HIGH TO LOW FRESH GAS FLOW SEQUENCE DURING INDUCTION: COMPARISON OF DESFLURANE/N20 AND ISOFLURANE/N20

A. BENGTSSON, W. STEVENS.

Portland, Oregon, USA.

When using a circle system to deliver inhaled agents, it is ideal to change to low fresh gas flows (FGF) quickly to maximize the advantages of the system. Poorly soluble agents allow rapid change without decrease of their end-tidal concentrations (Fet). We compared the rate at which we could achieve an anesthetizing Fet of desflurane (DES) or isoflurane (ISO) delivered with high flow, 4L/min, to a circle system and the allowable rate of decrease to low flow, 0.5 L/mm, without causing a sag in Fet

After IRB approval, we studied 28 ASA I or II patients, divided equally into DES/N₂O and ISO/N20 groups. After tracheal intubation and with Pe(C02 controlled between 35-40 mmHg, inhaled induction was done with FGF of 2.8 L/min N20, 1.2L/min02 and 1.5 MAC delivered (Fd DES or ISO via a standardized circle system. When Fei DES or ISO reached 1.1 MAC (6.6% for DES, 1.3% for ISO), FGF was decreased by 0.5 L/min to 3.5 L/min, keeping the N20 to 02 ratio constant and Fd DES or ISO at 1.5 MAC. Successive 0.5 L/min decreases were made until total FGF was 0.5 L/mm. We compared DES and ISO in time to achieve Fei = 1.1 MAC and rate of change to low How. Median values and 25-75 percentiles were calculated. Wilcoxon's test was used for statistical analysis.

Fet = 1.1 MAC was reached in 3.8 min (2-5.5) with DES, and 15.5 min (10-28) with ISO. FGF could be decreased to low flow within 10 minutes without sag in Fet DES. Longer times at each FGF were required with ISO to allow Fet to rise to the target level before  making the next increase in FGF (p<0.05)

 

The times to achieve an anesthetizing Fet of DES or ISO were similar to reported values

obtained under similar conditions. Only modest Fd overpressure and FGF lead to rapid induction and sustained Fet as FGF is decreased with DES whereas longer periods of higher FGF or Fd would be needed with ISO for rapid induction and stable Fet.

SERUM FLUORIDE LEVELS FOLLOWING LOW-FLOW SEVOFLURANE COMPARED WITH ISOFLURANE

J MacHale, M. Carroll, V. Hannon

Dublin

Introduction: Sevoflurane is an inhaled anaesthetic agent with recent approval for use in Ireland. Concern has been raised over increased fluoride levels and possible detrimental effects on renal function associated with the use of Sevoflurane. The aim of this study was to determine the effect of Sevoflurane compared with Isoflurane on serum fluoride levels using low-flow rates (^1 L/min) in a circle absorber system.

Methods: Twenty-two ASA physical status I-III patients were prospectively studied. Patients were randomly assigned (1:1) to receive either Sevoflurane or Isoflurane during low-flow (≤ 1 L/min) anaesthesia for a minimum two hour exposure with fresh Baralyme as the carbon dioxide absorbent in each case. Plasma inorganic fluoride ion concentrations were measured at pre-induction, emergence, and at 2, 24, and 72 hours post-operatively. Renal function was also assessed by monitoring BUN, serum creatinine, urinary glucose and urinary protein pre- and post-anaesthesia (24 and 72 hours). Safety was also assessed through the collection of adverse experience data. p values ^0.05 were considered statistically significant.

Results: Demographic data were similar between the two groups. MAC concentrations of the two anaesthetic agents were also similar between the two groups with Sevoflurane 0.83 (±0,021) MAC and Isoflurane 0.83 (±0.025) MAC. Flow rates used were 0.5 - 1.0 L/min, with the majority of patients in the range of 0.8 to 1.0 L/min. Baseline serum fluoride concentrations were similar for the two groups (1.79 for the Sevoflurane group and 1.42 for the Isoflurane group). There was a statistically significant difference (p<0.05) between the emergence, 2, 24, and 72 hour serum fluoride levels in the two groups with the levels in the Sevoflurane group being significantly higher than the levels in the Isoflurane group (32.95, 32.9, 13.6, and 3.87 compared with 2.378, 3.29, 3.637 and 2.158). There were no statistically significant differences between the groups for any of the renal function variables. The incidence of adverse experiences was similar for the two treatment groups.

