Measurement of the Second Gas Effect on Sevoflurane in Anaesthetised Patients

Study Overview
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Brief Summary
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This study is investigating the second gas effect, a phenomenon produced by the uptake of nitrous oxide (N2O) by the lungs, during the course of a typical anaesthetic. The effect is to increase the concentration of other breathed gases in the lung. These include oxygen and volatile anaesthetic agents such as sevoflurane, which are also normally administered along with N2O. We wish to i) measure the magnitude of the second gas effects on both blood and expired concentrations of sevoflurane (Part 1), and ii) see if a demonstrable difference exists between the effects on blood and expired concentrations.

Detailed Description
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This study is investigating the second gas effect, a phenomenon produced by the uptake of nitrous oxide (N2O) by the lungs, during the course of a typical anaesthetic. The effect is to increase the concentration of other breathed gases in the lung. These include oxygen and volatile anaesthetic agents such as sevoflurane, which are also normally administered along with N2O. While the second gas effect has been demonstrated previously, by measuring the concentration of volatile anaesthetic in the expired breath, no study has yet shown that it has a significant effect on the concentrations in the blood. The blood concentration is in fact more important, as it directly determines the concentration of anaesthetic reaching the brain, and therefore the effect on the depth of anaesthesia. The second gas effect on blood concentrations may be more powerful than that on expired concentrations, due to the detrimental effect of anaesthesia on the evenness of distribution of ventilation and blood flow in the lung. The proposed study will have two parallel components or Parts. We wish to i) measure the magnitude of the second gas effects on both blood and expired concentrations of sevoflurane (Part 1), and ii) see if a demonstrable difference exists between the effects on blood and expired concentrations.

Official Title
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Measurement of the Second Gas Effect on Sevoflurane in Anaesthetised Patients

Conditions
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Anesthesia

Intervention / Treatment
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* Drug: No nitrous oxide
* Drug: Administration of N2O


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Patients scheduled for elective surgery requiring relaxant general anaesthesia with arterial blood pressure monitoring via an arterial line. Exclusion criteria: Patients with moderately or severely impaired respiratory disease (FEV1 < 1.5L, or FVC < 2.0L). Patients under 18 years of age. Morbidly obese patients: BMI > 30 Patients unable to give informed consent.

Ages Eligible for Study
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Minimum Age: 18 Years

Sexes Eligible for Study
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All

Accepts Healthy Volunteers
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No

Study Plan
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How is the study designed?
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Design Details

Allocation: Randomized
Intervention Model: Crossover Assignment
Masking: None (Open Label)


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Experimental: N2O<br> | Drug: Administration of N2O<br> <br> |
| Placebo Comparator: No N2O<br> | Drug: No nitrous oxide<br> <br> |

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Fa/FI sevoflurane in 100% O2 vs in 70% N2O and 30% O2 during 1st 60 min anaesthesia |  |  | 
| Change in Fa/FI sevoflurane in 100% O2 vs in 70% N2O and 30% O2 after 60 minutes anaesthesia |  |  | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Fa/FI and FA/FI for N2O will also be recorded. |  |  |