The Effect of N2O Administration on Arterial Oxygenation

Study Overview
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Brief Summary
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Nitrous oxide is a standard part of most anaesthetic breathing mixtures. It has often been assumed that it reduces the amount of oxygen reaching the lower parts of the lung. However the results of previous studies and of some computer modelling suggests the opposite may be true, and that oxygen levels in the blood are actually higher with nitrous oxide. This study seeks to determine whether nitrous oxide increases or decreases blood oxygen levels in anaesthetized patients.

Detailed Description
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The concentrating and second gas effects of nitrous oxide (N2O) uptake have been well described, and are produced by rapid uptake of N2O during the first several minutes of an inhalational anesthetic. This significantly increases alveolar and arterial O2 partial pressure. Theoretical modelling of alveolar gas exchange has predicted that these effects may not be transient but may be a persisting phenomenon. However, N2O is known to promote absorption atelectasis in poorly ventilated lung units. This study, carried out pre-cardiopulmonary bypass in 20 patients undergoing coronary artery surgery, measures change in PaO2 after a minimum of 30 minutes of relaxant general anesthesia with a FIO2 of 30%. Patients are randomised to two groups. The intervention group has N2O introduced following baseline blood gas measurements, while the control group continued breathing an identical FIO2 in N2. The change in PaO2 in the two groups is the primary endpoint for comparison.

Official Title
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Relationship of Oxygenation During Anaesthesia in the Presence of N2O or N2 to Mode of Ventilation

Conditions
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Anesthesia

Intervention / Treatment
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* Drug: Nitrous oxide


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Patients undergoing elective coronary artery bypass surgery Exclusion Criteria: Age under 18 years, emergency surgery

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

Primary Purpose: Treatment
Allocation: Randomized
Intervention Model: Parallel Assignment
Masking: Single


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Experimental: Nitrous oxide<br>Nitrous oxide 70% as balance gas in inspired mixture | Drug: Nitrous oxide<br>* Substitution of Nitrous oxide for Nitrogen as balance gas in inspired gas mixture<br>* Other names: N2O;|
| Experimental: Nitrogen<br>Nitrogen instead of nitrous oxide 70% as balance gas in inspired mixture | Drug: Nitrous oxide<br>* Substitution of Nitrous oxide for Nitrogen as balance gas in inspired gas mixture<br>* Other names: N2O;|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| PaO2 | Measured change in PaO2 after a minimum of 30 minutes of general anaesthesia, following sternotomy, in the two groups. | Minimum of 30 minutes after induction of anaesthesia | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Change in pulmonary deadspace and shunt fractions | Change in pulmonary deadspace and shunt fractions, as calculated by the Bohr and shunt equations. | Minimum of 30 minutes after induction of anaesthesia |