Blood:gas solubility coefficient

Xenon 131 kDa boiling point —108 °C blood gas solubility coefficient 14 oibgas solubility coefficient 1.9 MAC 71 odourless. 

It is a isomer of enflurane and its chemical and physical properties are similar to enflurane, but it is approximately Vi times more potent, more volatile. It has a lower blood gas solubility coefficient than enflurane. It produces rapid induction and recovery. 

Sevoflurane is a fluorinated methyl isopropyl ether (Figure 3.2). It was first released for clinical use in Japan in 1990 and it is now available in many other countries worldwide. Being relatively insoluble (blood/gas solubility coefficient, 0.68) it has the potential to provide rapid anaesthetic induction and recovery. Unlike isoflurane it is non-irritant to the airway and can be given in high concentrations for anaesthetic induction. Its MAC ranges from 3.3 in infants to 2.5 in older children and 1.8 in adults. 

Four highly fluorinated ethers with low boiling points are currently used in anesthesia enflurane, isoflurane, sevoflumne, and desflurane (Figure 8.89). Des-flurane and sevoflurane are now the most used (sevoflurane is especially used in pediatrics). They exhibit the lowest blood-gas partition coefficients, the lowest ratio of toxic metabolites, and the lowest solubilities in lipids. These features limit the retention and, consequently, the metabolism is delayed (Table 8.2). 

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. 

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. 

An anaesthetic that has high solubility in blood, i.e., a high blood/gas partition coefficient, will provide a slow induction and adjustment of the depth of anaesthesia. This is because the blood acts as a reservoir (store) for the drug so that it does not enter the brain easily imtil the blood reservoir has been filled. A rapid induction can be obtained by increasing the concentration of drug inhaled initially and by hyperventilating the patient. 

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. 

Sevoflurane is a relatively new inhalation anesthetic in veterinary medicine. It has a low solubility in the blood. With a blood gas partition coefficient that is half that of isoflurane, sevoflurane should produce a more rapid induction, allow rapid change of anesthetic plane during surgery and result in a more rapid recovery in horses (Aida et al 1996). In one study in horses. 

The blood-gas partition coefficient is an index of the solubility of an anesthetic or its induction time, as depicted by the following examples. 

The solubility of the vapor in the blood. The higher flic blood/gas partition coefficient, the more rapidly the vapor will diffuse into the blood, until equihbrium is achieved. At equilibrium the net diffusion between blood and air ceases, but the concentrations in air and blood may still be different. A highly soluble vapor will therefore demonstrate a lower alveolar air concentration relative to the inhaled air concentration during exposure, and a higher alveolar concentration relative to the inhaled air concentration after exposure. 

When the inhalational agents are delivered by arterial blood to the tissues, the partial pressure rises in tissues to approach that in arterial blood. The rate at which a gas diffuses into tissues depends on (1) the solubility of the gas in the tissues, (2) the rate at which the gas is delivered to the tissues, and (3) the partial pressures of the gas in arterial blood and tissues. The solubility of the gas in the tissues is expressed as a tissue-blood partition coefficient, a concept analogous to the blood-gas partition coefficient discussed previously. 

Chloroform is lipid soluble and readily passes through cell membranes, causing narcosis at high concentrations. Blood chloroform concentrations during anesthesia (presumed concentrations 8,000-10,000 ppm) were 7-16.2 mg/mL in 10 patients (Smith et al. 1973). An arterial chloroform concentration of 0.24 mg/mL during anesthesia corresponded to the following partition coefficients blood/gas, 8 blood/vessel rich compartment, 1.9 blood/muscle compartment, 1.9 blood/fat compartment, 31 blood/vessel poor compartment, 1 and blood/liver, 2 (Feingold and Holaday 1977). Recently, partition coefficients were calculated for humans based on results in mice and rats, and in human tissues in vitro blood/air, 7.4 liver/air, 17 kidney/air, 11 and fat/air, 280 (Corley et al. 1990). 

The inhalational anesthetics have distinctly different solubility (affinity) characteristics in blood as well as in other tissues. These solubility differences are usually expressed as coefficients and indicate the number of volumes of a particular agent distributed in one phase, as compared with another, when the partial pressure is at equilibrium (Table 25.3). For example, isoflurane has a blood-to-gas partition coefficient (often referred to as the Ostwald solubility coefficient) of approximately 1.4. Thus, when the partial pressure has reached equilibrium, blood will contain 1.4 times as much isoflurane as an equal volume of alveolar air. The volume of the various anesthetics required to saturate blood is similar to that needed to saturate other body tissues (Table 25.3) that is, the blood-tissue partition coefficient is usually not more than 4 (that of adipose tissue is higher). 

For blood at 37 C, the normal mean value is p/C P) = 6.103, with a normal biological standard deviation (SD) of about 0,0015, mainly caused by variations in ionic strength. The solubility coefficient for CO2 gas, a, also varies with the composition of the solution. For pure water at 37 °C, the solubility coefficient a = 0.0329 mmol x L x mmHg , and for normal plasma at 37 °C it is 0.0306 mmol x L X mmHg", with a biological SD of about 0.0003 mmol x L X mmHg ... 

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