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BASIC PRINCIPLES

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According to Henry’s law, gas solubility describes the tendency of a gas to equilibrate with a solution. When a gas comes into contact with a solution, the gas molecules will move into and dissolve in the liquid. Some of the gas molecules will then move back from the liquid phase to the gas phase. At equilibrium, the number of gas molecules moving from the gas phase to the liquid phase will equal the number of molecules moving from the liquid phase to the gas phase. For example, equilibrium exists when arterial blood with a Pao2 of 100 mm Hg contacts an alveolar gas mixture also with PO2 of 100 mm Hg (Figure 9-1). There is no net gain or loss of O2 between the two phases.

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FIGURE 9-1

At equilibrium, the partial pressure of O2 in both the liquid and the gas phases is equal.

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The pharmacologic effect of an inhalation agent is determined by the partial pressure of the anesthetic in the brain. At equilibrium, brain partial pressure equals the anesthetic partial pressure in arterial blood. In the absence of transpulmonary shunt, alveolar gases equilibrate with pulmonary capillary and arterial blood gases. The partial pressure of the anesthetic in arterial blood, therefore, will equal its alveolar partial pressure.

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According to Dalton’s law, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of individual gases. By applying Dalton’s law, it is possible to calculate the alveolar partial pressure of an inhalation agent: the product of the total gas pressure (atmospheric pressure) multiplied by the fraction of alveolar concentration (FA) of the anesthetic. For example, if the FA of desflurane is 5% and the atmospheric pressure is 760 mm Hg, then the alveolar partial pressure of desflurane will be 760 × 0.05 = 38 mm Hg. In the absence of transpulmonary shunt, desflurane partial pressure in arterial blood and in the brain will also be 38 mm Hg (Figure 9-2).

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FIGURE 9-2

Desflurane equilibrium across tissues.

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The partial pressure of a gas in a liquid at a certain temperature is primarily determined by the solubility of that gas in the liquid. Higher gas solubility within a liquid results in lower gas partial pressures. Gas exerts more pressure in a liquid if the gas molecules exist in a free kinetic form within the liquid. Greater solubility means that the gas molecules are more tightly bound to the liquid molecules, resulting in less free, active gas molecules available to exert pressure. This relationship explains the counterintuitive phenomenon that the partial pressure of a highly soluble inhalation agent rises slowly in the blood despite the fact that it ...

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