Adequacy of Anesthesia

Clinical value of hypnosis monitoring

Arvi Yli-Hankala, MD, PhD
Tampere University Hospital
Department of Anesthesiology
Tampere, Finland

Email: ylihanka@dlc.fi

The article also available in PDF: 48 KB

Introduction

In general anesthesia, muscle paralysis complicates assessment of consciousness, as neuromuscular blocking agents are used very frequently. In addition, the typical "clinical signs" to adjust level of anesthesia, including blood pressure and heart rate, can be exceptionally unreliable. On the other hand, pharmacokinetic modeling is not very precise on an individual patient’s level either; the minimum alveolar concentrations of inhalation agents (MAC) or other ED50 concepts may pose a rather notable inter-individual variability. Therefore, the only objective way to optimize anesthesia is pharmacodynamic measuring of anesthetic effect.

Figure 1. The concept of modern anesthesia can be based on a group of parallel effects; ideally, each of them should be monitored by special technique.

Target organ, the brain

The brain is the target organ for all hypnotic drugs. The anesthesiologist can monitor the hypnotic component of anesthesia by measuring the effect of these pharmaceuticals on brain electrical activity (electrocardiogram, EEG). Certain features of spontaneous EEG, or auditory evoked potentials (AEP) can indicate the state of brain during anesthesia [1,2]. As such, however, electrical waves recorded from the brain are rather cumbersome to analyze.

What is the value of processed EEG

Great effort has been placed to detect the reliable information hidden in the EEG, and to convert it into an easily understandable form, still trying to avoid excessive oversimplification. Presumably, some clinicians would be delighted just to have "Yes/No" type of hypnotic measure or, say, some sort of traffic lights. Unfortunately, reality is a bit more complicated.

During hypnotic medication administration, an easily understandable value derived from the EEG might just be a single number indicating the "depth" of hypnosis. However, in that kind of simplified display, other valuable information available on the EEG would be unavailable. A single number would only give an estimate of the current state of hypnosis, but it could not predict any change in the patient’s condition.

One additional point deserves special mention. When the EEG derived information of hypnosis monitors is compared to clinical signs or monitored hemodynamic variables, one should bear in mind that e.g. movement response or increasing heart rate merely indicate nociception. These findings may have very little to do with consciousness. Therefore, the monitoring devices which have been designed to measure the degree of pure hypnosis are not predictive in nature. They are not movement monitors, heart rate monitors, or any other kind of nociception monitors.

Currently, there are two models of FDA cleared hypnosis monitors available for clinical use, BIS™ by Aspect Medical Systems and PSA4000™ (Patient State Index, PSI) by Physiometrix. The vast majority of the scientific knowledge of hypnosis monitoring, including validation information to show its clinical applicability comes from studies based on the bispectral index (BIS) method. Hence, this review refers almost exclusively to those studies.

Bispectral index

Peter Glass and his colleagues studied memory function, sedation scores (OAA/S), and BIS levels during isoflurane, propofol, midazolam, or alfentanil sedation in volunteers [3]. They demonstrated that with increasing isoflurane, propofol, or midazolam concentrations, memory function impairs, together with decreasing BIS levels. Loss of consciousness prediction probability values for BIS ranged from 0.89 to 0.98. In their study, the probability of any recall at BIS value of 60 was extremely low. That study gave a solid scientific background for clinical BIS monitoring when hypnotic anesthetics were used. Later on, Johansen and others [4] concluded that the real benefits of BIS monitoring could be obtained only when BIS values during surgery were maintained between 50 to 65. Today’s general understanding of the value of BIS monitoring is that it helps the clinician to titrate hypnotic medication to avoid unnecessarily high or low concentrations of the drug. Overdosing would be indicated by low BIS values (below 50 to 40). On the contrary, high BIS (over 60-65) would indicate too low concentrations of the drug.

Less drugs, faster recovery

Hypnosis monitoring decreases the consumption of drugs during maintenance of anesthesia. This has been reported with various hypnotic drugs and two different monitoring methods in many publications. With BIS monitoring in propofol anesthesia, the reduction of 15 % has been reported [5]. Recently, a preliminary finding utilizing PSA 4000 monitor appeared [12], revealing a 13 % decrease in propofol consumption in monitored patients compared to historic controls. In studies of sevoflurane and desflurane, the anesthetic-sparing effect of BIS monitoring has been 30 to 40 % [6,7].

Hypnosis monitoring (i.e. titrating hypnotic medication along with the BIS level) shortens immediate recovery time. This was first shown by Gan et al. [5] and Song et al. [6], and confirmed by others later [7]. In a study with propofol, a respond to commands was facilitated in four minutes in the group where BIS was used [5]. In another study using volatile agents, orientation was reached two to three minutes earlier in the group where BIS was monitored [6]. In addition to shorten awakening times, faster recovery can be associated with BIS monitoring. Times when the patient achieves criteria for bypassing the recovery room (PACU 1) correlate with BIS values in the end of surgical procedure [8]. In general, BIS-monitored patients are more alert in the in the post anesthesia care unit, also having better nursing assessment scores [5]. However, the readiness for discharge seems not to be influenced with BIS monitoring. That finding suggests a rather small role of residual anesthetic effect on discharge times.

There is also some evidence that the overall cost consumption may decrease when BIS monitoring is applied. Prices for the duration of stay in the OR and PACU are not easy to compare, as they may depend on the unique cost structure of each hospital. Hence, such estimates can never be universal. In Emory University, Johansen and Sigl [9] made an interesting study that was based on treatment costs. Their preliminary report has associated a clear reduction in the overall costs of treatment with the intraoperative use of BIS.

Controversial findings on the anesthetic-sparing effect

Not all studies have demonstrated an anesthetic-sparing effect of BIS monitoring. In some reports, the consumption of hypnotic drugs has even increased with the use of BIS. That may be a consequence of erroneous interpretation of traditional signs of "anesthetic depth". When hypnotic monitoring is evaluated, the subcortical signs of inadequate anesthesia (autonomic signs, motor responses) should not be taken as implications of awareness. Furthermore, studies to elucidate the question of unintentional awareness [10] are of great clinical importance. If an increased consumption of hypnotics is seen and BIS monitoring has been utilized, one should always critically evaluate anesthesia departments’ "standard" care practice.

Even when BIS monitoring does not necessarily have any impact on the consumption of general anesthetics at group level, it may be helpful for an individual patient. When a patient’s hypnotic concentrations are being adjusted according to the individual needs, the side effects of anesthesia can be reduced. A recent study demonstrated a BIS-associated decrease in the incidence of postoperative vomiting, although the anesthetic consumption in the monitored vs. unmonitored group was not reduced [11]. The explanation can be better targeting of anesthetic care, as the variability was less in the BIS monitored group.

Conclusion

Hypnosis monitoring offers many advantages for modern anesthesiology. Further studies are indicated to discover new potential application areas and patient groups. I feel that in hypnosis monitoring, there are important clinical aspects which will show their value in the future. First, hypnosis monitoring facilitates a clinician’s better understanding of the concept of modern anesthesia. That concept is based on a group of parallel effects; ideally, each of them should be monitored by special techniques (Fig. 1). Second, the use of EEG as a measure of anesthetic effect can encourage anesthesiologists for other applications of perioperative brain function monitoring. Personally, I hope that the day will come when intraoperative monitoring of brain function would not be any bigger event than cardiac monitoring is today.

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Last updated: 1 November 2001Created
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