Isoflurane During Cardiopulmonary Bypass

Study Overview
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Brief Summary
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There is no clinical way of assessing the depth of anaesthesia while patients are on the heart-lung machine. A new method of measuring the depth of anaesthesia using brainwaves called the Bispectral index (BIS) has been developed and its use in cardiac surgery is now widespread. However BIS is also altered by patients body temperature. As cooling is common during heart surgery the use of BIS to measure the depth of anaesthesia during heart-lung bypass remains controversial. This study aims to find out what depth of anaesthesia is produced according to BIS during heart lung bypass using a standard anaesthetic technique that utilises the anaesthetic isoflurane.

Detailed Description
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There is no clinical way of assessing patients' depth of anaesthesia on the heart-lung machine. For this reason, the amount of volatile anaesthetic agent like isoflurane, that is administered has traditionally been determined by anaesthetists' clinical experience. Recently, a device that assesses depth of anaesthesia by analysing the electrical brain waves called the Bispectral index (BIS) has been introduced. The study aims to determine whether the level of anaesthesia produced by isoflurane using the traditional approach is more than required simply to maintain unconsciousness according to BIS. In addition, the study will determine whether the concentration of isoflurane in exhaust gases from the heart-lung machine can be used as an estimate arterial blood concentration and so, depth of anaesthesia. The study will recruit patients who are scheduled for heart surgery using a heart-lung machine. Patients will be anaesthetised using a long established balanced technique including the administration of isoflurane 2.5% whilst supported on the heart-lung machine. Measurements of exhaust gas concentration of isoflurane from the heart-lung machine and level of BIS will be recorded at three time points. Simultaneously, three arterial blood samples will be taken and the isoflurane concentration measured in these samples. The BIS levels will be averaged and the proportion of patients with a BIS below 40 (lower limit of normal) will be calculated. Concentrations of isoflurane in the exhaust gas of the heart-lung machine will be compared with blood concentrations of isoflurane and the both concentrations will be compared with the level of BIS to establish whether they can be used to assess depth of anaesthesia. If this study finds that a large proportion of patients have more than adequate levels of anaesthesia, then future studies could explore whether administering lower concentrations of isoflurane improves outcome whilst maintaining anaesthesia.

Official Title
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The Depth of Anaesthesia Associated With the Use of 2.5% Isoflurane During Cardiopulmonary Bypass

Conditions
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Cardiopulmonary Bypass, Consciousness Monitors, Isoflurane

Intervention / Treatment
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* Procedure: 2.5 % Isoflurane administration


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Participants over the age of 18 Scheduled for elective cardiac surgery under the care of trial anaesthetists Patient has provided informed consent to participate Exclusion Criteria: Patients presenting for emergency cardiac surgery Patients who will have cardiac surgery that does not require cardiopulmonary bypass Unwilling/unable to provide informed consent Patients under the age of 18 Anaesthesia undertaken by anaesthetists not involved in trial

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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Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Elective Cardiac surgery<br>Those patients scheduled for elective heart surgery requiring cardiopulmonary bypass who will be given 2.5% isoflurane while on bypass. | Procedure: 2.5 % Isoflurane administration<br>* Anaesthesia maintained during cardiopulmonary bypass with 2.5% isoflurane administered via the oxygenator of the bypass circuit.<br>|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Depth of anesthesia according to BIS | BIS reading will be recorded at three time points during cardiopulmonary bypass | Intraoperative | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Comparison of exhaust gas and blood concentration of isoflurane | Exhaust gas and blood concentrations of isoflurane will be recorded at three time points during the operation. These values will be compared to see if their values are are related. | Intraoperative | 
| Comparison of blood isoflurane concentration and BIS score | Blood isoflurane concentration and BIS will be recorded at three time points and compared to see if they are related | Intraoperative | 
| Comparison of exhaust isoflurane Concentration and BIS score | Exhaust isoflurane and BIS will be recorded at three time points and compared to see they are related. | Intraoperative |