High initial concentration versus low initial concentration sevoflurane for inhalational induction of anaesthesia

Sevoflurane induction for general anaesthesia has been reported to be safe, reliable and well accepted by patients. Sevoflurane induction uses either low or high initial concentrations. The low initial concentration technique involves initially administering a low concentration then gradually increasing the dose until the patient is anaesthetized. The high initial concentration technique involves administering high concentrations from the beginning, continuing until the patient is anaesthetized.

We aimed to compare the induction times and complications between high and low initial concentration sevoflurane induction in patients who received inhalational induction for general anaesthesia. We defined 'high' as greater and 'low' as less than a 4% initial concentration.

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2011, Issue 9); MEDLINE (1950 to September 2011); EMBASE (1980 to September 2011); LILACS (1982 to September 2011) and ISI Web of Science (1946 to September 2011). We also searched the reference lists of relevant articles, conference proceedings; and contacted the authors of included trials.

We sought all published and unpublished, randomized controlled trials comparing high versus low initial sevoflurane concentration inhalational induction. Our primary outcomes were two measures of anaesthesia (time to loss of the eyelash reflex (LOER) and time until a weighted object held in the patient's hand was dropped), time to successful insertion of a laryngeal mask airway (LMA), and time to endotracheal intubation. Other outcomes were complications of the technique.

We used the standardized methods for conducting a systematic review as described by the Cochrane Handbook for Systematic Reviews of Interventions. Two authors independently extracted details of trial methodology and outcome data from reports of all trials considered eligible for inclusion. All analyses were made on an intention-to-treat basis, where possible. The overall treatment effects were estimated by using a fixed-effect model when there was no substantial heterogeneity, whereas the random-effects model was applied in the presence of considerable heterogeneity.

We used data from 10 studies with 729 participants in the review, though most analyses were based on data from fewer participants. There was substantial heterogeneity in the trials. Thus, our results should be read with caution. It was not possible to combine the trials for the primary outcome (LOER) but individual trials found faster induction times (typically 24 to 82 seconds faster) with high initial concentration sevoflurane. Apnoea appeared to be more common in the high initial concentration sevoflurane group (two trials,160 participants). There was no evidence of a difference in the incidence of cough, laryngospasm, breath holding, bradycardia, salivation and hypotension between the two groups, with the overall incidence of complications being low.

A high initial concentration sevoflurane technique probably offers more rapid induction of anaesthesia and a similar rate of complications except for apnoea, which may be more common with a high initial concentration. However, this conclusion is not definitive.