Monitoring the Integrity of Somatosensory Pathways with Evoked Electroencephalographic Bursts K. M. Hartikainen, MS&, M. Rorarius, MD, PhD$, and V. Jbtti, MD, PhD§ *Department of Physiology, Tampere Brain Research Center, Tampere University Medical School, and Departments of tClinica1 Neurophysiology and SAnesthesiology, Tampere University Hospital, Tampere, Finland, and SDepartment of Clinical Neurophysiology, Oulu University Hospital, Oulu, Finland. During isoflurane-induced electroencephalographic (EEG) suppression, external stimuli evoke high- amplitude cortical responses (bursts). We tested whether bursts evoked by somatosensory stimuli would reliably distinguish intact somatosensory path- ways from pathways in which peripheral nerve con- duction had beenblocked by local anesthetic. Ten sub- jects were anesthetized with isoflurane until burst suppression was achieved. During EEG suppression, they were given somatosensory stimulation, consisting of 3-sepisodes of 60 electric pulses(20mA, 0.2 ms), to the tips of the left and right fifth fingers alternately for 10min. Onefinger was then anesthetized at the base of the proximal phalanx with prilocaine and the other fin- ger was injected with salinein a double-blind manner. The stimulation was continued for 20 min. In nine pa- tients, the disappearance of bursts in response to stim- uli applied to the anesthetized finger clearly indicated the side of the conduction block. After the injection of local anesthetic, there was a predominance of offset bursts over onsetbursts in response to stimuli applied to the anesthetized finger (P < 0.05) before the re- sponses disappeared. We conclude that evoked bursts merit further investigation for potential use in monitor- ing the integrity of neural pathways. (Anesth Analg 1996;83:354-8) B urst suppression pattern in the electroencepha- logram (EEG) occurs during moderate to deep anesthesia using numerous anesthetics (l-4). In- creasing the concentration of isoflurane changes the continuous EEG pattern of light surgical anesthesia to burst suppression (2), in which high-amplitude activity (bursts) alternates with periods of EEG suppression. Intraoperative monitoring of EEG and evoked po- tentials informs the anesthesiologist about the depth of anesthesia, the function of the cerebral cortex, or the integrity of specific neural pathways (5-9). We have shown that during isoflurane-induced EEG suppres- sion, somatosensory, auditory, and visual stimuli evoke constant, high-amplitude cortical responses (bursts) (10,ll). These observations prompted the hy- pothesis that evoked bursts might prove useful in monitoring the integrity of neural pathways in oper- ations in which neural pathways are at risk. In this This study was supported by the Medical Research Fund of Tampere University Hospital and the Instrumentarium Science Foundation. Accepted for publication April 16, 1996. Address correspondence and reprint requests to Kaisa Hartikainen, MSc, Department of Clinical Neurophysiology, Tampere University Hospital, P.O. Box 2000, FIN 33521, Tampere, Finland. study, we tested whether bursts evoked by somato- sensory stimuli could reliably distinguish an intact somatosensory pathway from a pathway in which peripheral nerve conduction block had been induced by local anesthetic. Methods The study was approved by the ethics committee of Tampere University Hospital. Written, informed con- sent was obtained from 10 patients (ASA risk classifi- cation I) scheduled for elective gynecologic surgery. We excluded patients with suspected drug or alcohol abuse, those who smoked excessively, and those on medications known to alter the EEG. We used a ran- domized, double-blind study design. All patients were anesthetized between 7:00 and 8:00 AM. They were premeditated with oxazepam (15 mg per OS) 1 h before induction of anesthesia. Five hundred milliliters of 6% hydroxyethyl starch (Plas- mafucin@; Pharmacia, Uppsala, Sweden) was infused during the first 20 min, followed by 1000 mL of lac- tated Ringer’s solution. Anesthesia was induced with propofol 2.5 mg/kg intravenously, and muscle relax- ation was induced and maintained by rocuronium 0.6 mg/ kg intravenously (Esmeron; N.V. Organon, 354 Anesth Analg 1996;83:354-8 01996 by the International Anesthesia Research Society 0003-2999/96/$5.00