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Intraoperative Monitoring: The Eeg Monitor Can be a Window to the Brain
AORN JOURNAL
JUNE 1985, VOL 41, NO 6
Intraoperative Monitoring THEEEG MONITORCAN BE A WINDOW TO THE BRAIN
Margaret F. Fay, RN; Virginia R. Delyanis, RN
A
nesthesiology is a subtle, complex discipline, requiring early detection of significant events that could presage complications. Modern anesthesia advances allow the vast majority of patients to recover without untoward side effects. Nevertheless, a small but definitive risk of intraoperative complications remains. Today’s anesthesia team faces a wider patient age spectrum, more complex operative procedures, and newer and more potent narcotics and anesthetic agents than ever before. Each factor increases potential risks, including cardiac irregularities, adverse drug reactions, intraoperative strokes, and even death. According to an article in the Journal of the Hospital Corporation of America, While the practice of surgery is far safer than it ever has been before, 10,000 to 20,000 anesthesia-relateddeaths still occur every year in the United States. In 1979,300 anesthesia accident cases were settled in court for a total Margaret R Fay, RN, MA, is presia!ent and chief executive o&er of Five Pointr Communicntion COT, Minneapolir, and an OR day surgery staff nurse at SL Maiyk Hospital Minneapolis. She ir a graduate of SL Catherinek Hospital School of Nursing Omaha, and earned her MA in psychologyfromColumbh Pa@c University, and her BA in business from Columbia University, New York City. Virginia R. Lklyank, RN, is a director of clinical development and northwest regional sales manager for Neurologics, Inc. She earned her diploma from Cen&al Maine Medical Center School of Nursing, Lewiston, Me. She ir also a neumsurgical nurse consultant in Tacoma, Wash 1046
of $40 million in awards. A significant number of anesthesia accidents could be avoided if patients were more routinely monitored for EEG during surgery.’ Intraoperative monitoring, specifically the electroencephalogram, or EEG monitor, can reduce potential complications associated with anesthesia. It has been well documented that anesthesia risk can be minimized and complications reduced in direct proportion to the type, amount, and quality of information available to the anesthesia team. Information derived from the EEG is one of the best indicators of the physiological condition of the brain and its alteration by anesthetic agents. The central nervous system (CNS) has not routinely been monitored, even though the desired effects of anesthesia specifically impact it. But because the brain is the organ system most vulnerable to hypoxia and cerebral ischemia, measurement of functional CNS parameters should be a primary goal. Brain status is influenced by an extraordinary variety of interacting factors, including blood pressure, central venous pressure (CVP), intracranial pressure (ICP), cardiac output, temperature, blood concentratiodhernatocrit and viscosity, cerebral metabolic rate, pH, KO,, and PO,. Anesthetic agents further affect these already complex factors. The EEG provides a reliable and feasible technique for intraoperative neurologic assessment (seeFig 1). The EEG can be “a window to the brain,” and recent advances in medical technology have reduced its cost and equipment complexity. EEG monitoring is not only desir-
JUNE 1985, VOL 41, NO 6
AORN JOURNAL
Fig 1. The EEG provides intraoperative neurologic assessment.
able-it has become an essential adjunct to traditional monitoring systems.
Measurement Parameters
T
he newer, two-channel EEG monitor extracts key data from the EEG signalan irregular occilatory potential generated by the cerebral cortex. The data is visually translated through a series of LED (light emitting display) lights on the unit’s face. The processed information is described through two key measurement parameters: power and frequency. Each parameter has two channels, measuring right and left hemispheric status. The power measurement relates to detection of cerebral low blood flow or ischemia; frequency measurements relate to the approximate depth of anesthesia. Power is the amplitude or height of a wave squared. Frequency is the number of times a signal crosses a given baseline in a specified time period (cycles per second, or Hz-see Fig 2). EEG waves, defined by Greek letters, (see Table l), correlate with specific cycle parameters. Both power and frequency can be accurately measured and related to a patient’s neurologic status. For clarity, power can be thought of as the blood volume correlative, and frequency as the content within that blood volume. Because the brain’s repertoire of electrical behavior is limited, the brain will produce fastwave activity in response to small amounts of
an anesthetic agent. Ischemia abolishes the brain’s ability to produce this chemically evoked response. Loss of drug-induced fast-wave activity is displayed as a decline in the power scale (see Fig 3). This documented fact supercedes earlier theories that the presence of slow-wave activity is the crucial indicator of ischemia. A significant drop in power from an established baseline prompts immediate investigation of possible causes-was the patient’s position changed? Has there been a change in circulating volume? Is there carotid insufficiency? The frequency measurement-a modified baseline crossing technique-enables the user to determine if the patient’s activity is mainly slow, mainly fast, or mixed. Derivations in frequency
Traditional Terms Power = a2
Frequency Cycles Per Second (H,) Fig 2. Frequency is measured in cycles per second. 1047
AORN JOURNAL
JUNE 1985, VOL 41, NO 6
Table I
