Anesthetic  

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Not to be confused with Aesthetics

An anesthetic (or anaesthetic) is a drug that causes anesthesia—reversible loss of sensation. These drugs are generally administered to facilitate surgery. A wide variety of drugs are used in modern anesthetic practice. Many are rarely used outside of anesthesia, although others are used commonly by all disciplines. Anesthetics are categorized in to two classes: general anesthetics, which cause a reversible loss of consciousness (general anesthesia), and local anesthetics, which cause a reversible loss of sensation for a limited region of the body while maintaining consciousness.

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Local anesthetics

Local anesthetics are agents that prevent transmission of nerve impulses without causing unconsciousness. They act by binding to fast sodium channels from within (in an open state). Local anesthetics can be either ester or amide based.

Ester local anesthetics (e.g., procaine, amethocaine, cocaine) are generally unstable in solution and fast-acting, and allergic reactions are common.

Amide local anesthetics (e.g., lidocaine, prilocaine, bupivicaine, levobupivacaine, ropivacaine, mepivacaine and dibucaine) are generally heat-stable, with a long shelf life (around 2 years). They have a slower onset and longer half-life than ester anaesthetics, and are usually racemic mixtures, with the exception of levobupivacaine (which is S(-) -bupivacaine) and ropivacaine (S(-)-ropivacaine). These agents are generally used within regional and epidural or spinal techniques, due to their longer duration of action, which provides adequate analgesia for surgery, labor, and symptomatic relief.

Only preservative-free local anesthetic agents may be injected intrathecally.

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Adverse effects

Adverse effects of local anesthesia are generally referred to as local anesthetic toxicity.

Effects may be localized or systemic.

Examples of systemic effects of local anesthesia:

Local anesthetic drugs are toxic to the heart (where they cause arrhythmia) and brain (where they may cause unconsciousness and seizures). Arrhythmias may be resistant to defibrillation and other standard treatments, and may lead to loss of heart function and death.

The first evidence of local anesthetic toxicity involves the nervous system, including agitation, confusion, dizziness, blurred vision, tinnitus, a metallic taste in the mouth, and nausea that can quickly progress to seizures and cardiovascular collapse.

Toxicity can occur with any local anesthetic as an individual reaction by that patient. Possible toxicity can be tested with pre-operative procedures to avoid toxic reactions during surgery.

An example of localized effect of local anesthesia:

Direct infiltration of local anesthetic into skeletal muscle will cause temporary paralysis of the muscle. Peripheral nerve blocks are when a nerve block is a shot of anesthetic near a specific nerve or group of nerves. It blocks pain in the part of the body supplied by the nerve. Nerve blocks are most often used for procedures on the hands, arms, feet, legs, or face. Epidural and spinal anesthesia is a shot of anesthetic near the spinal cord and the nerves that connect to it. It blocks pain from an entire region of the body, such as the belly, hips, or legs.

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General anesthetics

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Inhaled agents

Volatile agents are specially formulated organic liquids that evaporate readily into vapors, and are given by inhalation for induction and/or maintenance of general anesthesia. Nitrous oxide and xenon are gases at room temperature rather than liquids, so they are not considered volatile agents. The ideal anesthetic vapor or gas should be non-flammable, non-explosive, and lipid-soluble. It should possess low blood gas solubility, have no end-organ (heart, liver, kidney) toxicity or side-effects, should not be metabolized, and should not be an irritant to the respiratory pathways of the patient.

No anesthetic agent currently in use meets all these requirements. The agents in widespread current use are isoflurane, desflurane, sevoflurane, and nitrous oxide. Nitrous oxide is a common adjuvant gas, making it one of the most long-lived drugs still in current use. Because of its low potency, it cannot produce anesthesia on its own but is frequently combined with other agents. Halothane, an agent introduced in the 1950s, has been almost completely replaced in modern anesthesia practice by newer agents because of its shortcomings. Partly because of its side effects, enflurane never gained widespread popularity.

