Nitrous oxide coefficients

The narcotic potency and solubiUty in oHve oil of several metabohcaHy inert gases are Hsted in Table 10. The narcotic potency, ED q, is expressed as the partial pressure of the gas in breathing mixtures requited to produce a certain degree of anesthesia in 50% of the test animals. The solubiUties are expressed as Bunsen coefficients, the volume of atmospheric pressure gas dissolved by an equal volume of Hquid. The Hpid solubiHty of xenon is about the same as that of nitrous oxide, a commonly used light anesthetic, and its narcotic potency is also about the same. As an anesthetic, xenon has the virtues of reasonable potency, nonflammability, chemical inertness, and easy elimination by the body, but its scarcity and great cost preclude its wide use for this purpose (see Anesthetics). 

Sevoflurane. Sevoflurane, l,l,l,3,3,3-hexafluoro-2-propyl fluromethyl ether [28523-86-6] is nonpungent, suggesting use in induction of anesthesia. The blood/gas partition coefficient is less than other marketed products (Table 1) yet similar to nitrous oxide, suggesting fast onset and recovery. In animal studies, recovery was faster for sevoflurane than for isoflurane, enflurane, or halothane (76). Sevoflurane is stable to light, oxygen, and metals (28). However, the agent does degrade in soda lime (77). 

The solubility coefficient S is used as a measure of water solubility. It is the ratio between the concentrations in water and air phases at equilibrium. Ethanol, a very soluble gas, has a solubility coefficient of 1 100 at, 37 C while the coefficient for nitrous oxide, a poorly soluble gas, is 0.1.5. 

Nitrous oxide is rapidly absorbed through inhalation, and it is distributed predominantly in blood with a blood/gas partition coefficient of 0.5 (Sten-qvist 1994). It is rapidly eliminated through the lungs, with small amounts being eliminated through the skin (Stenqvist 1994). 

By pulse radiolysis of nitrous oxide-saturated aqueous solutions of ferricyanide (2 X 10 " M) and various alcohols (0.1 M), Adams and Willson " were able to obtain absolute rate coefficients for the ferricyanide oxidation of the radicals derived from the alcohols by attack of the solvent irradiation product, OH-. 

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Tamimi K., Rinker E.B., and Sandall O.C. (1994) Diffusion coefficients for hydrogen sulfide, carbon dioxide, and nitrous oxide in water over the temperature range of 293-368 K. /. Chem. Eng. Data 39, 330-332. 

These are shown in Table 3.3. Xenon (MAC 70%) has a potency of about twice that of nitrous oxide (MAC 104%). Thus, it can be given in anaesthetic concentrations in o> gen with less risk of hypoxia. It is highly insoluble in all body tissues with a blood/gas partition coefficient of 0.14 (nitrous oxide, 0.47 sevoflurane, 0.65 desflurane, 0.42). 

One of the most important factors influencing the transfer of an anesthetic from the lungs to the arterial blood is its solubility characteristics (Table 25-2). The blood gas partition coefficient is a useful index of solubility and defines the relative affinity of an anesthetic for the blood compared with that of inspired gas. The partition coefficients for desflurane and nitrous oxide, which are relatively insoluble in blood, are extremely low. When an anesthetic with low blood solubility... 

Tensions of three anesthetic gases in arterial blood as a function of time after beginning inhalation. Nitrous oxide is relatively insoluble (blood gas partition coefficient = 0.47) methoxyflurane is much more soluble (coefficient = 12) and halothane is intermediate (2.3). 

Inhaled anesthetics that are relatively insoluble in blood (ie, possess low blood gas partition coefficients) and brain are eliminated at faster rates than the more soluble anesthetics. The washout of nitrous oxide, desflurane, and sevoflurane occurs at a rapid rate, leading to a more rapid recovery from their anesthetic effects compared with halothane and isoflurane. Halothane is approximately twice as soluble in brain tissue and five times more soluble in blood than nitrous oxide and desflurane its elimination therefore takes place more slowly, and recovery from halothane- and isoflurane-based anesthesia is predictably less rapid. 

The heterogeneous decomposition of nitrous oxide in contact with a heated gold wire is interesting, though undoubtedly rather exceptional. The reaction is unimole-cular with respect to nitrous oxide. The heat of activation, obtained from the temperature coefficient of the reaction velocity, is 29,000 calories. 

Inhaled anesthetics that are relatively insoluble in blood (low blood gas partition coefficient) and brain are eliminated at faster rates than more soluble anesthetics. The washout of nitrous oxide, desflurane, and sevoflurane occurs at a rapid rate, which leads to a more rapid recovery from their anesthetic effects compared to halothane and isoflurane. Halothane is approximately twice as soluble in brain tissue and five times more soluble in blood than nitrous oxide and desflurane its elimination therefore takes place more slowly, and recovery from halothane anesthesia is predictably less rapid. The duration of exposure to the anesthetic can also have a marked effect on the time of recovery, especially in the case of more soluble anesthetics. Accumulation of anesthetics in tissues, including muscle, skin, and fat, increases with continuous inhalation (especially in obese patients), and blood tension may decline slowly during recovery as the anesthetic is gradually eliminated from these tissues. Thus, if exposure to the anesthetic is short, recovery may be rapid even with the more soluble agents. However, after prolonged anesthesia, recovery may be delayed even with anesthetics of moderate solubility such as isoflurane. 

Figure 11.10 Meyer-Overton correlation for volatile general anesthetics in mice. The slope of the regression line is -1.02 and the correlation coefficient, r2 = 0.997. CTF, carbon tetrafluoride NIT, nitrogen ARG, argon PFE, perfluoroethane SHF, sulfur hexafluoride KRY, krypton N02, nitrous oxide ETH, ethylene XEN, xenon DDM, dichlorodifluoromethane CYC, cyclopropane FLU, fluroxene DEE, diethylether ENF, enflurane ISO, isoflurane HAL, halothane CHL, chloroform MOF, methoxyflurane.

FIGURE 7.10 Wavelength dependent enrichment coefficients for the nitrous oxide isotopologues in the region of stratospheric photolysis. Temperature is set to 298 K. 

Hunter in 1905 investigated the thermal decomposition of nitrous oxide by flowing the gas through a porcelain bulb in a furnace and measuring the decomposition for different contact times no attempt was made to discover the effect of surface or to measure the effect ofa wide variation in pressure. Hunter concluded that the nitrous oxide decomposition was second order and expressed the rate coefficient as... 

Agents that have low solubility in blood, i.e., a low blood/gas partition coefficient (nitrous oxide, sevoflurane), provide a rapid induction of anaesthesia because the blood reservoir is small and agent is available to pass into the brain sooner. 

Nitric oxide concentrations at given points within the flame were measured mass spectrometrically with known small concentrations of nitrous oxide injected into the reaction mixture. Hence the atomic oxygen concentration could be determined using the rate coefficient for (27), k2 = 2x 10 exp (—32,000// r) l.mole" . sec" It was shown that nitric oxide itself would not decompose appreciably under the conditions of the experiment. 

Anaesthetic gases such as ether which have a high blood solubility (Ostwald solubility coefficient in blood is 12) are transported away from the lungs more rapidly than those such as halothane (Ostwald coefficient = 2.3) and nitrous oxide (Ostwald coefficient = 0.47). As... 

---