Bispectral Index in the Assessment of Depth of Anesthesia

Headnote

Background: Adequate depth of anesthesia is one of the modern anesthesia aims. Possibility for monitoring of anesthesia depth is one of the modern anesthesia performances. There are subjective and objective methods for assessment depth of anesthesia. PRST score (pressure, rate, sweating, tears) is based on physiological response and can be used for estimating anesthesia depth. The aim of this study was to estimate depth of anesthesia with BIS accompanied by clinical parameters monitoring and ascertain presence of possible intraoperative awareness.

Methods: Bispectral index (BIS) is numerical processed, clinical confirmed electroencephalographic parameter that can be used for assessment depth of anesthesia as objective method.

Results: The mean age of patients was 51, 43 ± 13, 77. Patients were male in 90%, and female in 10%. General anesthesia lasted 54,93 ± 2,14 min. Bispectral index value during general anesthesia was between 35 and 60. There is statistical significant difference of BIS values in all measured periods (p<0,05). PRST score value was not higher than 3 in every moment of measuring. The highest PRST score values were immediately after intubation, but there were lower in further general anesthesia process. There is statistical significant difference of PRST score values in t1 and t2, t1 and t3, t1 and t4, t2 and t3.We made conversation with all patients 24 hours after operative procedure. We asked them, following formerly made questionnaire, about their pre, intra and postoperative memory. There was not any patient who replied intraoperative conscious, and most of them had first postoperative remembering associated with operative room.

Conclusion: Bispectral monitoring is significant support for clinical assessment depth of anesthesia.

Keywords: depth of anesthesia; bispectral index; PRST score

INTRODUCTION

Surgical anesthesia should be harmless and reversible state of insensibility of the patient, whose characteristics are sleep, analgesia, muscle relaxation and loss of reflexes (1). One of the goals of modern anesthesia is to ensure adequate depth of anesthesia to prevent awareness, but without overloading the patient with strong medications. One of the achievements of modern anesthesia is the possibility of monitoring the depth of anesthesia (1, 2). Adequate depth of anesthesia is present when the concentration of drug is sufficient to provide comfort to patients and perform surgery. There are subjective and objective methods for assessing depth of anesthesia. Subjective methods are based on movement and sympathetic response to the stimulus and depend on the opinions and experiences of anesthesiologists. Objective methods are based on the sensitivity of the monitor (2, 3, 4). Scoring of the patient's response to surgical stimulus, based on autonomous changes occurring in response to surgical stimulation, is a weak indicator of the depth of anesthesia (2, 5). It has been shown that hemodynamic response to stimulus does not always signify vigilance, or the absence of hemodynamic changes guarantee unconsciousness (6, 7). In patients who experienced vigilance during anesthesia, hypertension is reported 15 % of cases, tachycardia in 7%, and only 2% of patients have made a movement (8, 9, 10). PRST score (pressure, rate, sweating, tears) can be used to estimate the depth of anesthesia and is based on physiological response. The control parameter value is the value that was before the induction. Adding up the points of all four parameters determine the total amount which can range from 0 to 8. There is inadequate depth of anesthesia if score is more than three (5).

Electroencephalogram (EEG) can be used to estimate the depth of anesthesia. Anesthetics act on brain physiology and lead to changes in cortical neural activity, resulting in changes in electrical brain activity with a reflection on the EEG. EEG is a noninvasive indicator of brain function when the patient is unconscious and without vulnerability (11, 12). During general anesthesia, EEG changes imply an increase of average amplitude and decrease in average rate (12, 13). Bispectral index (BIS index) is a numerical processed, clinically confirmed EEG parameter, obtained by combining the more advanced EEG techniques such as bispectral analysis, a powerful spectral analysis and time analysis. These components are combined to optimize the correlation between EEG and clinical effects of anesthesia. BIS index is a number between 0 and 100 arranged to correlate with clinical status during the application of anesthetics. BIS value near 100 is a clinical state of alert, while 0 means the greatest possible effect on the EEG (isoelectric EEG). Administration of hypnotics leads to the fall of the BIS index value of 100 in the waking state. Loss of consciousness occurs at BIS values between 70 and 80. BIS index of 40-60 indicates adequate hypnotic effect of general anesthesia with postoperative rapid return of consciousness. BIS value below 40 indicates a deep hypnotic state. BIS values decline below 70, the possibility of explicit survival is less. With the BIS index values of less than 60 is very small chance of awareness (14, 15).

