Anaesthetics

Anaesthesia- Reversible loss of awareness or consciousness by induced drug to facilitate surgery. This can be either general anaesthesia, a total loss of awareness or a partial loss of awareness of body part such as a spinal anaesthetic or nerve block(local anesthesia). Anesthesia differs from analgesia in blocking all sensation, not only pain. Anesthesia is pharmacologically induced reversible state of amnesia, analgesia, loss of consciousness, loss of skeletal muscle reflexes and decreased stress response. Anaesthetics affect the cerebral and spinal centres thereby simultaneously reducing pain, sensation and reflex.

Anaesthetics may be divided into local and general. The local anaesthetics abolishes the sensibility of the peripheral nerves of a particular area.The general anaesthetics acts on the central nervous system and abolishes sensation throughout the whole body.

Local Anaesthetics

Injectable Local Anaesthetic:

There are 2 classes of local anaesthetic drugs defined by the nature of the carbonyl-containing linkage group.

• The ester agents include cocaine, procaine, amethocaine and chloroprocaine. Esters are relatively unstable in solution and rapidly hydrolyse in the body by plasma cholinesterase (and other esterases). One of the main breakdown products is para- amino benzoate (PABA) which is associated with allergic phenomena and hypersensitivity reactions.
• The amides include lignocaine, prilocaine, mepivacaine and bupivacaine. There are practical differences between these two groups of local anaesthetic agents. Amides are relatively stable in solution, are slowly metabolised by hepatic amidases and hypersensitivity reactions to amide local anaesthetics are extremely rare. In current clinical practice esters are superseded by the amides.

Factors affecting the onset of local anaesthetic action

• pH of tissue.
• pKa of drug.
• Time of diffusion from needle tip to nerve.
• Time of diffusion away from nerve.
• Nerve morphology.
• Concentration of drug.
• Lipid solubility of drug.

Expected duration of action of local anaesthesia

Recommended maximum doses of local anesthetics with vasoconstrictor

Adverse reactions of commonly used local anesthetics

Psychogenic

• Syncope (most common).
• Hyperventilation.
• Nausea and vomiting.
• Alterations in heart rate or blood pressure.

Allergic (potential allergens)

• Esters (true amide allergy is very rare).
• Metabisulfite (present with epinephrine and with levonordefrin).
• Methylparaben (no longer added to dental cartridges).

Toxic effects

• Primarily neurologic signs.
• May initially manifest as sedation, light headedness, slurred speech, mood alteration, diplopia, sensory disturbances, disorientation and muscle twitching.
• Higher blood levels may result in tremors, respiratory depression and tonic–clonic seizures.
• If severe, may result in coma, respiratory arrest and cardiovascular collapse.

Metheamoglobinemia

• Associated with prilocaine, articaine and benzocaine.

Paraesthesia

• Apparently more common with articaine and prilocaine.

Treatment modifications to consider if there are concerns regarding vasoconstrictors

• Monitor blood pressure and heart rate preoperatively.
• Minimize administration of epinephrine or levonordefrin.
• Monitor blood pressure and heart rate 5 minutes after injection.
• May re-administer epinephrine or levonordefrin if blood pressure and heart rate are stable.
• Monitor as required.
• Consider limiting epinephrine to 0.04 mg, levonordefrin to 0.2 mg.
• Avoid epinephrine 1:50,000.
• Never use epinephrine-impregnated retraction cord.

Use of local anesthetics during pregnancy

The above mentioned norms are laid down by U.S. Food and Drug Administration (FDA).

Topical local anaesthetic

Dentists use these topical anaesthetics for several reasons including:

• Preventing or reducing the pain that is caused by anaesthetic injections.
• Preventing the gag reflex. This is the reflex when something touches the back of your throat. Gag reflexes happen when you are X-rayed. Trays are put in your mouth for impressions and sometimes when you get fluoride treatment.
• Decreasing the unease during scaling and root planing.
• Decreasing the discomfort when stitches are removed.
• The relief of pain from complications like dry socket in tooth extractions.

