Department of Anaesthetics, Critical Care, and Pain Medicine, Royal Infirmary, Edinburgh EH3 9YW, UK*Corresponding author
Published in abstract form: Br J Anaesth 2000; 85: 156P.
Accepted for publication: March 29, 2001
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
Br J Anaesth 2001; 87: 2868
Keywords: anaesthetics volatile, sevoflurane; anaesthetic gases, nitrous oxide; anaesthetic techniques, inhalation; anaesthetic techniques, induction
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Methods and results |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
A disposable Mapleson D co-axial breathing circuit (Bain circuit) was used. Gas for carbon dioxide analysis (Datex Cardiocap 3) was sampled from the gas source side of a filter fitted to the mask. The flow rate was first set at 10 litre min1 of oxygen. A well fitting facemask was applied and the patient was instructed to breathe normally. The reservoir bag and the capnograph were carefully observed to ensure that no leaks were present, and that a secondary peak of carbon dioxide concentration was present during inspiration, indicating partial rebreathing. After the patient had taken three satisfactory breaths, the gas flow was reduced, to either oxygen of 3 or 1.5 litre min1 and nitrous oxide 1.5 litre min1 according to the random allocation. At the same time 0.5% sevoflurane was set on the vapourizer. After each subsequent three breaths, the sevoflurane concentration was doubled until 8% was reached. If coughing occurred, the concentration increase was not applied until breathing was settled for three breaths. After the sevoflurane concentration had been maintained at 8% for 15 s, fresh gas flow was increased to 6 litre min1. Any excitatory events such as vocalization, or movements of the head, neck, body or limbs, or reduction of the pulse oximeter reading to less than 94%, was noted. We also noted any cessation of breathing movements for more than 10 s, which was considered to be apnoea.
The study stopped when the arm had come down, and anaesthesia was continued as appropriate clinically. Values are expressed as mean (SD) and compared between the groups with Students t-test (induction time) and the chi-squared test (excitation and apnoea) (Minitab v. 13.1). We accepted P<0.05 as significant.
Twenty-six patients received nitrous oxide and oxygen, and 36 patients received oxygen only, as carrier gas. The features and the induction characteristics of the groups are shown in Table 1. The two groups were similar in age and weight. Induction times were very similar: 102 (24) s with nitrous oxide/oxygen, and 102 (27) s with oxygen. The 95% confidence limits for a difference in these times are ±13 s.
|
![]() |
Comments |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
The time to induction of anaesthesia in our study was greater than in other studies of inhalation induction, which report times of about 1 min3 6 presumably because we increased the fresh gas vapour concentration gradually, over about 30 s. However the increased time was offset by the small incidence of excitation and apnoea, which was less than reported by others.3 5 6
We did not conceal the use of the carrier gas from the anaesthetist giving the anaesthetic. Although studies that involve an observer assessment of quality should be double blind, we limited our assessment to features that we considered unambiguous: pulse oximeter readings, breathing movements, arm position, and any movement of the head or limbs. Different measures are used to define the induction of anaesthesia, but we have found that the measure we used in this study is equivalent to others, such as loss of voluntary finger tapping.7 Arm position gave an easily visible endpoint without needing intermittent stimulation of the patient as would occur with finger tapping or loss of lash reflex. It can be affected by excitatory side effects, and in two patients, excitatory effects made the endpoint of arm lowering equivocal. Exclusion of these times from analysis did not affect the statistical conclusion. We studied a sufficient number of patients to find a significant difference in minor and generally clinically insignificant excitation during induction, and found that nitrous oxide had no effect on the time for induction of anaesthesia.
In theory, nitrous oxide can speed induction of anaesthesia by adding its effect to that of sevoflurane, and also perhaps by the second gas effect. However, the concentrations used in this study would limit the importance of the latter mechanism. We cannot be certain what the inspired concentration of nitrous oxide would have been. It would have been less than 50% because of the limited fresh gas flow, and would have increased progressively during the induction process, perhaps reaching excitatory concentrations towards the time at which sevoflurane would be affecting consciousness as well. The system would also have contained some residual nitrogen from the patients lungs. By using nitrous oxide in the carrier gas, the benefits of pre-oxygenation will be reduced or abolished, with no clear benefit on the process of induction.
We conclude that in young unpremedicated patients, nitrous oxide confers no advantage when added to sevoflurane for inhalation induction.
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
2 Yurino M, Kimura H. A comparison of vital capacity breath and tidal breathing techniques for induction of anaesthesia with high sevoflurane concentrations in nitrous oxide and oxygen. Anaesthesia 1995; 50: 30811[ISI][Medline]
3 Hall JE, Stewart JIM, Harmer M. Single-breath inhalation induction of sevoflurane anaesthesia with and without nitrous oxide: a feasibility study in adults and comparison with an intravenous bolus of propofol. Anaesthesia 1997; 52: 4105[ISI][Medline]
4 Guracha Boru K, Drummond GB. Comparison of breathing methods for inhalation induction of anaesthesia. Br J Anaesth 1999; 83: 6503
5 Thwaites A, Edmends S, Smith I. Inhalational induction with sevoflurane: a double-blind comparison with propofol. Br J Anaesth 1997; 78: 35665
6 Philip BK, Lombard LL, Roaf ER, Drake AF, Calalang I, Philip JH. Comparison of vital capacity induction with sevoflurane to intravenous induction with propofol for adult ambulatory anesthesia. Anesth Analg 1999; 89: 6237
7 Strickland TL, Drummond GB Comparison of signs of anaesthesia using propofol, methohexital and sevoflurane. Br J Anaesth 1999; 83: 180P1P