Neurophysiology NeuroReport 0959-4965 # Lippincott Williams & Wilkins Monitoring of anaesthesia in neurophysiological experiments A. Kral, 1,2 J. Tillein, 1 R. Hartmann 1 and R. Klinke 1,CA 1 Physiologisches Institut III, J.W.Goethe- Universita Èt, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main, Germany; 2 Institute of Pathological Physiology, Comenius University Sasinkova 4, SK-81108 Bratislava, Slovak Republic CA Corresponding Author CORTICAL activity can be substantially changed by the type of anaesthetic used, and by its dose level. For easy monitoring of depth of anaesthesia we describe the changes in electroencephalogram and electrocardiogram accompanying changes in depth of anaesthesia in the cat. Anaesthesia was induced by the volatile anaesthetic iso¯urane. The high-frequency components (around 30 Hz) in the electroencephalogram disappear in deep anaesthesia. The electrocardiogram also shows substan- tial changes in contamination due to muscle fascicula- tions with anaesthesia level. Fasciculations appear as noise in the electrocardiogram. The amplitude of the electrical muscle activity contaminating the ECG can be easily used for the maintainance of a constant level of anaesthesia during a neurophysiological experiment. NeuroReport 10:781±787 # 1999 Lippincott Williams & Wilkins. Key words: Anaesthesia; Cortex; ECG; EEG; Field poten- tials; Neurophysiology; Muscle activity Introduction Neurophysiological experiments are (with rare ex- ceptions) conducted under general anaesthesia. Anaesthetics depress neural activity. For recordings of cortical neural activity, it is of crucial importance to maintain the anaesthesia level in a de®ned (con- stant) stage and as light as possible. Application of the anaesthetics must be fast and accurate to allow control of the level of anaesthesia adequately. Vola- tile anaesthetics are most suitable, as the application is continuous and the effect is rapid. The application can be controlled easily and the depth of anaesthesia can be held constant over hours. Depth of anaesthesia [1] is normally controlled via clinical signs such as spontaneous movements, reac- tions to noxious stimuli (withdrawal re¯ex), blood pressure (BP), heart rate (HR), body core tempera- ture, eyelid re¯ex and end-tidal CO 2 concentration. In humans, sweating and tear production are also used [2]. For volatile anaesthetics, the end expiratory concentration of the anaesthetic is monitored. The electroencephalogram (EEG) is often used for monitoring depth of anaesthesia. The deeper the anaesthesia, the smaller is the representation of high- er frequencies in the EEG [3±7]. With iso¯urane, an anaesthetic used in combination with N 2 O, a change from fast wave to slow wave pattern has been observed with increasing anaesthetic concentration [7,8]. The amplitude and latency of middle latency auditory and somatosensory evoked potentials are also dependent on the depth of anaesthesia. Deepen- ing the anaesthesia decreases the amplitudes, in- creases the latencies and decreases the content of high-frequency components of the evoked responses [4,9±12]. These responses are sometimes used as an additional control of patient vigilance. For our experimental studies concerned with the maturation of the central auditory system triggered by early cochlear implantation in congenitally deaf animals, we investigated long-latency waves in the auditory evoked responses [13±17]. Such long- latency responses have been described in the motor cortex of the cat after electrical stimulation of the thalamus [18]. It was stated that these responses re¯ect low-frequency oscillations (, 20 Hz) of the cortical units in visual, auditory and somatosensory cortices [19], which are also dependent on the depth of anaesthesia [19]. It is also known that the long-latency responses of the auditory evoked potentials are more sensitive to anaesthesia than middle-latency responses [5]. It is, therefore, of crucial importance to maintain the level of light anaesthesia constant throughout neurophysiological experiments. It will be shown in the present paper how the muscle activity contaminating the electrocardiogram (ECG) can be used as a simple criterion of depth of anaesthesia. NeuroReport 10, 781±787 (1999) Vol 10 No 4 17 March 1999 781