To the Editor:
We read with interest the letter to the editor from Labaste et al.1 We welcome the initiative of the authors to look for an alternative to intravenous drugs to sedate patients undergoing cardiac surgery using cardiopulmonary bypass (CPB) with volatile anesthetics (VA), using the Anesthetic Conserving Device (AnaConDa) (Sedana Medical, Uppsala, Sweden), in times of drug shortages. AnaConDa initially was created for inhaled sedation of patients ventilated with intensive care ventilators. Its safe use in the operating room has been described for minor gynecologic surgery.2
Unfortunately, information about the material composing the oxygenator, the sweep gas flow used, and the exact location of the gas monitoring, which are crucial in this setup, are lacking.
The use of VA with oxygenators composed of polypropylene previously has been described.3 However, most oxygenators used in cardiopulmonary bypasses consist of polymethylpentene,4 , 5 so-called plasma-tight membranes, which are impermeable to VA.6 , 7 In that case, applying VA by placing the AnaConDa just before the oxygenator of the CPB will not lead to an uptake by the patient. Thereby, the sevoflurane concentration measured at the gas outlet of the oxygenator does not match the end-tidal concentration of the patient, but rather the sevoflurane concentration existing in the oxygen line between the AnaConDa and oxygenator. During CPB, the increase of the sevoflurane pump rate observed may correlate with variations in the sweep gas flow. Adapting the depth of sedation to that concentration may lead to a high risk of awareness for the patient.8 , 9 Since the sedation was maintained with sevoflurane before start of the CPB, sevoflurane cannot diffuse out of the patient and, thus, sedation could be maintained to some degree. The metabolic rate of sevoflurane also may be lower than 5% because of the induced hypothermia. This may explain the sufficient depth of sedation measured with the bispectral index, which would not be the case if total intravenous anesthesia had been used prior to the CPB. After transporting the patient to the intensive care unit, connecting the gas outlet of the ventilator to another FlurAbsorb anesthetic gas filter is only necessary if more than 500 mL of VA have been applied.10 Otherwise the same FlurAbsorb filter could have been used.
Finally, it is also important to use tubing composed of materials impermeable to VA (such as polyvinyl chloride internally lined with low-density polyethylene) to prevent VA from leaking through the wall of the tube.11
Conflict of interest
A. Kermad: None. T. Volk received consulting fees from Sedana Medical (Danderyd, Sweden). A. Meiser received consulting fees from Sedana Medical (Danderyd, Sweden) and honoraria for lectures from Pall Corporation (Dreieich, Germany).
References
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