Optimal sedative selection for various locoregional techniques.

 

Nick Sutcliffe

Glasgow

 

Introduction

The use of locoregional anaesthesia for a variety of surgical procedures is increasing as it provides not only satisfactory operating conditions and good intra and post operative analgesia, but also has advantages in terms of health economics. Sedation may be preferred by the patient during the procedure and commonly hypnotics such as Midazolam and Propofol are used for this purpose. However, the placement of some peripheral nerve blocks can be painful and the commonly used hypnotic agents have no analgesic effect when given in sedative doses. This has led some workers to combine these agents with opioid drugs to cover this discomfort. Unfortunately the addition of opioids to a sedation regimen often leads to an increase in the incidence of respiratory depression. Ideally during sedation, the patient should be relaxed, comfortable and co-operative throughout the procedure including the placement of the regional block. In practice, achieving this ideal may be the most challenging aspect of anaesthesia. The ideal technique would provide independent easily titrateable control of both analgesia and hypnosis. In this way the balance of these two aspects could be altered as appropriate providing a higher level of analgesia initially during the regional procedure and then reducing the analgesic component once the regional blockade is established, thus avoiding unnecessary respiratory depression

 

Hypnotic Component

An ideal sedative agent should produce a rapid and smooth onset of action and rapid offset and allow easy control of the level and duration of sedation. Of the currently available drugs propofol and midazolam are the two most suitable agents with midazolam being particularly popular with non-anaesthetists. However, the pharmacokinetics properties and recovery characteristics of propofol make it potentially better suited for short-term sedation; this is supported by available studies comparing the 2 drugs (1,2). The problem with propofol particularly for the non-anaesthetist is its short duration of action requiring repeated bolus dosing infusions or complex infusion regimens. An alternative approach to the delivery of this drug is the use a target controlled infusion (TCI) as pioneered by Kenny and colleagues (3). TCI has been successfully used to delivery sedation in a number of areas (4,5). Currently this is arguable the best way to deliver physician controlled sedation.

 

A number of studies have shown the benefits of patient controlled sedation, this technique has a high degree of patient satisfaction, and often results in a less sedated patient compared with physician controlled sedation (1,2,6). A further development of this technique is the combination of patient control and TCI (7).

 

Analgesic Component

A number of opioids have been used in combination with hypnotics to provide a component of analgesia during sedation. In general the use of such regimens results in more respiratory depression than regimens without opioids. However many of these studies have used longer acting agents or infusions of opioids throughout the procedure rather than titrating the analgesic component to the stimulating part of the procedure. The use of remifentanil by infusion has been described with some success (ref). A further refinement would be the use of TCI remifentanil which would allow accurate control of the analgesic component. This would allow accurate titration of this effect and thus potentially avoid any excessive respiratory depression whilst still allowing the anaesthetist to obtund any painful stimulus during the placement of his regional block.

 

Effect site Targeted TCI

A further refinement of the TCI technique is the use of effect-site targeting rather than blood concentration targeting. It is the effect-site (or brain) concentration that determines the effect of a hypnotic drug rather than the blood concentration. As the blood concentration changes during induction of and titrating sedation the effect-site concentration (and thus the effect on the patient) lags behind (fig1). Using effect-site targeting the blood concentration is allowed to rise above the target during an increase in the set effect-site target, thus driving drug into the brain faster and achieving the desired effect faster than using standard TCI. When titrating the target down the algorithm allows the blood concentration to fall below the set target to allow a more rapid fall in the effect site concentration (fig2). Such a system allows more rapid titration of sedation with less potential for overdose. This system can be further enhanced by allowing the patient to control the target via a push button; this type of system has been evaluated in volunteers (9) and is currently under investigation for use in dental sedation.

 

Conclusion

The combination of regional anaesthesia an sedation provides the potential for good postoperative pain control, and rapid good quality recovery, whilst still providing patients with anxiolysis and comfort during the procedure. The method of delivery of such sedation is a rapidly developing area. We now have sophisticated techniques which allow rapid titration of both the hypnotic and analgesic components of the sedation. These techniques should ultimately allow physicians to tailor these individual components to each particular patient and procedure and achieve the goal of good quality sedation and analgesia without undue over-sedation or respiratory depression.  

 

References

  1. Rudkin et al “Intra-operative patient controlled sedation” Anaesthesia 1992, 47:376-81.
  2. Pac-soo et al “Patient controlled sedation for cataract surgery using peribulbar block” British Journal of Anaesthesia 1996, 77(3) 370-374.
  3. White, M. and Kenny, G.N. (1990) Intravenous propofol anaesthesia using a computerised infusion system. Anaesthesia 45, 204-209.
  4. Skipsey IG, Colvin JR Kenny GN Sedation with propofol during surgery under regional blockade (1991) Anaesthesia1993 48:210-213.
  5. Church JA, Stanton PD, Kenny GN & Anderson JR. Gastrointest.Endosc. 1991:37:175-179

 

  1. Osborne et al “Intra-operative patient controlled sedation and attitude to control” Anaesthesia 1994,47:287-92.
  2. Irwin et al” Patient maintained propofol anaesthesia” Anaesthesia 1997,52,(6)525-30.
  3. Leitch JA. Sutcliffe N. Kenny GN. Patient-maintained sedation for oral surgery using a target-controlled infusion of propofol - a pilot study. British Dental Journal. 2003:194(1):43-5.
  4. Holas A; Krafft P; Marcovic M; Quehenberger “Remifentanil, propofol or both for conscious sedation during eye surgery under regional anaesthesia.” European Journal of Anaesthesiology 1999,16:741-8
  5. Absalom AR, Sutcliffe N “Effect-site targeted patient maintained sedation with propofol” Anesthesiology 2000: 93(sup)-A291

 

Blood Concentrationtion
 

Effect-site Concentration
 

 

 

Figure 1. Blood and effect-site concentration of propofol during a standard blood targeted TCI set at 1 µg/ml then reduced to 0.5 µg/ml after 10 minutes.

 

 

 

 

 

 

 

Effect-site Concentration
 

Blood Concentrationtion
 

 

 

 

Figure 2. Blood and effect-site concentration of propofol during an effect-site targeted TCI set at 1 µg/ml then reduced to 0.5 µg/ml after 10 minutes.