Chapter 19 Depth of Anesthesia Monitoring by Bispectral Analysis in Zoo Animals
General anesthesia is required in zoological medicine for a variety of reasons, including painful (e.g., surgery) and nonpainful interventions (e.g., radiographic examination). Typically, induction will be achieved via an injectable anesthetic, alone or as a combination. In addition to achieving analgesia and hypnosis, anesthesia in zoological medicine aims at the safe immobilization of the patient over a determined time period. Recovery time is expected to be quick, especially under field conditions, and safe both for the animal and the personnel involved. Monitoring of the patient under general anesthesia is done by a variety of tests. Some of these evaluate hemodynamic functions and autonomic nervous responses, such as heart rate, respiratory rate, blood pressure, and oxygen saturation. Others are intended to measure the depth of anesthesia (DoA) with respect to the degree of hypnosis and analgesia, including measurement of the patient’s reactions to nonphysiologic positions such as dorsal recumbency (righting reflex), corneal and pupillary reflexes, and toe pinch stimulation.
In human medicine, DoA measurement is of special importance to avoid occurrence of awareness—that is, the postoperative recollection of events occurring during general anesthesia. In a recent review, incidences of awareness up to 0.2% in adults and up to 0.8% in children were reported.3
In veterinary medicine, the role of awareness cannot be assessed because animals cannot report their postanesthetic experiences. Measurement of DoA is nevertheless of interest from an animal welfare point of view. In zoological medicine, DoA monitoring is of further interest from a personnel and equipment safety point of view. However, the reliability of a stimulation test with regard to the large number of species encountered by the clinician in zoological medicine is at best adequate in relation to safety.
For safety reasons, often a lower than necessary DoA will be chosen, which only prolongs recovery but increases the dose-dependent cardiopulmonary impairment, resulting in an increase of postanesthetic morbidity and mortality. This is reflected by the high anesthetic- and sedative-related risk of death, which ranges in small mammals from 1.4% to 3.6%, in birds from 1.8% to 16.3%, and in reptiles it is 1.5%, compared with dogs and cats (0.1% to 0.2%) and humans (0.02% to 0.01%2). In wildlife anesthesia, mortalities up to 3% have been reported and it was proposed that mortalities above 2% should not be acceptable.1
Established DoA monitors include the bispectral index (BIS, Aspect Medical Systems, Norwood, Mass), the Narcotrend index (Narcotrend Monitor, Schiller AG, Baar, Switzerland), and the state entropy (SE) and response entropy (RE), derived from the spectral entropy from the electroencephalogram (EEG; M-Entropy module, GE Healthcare, Helsinki, Finland).
These monitors process the level of corticocerebral activation measured by analog EEGs into a signal that reflects the DoA. The most widely used monitor is the BIS, which in 2004 was used in approximately 34% of all hospital operating rooms in the United States and 78% of teaching institutions, and had a worldwide installation base of over 25,000 units.8 Numerous studies have investigated the BIS. An Internet search on the term bispectral index anesthesia in August 2010 produced over 138,000 results, including a report on the use of the BIS in animals, including the dog, cat, horse, and goat.9
Just like any monitor, the BIS offers possibilities but also has limitations, especially with respect to various drugs. The transposition from a single-species environment such as human medicine to the multitude of species encountered in zoological medicine needs to be done with appropriate care, as with pulse oximetry.
Bispectral analysis is based on a complex statistical evaluation of human electroencephalographic data that was developed to obtain an index of the level of hypnosis. It uses a Fourier transform, an operation that transforms one complex-valued function of a real variable into another, such as time into frequency. The BIS value is represented as a dimensionless value from 0 (cortical silence) to 100 (awake) (Fig. 19-1). In humans, an optimal degree of general anesthesia is defined as that associated with a BIS within the range of 40 to 60.
Figure 19-1 The bispectral index (BIS) scale is a dimensionless scale from 0 (flatline) to 100 (awake). We used the A-2000-XP Platform Bispectral Index Monitoring System (Aspect Medical Systems, Norwood, Mass). BIS values of 65 to 85 have been recommended for a light hypnotic state, whereas values of 50 to 65 for deep sedation and values of 40 to 50 for general anesthesia are used. BIS values from 40 to 25 have been seen to produce a deep hypnotic state and, with the BIS less than 25, cortical suppression becomes more and more manifest.
(From Martin Jurado O: Determination of the Anaesthesia Depth in Chickens with Bispectral Index [BIS]. Ph.D. Thesis, University of Zurich, Zurich, 2008.)
In 1996, the U.S. Food and Drug Administration approved the BIS monitor as an accepted measure of the hypnotic effect of anesthetics and sedative drugs in humans. Since its introduction, BIS monitoring has gained increasing popularity in daily anesthesia practice. However, the current evidence indicates various cases of paradoxical BIS changes and inaccurate readings, which also need to be to be taken into consideration when applying the BIS in veterinary medicine (Table 19-1).4,6
The equipment includes a monitor, a digital signal processing cable, and three electrodes with sensors. Newer models include four sensors. It is important to note that the algorithm has been constantly adapted from one model to the other. Therefore, every BIS system will yield different values and the anesthetist needs to be familiar with the system that is being used to interpret changes in values adequately. As with the use of pulse oximetry in zoo animals, the most valuable types of information are the trends indicated by the BIS.
The monitor is available as stand-alone system or as an add-on module for most comprehensive patient monitoring systems. Our studies were performed with an A-2000-XP Platform Bispectral Index Monitoring System (Aspect Medical Systems). The sensors were fitted with 24-gauge needles to allow subcutaneous placement instead of the need to shave or pluck the areas, even in small animals. The impedances for sensors 1 and 3 and for sensor 2 were always <7.5 kΩ and <30 kΩ, respectively. Figure 19-2 graphically displays the location of the sensors.
Figure 19-2 Placement of sensors for bispectral index monitoring in a bird (A, B) and mammal (C, D). Sensor 1 (dot) is placed between the eyes in the frontal area. Sensor 2 (circle) is placed over the temporal musculature. Sensor 3 (X) is placed immediately caudal to the eye angle.
(Courtesy Jeanne Peter, Institute of Veterinary Anatomy, University of Zurich, Zurich, Switzerland.)
Based on the system we used, the different results are displayed in Figure 19-3. The main value is the BIS value, ranging from 0 (flatline or isoelectric EEG) to 100 (awake), which is displayed every 5 seconds. This represents the mean of the maximum and minimum indices of the last 15 or 30 seconds. This smoothing rate needs to be selected in the main menu. In cases of high interference, a longer smoothing rate (30 seconds) is chosen.
Figure 19-3 Bispectral index monitor.
(A-2000-XP Platform Bispectral Index Monitoring System, Aspect Medical Systems, Norwood, Mass.)