Enflurane-halothane mixture

The concentration of an inhaled anesthetic in the inspired gas mixture has direct effects on both the maximum tension that can be achieved in the alveoli and the rate of increase in its tension in arterial blood. Increases in the inspired anesthetic concentration increase the rate of induction of anesthesia by increasing the rate of transfer into the blood according to Fick s law (see Chapter 1). Advantage is taken of this effect in anesthetic practice with inhaled anesthetics that possess moderate blood solubility (eg, enflurane, isoflurane, and halothane). For example, a 1.5% concentration of isoflurane may be administered initially to increase the rate of rise in the brain concentration the inspired concentration is subsequently reduced to 0.75-1% when an adequate depth of anesthesia is achieved. In addition, these moderately soluble anesthetics are often administered in combination with a less soluble agent (eg, nitrous oxide) to reduce the time required for loss of consciousness and achievement of a surgical depth of anesthesia. 

Uptake curves for inhaled anesthetics Figure 11.6 illustrates the uptake curves for four inhalation anesthetics. The solubility in blood, as well as tissues, is in the following order halothane > enflurane > isoflurane > nitrous oxide. Because of its low solubility, the partial pressure of nitrous oxide in the inspired mixture and the body most rapidly achieves a steady-state. 

A. Classification and Pharmacokinetics The agents currently used in inhalation anesthesia are nitrous oxide (a gas) and several easily vaporized liquid halogenated hydrocarbons, including halothane, desflurane. enflurane, isoflurane, sevoflurane, and methoxyflurane. They are administered as gases their partial pressure, or tension, in the inhaled air or in blood or other tissue is a measure of them concentration. Since the standard pressure of the total inhaled mixture is atmospheric pressure (760 mm Hg at sea level), the partial pressure may also be expressed as a percentage. Thus 50% nitrous oxide in the inhaled air would have a partial pressure of 380 mm Hg. The speed of induetion of anesthetic effects depends on several factors ... 

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