Flurans are polyhalogenated hydrocarbons with an oxygen bridge (ether bridge) as a functional group. All five known flurans belong to the group of inhalation narcotics and are characterized by a very good hypnotic, i.e. soporific, effect. Their analgesic (pain-relieving) effect, on the other hand, is weak, so that fluranes are usually used in anesthesia together with other preparations with a higher analgesic effect.
What are flurans?
Five different, multiply halogenated, hydrocarbons are grouped under the term fluranes. As a characteristic feature, they all have a so-called ether bridge, an oxygen bridge with two organic residues (organyl groups) each. They are all colorless, non-flammable liquids with a boiling point of around 45 to 60 degrees Celsius. In the case of desflurane, which is used almost as standard as an inhalation anesthetic, the boiling point is only 23.5 degrees at normal pressure. All five fluranes are characterized by inertness and by the fact that they are light-stable and do not react with metals or plastics. With the exception of sevoflurane, which has a pleasant odor, the other four fluranes are characterized by a pungent odor with irritation of the upper respiratory tract. The three fluranes iso-, sevo- and desflurane, together with nitrous oxide (laughing gas), are among the most important inhalable anesthetics. Halothane, which was frequently used until the 1970s, is no longer of any importance because of the possible side effects it has been found to have and has largely been replaced by flurane. To exclude possible confusion, the containers of the individual fluranes are labeled with specific colors as standard.
Pharmacological effects on the body and organs
The various flurans used as volatile anesthetics have different effects and are therefore listed separately. Common to all flurans are their good hypnotic and, in some cases, muscle-relaxant properties combined with weak analgesic effects. Therefore, flurans are usually used in combination with appropriate pain-reducing agents. Isoflurane is a commonly used anesthetic from the flurane group. It causes vasodilatation, which lowers arterial blood pressure. The particular advantage of the anesthetic is its low metabolism of only about 0.2 percent. This means that the active ingredient is largely exhaled again, so isoflurane can also be used in patients with damaged livers. Another commonly used anesthetic from the flurane group is sevoflurane, which, unlike the other fluranes, is not irritating to the mucous membranes and has a pleasant odor. Its properties have led to its widespread use in pediatric anesthesia. Desflurane, another anesthetic from the flurane group, has become a kind of standard anesthetic. A special feature is its rapid onset and release of anesthesia, which results in its good controllability. However, because of its irritant effect on the respiratory mucosa, the agent is not suitable for inhalation induction of anesthesia. While all fluranes consist of non-flammable liquids, methoxyflurane is the only representative that is easily flammable and combustible in the temperature range minus 35 degrees to 104.5 degrees in the liquid aggregate state. Methoxyflurane was only used as an anesthetic until the 1970s. Enflurane, which also belongs to the group of five furans, is hardly used as an anesthetic anymore.
Medical application and use for treatment and prevention.
From the group of five known fluranes, only isoflurane, desflurane and sevoflurane played a significant role in modern anesthesia. The three flurans are used as so-called volatile anesthetics, which are used as inhalation anesthetics via special vaporizers. Flurans are well suited to the vaporization technique due to their low molecular weight, high vapor pressure and low boiling point. However, they are less suitable as sole anesthetics because of their only weak analgesic properties. They are usually combined with pain-reducing agents to form a so-called balanced anesthesia. A major reason for the use of fluranes as inhalation anesthetics is their high efficacy and their good controllability of the depth of anesthesia via admixture to the inspiratory air.The speed with which the anesthetic responds to changes in concentration in the inhaled gas mixture depends mainly on the solubility of the anesthetic in the blood. Poor solubility results in rapid efficacy, i.e., rapid “falling asleep,” but also a short elimination time. A measure of the solubility of the volatile agent is the blood-gas partition coefficient. Coefficients less than one indicate that the partial pressures between the gas in the alveoli and the blood can equalize quickly, and thus have a rapid effect. This is true in both directions, for the “flooding in” of anesthesia and for the draining out when the inspiratory gas mixture no longer contains an anesthetic.
Risks and side effects
The three main fluranes used, iso-, des-, and sevoflurane, also differ in terms of potential side effects. Desflurane is characterized by the fewest side effects. This is mainly due to its low metabolization of less than 0.1 percent. This means that the likelihood of liver damage from breakdown products of the drug is extremely low. In very rare cases, desflurane – as well as other inhalation anesthetics – can trigger malignant hyperthermia if there is a corresponding genetic predisposition to this. Malignant hyperthermia is a life-threatening condition that can lead to temperature elevation, muscle rigidity, and metabolic derangement if immediate countermeasures are not initiated. Failure to properly maintain the carbon dioxide absorbers in anesthesia machines can result in the development of dangerous carbon monoxide. Sevoflurane, which is commonly used primarily in pediatric anesthesia, has a somewhat higher metabolization rate of 3 to 5 percent, resulting in the release of certain organic fluorine products and inorganic fluorine, which, however, have not been observed to cause renal toxicity. Sevoflurane is also approved in Germany for low-dose long-term anesthesia (artificial coma) without time limit.
