2005 Annual Scientific Meeting - CLICK FOR PROGRAMME

REMIFENTANIL AND TCI

 Gavin NC Kenny, BSc(Hons), MB ChB, MD, FRCA, FANZCA

Professor and Head of Department

Glasgow University Department of Anaesthesia

The value of target controlled infusion (TCI) for the administration of propofol has been demonstrated by many studies and about 15,000 systems are now in use world-wide. The aim of any dosage regimen in anaesthesia is to titrate the administration of a drug to achieve the desired clinical effect for any individual patient while minimising the unwanted and toxic side effects. This requirement also applies to remifentanil and a TCI system was developed to enable anaesthetists to transfer directly their skills learnt with inhalational agents to the administration of remifentanil.

WHY IS THERE A NEED FOR TCI REMIFENTANIL?

Remifentanil is a rapidly acting opioid but its unique feature is the lack of any context sensitive half time. This means that the concentration will always half in 3-5 minutes independent of any pathological changes in the patient. However, if the infusion rate of remifentanil is increased, only 57% of the final concentration at the effect site is achieved after 5 minutes. If the infusion rate is halved or doubled, then a stable, steady state is not achieved even after 15 minutes. In anaesthesia, we wish to achieve the clinical effect as rapidly as possible so that we can titrate the drug to produce the optimum dose for the individual patient.

In contrast to using a manual infusion, rapid changes are obtained with TCI when increasing and decreasing the blood concentration and the system will achieve the most accurate and rapid change possible with the drug.

A further issue to consider is the pharmacokinetic variation with age and gender. If a dose of 1 µg/kg remifentanil is given to a young, tall, slim female, the peak blood concentration will be approximately 13 ng/ml. The patient would be expected to breathe at an effect site concentration of about 2 ng/ml and respiration will start at about 4 minutes after the administration of the single dose.

In contrast, the administration of the same dose to an elderly obese and short female will achieve a peak blood concentration of about 28 ng/ml and the effect site concentration of 2 ng/ml would be achieved about 10 minutes after the dose was given. There is a greater sensitivity to the action of remifentanil in the elderly and this means that respiration may not start until the effect site concentration has decreased to about 1 ng/ml which would take around 14 minutes after the administration of the remifentanil dose.

BENEFITS OF TCI REMIFENTANIL

TCI remifentanil allows the blood concentration to be targeted as accurately as possible for the individual patient and so allows the titration of dose to achieve the required clinical effect as simply as possible. Like other opioids, remifentanil administration can result in marked respiratory depression. While remifentanil can be used during spontaneous respiration, this technique does not exploit the unique properties of the drug. Low dose infusion regimens during which spontaneous respiration persists can be used but may not confer benefits when compared with other opioids.

The beneficial effects of remifentanil including cardiovascular stability occur at blood concentrations greater than that at which spontaneous respiration is depressed. Consequently, most patients require their lungs to be ventilated. This is not usually a problem and can be performed via a laryngeal mask airway if intubation is deemed unnecessary. Heart rate is slowed by remifentanil and there may be a marked bradycardia in some patients. The blood pressure is usually well maintained but hypotension can occur. It is in these situations that the Remifusor is of great value since the target concentration can be titrated precisely to the requirements of the individual patient.