ANALGESIA AND SEDATIONCHAPTER 1

Local anesthetics are chemical agents that produce loss of sensation to pain in a specific area of the body without the loss of consciousness by the depression or excitation in the nerve endings. In clinical dental practice, a localized loss of pain is desired. Although dental analgesia and dental anesthesia are used synonymously in dentistry, local analgesia is more accurate. Local anesthesia can be achieved by a number of different mechanisms including the use of neurolytic agents in addition to the traditional agents. However, in clinical dental practice, only reversible local anesthetic agents are used. These agents block the nerve conduction impulses reversibly and can be administered via a spray, drops or an injection. The most common route of administration is by injection to the area requiring numbing. This works as the anesthetic selectively targets nerves to block them transmitting pain signals and sending them to the brain.

Local anesthetics can be classified according to their (Table 1.1):

All local anesthetics agents used in dentistry work by obstructing the exchange in Na⁺ permeability which is essential for the initial phases of a neuronal action potential. In general terms, these agents prevent nerve conduction by obstructing the depolarization which is effected by the influx of sodium ions. In the axon wall there are numerous pores which allow the ionic changes (influx of sodium ions). Local anesthetics (LA) cations block these channels. As a result, the nerve loses the capacity to create the impulse and the patient loses the sensation in the area supplied by the nerve.

The injectable local anesthetics used in dentistry have a common core structure consisting of:

All local anesthetics are weak bases. Chemical structure of the local anesthetic molecule have an amine group on one end connected to an aromatic ring on the other and an amine group on the right side (Fig. 1.2). The amine end is hydrophilic (soluble in water) and the aromatic head is lipophilic (soluble in lipids). Therefore, a local anesthetic molecule can either have an amide group (Fig. 1.3) or an ester group (Fig. 1.4).

Lipid Solubility: All local anesthetics are weak bases. Increasing the lipid solubility leads to faster nerve penetration and increases the onset of action. Onset of action is also dependent upon the proximity of the site of injection to the nerve to be anesthetized and the diameter of the nerve fibers. Thin fibers are anesthetized more rapidly as compared to thick fibers possibly because the nodes of Ranvier are closer together.

pH Influence: pH of the local anesthetics ranges from 7.6 to 8.9. Decrease in the pH shifts the equilibrium towards the ionized form of the local anesthetics thus delaying the onset of action.

Duration of Action: This is dependent on the rate of diffusion along a concentration gradient away from the site of injection. Lower vasodilator activity of a local anesthetic leads to slower absorption and longer duration of action (Table 1.3). Other factors affecting the duration and depth of the local anesthesia can be:

Effects on Other Tissues Including Toxicity: The functions of lipid containing organs and tissues such as the brain and heart may be affected by high levels of local anesthetics. Toxicity is the peak circulation levels of local anesthetics. Levels of local anesthetics administered to patients are varied according to age, weight and health. Some common toxic effects are light headedness, shivering or twitching, seizures, hypotension and numbness (Table 1.4).

Rate of Degradation: Amide local anesthetics are broken down by hepatic dealkylation and hydrolysis and excreted in the urine. Esters are metabolized by esterases.

Failure to achieve profound analgesia during treatment procedure may be related to a variety of factors, like:

These include both local and systemic effects.

Spread of Infection: Infection may spread into the tissues due to the needle passing through a contaminated tissue or the needle being contaminated before use.

Hematoma: Damage of a blood vessel by the tip of the needle may cause bleeding into the tissues. Small hematomas (in sulcus) are of little significance but significant bleeding may produce swelling and cause pain and trismus.

Needle Fracture: The chances are less likely due to the disposable use of needles these days.

Needle Stick Injury: The risk can be minimized by needle sheathing or the use of safe syringes.

Trismus: This occurs after inferior dental (ID) block and is attributed to damage in the medial pterygoid muscle. Either injection given too low or faster rate of deposition are the common factors. Management usually involves reassurance and encouragement to open the mouth slowly. If needed, antibiotics can be prescribed.

Blockage of the Facial Nerve: If the injection is given in close proximity to the facial nerve, a motor blockage causing temporary paralysis of the facial muscles may occur. The effect can last for 1–2 hours. In these cases, the desired branch of the trigeminal nerve will not be anesthetized.

Nerve Damage: Very rarely, the tip of the needle may pierce or hit a nerve bundle during injection causing an immediate electric shock like sensation to the patient. A complete return to normal sensation occurs soon.

Syncope: This is the most common systemic complication. It can be minimized by administering LA with patient in a supine position.8

Allergy: Some drugs may react with the anesthetic solution that causes allergies to the patient. For this, dentists should be careful keeping the medical history of the patient in mind.

Regional Infection: Infection can spread within the perioral tissues through planes of the head and neck by the passage of the needle through an infected area.

Cardiovascular Risk/Collapse: This may be related to stress in a susceptible patient (patients with angina pectoris, previous myocardial infarction or who have had previous cardiac surgery or circulatory dysfunction like cardiac failure. Excessive amounts of LA, faster delivery of the anesthetic solution and lack of aspiration can be contributing factors. The maximum safe dose of the lidocaine should be halved in such patients.

Liver Disease: Patients with reduced hepatic function may exhibit an increased rate of metabolism of the amide type of local anesthetics. Dosage levels should therefore be adjusted for these patients.

Methemoglobinemia: This is caused by a metabolite of prilocaine which reduces their oxygen delivering capacity and results in hypoxia. This is a rare complication.

The drugs used for anesthesia in dentistry are (Table 1.5).