In theory, any inhaled anesthetic agent can be used for induction of general anesthesia. However, most of the halogenated anesthetics are irritating to the airway, perhaps leading to coughing, laryngospasm and overall difficult inductions. For this reason, the most frequently used agent for inhalational induction is sevoflurane Template:Citation needed. All of the volatile agents can be used alone or in combination with other medications to maintain anesthesia (nitrous oxide is not potent enough to be used as a sole agent).

Volatile agents are frequently compared in terms of potency, which is inversely proportional to the minimum alveolar concentration. Potency is directly related to lipid solubility. This is known as the Meyer-Overton hypothesis. However, certain pharmacokinetic properties of volatile agents have become another point of comparison. Most important of those properties is known as the blood: gas partition coefficient. This concept refers to the relative solubility of a given agent in blood. Those agents with a lower blood solubility (i.e., a lower blood–gas partition coefficient; e.g., desflurane) give the anesthesia provider greater rapidity in titrating the depth of anesthesia, and permit a more rapid emergence from the anesthetic state upon discontinuing their administration. In fact, newer volatile agents (e.g., sevoflurane, desflurane) have been popular not due to their potency (minimum alveolar concentration), but due to their versatility for a faster emergence from anesthesia, thanks to their lower blood–gas partition coefficient.

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Intravenous agents (non-opioid)

While there are many drugs that can be used intravenously to produce anesthesia or sedation, the most common are:

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Intravenous opioid analgesic agents

The following agents have longer onset and duration of action and are frequently used for post-operative pain relief:

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Muscle relaxants

Muscle relaxants do not render patients unconscious or relieve pain. Instead, they are sometimes used after a patient is rendered unconscious (induction of anesthesia) to facilitate intubation or surgery by paralyzing skeletal muscle.

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Adverse effects

  • Depolarising Muscle Relaxants i.e. Suxamethonium
    • Hyperkalaemia – A small rise of 0.5 mmol/l occurs normally, this is of little consequence unless Potassium is already raised such as in Renal Failure
    • Hyperkalaemia – Exaggerated potassium release in burn patients (occurs from 24 hours after injury, lasting for up to 2 years), neuromuscular disease and paralyzed (quadraplegic, paraplegic) patients. The mechanism is reported to be through upregulation of acetylcholine receptors in those patient populations with increased efflux of potassium from inside muscle cells. May cause life threatening arrhythmia
    • Muscle aches, commoner in young muscular patients who mobilize soon after surgery
    • Bradycardia, especially if repeat doses are given
    • Malignant hyperthermia, a potentially life threatening condition in susceptible patients
    • Suxamethonium Apnoea, a rare genetic condition leading to prolonged duration of neuromuscular blockade, this can range from 20 minutes to a number of hours. Not dangerous as long as it is recognised and the patient remains intubated and sedated, there is the potential for awareness if this does not occur.
    • Anaphylaxis
  • Non-depolarising Muscle Relaxants
    • Histamine release e.g. Atracurium & Mivacurium
    • Anaphylaxis

Another potentially disturbing complication where neuromuscular blockade is employed is 'anesthesia awareness'. In this situation, patients paralyzed may awaken during their anesthesia, due to an inappropriate decrease in the level of drugs providing sedation and/or pain relief. If this fact is missed by the anaesthesia provider, the patient may be aware of his surroundings, but be incapable of moving or communicating that fact. Neurological monitors are increasingly available that may help decrease the incidence of awareness. Most of these monitors use proprietary algorithms monitoring brain activity via evoked potentials. Despite the widespread marketing of these devices many case reports exist in which awareness under anesthesia has occurred despite apparently adequate anesthesia as measured by the neurologic monitor.

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Intravenous reversal agents

  • Flumazenil, reverses the effects of benzodiazepines
  • Naloxone, reverses the effects of opioids
  • Neostigmine, helps reverses the effects of non-depolarizing muscle relaxants
  • Sugammadex, new agent that is designed to bind Rocuronium therefore terminating its action


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