Topical anaesthetics can be applied as ointments, gels, sprays or through an adhesive patch. Some topical anaesthetics are flavoured (cherry, banana, watermelon, pina colada, bubble gum, mint, strawberry and raspberry). Most of the topical anaesthetics use the same drugs as injectable local anaesthetics although topical anaesthetics are given in higher concentrations. This is because only some of it will penetrate through the tissue.

Topical anaesthetics are applied to the mucous membrane and penetrates to a depth of 2-3mm. The drug must cross epidermal barrier with the stratum corneum presenting the major obstacle, to reach the endings of A-delta and C fibres.

When applying anaesthetics, dry the area with gauze pads. Apply the anaesthetic using an applicator such as a cotton swab, held in the area for about two to three minutes until it becomes numb. The topical anaesthetic can be applied using a spray or using an adhesive patch.

Topical anaesthetics will numb the nerves for about two to three millimetres below the surface. They are effective for about fifteen to thirty minutes which is just enough time to get an injection or have stitches removed.

General Anaesthetics

General anesthesia is the induction of a balanced state of unconsciousness, accompanied by the absence of pain sensation and the paralysis of skeletal muscle over the entire body. It is induced through the administration of anesthetic drugs and is used during major surgery and other invasive surgical procedures.

General anesthesia is intended to bring about five distinct states during surgery:

• Analgesia or pain relief.
• Amnesia or loss of memory of the procedure.
• Loss of consciousness.
• Motionlessness.
• Weakening of autonomic responses.

General anesthesia in dental procedures

The use of general anesthesia in dental and oral surgery patients differs from its use in major surgery because the patient's level of fear is usually a more important factor than the nature of the procedure. High levels of preoperative anxiety, lengthy and complex procedures and the need for a pain-free operative period may be indications for general anesthesia in healthy adults and very young children. At least three professionals are required when general anesthesia is used during dental procedures: one is the operating dentist; the second is a professional responsible for observing and monitoring the patient; the third person assists the operating dentist.

Stages of anaesthesia

Stage 1

Stage 1 anaesthesia, also known as the analgesia, is the period between the initial administration of the induction medications and loss of consciousness. During this stage the patient progresses from analgesia without amnesia to analgesia with amnesia. Patients can carry on a conversation at the time.

Stage 2

Stage 2 anaesthesia, also known as the delirium , is the period following loss of consciousness and marked by excited and delirious activity. During this stage, respirations and heart rate may become irregular. In addition, there may be uncontrolled movements, vomiting, breath holding and pupillary dilation. Since the combination of spastic movements, vomiting and irregular respirations may lead to airway compromise, rapidly acting drugs are used to minimize time in this stage and reach stage 3 as fast as possible.

Stage 3

Stage 3, "surgical anaesthesia". During this stage, the skeletal muscles relax and the patient's breathing becomes regular. Eye movements slow, then stop and surgery can begin. It has been divided into 3 planes:

• Plane 1 : Roving eye balls which ends with fixed eyeballs.
• Plane 2 : Loss of corneal and laryngeal reflexes.
• Plane 3 : Pupil dilate and light reflex is lost.
• Plane 4 : Intercostal paralysis, shallow abdominal respiration, dilated pupil.

Stage 4

Stage 4 anaesthesia, also known as "overdose", is the stage where too much medication has been given and the patient has severe brain stem or medullary depression. This results in a cessation of respiration and potential cardiovascular collapse. This stage is lethal without cardiovascular and respiratory support.

Adverse effects

Nausea and vomiting

Post-operative nausea and vomiting is a common problem during recovery from general anesthesia. In addition, patients may feel drowsy, weak or tired for several days after the operation, a combination of symptoms sometimes called the hangover effect. Fuzzy thinking, blurred vision and coordination problems are possible. For these reasons, anyone who has had general anesthesia should not drive, operate machinery or perform other activities that could endanger themselves or others for at least 24 hours or longer if necessary.

Anesthetic toxicity

Inhalation anesthetics are sometimes toxic for the liver, the kidney or to blood cells. Halothane may cause hepatic necrosis or hepatitis. Sevoflurane may react with the carbon dioxide absorbents in anesthesia machines to form compound A, a haloalkene that is toxic to the kidneys. The danger to red blood cells comes from carbon monoxide formed by the breakdown products of inhalation anesthetics in the circuits of anesthesia machines.

