Anaesthetic gases and vapours

Abstract

Administration Volatile anaesthetic agents are liquids with a low boiling point (BP) and high saturated vapour pressure (SVP) so that they evaporate easily. The physical properties of each agent influence vaporisation. Volatile agents have higher saturated vapour pressures and lower boiling points than water. Most have a characteristic smell, which can also be pleasant. Volatile agents are administered via inhalation through the lungs and so enter the circulation via the pulmonary alveolar capillaries. Intravenously administered agents are injected into a small part of the venous system and are then diluted by mixing with other sources of the venous blood. The injected agent then passes through the right heart before reaching the pulmonary circulation. Inhaled agents bypass this venous phase and are fairly evenly spread through the ventilated alveoli. However, there is some delay in achieving sufficiently high alveolar concentrations for induction of anaesthesia. Uptake of inhaled anaesthetic agents Inhaled anaesthetics are administered so as to achieve levels in central neural tissue sufficient to produce anaesthesia without detrimental effects on other organs. At equilibrium, the partial pressures of the agents will be identical throughout the body, but the concentrations in different tissues will be determined by the partition coefficients. When a difference in partial pressure exists between two compartments, there will be a movement down the pressure gradient until equilibrium is achieved.