Ulrich Lips

Stages of Anesthesia

Narcotic, analgetic, and neuroleptic substances used in anesthesiology for general anesthesia cause a continuous reversible suppression of all or individual structures of the CNS (holencephalic and telencephalic narcotics). The changes in the level of consciousness and in the vegetative functions during the induction, maintenance, and termination of anesthesia are at all times related to the actual depth of anesthesia. The various stages of anesthesia can thus be clinically classified, as was first described by Guedel in 1920 [5]. In the course of anesthesia with holencephalic drugs, these stages are basically passed through in a similar sequence — analgesia with complete alertness, loss of consciousness, coma due to narcotic overdose, and intoxication. The time leading up to the individual stages and their duration depends on the type of narcotic agent and its specific appearance and disappearance in the CNS. In modern anesthetic techniques with combinations of several anesthetic-narcotic relaxing substances, the individual stages of anesthesia are frequently varied in their clinical manifestations and thus can be less easily distinguished than with the monoanesthetic formerly used.

Special Considerations in Patients with Seizure Disorders

In accordance with the proportion found in the population at large, about 3% of anesthesiological/surgical patients are predisposed to seizure disorders (manifest or latent epilepsies, fits as residues of trauma or as symptoms of displacing processes).

Perioperative Disturbances with Cerebral Effects

Disorders of the physiological balance result in EEG changes. The EEG has proven to be a highly sensitive indicator of alterations in cerebral function. Since various noxae may cause similar encephalographie phenomena, the EEG allows an evaluation of the extent and degree of cerebral alteration. Conclusions as to the actual cause are not possible from the EEG alone but can be made in conjunction with the clinical information. The course of EEG changes due to clear-cut alterations in cerebral perfusion and metabolism has been studied and is known to a certain extent.

Grundlagen der Analyse des Elektroenzephalogramm

Die computergestutzte EEG-Analyse hat bislang in begrenztem Umfang in der klinischen EEG-Diagnostik [9, 14, 22] Anwendung gefunden. Einen festen Platz hat sie sich in der Schlafforschung und in der Analyse cerebraler Medikationswirkungen erworben. Fur die Narkoseuberwachung sowie fur die Uberwachung im Rahmen der Herzchirurgie (PRONK 1982) gewinnt sie zunehmend an Bedeutung. Ein sinnvoller Einsatz und eine kritische Bewertung der EEG-Analyse, insbesondere in diesen Bereichen, setzt das Verstandnis von Grundprinzipien und Aussagemoglichkeiten der verschiedenen Analyseverfahren voraus.

Cerebral Effects of Different Anesthesiological and Surgical Measures During Anesthesia

Unusual stress by painful stimuli (e.g., start of surgery or peritoneal irritation) or routine anesthesiological procedures (e.g., intubation, insertion of a gastric tube, central venous or arterial puncture) can cause a rise in the EEG frequencies of a patient under narcosis. This indicates a reduction of the depth of anesthesia caused by irritation. Sudden drops of EEG frequencies in the steady state of anesthesia are occasionally seen after introduction of large amounts of cold infusions or transfusions (no example is shown). This can be interpreted as a sign of insufficient cerebral supply (hypoxia).

EEG Monitoring During Intensive Care

The clinical situation and the long-term therapy often necessary for patients with life-threatening diseases or markedly reduced vital functions are indications for regular or continuous monitoring of cerebral activity, as the highly variable pathophysiological situations and immediate therapeutic reactions connected with them constitute a risk factor for the patients’ cerebral function.

Perioperative Factors Influencing Cerebral Function

Stressful situations lead to alterations in the basic EEG activity. They manifest themselves in increasing beta frequencies or else in a general reduction of voltage. There are periods of primary cerebral depression or ineffective supply to the brain secondary to critical situations during the operation and anesthesia, or because of postoperative complications. These events may be caused by intoxication, by pulmonary disturbances, cardiovascular crisis, or direct oxygen deficiency. If the corresponding parameters deviate only minimally from the norm, EEG desynchronization is seen. In case of marked deviations, one finds synchronization. The degree of frequency reduction correlates with the extent of the disturbance.

Technical Requirements for EEG Recording in the Operating Room and Intensive Care Unit

Electrical brain activity can be recorded from the intact scalp. Voltage is low (5–500 μV), so registration requires high amplification. Artefacts by interference from other, insufficiently shielded electrical equipment in the operating room or the intensive care unit render EEG difficult. With modern EEG equipment, however, it is generally possible to record with the recommended calibration of 50 μV= 7 mm, a time constant of 0.3 s, and a high-frequency filter of 70 Hz. For diagnosis and subsequent therapy a simultaneous recording of 12 leads is required, while for sample monitoring purposes a restriction to smaller, 2-lead equipment is sufficient. To minimize electrical interference with the extremely sensitive unamplified signals, input amplifiers are placed as close to the electrodes as possible, usually in the connection box. A combination of electrode and miniaturized input amplifier directly on the scalp would be particularly favorable. Ag/AgCl adhesive electrodes have proved the best for use during surgery. They are fixed to the scalp with adhesive paste after appropriate preparation (reduction of skin resistance by removal of keratinous tissue with a fiberglass pen, degreasing with alcohol, and filling the electrode with electrode paste; Fig. 1). For hairless skin areas, foam rubber electrodes prepared with electrode paste are available, as are needle electrodes for the anesthetized patient.

Advantages of EEG in Anesthesiology

Electrical activity is a characteristic of every living cell. Electrical potentials of the intact human cortex were first recorded on the skull by Hans Berger in 1924. He found sinusoidal waves with a frequency of 1–60 Hz. The normal frequency in adults was found to be 10 Hz. The voltage and frequency of cortical potentials vary with changing levels of cerebral function, and are particularly influenced by the state of the metabolism at any given moment. Growth and aging processes, fluctuations of vigilance, every kind of illness and drugs acting on the central nervous system all cause metabolic-functional alteration. The electroencephalogram (EEG) makes these changes visible and therefore also interpretable.

EEG Monitoring During Anesthesia

EEG monitoring of cerebral functions immediately before and during anesthesia provides the anesthetist with most instructive information. Even before induction, the initial frequency and amplitude show whether the patient’s physiological condition is normal or pathologically altered, and indicate the nature and strength of the effects of sedative premedication.