Minimal and medium flow anaesthesia with isoflurane and desflurane: Effects on inspired and expired oxygen and anaesthetic gas concentrations

摘要

Background: In this study we aimed to compare haemodynamics, body temperature, inspired and expired oxygen and anaesthetic gas concentrations in minimal and medium flow anaesthesia with isoflurane and desflurane. Methods: We studied 60 ASA 1-2 patients undergoing elective surgical procedures Patients were randomly divided to two equal main groups to receive isoflurane and desflurane. Then these main groups were randomly divided to 3 equal sub-groups such as to receive isoflurane or desflurane in 500, 1000 and 2000 ml.min-1 fresh gas flow (FGF) respectively. FGF was applied 4 L min-1 in initial phase (10 min) after standard anaesthetic induction, then isoflurane and desflurane concentrations were adjusted as 1.5 % and 6 % respectively and FGF was adjusted according to groups. HR, MAP, SpO2 oesophageal temperature, vaporizer settings, inspired and expired anaesthetic gas concentrations were recorded at regular intervals throughout the study.Results: Inspired and expired anaesthetic concentrations were decreased significantly in minimal flow groups compared to medium and high flow groups. FiO2 values were decreased parallel to duration of anaesthesia. Low FiO2 occurred in 2 cases in minimal flow isoflurane group and 8 cases in minimal flow desflurane group. Conclusion: Isoflurane and desflurane could be used safely with minimal and medium FGF, but we decided that isoflurane was superior to desflurane regarding oxygenation. We thought that there was hypoxia risk in cases which desflurane was used as inhalant agent in minimal flow with 50 % N2O in O2. However, we concluded that the increasing FiO2 % ratio can prevent hypoxia. Introduction There is increasing interest in the subject of inhalation anaesthesia using low rates of fresh gas flow. The reasons for this interest are not hard to discover. The lower the fresh gas flow, the less do anaesthetic agents pollute the environment. Very considerable reductions in the cost of expensive agents can be made (1).The major risks which have been identified if low-flow anaesthesia is used inappropriately are accidental hypoxia, over- or under dosage of volatile anaesthetics, hypercapnia, and accumulation of potentially toxic trace gases (2). In low-flow anaesthesia, there is a marked difference between the fresh gas concentration of the anaesthetic and its concentration within the breathing system, and this difference increases with the decreasing fresh gas flow, but decreases with decreasing solubility of the anaesthetic agent such as desflurane. If the concentration of the volatile anaesthetic shall be changed, the vaporizer has to be adjusted to a concentration considerably exceeding the aspired nominal value (3).Isoflurane is considered to be the most suitable of the group of older anaesthetic agents to be used in low flow systems (4). It was reported that desflurane is suitable for low flow techniques with maximum output vaporizer. In addition, due to low blood/gas solubility of desflurane, it may be superior to isoflurane when used in low flow anaesthesia (5). In this study, we aimed to compare haemodynamics, body temperature, inspired and expired oxygen and anaesthetic gas concentrations in minimal and medium flow anaesthesia with isoflurane and desflurane. Methods With institutional ethics committee approval and written, informed consent from the subjects, we studied sixty ASA 1-2 patients undergoing elective surgical procedures. Patients with acute or chronic pulmonary, cardiac or metabolic disease, hepatic or renal dysfunction, anaemia, weight over ideal body weight of 30% or under 50 kg, history of current or past smoking or alcohol addiction, and younger than 18 years old were excluded from the study. Patients were randomly divided to two equal main groups to receive isoflurane (group I, n: 30) and desflurane (group D, n: 30). Then these main groups were randomly divided to three equal sub-groups such as to receive isoflurane or desflurane in 500 ml.min -1 fresh gas flow (group