In 1940, the first modern local anesthetic agent was lidocaine, trade name xylocaine. Used for blocks and infiltrations; however, effectiveness of analgesia is limited and of brief duration. This belongs to the amide class and causes little allergic reaction.

The addition of 1:80,000 epinephrine prolongs effectiveness to over 90 minutes.

This is less toxic in higher doses than lidocaine because of the small vasodilatory activity. Used for blocks and infiltrations, effective analgesia over 90 minutes, but predisposes to methemoglobinemia. It should be avoided in pregnancy.

This is the newest local anesthetic drug approved by the FDA in 2000. Its half-life is less than about one-fourth of lidocaine. This is used with a vasoconstrictor and enters the blood barrier smoothly. It is currently recommended for infiltration use only as it has rapid onset (<2 minutes) with exceptional ability to penetrate dense mandibular cortical bone. It is therefore ideal where blocks are contraindicated.

This drug produces long anesthetic affect and is used for blocks and infiltrations where up to 8 h of anesthesia is required. This is available as 0.5% solution with a vasoconstrictor (Table 1.6).10

There are various types of dental anesthetic techniques used. These are mentioned in brief below:

The anesthetic solution is deposited at/near the apex of the tooth and diffuses through the bone to affect the peripheral nerves and those nerves serving the periodontal ligament, adjacent bone and soft tissues. This may be used for minor procedures such as fillings.

This is a common form of local dental anesthesia which blocks the reception of pain for one quadrant of the mouth at a time. This type of block is typically given in the buccal surface cheek. The anesthetic solution 11is deposited around a nerve trunk and causes anesthesia to tissues within the distribution of the nerve peripheral to the point of administration (Table 1.7). The different types of nerve blocks used in dentistry are:

An injection of local anesthetic is given directly into the osseous (bone) structure of the tooth. This provides profound single tooth analgesia.

The anesthetic solution is deposited along the periodontal membrane of individual teeth (to be anesthetized). This technique has the advantage of rapid onset and specific analgesia to isolated teeth and is a useful adjunct to conventional local anesthesia. The disadvantages include post injection discomfort due to temporary extrusion.

Benzocaine (10–20%), eugenol, and forms of xylocaine (5–10%) are used topically to numb various areas before injections or other minor procedures. They produce their effect by acting on the free nerve endings (spray, gel or ointment). Ice bags can also be used for this purpose and act by means of refrigerant action on the surface of the mucous membrane. Another example is ethyl chloride.

Drugs such as midazolam, ketamine, propofol and fentanyl are used to put the patient in a twilight sleep or render them completely unconscious and unaware of pain.

Conscious sedation is defined as a technique in which the use of a drug or drugs produces a state of depression of the central nervous system enabling treatment to be carried out, but during which verbal contact with the patient is maintained. The patient is able to retain their protective airway reflexes, and is able to respond to and understand verbal communication. It is a means to control an unmanageable child where all other means of restraint 13have failed. Patients should be carefully selected and medically pre-assessed and written consent must be obtained. The principal methods currently in use are inhalation, intravenous and oral although other methods are occasionally used.

A titrated mixture of up to 70% nitrous oxide may be administered. Clinical monitoring of patient color, respiration and pulse is required. It can be combined with an oral anxiolytic drug. Adult patients need not be accompanied home afterwards. This is the technique of choice in children felt to be candidates for sedation with mild and moderate anxiety and anxiety related gagging reflex. Works best in children aged 5 and above. Nitrous oxide (N₂O), also known as “laughing gas”, easily crosses the alveoli of the lung and is dissolved into the passing blood, where it travels to the brain, leaving a dissociated and euphoric feeling for most patients. Nitrous oxide is used in combination with oxygen. Often (especially with children) a sweet-smelling fruity scent similar to an auto scent is used with the gas to inspire deep inhalation. The most common delivery system for this is the Quantiflex system. Written and informed consent is required for inhalational sedation. Advantages of conscious sedation include safe to use with minimal side effects, extremely useful in fearful, uncontrollable patients and provide comfortable and efficient dental treatment without compromising on quality.

Contraindications to Inhalational Sedation Include: These could be local or systemic. This technique cannot be used in patients with nasal obstruction, intolerance to mask, cold. Systemic contraindications include severe respiratory disease or mental/physical handicap.

Various intravenous sedatives have been used in the past. Titrated benzodiazepine is used to achieve anesthesia. Pulse oximetry and blood pressure monitoring is a must. Facilities must be available to administer oxygen or ventilation if needed.

Any adverse drug interactions during pregnancy may affect either the mother or the fetus. Doses of dental local anesthetics are relatively small and are unlikely to cause any complications during pregnancy. All local anesthetics cross the placenta to some degree. Prilocaine has been found in highest concentration in the fetal circulation following its injection with bupivacaine as the lowest and lidocaine in between. Felypressin can cause uterine contractions; therefore it is best avoided in pregnancy although the dosage used in dental local anesthetics is very low to cause any unlikely effects. Lidocaine with epinephrine is most commonly used for pregnant patients. The anesthetics should have minimum epinephrine concentrations. Acetaminophen is the best analgesic used in pregnancy for dental patients. Penicillin, cephalosporin and erythromycin are the antibiotics of choice if needed. In the first trimester of pregnancy, radiographs should be limited to a minimum and performed only if deemed absolutely necessary.

Maximum recommended doses of local anesthetics are based on the weight of the child (expressed as mg/kg of the body weight). For obese children, maximum doses are calculated on the basis of ideal body weight and not the true body weight.

The level of anesthesia can be controlled to regulate how long the pain block lasts for as well as the level of numbness. This is meticulously controlled by concentration, amount administered as well as the actual solutions used in the anesthetic.