Eger E I, Ionescu P, Gong D
Department of Anesthesia, University of California-San Francisco, 94143-0464, USA.
Anesth Analg. 1998 May;86(5):1070-4. doi: 10.1097/00000539-199805000-00032.
Uptake of inhaled anesthetics may be measured as the amount of anesthetic infused to maintain a constant alveolar concentration of anesthetic. This method assumes that the patient absorbs all of the infused anesthetic, and that none is lost to circuit components. Using a standard anesthetic circuit with a 3-L rebreathing bag simulating the lungs, and simulating metabolism by input of carbon dioxide, we tested this assumption for halothane, isoflurane, and sevoflurane. Our results suggest that after washin of anesthetic sufficient to eliminate a material difference between inspired and end-tidal anesthetic, washin to other parts of the circuit (probably the ventilator) and absorbent (soda lime) continued to remove anesthetic for up to 15 min. From 30 min to 180 min of anesthetic administration, circuit components absorbed trivial amounts of isoflurane (12 +/- 13 mL vapor at 1.5 minimum alveolar anesthetic concentration, slightly more sevoflurane (39 +/- 15 mL), and still more halothane (64 +/- 9 mL). During this time, absorbent degraded sevoflurane (321 +/- 31 mL absorbed by circuit components and degraded by soda lime). The amount degraded increased with increasing input of carbon dioxide (e.g., the 321 +/- 31 mL increased to 508 +/- 48 mL when carbon dioxide input increased from 250 mL/min to 500 mL/min). Measurement of anesthetic uptake as a function of the amount of anesthetic infused must account for these findings.
Systems that deliver inhaled anesthetics may also remove the anesthetic. Initially, anesthetics may diffuse into delivery components and the interstices of material used to absorb carbon dioxide. Later, absorbents may degrade some anesthetics (e.g., sevoflurane). Such losses may compromise measurements of anesthetic uptake.
吸入麻醉药的摄取量可通过输注麻醉药的量来衡量，以维持麻醉药的肺泡浓度恒定。该方法假定患者吸收了所有输注的麻醉药，且无麻醉药流失到回路组件中。我们使用带有3升复吸袋模拟肺部的标准麻醉回路，并通过输入二氧化碳来模拟代谢，对氟烷、异氟烷和七氟烷进行了这一假设的测试。我们的结果表明，在麻醉药冲洗至消除吸入麻醉药与呼气末麻醉药之间的显著差异后，向回路其他部分（可能是呼吸机）和吸收剂（碱石灰）的冲洗在长达15分钟内持续去除麻醉药。在麻醉给药30分钟至180分钟期间，回路组件吸收的异氟烷量极少（在1.5最低肺泡麻醉浓度下为12±13毫升蒸气），七氟烷略多（39±15毫升），氟烷更多（64±9毫升）。在此期间，吸收剂使七氟烷降解（回路组件吸收并被碱石灰降解321±31毫升）。降解量随二氧化碳输入量增加而增加（例如，当二氧化碳输入量从250毫升/分钟增加到500毫升/分钟时，321±31毫升增加到508±48毫升）。将麻醉药摄取量作为输注麻醉药的量的函数进行测量时必须考虑这些发现。
输送吸入麻醉药的系统也可能去除麻醉药。最初，麻醉药可能扩散到输送组件和用于吸收二氧化碳的材料间隙中。后来，吸收剂可能会降解一些麻醉药（如七氟烷）。这种损失可能会影响麻醉药摄取量的测量。