Malignant hyperthermia

Malignant hyperthermia is a rare condition caused by an allergic response to a general anesthetic. The signs of malignant hyperthermia include rapid, irregular heartbeat; breathing problems; very high fever and muscle tightness or spasms. These symptoms can occur following the administration of general anesthetics, especially halothane.

Inhalation Anaesthetic

Gas

• Nitrous oxide

Liquids

• Ether
• Halothane
• Enflurane
• Isoflurane
• Desflurane

Nitrous oxide N2O

It is used in surgery and dentistry for its anaesthetic and analgesic effects. Nitrous oxide is a weak general anesthetic and so is generally not used alone in general anesthesia. In general anesthesia it is used as a carrier gas in a 2:1 ratio with oxygen for more powerful general anesthetic agents such as sevoflurane or desflurane.

It is non toxic to liver, kidney and brain.

Ether

Ether is a potent anaesthetic, produces good anlgesia and marked muscle relaxation by reducing ACh output from motor nerve endings.

Today, ether is rarely used. The use of flammable ether waned as nonflammable anesthetic agents such as halothane became available. Additionally, ether had many undesirable side effects, such as postanesthetic nausea and vomiting. Modern anesthetic agents, such as methyl propyl ether(Neothyl) and methoxyflurane (Penthrane) reduce these side effects.

Halothane

Halothane is a well tolerated, non-irritant potent agent giving rapid induction, low dose maintenance and rapid recovery. It is a potent anaesthetic -precise control of administration concentration is essential.

As halothane is a very potent agent it is not suitable for use by untrained anaesthetic staff. Its poor analgesic properties necessitate deep planes of anaesthesia before surgery can be tolerated. For this reason it is generally not suitable as a sole agent without an analgesic supplement e.g. nitrous oxide, trichloroethylene, local anaesthetic block or other analgesic, especially during spontaneous respiration.

It provides no post-operative analgesia and causes uterine relaxation and haemorrhage particularly if greater than 0.5% halothane is used.

The depression of the cardiovascular system may cause bradycardia, hypotension and a reduction in cardiac output. Halothane sensitizes the heart to adrenaline and predisposes the patient to developing arrhythmias. These arrhythmias occur most commonly in patients who are retaining CO2 or who have an inadequate analgesic component in their anaesthetic.

They can usually be managed by supporting the ventilation, reducing the amount of halothane in the inspired gases and supplementing with another analgesic e.g. a small dose of I.V. opiate (be sure to support respiration due to the extra respiratory depression that may result). If this is not effective I.V. lignocaine or propranolol (avoid in asthma) are generally effective. Injection of adrenaline by the surgeon during anaesthesia with halothane may be dangerous and the doses need to be carefully monitored. The surgeon should never inject more than 10ml of 1:100,000 in any ten minute period and never exceed 30ml of 1:100,000 in an hour. If possible avoid adrenaline altogether; if it has to be used monitor the pulse closely and support ventilation.

Isoflurane

Though irritant, isoflurane has many advantages, i.e better adjustment of the depth of anaesthsia and low toxicity. It does not provoke seizures and is preferred for neurosurgery.

Intravenous Anaesthetic

Inducing agent

Thiopentone sodium

It is an intravenous ultra-short-acting general anaesthetic.

Thiopentone sodium is an ultra-short-acting barbiturate and is most commonly used in the induction phase of GA . Following intravenous injection the drug rapidly reaches the brain and causes unconsciousness within 30–45 seconds. At one minute, the drug attains a peak concentration of about 60% of the total dose in the brain. Thereafter, the drug distributes to the rest of the body and in about 5–10 minutes the concentration is low enough in the brain such that consciousness returns.

Thiopentone sodium is not used to maintain anesthesia in surgical procedures because in infusion, it displays zero order elimination kinetics, leading to a long period before consciousness is regained. Instead, anesthesia is usually maintained with an inhaled anaesthetic (gas) agent.

