De-mystifying the “Mixifusor”.
Total Intravenous Anaesthesia (TIVA) using a combination of propofol and remifentanil in the same syringe has become a common and accepted technique in Paediatric Anaesthesia. This commentary outlines a recent publication in Pediatric Anesthesia from a group of UK paediatric anaesthetists who performed a service evaluation assessing the safety and efficacy of a propofol/remifentanil mixing technique in around 900 cases looking at complications and adverse reactions. The commentary also discusses potential issues from a pharmaceutics, pharmacokinetic and also medico-legal standpoint.
Potential issues from a pharmaceutic perspective included physical stability of the emulsion, changes in drug concentration over time, nonuniform mixing and risk of bacterial contamination.
From a pharmacokinetic point of view, when delivered as a propofol target controlled infusion (TCI), remifentanil delivery is not targeted and passively follows the predicted plasma or effect site concentrations of propofol. Consequences include potential rapid increase and peak in remifentanil concentrations risking apnoea, bradycardia and hypotension, particularly with concentrated doses (>10mcg/ml), and a faster decline risks inadequate analgesia and patient movement.
Medicolegal considerations include creating a new, unlicensed drug taking responsibilities of the manufacturer and thus consequences of its administration. If a critical incident or harm does occur, then the clinician’s practice may result in criticism.
Take home message
If you choose to mix propofol and remifentanil, adverse reactions are unlikely particularly if lower concentrations of remifentanil (5mcg/ml) are used. If using a TCI model, perhaps consider a staged approach to the initial TCI target rather than achieving it in a single step to minimise the transient remifentanil overdose at the commencement of the infusion. If an adverse event does occur, the individual must be cautious regarding liability and potential medicolegal consequences.
Reviewed by Dr. Justin Hii