Chloroform is a colorless, sweet-smelling, dense liquid and widely used industrial and laboratory solvent. The total global flux of chloroform through the environment is approximately 660 000 tonnes per year, and about 90% of emissions are natural in origin. Many kinds of seaweed produce chloroform, and fungi are believed to produce chloroform in soil. Chloroform is used as an industrial solvent and as an intermediate in the manufacture of polymeric materials. The major use of chloroformtoday is in the production of the refrigerant R-22, commonly used in the air conditioning business. Inhaled chloroform anesthesia was introduced in 1847 and Chloroform subsequently became the most widely used volatile anaesthetic, and was used in horses before the end of the 19th century. Pure chloroform is known to be decomposed by the air with the formation of hydrochloric acid, phosgene and carbon dioxide. Phosgene is also generated metabolically from chloroform, and liver and kidney damage can ensue from its production.

Diethyl ether (ether) had been widely used for anesthesia until the 1960s despite its explosive properties and toxicity to both humans and animals. Diethyl ether still serves a role today as an effective inhalation agent. Newer inhalation agents have replaced ether completely and open drop delivery systems have been exchanged for complicated vaporizers and monitoring systems. Anesthesia in the developing world, however, still closely resembles primitive anesthetics.

Azaperone (Stresnil, Fluoperidol) is a pyridinylpiperazine and butyrophenone neuroleptic drug with sedative and antiemetic effects. It is mainly as a tranquilizer in veterinary medicine. Azaperone is officially indicated for the “control of aggressiveness when mixing or regrouping weanling or feeder pigs weighing up to 36.4 kg”. It is also used clinically as a general tranquilizer for swine, in particular with aggressive sows to allow piglets to be accepted, and as a preoperative agent prior to general anesthesia or cesarian section with local anesthesia. Azaperone has also been used as a neuroleptic in horses, but some horses develop adverse reactions (sweating, muscle tremors, panic reaction, CNS excitement) and IV administration has resulted in significant arterial hypotension in the horse; because of these effects, most clinicians avoid the use of this drug in equines. Azaperone appears to have minimal effects on respiration and may inhibit some of the respiratory depressant actions of general anesthetics. A slight reduction of arterial blood pressure has been measured in pigs after IM injections of azaperone, which is apparently due to slight alpha-adrenergic blockade. Azaperone has been demonstrated to prevent the development of halothane-induced malignant hyperthermia in susceptible pigs. Preliminary studies have suggested that the effects of butyrephenones may be antagonized by 4-aminopyridine. Azaperone acts primarily as a dopamine antagonist but also has some antihistaminic and anticholinergic properties as seen with similar drugs such as haloperidol. Azaperone may cause hypotension and while it has minimal effects on respiration in pigs, high doses in humans can cause respiratory depression which may be why it is rarely used in humans. Higher doses are used for anesthesia in combination with other drugs such as xylazine, tiletamine and zolazepam. Azaperone is also used in combination with strong narcotics such as etorphine or carfentanil for tranquilizing large animals such as elephants.

Alfaxalone is a rapidly acting hydrophobic synthetic neurosteroid. It is indicated for the induction and maintenance of anesthesia and for induction of anesthesia followed by maintenance with an inhalant anesthetic, in cats and dogs. Alfaxalone induces anaesthesia through activity at the gamma amino butyric acid sub-type A receptor (GABAA) present on cells in the Central Nervous System (CNS). Alfaxalone enhances the effects of GABA at the GABAA receptors resulting in opening of channels into the cells and an influx of chloride ions. This causes hyperpolarisation of the cells and inhibition of neural impulse transmission. Alfaxalone can be safely combined with premedicants (xylazine, (dex)medetomidine, acepromazine, midazolam), opioids (morphine, methadone, hydromorphone, butorphanol, nalbuphine, buprenorphine, fentanyl), and NSAIDs. Alfaxalone’s adverse reactions are: hypotension, tachycardia, apnea, hypertension, bradypnea and others.