Adverse effects

• Laryngospasm occurs generally when respiratory secretions or other irritants are present.
• Headache, emergence delirium, prolonged somnolence and nausea.

Propofol

Propofol is a short-acting intravenous sedative agent used for the induction of general anaesthesia in adults and children, maintenance of general anesthesia and sedation in medical contexts, such as ICU sedation for intubated, mechanically ventilated adults and in procedures such as endoscopies and dental surgeries.

When used for IV sedation propofol typically wears off in minutes. Propofol is versatile; the drug can be given for short or prolonged sedation as well as for general anesthesia. Its use is not associated with nausea as is often seen with opioid medications. These characteristics of rapid onset and recovery along with its amnestic effects have led to its widespread use for sedation and anesthesia.

Adverse effects

• Apart from the hypotension (mainly through vasodilatation) and transient apnoea following induction doses, one of propofol's most frequent side effects is pain on injection, especially in smaller veins.

Slower acting drugs

Benzodiazepines

These are now preferred drugs for endoscopies, cardiac catheterization, angiographies, sedation during local/regional anaesthesia, fracture settings etc.

Diazepam

0.2-0.5mg/kg by slow undiluted injection in a running drip.This reduces burning sensation in the veins.

Lorazepam

It is more potent, slower acting and less irritating than diazepam 2-4mg I.V.

Midazolam

1-2.5 mg I.V. followed by 1/4 supplemental doses.

It is used for intubated and mechanically ventilated patients

Dissociative anaesthesia

Ketamine

Ketamine has a wide range of effects in humans, including analgesia, anaesthesia, hallucinations, elevated blood pressure and bronchodiltion. Ketamine is primarily used for the induction and maintenance of GA, usually in combination with some sedative drug.

Ketamine is usually injected intravenously or intramuscularly but it is also effective when insufflated, smoked or taken orally. Because ketamine tends to increase or maintain cardiac output, it is sometimes used in anesthesia for emergency surgery when the patient's state of fluid volume status is unknown. It is alsorecommended for head and neck operations, in asthamatics and in those who do not want to loose consciousness for short operations.

Opioid analgesics

Fentanyl

Intravenous fentanyl is extensively used for anaesthesia and analgesia.

After I.V. fentanyl the patient remains drowsy but conscious respiratory depression is marked, heart rate decreases but fall in BP is less and heart is not sensitized to Adrenaline. Nausea, vomiting and itching often occurs during recovery.

Trofentyl, Fent 50ug/ml in 2 ml amp, 10 ml vial.

Conscious Sedation and Nitrous Oxide

All types of dental care- rendered with no anaesthesia to treatment rendered with general anaesthesia - require accurate diagnosis, proper treatment and effective patient monitoring.

The term conscious sedation is used to describe the amelioration of patient anxiety, the blunting of the stress response and often some degree of amnesia produced by a combination of psychological techniques and drugs.

Appropriately trained dentist may use any number of preoperative and operative pharmacological conscious sedation techniques to achieve the goals of anxiety reduction and pain control. Such techniques may include sedation by enteral, inhalation and parenteral route means.

Dentists administering sedation and anesthesia should be familiar with the Guidelines for the Use of Sedation and General Anesthesia by Dentists. Dentists who are qualified to utilize sedation and general anesthesia have a responsibility to minimize risk to patients undergoing dental treatment by:

• Using only those drugs and techniques in which they have been appropriately trained.
• Limiting use of these modalities to patients who require them.
• Conducting a preoperative evaluation of each patient consisting of at least a thorough review of medical and dental history, a focused clinical examination and consultation, when indicated, with appropriate medical and dental personnel.
• Conducting physiologic and visual monitoring of the patient.
• Having available appropriate emergency drugs, equipment and facilities and maintaining competency in their use.
• Maintaining fully documented records of drugs used, dosage, vital signs monitored, adverse reactions, recovery from the anesthetic and, if applicable, emergency procedures employed.
• Utilizing sufficient support personnel who are properly trained for the functions they are assigned to perform.
• Treating high-risk patients in a setting equipped to provide for their care.

The Association expects that patient safety will be the foremost consideration of dentists who use sedation and general anesthesia.