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The Role of Sedation During Regional Anaesthesia: Original Papers

Monitored patient-controlled sedation: practical technique or academic research tool?

Osborne, G. A.

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European Journal of Anaesthesiology - Supplement: July 1996 - Volume 13 - Issue - p 13-17

Abstract

Introduction

Monitored patient-controlled sedation (MPCS) is a technique that allows carefully monitored patients to administer intravenous sedatives to themselves, enabling them to achieve a level of conscious sedation that meets their individual requirements. This technique has a number of potential applications, including long or stressful surgical procedures, radiological or endoscopic diagnostic procedures, and certain therapeutic procedures, such as the dressing of burns. In addition, MPCS may provide a useful research tool to investigate the complex relationships between hypnosis, anxiolysis and amnesia during sedation. This paper reviews the clinical experience with MPCS and the benefits that this technique may offer to patients and theatre staff.

Technical considerations

Careful monitoring by an anaesthetist is mandatory during MPCS. Indeed, the use of MPCS substantially increases the anaesthetist's responsibilities; in addition to being responsible for the patient's general safety and well-being during surgery, the anaesthetist has to be familiar with extra equipment and is responsible for instructing the patient in the use of that equipment. The anaesthetist remains responsible for drug administration and the control of the level of sedation.

MPCS offers a number of potential advantages over other sedation regimens. Patients may vary widely in their need for sedation, and the degree of stress experienced during any one procedure can also vary markedly; there may also be significant pharmacokinetic and pharmacodynamic variations between patients. The MPCS approach is flexible, and can accommodate this wide variability. Additional, but less quantifiable, benefits can arise from the psychological effect on the patient of having the control to modify an unpleasant stimulus. An additional advantage is that the MPCS system is designed to act as a feedback mechanism that stops drug delivery when the desired level of sedation is reached; the aim is to produce conscious sedation, with the patient remaining able to respond to the anaesthetist.

The MPCS system used in the studies described in this paper is shown schematically in Fig. 1. This system, which is similar to those used for patient-controlled analgesia (PCA), delivers a fixed bolus of drug when the patient presses the demand button: effective delivery is facilitated by flushing the drug through the intravenous line. The variables that need to be considered are the same as those that apply in PCA, and include drug concentration, bolus dose, infusion rate, and lock-out interval. MPCS, however, requires a faster response to patient demands than PCA. Ideally, the system should respond quickly to a need for a single bolus dose, but should also respond well to a need for repeated doses that may be required during the induction of sedation. Moreover, it should also allow the level of sedation to be reduced if required. Similarly, it is essential that the system prevents both serious overdosage and deeper sedation, which can lead to loss of consciousness and, therefore, anaesthesia.

Fig
Fig:
1. The MPCS system used at the Royal Adelaide Hospital, Adelaide, Australia.

The response to a single bolus dose depends on the dose of drug and the time to effect. The time to effect depends on the time taken to infuse the bolus, the circulation time between arm and brain and the time to reach and influence the receptor site. Both propofol and midazolam are suitable for MPCS; however, propofol offers the advantage of a shorter time to effect. This produces a good response to single bolus doses. The onset of response is less clearly defined with midazolam than with propofol, and the time to full effect may be longer [1]. Another important consideration for any system is the lock-out period necessary to prevent over-sedation. The use of single boluses that are large enough to produce a clear effect requires a lock-out period that is longer than the full onset time, and thus agents with long onset times will require longer lock-out intervals. This will impair the response to repeated doses with slow induction of sedation and may reduce the confidence of the patient.