Propanidid (Epontol) is an analgetically potent and shortterm anesthetic, widely used in the 1960s. It was originally introduced by Bayer in 1963. Epontol, an injectable emulsion formulation of propanidid, provided by Bayer, was withdrawn from the market in Great Britain in 1983 because of concern over anaphylactoid reactions. Thus, in spite of the fact that propanidid provides shorter and more predictable recovery times than propofol, it has not been accepted widely as an injectable anesthetic. Even though Cremophor EL has been shown to cause anaphylactic reactions in humans in several cases (both when given intravenously and orally), it is still debated whether or not propanidid itself may have contributed to the reactions. It has been argued that the toxic effects or reactions to propanidid (and Althesin) were due to the drugs themselves. Several cases of negative reactions have been recorded for different drugs using Cremophor EL as solubilizer. This suggest that the negative reactions were mainly caused by Cremophor and not by the drug substances themselves. Propanidid is presumed to work as a GABA receptor agonist.

PHENOPERIDINE is an opioid analgesic partly metabolized to meperidine in the liver. It is derived from pethidine by replacing the N methyl by a phenyl propanol chain. It is reputed to be a typical morphine-like analgesic characterized by its high potency, rapid onset of action, the intensity of its peak effect and the short duration of its pharmacological effects. It is used in general anesthesia.

Enibomal (narcobarbital) is a barbiturate drug. By its chemical structure, it is N-methylated derivative of propallynolal. Narcobarbital was discovered in 1930s and later investigated as an anesthetic agent in gynecological surgery and electroconvulsive treatment. In the United States, narcobarbital is classified as a schedule III drug and is not found in pharmaceutical preparations. In Europe, narcobarbital was used in veterinary as a general anesthetic for pigs, but by 2000 it was withdrawn and no longer available at the market.

Xenon is a noble gas used as an anesthetic agent. Xenon can associate with amino acid side chains of the active site of enzymes such as serine proteinases (including elastases and collagenases); these enzymes can form a specific binding cavity for single xenon atom without inducing major changes in protein structure. It has been demonstrated that xenon binds within the heme cavity of cytochrome P-450 monooxygenases and is capable of inhibiting the catalytic activity of some enzymes in vitro. Current evidence suggests that inhibition of N-methyl-D-aspartate (NMDA) receptor signaling is the mechanism by which xenon induces anesthesia. Although approved for use in anesthesia in Russia in 2000, it was unavailable for use in western countries until October 2005 when approval for its use was granted in Germany.

Sodium oxybate is the sodium salt of gamma-hydroxybutyrate (GHB), an endogenous metabolite of gamma-aminobutyric acid (GABA) a major inhibitory neurotransmitter. Evidence suggests a role for GHB as a neuromodulator/neurotransmitter. Under endogenous conditions and concentrations, and depending on the cell group affected, GHB may increase or decrease neuronal activity by inhibiting the release of neurotransmitters that are co-localised with GHB. After exogenous administration, most of the observed behavioural effects appear to be mediated via the activity of GHB at GABA(B) receptors, as long as the concentration is sufficient to elicit binding, which does not happen at endogenous concentrations. Xyrem (sodium oxybate) oral solution is indicated for the treatment of cataplexy in narcolepsy and excessive daytime sleepiness (EDS) in narcolepsy.

Propofol (2,6-diisopropylphenol) is an intravenous anaesthetic agent used for induction and maintenance of general anaesthesia. IV administration of propfol is used to induce unconsciousness after which anaesthesia may be maintained using a combination of medications. It is extensively metabolized, with most of the administered dose appearing in the urine as glucuronide conjugates. Favorable operating conditions and rapid recovery are claimed as the main advantages in using propofol, whereas disadvantages include a relatively high incidence of apnea, and blood pressure reductions. The action of propofol involves a positive modulation of the inhibitory function of the neurotransmitter gama-aminobutyric acid (GABA) through GABA-A receptors. Due to its high lipid-solubility, propofol was initially formulated as a solution with the surfactant Cremophor EL, but the occurrence of pain on injection and anaphylactoid reactions prompted to search for alternative formulations. Results from using cyclodextrins, water-soluble prodrugs, and adopting Bodor's approach to the site-specific chemical delivery system (CDS), as well as the advantages provided by computer-controlled infusion systems, are examined in some detail.