Gaseous anesthetic

Yamakura T, Harris RA Effects of gaseous anesthetics nitrous oxide and xenon on ligand-gated ion channels comparison with isoflurane and ethanol. Anesthesiology 93 1093-1101,2000... 

Inhaled anesthetics currently in use include halo-genated volatile liquids such as desflurane, enflurane, halothane, isoflurane, methoxyflurane, and sevoflurane (Table 11-1). These volatile liquids are all chemically similar, but newer agents such as desflurane and sevoflurane are often used preferentially because they permit a more rapid onset, a faster recovery, and better control during anesthesia compared to older agents such as halothane.915 These volatile liquids likewise represent the primary form of inhaled anesthetics. The only gaseous anesthetic currently in widespread use is nitrous oxide, which is usually reserved for relatively short-term procedures (e.g., tooth extractions). Earlier inhaled anesthetics, such as ether, chloroform, and cyclopropane, are not currently used because they are explosive in nature or produce toxic effects that do not occur with the more modern anesthetic agents. 

Inhaled anesthetics are delivered to the lungs in gas mixtures in which concentrations and flow rates are easy to measure and control. However, dose-response characteristics of gaseous anesthetics are difficult to measure. Although achievement of an anesthetic state depends on the concentration of the anesthetic in the brain, that concentration is impossible to measure under clinical conditions. Furthermore, neither the lower nor the upper ends of the graded dose-response curve can be ethically determined, since at very low concentrations severe pain might be experienced while at high concentrations there would be a high risk of fatal cardiovascular and respiratory depression. 

The lungs are the primary site of elimination for gaseous anesthetics and any other compounds that are volatile. For example, certain aromatic hydrocarbons are largely eliminated in the expired air. The major pathway for the elimination of ethanol, of course, is metabolism by the liver. However, approximately 2 percent is eliminated via the lungs. The equilibrium partition coefficient for ethanol between blood and alveolar air in humans is approximately 2100 1. Therefore, the ethanol concentration in end-expiratory air can be measured and multiplied by 2100 (e.g., by the Breathalyzer machine) to provide a fairly accurate estimate of ethanol concentration in the blood. 

Dentists were instrumental in the introduction of gaseous anesthetics because they came in daily contact with persons suffering from excruciating pain, often of their own making. It was during a theatrical production that Horace Wells, a dentist,... 

Perhaps the most unusual aspect regarding the structure of gaseous anesthetics is the lack of a basic chemical structure that can produce anesthesia. Table 11.3 compares the diversity of chemical classes that can produce anesthesia. 

Which of the following receptors is presently the leading candidate to interact with gaseous anesthetics ... 

Since 1,1-dichloroethane was once used as a gaseous anesthetic, it can be inferred that it causes central nervous system depression upon actue exposure. No information is available on the longterm neurologic effects of inhaled 1,1-dichloroethane in humans. 

No studies were located in humans or animals regarding the absorption of inhaled 1,1-dichloroethane. However, its use as a gaseous anesthetic agent in humans provides evidence of its absorption. Furthermore, the volatile and lipophilic nature of 1,1-dichloroethane favors pulmonary absorption. Structurally related chlorinated aliphatics and gaseous anesthetics are known to be rapidly and extensively absorbed from the lung. The total amount absorbed from the lungs will be directly proportional to the concentration in inspired air, the duration of exposure, the blood/air partition coefficient of 1,1-dichloroethane, its solubility in tissues, and the individual s ventilation rate and cardiac output. One of the most important factors controlling pulmonary absorption is the blood/air partition coefficient of the chemical. The concentration of the chemical and the duration of exposure are also important determinants of the extent of systemic absorption. 

No studies were located in humans or animals regarding the distribution of 1,1-dichloroethane following inhalation exposure. However, since this chemical was once used as a gaseous anesthetic, it can be assumed that it is distributed to the central nervous system as well as to the other tissues of the body. Tissue uptake of halocarbons such as 1,1-dichloroethane is governed by the affinity of each tissue for the lipophilic chemical (i.e. the higher the lipid content of a tissue, the greater its uptake of 1,1-dichloroethane) (Sato and Nakajima 1987)... 

C6. Carpenter, F. G., Alteration in mammalian nerve metabolism by soluble and gaseous anesthetics. Am. J. Physiol. 187, 573-578 (1956). 

Start with a binary molecular compound. Note that a binary molecular compound is composed only of two nonmetal atoms—not metal atoms or ions. An example is dinitrogen monoxide (N2O), a gaseous anesthetic that is more commonly known as nitrous oxide or laughing gas. The naming of nitrous oxide is explained in the following rules. 

The minimum alveolar concentration (MAC) is defined as the conoentration at 1 atmosphere of anesthetic in the alveoli that is required to produce immobility in 50% of adult patients subjected to a surgical incision. A further inorease to 1.3 MAC frequently will oause immobility in 99% of patients. At equilibrium, the conoentration (or partial pressure) of an anesthetic in the alveoli is equal to that in the brain, and it is this concentration in the brain that probably most olosely reflects the concentration at the site responsible for the anesthetio aotions. Thus, the MAC often is used as a measure of the potenoy of individual anesthetic agents. The MAC of many of the volatile and gaseous anesthetics in use today is shown in Table 18.2. 

A corresponding pseudo-steady-state partitioning takes place between the toxicant vapor and this same site of action for gaseous anesthetic... 

Gaseous or highly volatile liquid anesthetics are often preferred in surgical procedures because once inhaled, these vapors can quickly enter the bloodstream through the alveoli and then enter the brain. Several common gaseous anesthetics are shown with their boiling points ... 

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