Propofol Target-Controlled Infusion in Emergency Department Sedation

Procedural sedation and analgesia (PSA) has long been a core skill of the Emergency Physician, although over the last decade, developments in patient monitoring and the use of newer sedative and analgesic agents have served to improve both safety and efficacy. Despite these advances and their consolidation into well-designed guidelines concern regarding the safety of ED PSA persists.

More specifically, concerns have been raised regarding the use of propofol, an ultra-short acting anaesthetic agent. The practice of using sub-anaesthetic doses of propofol to achieve sedation in ED PSA originated around the turn of the millennium and has since become the most common choice of sedative in the ED. Propofol offers a number of advantages as a sedative agent, including; a short onset and recovery time, amnesiac properties and good efficacy. The cost of these properties is in part, a narrow therapeutic range.

When given as a bolus, propofol will induce a spectrum of states up to and including general anaesthesia, dependent on the dosage. The correlation between dose and effect varies based on several patient factors. Whilst targeting a state of sedation it is possible for the operator to 'overshoot' moving rapidly from Conscious Sedation to Deep Sedation to General Anaesthesia. The inadvertent induction of general anesthesia may result in unanticipated complication.

The principle complications associated with accidental over-sedation or general anesthesia relate to the patency of the patient's airway. The reported frequency of airway complications during ED PSA with propofol range from 5.0% to 9.4%7, this includes a rate of supplemental ventilation of between 3.0% and 9.4%, with oxygen desaturation occurring in between 5% and 7% of cases12. In addition, the bolus administration of propofol may result in transient hypotension. Despite being a relatively short lived effect, this may be pronounced in those with intravascular volume depletion or in the elderly, and in some may be profound, with one series demonstrating that 3.5% of those undergoing PSA with propofol experience ≥ 20% falls in blood pressure. At first inspection the reported rate of complication may appear low, and has by some been interpreted as evidence of the relative safety of ED PSA, however alternative views have been expressed in the context of much lower rates of adverse events seen in elective painful procedures requiring conscious sedation. This is compounded by the non-standard way in which adverse events have been reported across studies.

A potential solution to the adverse events experienced with the bolus administration of propofol is the use of a target-controlled infusion (TCI). This method is widely used in anaesthetic practice and whilst the PK remains the same irrespective of if you give a bolus or infusion of propofol, TCI allows the titration in perhaps a more controlled manner.

The aim when administering propofol, as with any drug, is to induce a desired clinical effect, in this context a level of sedation. As previously alluded to, when administering propofol in a bolus fashion or as a fixed rate infusion, without regard to those factors which cause biologic variability (e.g. age, gender, weight), it is difficult to accurately and consistently predict clinical effect. The development, in the early 1990's, of computer assisted infusion devices, for the first time, allowed the clinician to target a plasma concentration, with the pump automatically altering the rate of infusion based on a pre-programmed pharmacokinetic model. These target-controlled infusion (TCI) devices have undergone significant development and are now in widespread clinical use. Their operation uses a mathematical mode that reflects a theoretical 'three-compartment' model that may comprise the central compartment (plasma), and two peripheral compartments (highly perfused tissue e.g. brain and poorly perfused tissue e.g. adipose). In a state of equilibrium, propofol will diffuse between compartments at a constant rate. These rate constants have been used in pharmacokinetic models to mathematically predict the plasma concentration and latterly the effect site concentration, in this case that in the brain.

In practical terms, TCI allows the operator to more accurately target a specific clinical effect. When propofol is administered as a bolus, the operator is likely to either under-dose, delivering an insufficient effect site concentration, or over-dose, exceeding the desired effect site concentration. TCI allows the operator to titrate to effect and then to maintain a steady state, potentially eliminating the risk of 'over shooting' and reducing the rate of adverse events. TCI is not without limitation. PK models are an estimate as they have been derived from a healthy population but any inherent inaccuracy is consistent in different populations and accounted for by careful titration.

The use of propofol TCI has been studied in a number of settings, including gastrointestinal endoscopy, dental surgery, oocyte retrieval and bronchoscopy. To our knowledge propofol TCI in sedation has not been studied in an ED setting. Trials have demonstrated a good safety profile for propofol TCI, with at least one large randomised-controlled trial in an endoscopy setting, showing a reduction in both respiratory and cardiovascular adverse events in comparison to the bolus administration of propofol. Unfortunately, trials to date have suffered from a high degree of heterogeneity, leading the Cochrane review, on the subject of propofol TCI versus manually controlled infusion in both general anaesthesia and sedation, to conclude that there was insufficient evidence to make firm recommendations regarding its use in clinical practice.