The kinetics of inhaled anesthetics is fundamental to the clinical practice of anesthesia.1 This subject is often called uptake and distribution of anesthetics.2 It explains the time course of anesthetic movement from the delivery system to the site of action, the patient's central nervous system. Although the site of action of inhaled anesthetic agents includes the brain and spinal cord, “brain” will be used alone in the remainder of this chapter. See Chapter 1 for further discussion of anesthesia site of action.
Inhaled agents are either gases or vapors, depending on their physical state at room temperature and pressure. Nitrous oxide, cyclopropane, and xenon are gases. For these agents, the anesthetic source is a flow controller with flow meter. Halothane, isoflurane, sevoflurane, and desflurane, as well as the historical agents diethyl ether, methoxyflurane, fluroxene, and enflurane are vapors. In this chapter, the agent source for all of these will be called a vaporizer. The physical properties of most of these agents are listed in Table 2–1.
Table 2–1Physical and kinetic properties of anesthetic agents. ||Download (.pdf) Table 2–1 Physical and kinetic properties of anesthetic agents.
| ||Ether ||Halothane ||Enflurane ||Isoflurane ||Sevoflurane ||Desflurane ||N2O ||Xenon |
|MAC (%) ||1.9 ||0.8 ||1.7 ||1.1 ||2.1 ||6.0 ||110 ||70 |
|Blood/gas solubility ||12.1 ||2.47 ||1.90 ||1.30 ||0.65 ||0.42 ||0.47 ||0.13 |
|VRG/blood solubility ||1.10 ||1.94 ||1.47 ||1.62 ||1.69 ||1.29 ||0.89 ||1.30 |
|Mus/blood solubility ||0.90 ||4.01 ||2.42 ||3.46 ||3.69 ||2.31 ||1.15 ||2.00 |
|Fat/blood solubility ||5.00 ||60.7 ||33.2 ||53.8 ||52.3 ||31.0 ||2.30 ||10.0 |
|Alveolar plateau height ||0.06 ||0.24 ||0.30 ||0.38 ||0.55 ||0.66 ||0.63 ||0.86 |
|1.7 ||3.1 ||2.3 ||2.6 ||2.7 ||2.0 ||1.4 ||2.1 |
|0.6 ||2.4 ||1.5 ||2.1 ||2.3 ||1.4 ||0.7 ||1.2 |
|4 ||49 ||27 ||43 ||42 ||25 ||2 ||8 |
The measure of anesthetic level in a compartment or location is the partial pressure. Partial pressure is also called tension. Tension is a generic term that applies to variables that equalize in connected locations. Examples are hydrostatic tension (water height) and electrical tension (voltage). The interchangeable terms “high-tension wires” and “high-voltage wires” are familiar examples.
Anesthetic partial pressure or tension could be expressed in common pressure units such as mm Hg, Pa, or kPa. However, the most commonly used unit for anesthetic partial pressure is % atm (percent of one sea level atmosphere). One percent partial pressure represents 1% × 760 mm Hg = 7.6 mm Hg. The anesthetic tension is then said to be 1%. Commercial vaporizers state their delivered tension in percent and thus are consistent with this description.
Equilibrium is achieved when the tensions in compartments are equal. The locations or compartments of interest for inhaled agents are breathing circuit (inspired gas), lungs (alveolar gas), arterial blood, ...