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An anesthetic (or anaesthetic, see spelling differences) is a drug that brings about a state of anesthesia - a reversible state of amnesia, analgesia, loss of consciousness, loss of skeletal muscle reflexes and decreased stress response. These drugs are generally administered by an anesthesia provider in order to induce or maintain anesthesia 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 categories: general anesthetics cause a reversible loss of consciousness (general anesthesia), while local anesthetics cause reversible local anesthesia and a loss of nociception.

[edit] Local anesthetics

Main article: Local anesthetic

Local anesthetics are agents which 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.

[edit] 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.

[edit] General anesthetics

Main article: General anaesthetic

[edit] Inhaled agents

Main article: Inhalational anaesthetic

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, lipid-soluble, and should possess low blood gas solubility, have no end organ (heart, liver, kidney) toxicity or side-effects, should not be metabolized, and should be non-irritant when inhaled by patients.

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.^[1] Partly because of its side effects, enflurane never gained widespread popularity.^[1]

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^{[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.

[edit] Intravenous agents (non-opioid)

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

Barbiturates
- Methohexital
- Thiopental (Previously known as Thiopentone in the UK)
Benzodiazepines
Etomidate
Ketamine
Propofol

The two barbiturates mentioned above, thiopental and methohexital, are ultra-short-acting, and are used to induce and maintain anesthesia.^[2] However, though they produce unconsciousness, they provide no analgesia (pain relief) and must be used with other agents.^[2] Benzodiazepines can be used for sedation before or after surgery and can be used to induce and maintain general anesthesia.^[2] When benzodiazepines are used to induce general anesthesia, midazolam is preferred.^[2] Benzodiazepines are also used for sedation during procedures that do not require general anesthesia.^[2] Like barbiturates, benzodiazepines have no pain-relieving properties.^[2] Propofol is one of the most commonly used intravenous drugs employed to induce and maintain general anesthesia.^[2] It can also be used for sedation during procedures or in the ICU.^[2] Like the other agents mentioned above, it renders patients unconscious without producing pain relief.^[2] Because of its favourable physiological effects, "etomidate has been primarily used in sick patients".^[2] Ketamine is infrequently used in anesthesia practice because of the unpleasant experiences which sometimes occur upon emergence from anesthesia, which include "vivid dreaming, extracorporeal experiences, and illusions."^[3] However, like etomidate it is frequently used in emergency settings and with sick patients because it produces fewer adverse physiological effects.^[2] Unlike the intravenous anesthetic drugs previously mentioned, ketamine produces profound pain relief, even in doses lower than those which induce general anesthesia.^[2] Also unlike the other anesthetic agents in this section, patients who receive ketamine alone appear to be in a cataleptic state, unlike other states of anesthesia that resemble normal sleep. Ketamine-anesthetized patients have profound analgesia but keep their eyes open and maintain many reflexes.^[2]

[edit] Intravenous opioid analgesic agents

While opioids can produce unconsciousness, they do so unreliably and with significant side effects.^[4]^[5] So, while they are rarely used to induce anesthesia, they are frequently used along with other agents such as intravenous non-opioid anesthetics or inhalational anesthetics.^[2] Furthermore, they are used to relieve pain of patients before, during, or after surgery. The following opioids have short onset and duration of action and are frequently used during general anesthesia:

Alfentanil
Fentanyl
Remifentanil
Sufentanil (Not available in the UK)

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

Buprenorphine
Butorphanol
Diamorphine, (diacetyl morphine, also known as heroin, not available in U.S.)
Hydromorphone
Levorphanol
Meperidine, also called pethidine in the UK, New Zealand, Australia and other countries
Methadone
Morphine
Nalbuphine
Oxycodone, (not available intravenously in U.S.)
Oxymorphone
Pentazocine

[edit] 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.

Depolarizing muscle relaxants
- Succinylcholine (also known as suxamethonium in the UK, New Zealand, Australia and other countries, "Celokurin" or "celo" for short in Europe)
Non-depolarizing muscle relaxants
- Short acting
  - Mivacurium
  - Rapacuronium
- Intermediate acting
- Long acting

[edit] 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 becoming increasingly available which 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.^{[citation needed]}

[edit] 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

[edit] References

^ ^a ^b Townsend, Courtney (2004). Sabiston Textbook of Surgery. Philadelphia: Saunders. Chapter 17 – Anesthesiology Principles, Pain Management, and Conscious Sedation. ISBN 0721653685.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ Miller, Ronald (2005). Miller's Anesthesia. New York: Elsevier/Churchill Livingstone. ISBN 0443066566.
^ Garfield JM, Garfield FB, Stone JG, et al.: A comparison of psychologic responses to ketamine and thiopental-nitrous oxide-halothane anesthesia. Anesthesiology 1972; 36:329–338.
^ Philbin DM, Rosow CE, Schneider RC, et al.: Fentanyl and sufentanil anesthesia revisited: how much is enough?. Anesthesiology 1990; 73:5–11.
^ Streisand JB, Bailey PL, LeMaire L, Ashburn MA, Tarver SD, Varvel J, Stanley TH: Fentanyl-induced rigidity and unconsciousness in human volunteers. Incidence, duration, and plasma concentrations. Anesthesiology 1993; 78:629–634.

[town-0] Townsend, Courtney (2004). Sabiston Textbook of Surgery. Philadelphia: Saunders. Chapter 17 – Anesthesiology Principles, Pain Management, and Conscious Sedation. ISBN 0721653685.

[miller-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ Miller, Ronald (2005). Miller's Anesthesia. New York: Elsevier/Churchill Livingstone. ISBN 0443066566.

[2] Garfield JM, Garfield FB, Stone JG, et al.: A comparison of psychologic responses to ketamine and thiopental-nitrous oxide-halothane anesthesia. Anesthesiology 1972; 36:329–338.

[3] Philbin DM, Rosow CE, Schneider RC, et al.: Fentanyl and sufentanil anesthesia revisited: how much is enough?. Anesthesiology 1990; 73:5–11.

[4] Streisand JB, Bailey PL, LeMaire L, Ashburn MA, Tarver SD, Varvel J, Stanley TH: Fentanyl-induced rigidity and unconsciousness in human volunteers. Incidence, duration, and plasma concentrations. Anesthesiology 1993; 78:629–634.

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Contents

[edit] Local anesthetics

[edit] Adverse effects

[edit] General anesthetics

[edit] Inhaled agents

[edit] Intravenous agents (non-opioid)

[edit] Intravenous opioid analgesic agents

[edit] Muscle relaxants

[edit] Adverse effects

[edit] Intravenous reversal agents

[edit] References