The practical aspects of propofol target controlled infusion for magnetic resonance imaging in children: An audit from the Royal Marsden Hospital

INTRODUCTION

There are several techniques described in the literature for anaesthetizing children for non-painful procedures like scans. The use of volatiles has been associated with more agitation in recovery and post operative nausea and vomiting than propofol.[1,2] Concerns regarding use of propofol for short procedures and association with propofol infusion syndrome have not yet been realised,[3] and its use has been supported in NICE guidelines published December 2010.

There are numerous models for propofol target controlled infusion (TCI), which is reflective of the heterogeneity of the pharmacokinetics and pharmacodynamics seen in children.[4] The accuracy of Paedfusor has been demonstrated previously in cardiac catheterization procedures.[5]

METHODS

Following approval by the hospital's Clinical Audit Department, data were collected prospectively from January to December 2011.

Day care patients undergoing magnetic resonance imaging (MRI) using propofol delivered by TCI as a sole agent were included. Questionnaires were given to parents to ascertain the incidence of nausea or vomiting, and time to discharge.

Anaesthesia was induced intravenously via existing tunnelled central line, where available, in the anaesthetic room adjacent to the scanner. In this area there was a fully-operational anaesthesia machine and full monitoring, and airway and resuscitation equipment were immediately available. In the absence of available intravenous access for induction, patients received an inhalational induction using sevoflurane. Anaesthesia was maintained using propofol TCI (Paedfusor model).

Oxygen was administered via Hudson mask and a shoulder roll was used to help maintain an open airway. Before entering the scanning room, the TCI pump was disconnected and placed in the control room.

In the scanning room, IV access was reconnected via 3-way tap to a propofol primed IV extension. The distal end of the extension was passed through a hole in the wall between the scanning room and control room, and connected to the TCI pump via a single anti-siphon Westcott infusion set.

Capnography, oxygen saturation, ECG and blood pressure monitoring were recommenced. An MRI compatible monitor relayed information to the control room monitor via radio control.

Patients were observed continuously by an anaesthetist. Changes to target plasma concentration (Cpt) were made at their discretion and recorded. At the end of the scan the infusion was discontinued and the patient transferred to the recovery room, adjacent to the scanning room. Patients left recovery once they fulfilled standard criteria based on the recommendations of the Association of Anaesthetists of Great Britain and Ireland.[6] Patients were discharged once they satisfied local protocols based on recommendations from British Association of Day Surgery.[7] The times from end of infusion to patient leaving recovery, and end of infusion to patient's discharge home were also recorded.

RESULTS

Thirty-five children having a single MRI scan from January to December 2011 were included. Patients re-presenting for further scans during this period had only their first scan included. There were no other exclusions. Twenty-one completed questionnaires were received from parents. The mean age of the cohort of 35 patients was 4.3 years (range 2-11 years). Table 1 illustrates variables related to the anaesthetic time and recovery.

Table 1.

The mean, median, standard deviation and interquartile range for each variable

In 71.4% cases the starting Cpt chosen by the anaesthetist was 6 μg/ml. Thirty-one children had existing IV access which was used for induction. The remaining four children received an inhalational induction using sevoflurane and oxygen and peripheral IV access secured once the child was asleep. Anaesthesia was then maintained using propofol TCI, commencing at a target Cpt of 4 μg/ml. Figure 1 illustrates changes made to Cpt as the anaesthetic progressed.

Figure 1.

Changes to target plasma concentration with time. Size of block directly proportional to number of patients

The mean infusion rate of propofol given to children <3 years and >3 years was 370.1 and 351.8 mcg/kg/min, respectively. Two children required an airway adjunct (5.7%) in the form of laryngeal mask airway, to treat airway obstruction. No children required intubation. There were no haemodynamic disturbances requiring treatment.

Five of 35 patients required a repeat scan sequence. In one instance this was due to patient movement (incidence 2.8%). Other causes included monitor battery failure, and inadequate imaging due to technical reasons related to the scanner. No patients required a repeat scan on another day.

One patient experienced PONV. All children were discharged within 3 h. Twenty-one of 35 questionnaires were returned by post from parents.

DISCUSSION

The success or failure of an anaesthetic technique for an MRI scan can be defined by the absence of adverse cardio-respiratory events and the prevention of movement. A technique using propofol infusion at 100 mcg/kg/min has been found to keep most children still during an MRI scan.[8] In our audit, a higher mean infusion rate was seen. Despite this, spontaneous respiration and airway patency were maintained without cardiovascular instability in all children or airway adjuncts in 33 of 35 children. In addition, the incidence of movement requiring repeat scan sequence was 2.8%, which is comparable to other studies.[8,9]

Without the availability of MRI compatible Bispectral Index (BIS) monitoring, or the use of sedation scores, the optimum Cpt of propofol required to maintain an adequate depth of anaesthesia is difficult to estimate. As such, selection of starting Cpt and subsequent timings of changes to Cpt in this study was based on the clinical picture and experience of the anaesthetist, but is ultimately subjective. The inability to define how deeply sedated or anesthetized children were represents a limitation of this study.

Other measures of adequacy of anaesthesia were incidence of PONV and rapidity of recovery, as measured by time spent in recovery and time to discharge home. All children went home within 3 h of the end of the propofol infusion and there were no readmissions. The one child who experienced PONV had features of raised intracranial pressure at the time of the scan.

The majority of patients in our cohort had established long-term IV access at the time of scan. This meant that IV induction was possible and pain on injection due to propofol may be reduced as the drug is delivered into a large central vein.

Our technique did not require MRI compatibility as the pump was situated in the control room. Data from Stamford University Medical Centre suggests that the hourly cost of delivering propofol by infusion is comparable to that of sevoflurane at 1 L/min. FGF in children.[10] The cost of propofol TCI may be lower still but has not yet been studied.

Propofol delivered by TCI may offer a more refined anaesthetic for children undergoing short surgical procedures or sedation for scans. The technique would benefit from formal depth of anaesthesia monitoring. Direct comparisons with Total Intravenous Anaesthesia may help anaesthetists decipher if TCI offers any meaningful advantage in terms of safety, recovery time and service provision.