SUBJECTS AND METHODS

Study was prospective and 30 patients who underwent inguinal hernia surgery at the Department of Surgery, University Clinical Centre Tuzla were analyzed. Subjects were of both sexes, aged 20-70 years, according to the classification of the American Society of Anesthesiologists (ASA) I and II operational risk. We used BIS XP System by Aspect Medical System to estimate the depth of anesthesia. Before the introduction of anesthesia unilateral BIS sensor that records the EEG waves was mounted on cleaned and dried head. BIS sensor is with the appropriate cable connected with the BIS monitor that displays the EEG waves and BIS index value. Using a sensor that is placed on the patient's forehead BIS monitoring translates information from the electroencephalogram in a simple number that is read on a monitor and represents a patient's state of mind. BIS index was monitored continuously, and we recorded the value at the time of intubation (t1), the first skin incision (t2), 20 min after the first incision (t3), and immediately after placing the last suture in the skin. During anesthesia, the BIS index values were maintained in the range 40- 60, which is considered adequate depth. With BIS index quotation, it was determined and noted PRST score, too. Before the introduction of anesthesia values of systolic blood pressure and heart rate were recorded (t0). At the time of intubation (t1), the first skin incision (t2), 20 min after the first incision (t3), and immediately after placing the last suture in

the skin (t4), values of blood pressure and heart rate were recorded, also the occurrence of tears in the closed or when opening the eye, and the degree of skin moisture. Each parameter is scored from 0 to 2 and adding up all the points obtained by the PRST score on the basis of which was estimated depth of anesthesia. For an introduction to anesthesia was used propofol (1.5 to 2.5 mg / kg) for muscle relaxation atracurim (0.6 mg / kg), while the anesthesia was mainatined with 02, N2O and sevoflurane, and analgesia with fentanyl (0, 15 to 0.25 mg). Postoperatively (24 hours after awakening from anesthesia), according to pre-made questionnaire we interviewed respondents to obtain information on whether they heard or felt anything during the surgery:

1. What was the last thing you remember before going to sleep before your surgery/procedure

2. What is the first thing you remember whwn waking up from your surgery

3. Do you remember anything between

4. Did you have any dreams while you were asleep during your surgery

5.What was the most unplesant thing you remember from your surgery and your anesthesia

Statistical analysis was performed by descriptive statistics to calculate the mean and standard deviation, and t-test, χ^sup 2^ tests for calculating the materiality established results. Statistical analysis was performed with a confidence interval of 95%, a value of p <0.05 was considered significant.

RESULTS

The average age of respondents was 51.43 ± 13.77. The male were represented in 90% of cases, a female at 10%. General anesthesia lasted 54.93 ± 2.14 min. Bispectral index value during general anesthesia was between 35 and 60. The mean value of BIS at t1 was 38.93, 46.56 at t2, the t3 49.66 and t4 60.43 (Figure 2).

There was a statistically significant difference in BIS index values in all periods of measurement, and is most intensive between t1 and t4, t2 and t4, and t3 and t4 (Table 4).

In all periods of measurement PRST score values were not higher than 3, which indicated an adequate depth of anesthesia. The highest PRST score values were immediately after intubation, whereas in the further course of general anesthesia were lower (Figure 3).

There was a statistically significant difference between the PRST score value at t1 and t2 (p = 0.00011), t1 and t3 (p = 0.00002), and t1 and t4 (p = 0.00001), t2 and t3 (p = 0.04340).

All respondents were interviewed 24 hours after surgery. According to a prepared questionnaire we asked questions about the immediate preoperative, intraoperative and postoperative memory. None of the respondents did not reply for the existence of consciousness during the intraoperative period, whereas most of them the first postoperative memories tied to the operating room.