The infusion rate is an important factor influencing both the risk of overdosage and the time to effect. High infusion rates necessitate a lock-out period to avoid overdosage during the onset time. In contrast, the risk of serious overdosage is lower with slower infusions systems. A high infusion rate also reduces the time to effect, and thus systems with high infusion rates can be regarded as increased response systems, while those with lower infusion rates can be regarded as reduced response systems. Early research into MPCS used pumps that were originally designed for PCA, and which had a bolus delivery rate of 1.6 ml min−1. These systems, however, took approximately 75 s (twice the arm-brain circulation time) to deliver a 20-mg (2-ml) dose of propofol. More recent commercially available pumps have infusion rates of up to 3.3 ml min−1; even with these, however, the delivery time for a 2-ml bolus is comparable to the arm-brain circulation time of propofol, and thus the time to full effect is doubled. At present, increased response systems with high infusion rates must, therefore, be specifically designed by the user. The pump used at the Royal Adelaide Hospital, Adelaide, Australia, has a maximum infusion rate of 20 ml min−1, which gives a delivery time of 6 s for a 2-ml dose.

The significance of the infusion rate is shown in Fig. 2. With an infusion rate of 20 ml min−1, repeated doses of 2 ml separated by lock-out periods of 1 min give a maximum delivery capacity of approximately 100 ml h−1. This is an important consideration, as too high a delivery capacity may result in oversedation, while too low a capacity will lead to delays in sedation. Some studies have used a pump designed for PCA, with an infusion rate of 3.3 ml min−1, to deliver small boluses of propofol 3 mg without a lock-out interval [2]. This system has a potential maximum delivery capacity of approximately 200 ml h−1; in practice, however, patients find it difficult to time their doses precisely and the actual delivery capacity is substantially less. Increasing the size of the dose to 2 ml would make it possible to achieve a capacity of approximately 200 ml h−1, but would require 36 s to infuse the bolus and would introduce the need for a lock-out interval to prevent oversedation.

Fig. 2
Fig. 2:
The effect of infusion rate on the delivery capacity of a MPCS system. At an infusion rate of 1200 ml h−1 (20 ml min−1), a 2-ml bolus can be delivered in 6 s; with a 1-min lock-out time, this gives a maximum capacity of approximately 100 ml h−1. The dotted line is the maximum delivery rate of the system at an infusion rate of 1200 ml h−1. The time interval between A and B is the duration of the arm-brain circulation time.

Clinical experience with MPCS

In a pilot study of MPCS, a Graseby pump, modified for use with patient-controlled sedation, was used to deliver boluses of propofol 0.7 mg kg−1, at an infusion rate of 1.7 ml min−1 with a lock-out interval of 1 min [3]. This approach was used in 23 patients undergoing third molar extraction under local anaesthetic. This procedure offers a good model for evaluating MPCS because it is stressful, requires sedation, and involves working in a shared airway and therefore sedation must be safe. All patients liked the idea of self-administering a sedative according to their requirements. No cardiovascular or respiratory depression, and no oversedation, occurred during MPCS in this study.

The same system was used to compare MPCS using propofol with sedation using midazolam and fentanyl delivered by an anaesthetist [4]. The mean dose of propofol was 5.3 mg kg−1 (range 2.1-12.0 mg kg−1) and that of midazolam was 0.11 mg kg−1 (range 0.05-0.16 mg kg−1). For patients using MPCS propofol, the mean number of successful demands for propofol was 6.7, and the mean number of unsuccessful demands (i.e. those made during lock-out periods) was 31.6. However, when patients were asked to score how they felt during the procedure using a simple subjective scale ('good', 'indifferent' or 'bad'), there were significant differences between propofol-treated patients and those who received midazolam and fentanyl. Overall, 19 of the 20 propofol MPCS patients reported feeling good during the procedure, compared with only 11 of the 20 midazolam-treated patients (P<0.01); one patient who received MPCS and eight who received midazolam were indifferent, and one midazolam-treated patient reported feeling bad. Similarly, all 20 propofol patients who received MPCS reported that they would be happy to undergo the same procedure with the same form of sedation, compared with only 15 midazolam-treated patients (P<0.05). In both groups, 15 patients could not remember the extractions and four could not remember receiving surgical local analgesia; six patients in the midazolam group could not remember their post-operative recovery, whereas all patients who received propofol could remember recovering from the procedure. No oversedation occurred during MPCS with propofol, but three patients in the midazolam group were sedated to the point where verbal contact with the anaesthetist was lost briefly. This suggests that the differences in patients' subjective feelings between the two groups were not due to inadequate sedation with midazolam.

