To the Editor:—
In a recent article, Iselin-Chaves et al. 1compared the prediction probability of two methods, i.e. , bispectral analysis of the electroencephalograph (BIS®) and peak amplitudes and latencies of middle-latency auditory-evoked potential (AEP). The authors list four inconveniences, which, in their opinion, may limit the use of AEP in clinical practice.
1. AEPs need considerable time to produce a response.
2. Difficulties exist in interpreting the waveforms and on-line measurement of Pa and Nb latencies.
3. Difficulties exist in obtaining and recording AEP.
4. An AEP needs auditory stimuli and, therefore, is applicable only to patients with preserved hearing.
Any anesthesia monitor requires a certain time to process its measured data. Therefore, a number presented on the screen will never correspond to the most recent data acquired. We define the difference between data acquisition and data presentation as total update delay . The total update delay is composed by the time needed to acquire the data and the time needed by the algorithm to calculate the number that should appear on the screen. The total update delay should not be confused with the update time , which is the period between the appearance of two subsequent numbers on the screen. The total update delay of an AEP is considerable when using the classic extraction technique, i.e. , the moving time average. However, the literature describes methods other than the moving time average, which can extract the AEP within a few sweeps. 2,3The method implemented in a monitor by our group is the autoregressive model with exogenous input, which facilitates extraction of the AEP within 15 sweeps, thus producing a total update delay of the AEP of 1.7 s. 4,5Iselin-Chaves et al. 1state that the method used by Davies et al. , 6the moving time average, has a total update delay of the AEP index within 3 s. This is not correct—the method used by Davies et al. 6displays a new index value every 3 s (the update time), but the total update delay is 36.9 s. The update time is independent of the total update delay; thus it is important that a method with a short update time is not interpreted mistakenly as a method with a short total update delay. Iselin-Chaves et al. 1state that the A-2000®monitor (Aspect Medical, Natick, MA) has a continuous display of the Bispectral Index; however, the total update delay is approximately 30 s.
According to the second issue, difficulties in the interpretation of the latencies of AEP peaks exist. However, to overcome these difficulties and simplify the interpretations, different research groups have defined indices that automatically quantify the peak changes into a single number. The objective of an AEP index is to substitute manual interpretation. 7–9
The third issue (setup time > 5 min) is solved by modern monitors, such as the A-line®AEP monitor (Danmeter, Odense, Denmark). The only difference between this monitor, and, for example, the A-2000 BIS®monitor is that besides electrodes, a set of headphones are needed.
The fourth issue is deafness. An AEP cannot be created in a patient who has 100% hearing loss. However, if a patient has only partial hearing capability, an AEP could be evoked by a click stimulus. Changes in hearing level in the range 20–30 dB do not have significant influence on AEP peak amplitudes and latencies. 10The percentage of people with total hearing impairment is low—0.074% of the total population. 11
*Poly- technic University of Catalonia, Barcelona, Spain. firstname.lastname@example.org