DISCUSION

One goal of anesthesiologists is to achieve adequate depth of anesthesia during surgical procedures, to prevent the occurrence of intraoperative awareness. Too deep anesthesia can lead to cardiovascular depression and prolonged awakenings, and light anesthesia to serious physiological consequences for the patient. Moerman et al. (1993) have conducted a study that analyzed twenty six patients who experienced intraoperative awareness (17). Almost 70% later had an uneasy feeling, insomnia, nightmares, re-living the events or feelings of anxiety during the day, while 6% took psychotherapeutic assistance. Before the introduction of muscle relaxants, to achieve adequate depth of anesthesia implied a balance between lack of movement to painful stimuli with maintaining adequate respiration. With the absence of movement on pain it could be safely assessed that the patient is not awake. However, when using muscle relaxants it is necessary to be sure that the applied anesthetic dose is sufficient to prevent awareness. The introduction of new anesthetic techniques, such as intravenous anesthesia, using potent opiate analgesics, newer volatile anesthetics, and the assessment of depth of anesthesia has gained even greater significance. In their daily work anesthesiologist assesses the depth of anesthesia based on clinical signs, which represent a response of the autonomic nervous system of the organism to light anesthesia - tachycardia, hypertension, sweating, tearing, and dilation of the pupil. Evans and Davies in 1984 introduced a scoring system for clinical assessment of depth of anesthesia (PRST score). This scoring system may indicate an adequate depth of anesthesia, but patients can be awake during it, or parameters can indicate light anesthesia but patients do not complain later on intraoperative awareness. The signs of increased autonomic activity may be absent when using opioids, cholinergcs a, beta blockers, vasodilators and antihypertensive drugs. Increasing the parameter values of PRST score can cause hypovolemia, inadequate analgesia, hypoxia or hypercapnia. Signs of a light anesthesia often occur, but their correlation with awareness is low. In our study, titration of anesthetics and anesthetic depth assessment were done on the basis bispectral index. Before administration of midazolam BIS index value was between 97 and 100, after administration of midazolam and fentanyl over 80, and after administration of intravenous anesthetics value was below the 60. With continuous monitoring of the BIS index the PRST score was assessed. The lowest mean BIS index was in t1, and the highest in t4, which was expected since we have an intravenous anesthetic for induction of anesthesia titrated to BIS index values below 40, and at the end of surgery reduced the concentrations of volatile anesthetic corresponding EEG and BIS .The greatest value PRST score was in t1. In the further course the mean PRST score values were lower, due to achieving higher concentrations of anesthetics in the central nervous system after turning volatile anesthetics and the administration of additional doses of opioid analgesics. During surgery, we titrated the anesthetic by the BIS index values, keeping it in the range 40-60, which according to an activity level of the central nervous system that is required for general anesthesia. In addition to this objective method nor clinical assessment of depth of anesthesia with PRST score in this group of respondents did not indicate the likelihood of being "shallow" of anesthesia and intraoperative awareness. In our study, 24 hours after surgery the subjects were interviewed according to pre-prepared questionnaires, in order to determine the existence of explicit memory for intraoperative events. All respondents gave a negative response to questions about whether something heard, seen or felt while they were under general anesthesia. Assessing the depth of anesthesia using BIS monitoring is a noninvasive method. The value of BIS index reflects the electrical activity of the brain and the patient's state of consciousness, so BIS monitoring reduces the possibility of intraoperative awareness of the patient and explicit memory for intraoperative events. Ekman and colleagues (2004) conducted a study aimed at analyzing the occurrence of explicit memory in patients who were monitored for BIS monitoring during general anesthesia. The study included 4945 patients who underwent elective surgery. They compared the results of their study with results from earlier studies, but where no cerebral monitoring was used. Two patients (0.04%) who were followed with BIS monitoring had explicit memory, while 0.18% of patients in studies without cerebral monitoring, had explicit memory (p <0.038). In conclusion, Ekman and colleagues have pointed out that the use of BIS monitoring during general anesthesia significantly reduces the incidence of intraoperative awareness (18). Bispectral index allows anesthesiologists directly and accurately monitoring the central nervous system during the application of anesthetics or sedatives, or assessment the hypnotic effect of anesthesia (19). Monitoring Bispectral index monitoring assesses the depth of anesthesia and facilitates titration of anesthetics. In the operating room changes in blood pressure and heart rate are not uncommon, and the task of anesthesiologists in these situations is to make a prompt diagnostic evaluation and timely intervention to eliminate the cause of these changes. BIS monitoring provides new information that can facilitate the anesthesiologist in making decisions and treatment of many of these situations (20). BIS monitoring is not a substitute for clinical assessment of depth of anesthesia. However, the use of BIS monitoring with clinical assessment allows anesthesiologists precise decision-making and balancing a dosage of anesthetics and other medicines such as analgesics and cardioactive agents, especially in patients with higher operative risk (21).

CONCLUSION

Evaluation of intraoperative depth of anesthesia is one of the main tasks of anesthesiologists. BIS monitoring is a useful addition to clinical assessment of anesthesia, especially in high-risk patients. BIS monitoring is not a substitute for clinical assessment depth of anesthesia. It is necessary for anesthesiologist to interview patients preoperatively and postoperatively, record the factors and prevent the consequences of intraoperative awareness. Intraoperative vigilance causes permanent damage to the psychic sphere in patients, and liability for anesthesiologists.

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Smajic J, Praso M, Duric A, Hodzic M. Bispectral Index in the Assessment of Depth of Anesthesia. Acta Med Sal 2011; 40(Suppl 1); S32-S36.

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