Does the Type of Oxygenator Influence Arterial Blood Concentration of Isoflurane During Cardiopulmonary Bypass?

Cardiopulmonary bypass (CPB) is the keystone of most of these surgeries, as it allows for the maintenance of patient circulation without the requirement for a beating heart or breathing lungs. However, in doing so, CPB creates a unique set of difficulties associated with administering and maintaining anaesthesia. Firstly, the usual route of administration of inhalational anaesthetics, such as isoflurane, via the lungs is impossible. Secondly, clinical signs used to assess depth of anaesthesia, such as heart rate and systemic arterial blood pressure, are obscured by the mechanical support of the circulation. As a consequence, maintaining and assessing the depth of anaesthesia achieved during CBP provides challenges for the anaesthetists.

One common approach to administering anaesthesia during CBP involves the delivery of an inhalational anaesthetic agent, such as isoflurane, into the sweep gas supply to the oxygenator. Whilst classic terminology describes agents such as isoflurane as inhalational, in CPB this may cause confusion due to the delivery of the agent through the oxygenator as opposed to the lungs. Therefore, the alternative name of volatile anaesthetic agents is utilised in this protocol. Isoflurane concentrations ranging between 1% and 2.5% have been used by several anaesthetists at the Royal Infirmary of Edinburgh (RIE) for many years. This pragmatic approach has been found to produce clinically adequate depth of anaesthesia when either of the two oxygenators in routine use at the RIE are employed. Indeed, it has been shown using an electro-encephalographic (EEG) technique of assessing depth of anaesthesia, known as the Bispectral Index (BIS), that administering 2.5% isoflurane produces a more than adequate depth of anaesthesia. However, as discussed below, the interpretation of this finding is limited by the profound effect of hypothermia, which is commonly induced on CPB, has on BIS.

BIS is frequently utilised by anaesthetists to monitor anaesthetic depth. It involves real-time monitoring brain electrical waves using an EEG to produce a dimensionless number between 0 (no cortical brain activity) and 100 (awake), with 40 to 60 being widely accepted as a score suitable for general anaesthesia. Whilst BIS is frequently used in practice, as it has been shown to reduce anaesthetic awareness and increase emergence time, it is not without controversy as there is conflicting evidence regarding its reliability, particularly during CPB as low body temperature is known to affect its readings.

Considering the limitations of BIS, other methods can be employed to estimate depth of anaesthesia. The arterial blood concentration of the volatile agent is a good surrogate, as the concentration of agent in the brain will equilibrate with that of the blood.

Whilst this measurement provides a definitive value of how much anaesthetic agent is reaching the brain, it is impractical for clinical use because of the long duration and complexity of laboratory analysis that is required. Therefore, another approach can use the concentration of the volatile agent in the gases exhausted from the oxygenator in order to estimate the arterial blood concentration. This approach is analogous to using the end-tidal concentration that is expired from the lungs during routine general anaesthesia. Oxygenator exhaust, unlike arterial blood concentration, would be practical in a clinical scenario as the oxygenator exhaust from CPB can be attached to a standard anaesthetic gas monitor. Indeed, there is evidence that suggests measuring the oxygenator exhaust concentration of volatile anaesthetic agent is a good surrogate of arterial blood levels.