Subsequent studies at the Royal Adelaide Hospital have used a custom-built pump to achieve an increased step response with rapid bolus infusion. This system was used in a comparative study of MPCS with bolus doses of propofol, 20 mg in 2 ml, and midazolam, 0.5 mg in 2 ml [5]. Both drugs were given over 6 s after an initial dose of fentanyl 0.7 μg kg−1, which was given before the dental local anaesthetic. A lock-out period of 1 min was used. The depth of sedation was scored on a five-point scale (1, fully awake; 2, drowsy; 3, eyes closed but rousable on command; 4, eyes closed but rousable by mild physical stimulation; 5, eyes closed and unrousable by mild physical stimulation).

Both drugs were equally well accepted by the patients. There were, however, marked differences between them in terms of efficacy. The mean number of successful demands for propofol was 8.0, and the mean number of unsuccessful demands was 3.2, whereas the corresponding figures for midazolam were 14.0 and 19.1, respectively. An analysis of the number of unsuccessful demands during individual lock-out intervals (Fig. 3) showed that few propofol-treated patients made more than one demand during any one lock-out interval, whereas multiple demands were relatively common in midazolam-treated patients. These findings may be at least partly attributable to the slower onset of action of midazolam. The slower onset of action of midazolam cannot be offset by increasing the bolus dose because this would lead to oversedation; even under the conditions used in this study, two midazolam-treated patients were oversedated, reaching a sedation score of 4. This suggests that repeated doses of midazolam may accumulate under these conditions. In contrast, no propofol-treated patient attained a sedation score greater than 3.

Fig. 3
Fig. 3:
Incidence of unsuccessful demands during the same lock-out period in patients receiving MPCS with either propofol or midazolam. Reproduced with permission from[5].

The quality of sedation provided by MPCS was investigated in a crossover study that compared MPCS with propofol with continuous infusion of the same drug [6]. Patients scheduled for third molar extraction were allocated at random to either MPCS or continuous infusion for the first procedure, in which two teeth were removed; the remaining two teeth were extracted 6 weeks later and patients received the alternative method of sedation for this procedure. The patients were subsequently asked whether they had any preference for either form of sedation. The dose of propofol used for MPCS was 18 mg, given at an infusion rate of 20 ml min−1 with a 1-min lock-out interval. Patients scheduled to receive continuous infusion 3.6 mg kg−1 h−1 also initially received a bolus of propofol 20 mg, followed by further 10-mg boluses at 1-min intervals until a sedation score of 3 was reached. In both cases, patients received an initial dose of fentanyl 0.7 μg kg−1 before dental local anaesthesia.

There was no significant difference between the use of MPCS for the first or second procedure. The mean number of successful demands was 7.5 in patients who received MPCS before continuous infusion, and 7.7 in those who used the technique after continuous infusion. The mean numbers of unsuccessful demands in the two groups were 4.4 and 2.8, respectively.

There were no significant differences in observer-based outcome measures between the two sedation techniques (Table 1). The depth of sedation, however, differed markedly between them. During continuous infusion, all patients reached a sedation score of 3; in contrast, during MPCS a significant number of patients were satisfied with a sedation score of 2. This difference in sedation was reflected in the patients' preferences. More patients expressed a preference for MPCS than for the infusion and those patients tended to have stronger preferences than those who preferred the infusion. The preference for MPCS was based on the control provided; patients reported that they could use the system to achieve specific desires, for example to be more sedated or to remain alert.

Table 1
Table 1:
Outcome of sedation with propofol, given by MPCS or continuous infusion. Results are presented as mean ± SD. Data from[6]

Benefits and limitations of MPCS

Some potential difficulties with MPCS have been raised, including the question of whether patients can adequately judge their sedation needs while already sedated [7]. The clinical experience obtained to date, however, suggests that this technique is effective and highly acceptable to patients. MPCS can accommodate wide variations in sedation requirements between patients, and allows patients to receive the level of sedation that they want. Patients may also derive psychological benefits from this method of control by being able to modify anticipated unpleasant stimuli.