Conclusion: Although use of Sevoflurane results in a significant elevation in serum fluoride levels compared with the use of Isoflurane, no difference in renal effects of the two agents was documented in this group of patients undergoing low-flow anaesthesia,   

This study was supported by a grant from Abbott Laboratories.

Vaporiser in Circuit: practicalities and potentials

GF Nunn

Leeds

The use of a vaporiser in the circle system (VIC) has a long and respected history, but has in the past been associated only with spontaneous breathing and high fresh gas flows. There is now wide availability of infrared gas analysers and considerable interest in the financial and environmental advantages of low fresh gas flows. This makes it appropriate to re-evaluate the use of VIC for ventilated patients.

The classic drawover design is the Goldman which dates from the introduction of Halothane. Initially produced with a 2% maximum output, it and its competitors were later revised to give a maximum output of around 2.8% once further experience had been gained with this new potent and relatively insoluble agent.

The Goldman vaporiser is no longer produced in the UK, though pattern Goldmans are still readily and cheaply available in the Asian market. Almost all other more sophisticated vaporiser designs are unsuitable for use in a circle system, due mainly to problems with high maximum output and the problems of water condensation within their wick systems.

There are currently two Goldman type vaporisers readily available in the UK. These are the McKesson Mark Three (Chesterfield, England) and the Komesaroff (Australia), sold mainly to the dental and veterinary markets respectively. Both are intended for use in the fresh gas supply and can deliver between 2% and 2.5% Isoflurane. Their characteristics are very similar and not ideal but contrast markedly with the McKesson Mark One whose maximum output of some 0.8% Isoflurane and more sophisticated control mechanism make it more suitable for use within a circle system.

The use of a vaporiser in the circle has long been known to pose a risk of rapidly rising vapour concentrations. This suggests that a lower maximum output would be advantageous. This, however, limits the maximum rate of rise of vapour concentration. Provided that gas flows and hence wastage are kept low this latter is not a serious problem.

The ideal drawover vaporiser for use in the circle may thus be a modification of the current McKesson or Komesaroff with its porting redesigned so as to reduce its maximum output.

Molecular sieves for low-flow xenon anaesthesia.

CW Renfrew, JM Murray, JPH Fee PhD FRCA.

Belfast.

Xenon is a non-explosive, inert gas with general anaesthetic properties which is non-teratogenic and does not undergo biotransformation(l). It has a blood-gas coefficient of 0.14(N₂0 is 0.47) and has a MAC of 71% (N₂0 is 110%) and is a suitable replacement for nitrous oxide. However, the expense of this gas dictates that its use be confined to low-flow or closed system anaesthesia. Problems exist however, with the accumulation of nitrogen inside the circle system causing an increased use of xenon (2,3). Zeolitic molecular sieves are aluminosilicates whose microporous structure enables them to selectively adsorb molecules of differing size. Zeolites 3A, 4A and 5A do not adsorb oxygen at atmospheric pressure. Aim: to investigate the adsorptive properties of three synthetic molecular sieves (3A.4A and 5A) with reference to nitrogen in the presence of oxvgen and xenon. Methods: using an in vitro system three gas mixtures were passed through 100 g of the three of zeolites. The following gas mixtures were used: (a)nitrogen 50% and oxygen 50%; (b)xenon 50% and oxygen 50%; (c) xenon 50% and nitrogen 50%.

Gas mixtures were passed through a canister containing 100g of zeolite at a total rate of 100ml/min. An initial gas sample was taken from the proximal sampling port of the canister

(time 0) and further gas samples were taken from the distal sampling port of the canister at 2,

4, 6 .8 and 10 minutes, into gas tight syringes. The samples were then analysed using gas chromatographv or infrared absorptive spectometry.

 

Results: Zeolite 3A did not adsorb nitrogen or xenon. Zeolite 5A adsorbed xenon and nitrogen. Zeolite 4A adsorbed nitrogen but not xenon (tables 1,2 and 3).

 

Conclusion: Zeolite 4A beads adsorb nitrogen in the presence of xenon, but the efficiency in nitrogen adsorption is decreased by the presence of the xenon gas.

 

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