitization and nonsteroidal anti-inflammatory drugs (NSAIDs) reduce the severity of the peripheral inflammatory response. The combined use of an opioid and an NSAID is more effective than using either drug alone. Local anesthetics (analgesics) can produce complete pain relief by blocking all sensory input from the affected area.

Basic dental analgesic plan:

• Include an opioid in the premedication.
• Use local anaesthetics prior to surgery and/or administer additional opioids intraoperatively.
• Give opioids and/or NSAIDs postoperatively. Local anesthesia (administered at the end of a procedure) will also provide postoperative analgesia.
• Administer NSAIDs during recovery.

Local Anaesthesia

Local anaesthesia (LA) can be used to provide intra- and postoperative analgesia. In contrast to human patients, dogs and cats are not amenable to LA if conscious, therefore the techniques are used when the animal is under GA. See dental anaesthesia and analgesia techniques for more information.

When given prior to the start of a procedure, the use of LA may reduce the requirement for general anaesthetic (GA) drugs during surgery. When given at the end of a procedure, prior to GA recovery, they will provide postoperative analgesia.

Useful techniques in the oral cavity include infiltration anaesthesia and regional nerve blocks.

All clinically used local anaesthetics are membrane-stabilizing agents. They prevent depolarization and thus stop or retard conduction of impulses. Sensation disappears in the following order: pain, cold, warmth, touch, joint and deep pressure. Procaine hydrochloride is the prototype of all local anesthetics. It is the standard drug for comparison of anesthetic effects. For LA in the oral cavity lidocaine, mepivacaine, bupivacaine and ropivacaine are all suitable. The local anaesthetic drug chosen for postoperative pain relief should ideally have a long duration of action, therefore bupivacaine (onset 15 minutes, duration 4–6 hours) is the drug of choice. Lidocaine can be used during surgery for a more immediate effect.

Mechanism of Action

The mechanism of action of all local anaesthetic drugs is similar. The salt of the anaesthetic base (RNH⁺Cl^–) is an ionizable quaternary amine with little or no anaesthetic properties of its own because it is not lipid soluble and therefore not absorbed through the nerve membrane. After deposition in tissue that is slightly alkaline and has considerable buffering capacity, the anaesthetic base is liberated as follows:

RNH⁺Cl^– <--> Cl^– + RNH⁺ <--> Cl^– + RN + H⁺

The free anaesthetic base (RN) is absorbed in the outer lipid nerve membrane, where anaesthetic action takes place. If sufficient local buffering capacity exists to remove the dissociated H⁺, this reaction proceeds to the right, and an active base is liberated which exerts an anaesthetic effect. In inflamed or infected tissue, however, the pH is acidic and the result is that only small amounts of free base dissociate from the anaesthetic salt, resulting in poor local anaesthesia.

In human dentistry and oral surgery, vasoconstrictors (adrenaline, L-noradrenaline) are routinely used in combination with the local anaesthetic. The main reason is to delay systemic absorption of the local anaesthetic, thus reducing the toxicity and increasing the margin of safety. Local anaesthetics produce analgesia when given in small doses intravenously, but are potent proconvulsants and can induce marked myocardial depression and cardiac dysrythmias when administered systemically. The addition of vasoconstrictors, by reducing systemic absorption of the local anaesthetic, also increase intensity and prolong anaesthetic activity. However, they may increase the risk of cardiac arrhythmias and ventricular fibrillation. In veterinary dentistry and oral surgery, local anaesthetics are generally used without the addition of vasoconstrictors.

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