MPCS also allows communication between the patient and theatre staff. An audible demand button reassures patients that they have pressed the button far enough to receive a dose, and also alerts the theatre staff to a patient's request for sedation. This indicates the possibility of patient discomfort, and thus the surgeon can modify his or her approach and other members of the team can offer appropriate support.

A further advantage of MPCS is that the amnesic properties of propofol can be optimized. Although some studies have suggested that amnesia only occurs during oversedation with propofol [8], the experience obtained with MPCS suggests that this agent does provide good amnesia during conscious sedation.

In addition to the benefits to the patient, MPCS can offer incidental advantages to the theatre staff. Patient co-operation is more likely to be maintained if over-sedation is avoided; this is an important consideration with certain procedures, such as oral surgery. Moreover, as described above, the theatre staff are aware of the patient's sedative needs and thus sedation becomes a team approach rather than being of interest to the anaesthetist alone.

The major limitation of MPCS is the need for additional equipment. In particular, no increased response systems are available commercially at present, and therefore such systems have to be custom-built. Furthermore, approximately 10% of patients may prefer not to have control of their sedation; good communication and education is, therefore, important to ensure that patients understand the technique and genuinely wish to receive it.

The induction of sedation may be delayed with MPCS. Typically, a sedation score of 3 is reached in approximately 9 min. Because of this delay in induction, MPCS is not suitable for very brief procedures. The technique must be used with caution in elderly or relatively unfit patients in whom the arm-brain circulation time is variable and the effect of bolus doses less predictable. Furthermore, MPCS is not a substitute for good analgesia, and thus should not be used with patients who are in extreme discomfort.

Conclusions

Due to the lack of suitable commercially available equipment, the place of MPCS in clinical practice remains to be established. The experience obtained to date, however, suggests that the technique is effective and could offer valuable benefits to patients and theatre staff.

Acknowledgments

Some of the work discussed in this paper was carried out in collaboration with Dr G. Rudkin and Dr D. Jarvis at the Royal Adelaide Hospital.

References

1 Greenblatt DJ, Ehrenberg BL, Gunderman J et al. Pharmacokinetic and electroencephalographic study of intravenous diazepam, midazolam and placebo. Clin Pharmacol Ther 1989; 45: 356-365.
2 Cook LB, Lockwood GG, Moore CM, Whitwam JG. True patient-controlled sedation. Anaesthesia 1993; 48: 1039-1044.
3 Rudkin GE, Osborne GA, Curtis NJ. Intra-operative patient-controlled sedation. Anaesthesia 1991; 46: 90-92.
4 Osborne GA, Rudkin GE, Curtis NJ, Vickers D, Craker AJ. Intra-operative patient-controlled sedation: comparison of patient-controlled propofol with anaesthetist-administered midazolam and fentanyl. Anaesthesia 1991; 46: 553-556.
5 Rudkin GE, Osborne GA, Finn BP, Jarvis DA, Vickers D. Intra-operative patient-controlled sedation. Comparison of patient-controlled propofol with patient-controlled midazolam. Anaesthesia 1992; 47: 376-381.
6 Osborne GA, Rudkin GE, Jarvis DA, Young IG, Barlow J, Leppard PI. Intra-operative patient-controlled sedation and patient attitude to control - a crossover comparison of patient preference for patient-controlled propofol and propofol by continuous infusion. Anaesthesia 1994; 49: 287-292.
7 Zimmerman JB, Glass PSA. Intravenous anaesthesia. Curr Opin Anaesth 1993; 6: 648-652.
8 Smith I, Monk TG, White PF, Ding Y, Propofol infusion during regional anaesthesia: sedation, amnesic and anxiolytic properties. Anesth Analg 1994; 79: 313-319.
Keywords:

SEDATION; patient-controlled; ANAESTHETICS INTRAVENOUS, propofol, midazolam

© 1996 European Society of Anaesthesiology