Probabk Causes
EEG Waves
(Power)
Delta: Slowest waves of the EEG. 0 to 4 cycles per second or hertz Theta: Faster waves. 5 to 7.5 cycles per second or hertz Alpha: Faster waves. 7.5 to 12 cycles per second or hertz Beta: Faster waves. 13 to 20 cycles per second or hertz are related to adequacy or inadequacy of oxygen, ventilation, carbon dioxide measurements, and/ or the effects of pharmacologic agents on the anesthetized patient. Like an electrocardiogram (EKG), or a single blood pressure reading, an EEG signal is not valuable in and of itself. An individual EEG wave provides no clinically useful information. A series of waves, however, shows a trend, a developing pattern, or a characteristic signature. These can help identify clinically meaningful data indicative of significant changes in the anesthetized patient’s condition. Signatures are a series of consistently repeated changes in power, frequency, or both. A signature changes with various anesthetic agents, but as a user becomes more knowledgeable and experienced with EEG monitoring, she can correlate signaturesto various clinical responses.It is dificult to describe an average signature, but the following examples are typical signatures of the average healthy anesthetized patient proceeding from an awake state through induction of anesthesia. In an awake state, the amplitude of abundant alpha activity can often be modulated simply by having a patient open or close his eyes. With the patient’s eyes closed, a prompt increase occurs in power and frequency. When the eyes are opened, power declines, but frequency changes little, because the amplitude of the alpha waves is attenuated. In a preanesthetic state, there is generally an abundance of low-voltage fast activity, with an integrated amplitude not much different than in an “eyes-open’’ alpha state. Preanesthetic medications abolish the alpha activity and replace 1048
I
Circulating Volume? By-Pass Pump Perfusion? Carotid Insuficiency? Patient Position?
Fig 3. Decline in power with deep anesthesia.
Probabk Causes (Frequency)
I
a Adequate Ventilation? a Anesthetic Level?
Narcotic Effect? Hypothermia?
JI Fig 4. Potential causes of a drop in frequency.
the background with abundant low-voltage fast activity. This drug-induced fast activity can not be modulated by opening and closing the eyes. During induction with a rapidly acting barbiturate like thiopental, the EEG changes. Depending on how rapidly the barbiturate is infused, the signature may change from lowvoltage fast activity to high-voltage slow activity. With a light level of anesthesia, the amplitude of drug-induced fast activity increases. Normally, all volatile anesthetic agents create these low levels of activity. As the anesthetic level deepens, EEG frequency progressively declines, particularly when narcotic supplements are employed. Generally, there is a corresponding drop in blood
AORN JOURNAL
JUNE 1985, VOL 41. NO 6
pressure, a condition that creates a decline in power as well. As the EEG slows, cardiac depressant effects are more likely. Cardiac depression can create ischemia, which reduces EEG frequency. This condition would first be indicated by the drop in frequency, and eventually by a drop in power. Frequency declines as anesthetic saturations deepen. Aside from these “normal” signatures during the stages of anesthesia, signatures are also affected by hypothermia, hypoxia, and high dosage narcotics. Each has a corresponding characteristic. In hypothermia, there is a gradual reduction in EEG frequency as the temperature declines. With low flow or CNS hypoxia, a sudden persistent symetric or asymetric drop in power and/or frequency occurs. With high dosage narcotics, like fentanyl, the EEG slows. At the same time, a rhythmic pattern not associated with low blood flow occurs (Fig 4). Signatures, loss of consciousness, and enflurane also correlate. Two grouping of waves are noted with this agent. First delta and low theta waves are noted with a slight gap in high theta. The alpha and low beta are also observed. As anesthetic levels lighten, the gap narrows markedly. Halothane generates dominant, large amplitude activity in the alpha and low beta bands. High concentrations of the agent shift the dominant activity to sub-alpha and slow theta activity. Each signature is reflected graphically by the LED lights on the monitor’s face. As the user becomes accustomed to various LED patterns, her interpretative skills increase. EEG monitoring can also aid early detection of many surgical problems-increased depth of anesthesia, stroke, and general hemispheric low blood flow condition. For instance, in carotid endarterectomy, or similar situations in which circulation is compromised, EEG signatures show a sudden decrease in amplitude and with increased severity, a total loss of power and frequency. Once the problem is rectified, there is a dramatic return of both. Damage created by cerebral ischemia or CNS hypoxia can be detected and managed by manipulating critical variables such as depth of anesthesia, increased circulating volume, or
systolic blood pressure.
Conclusion n spite of its ability to serve as an effective “reporter” on changes in the brain’s EEG activity, the EEG cannot replace the professional practitioner. The staff must integrate the etiology of changes with knowledge of EEG activity, and relate this information to the host of independent variables that can affect a patient’s overall condition. As OR and recovery room nurses acquire the interpretive skills necessary to assess changes in the signatures or patterns associated with independent variables, the EEG monitor will be a critical adjunct to the nursing 0 diagnostic process.
I
Note 1. Journal of the Hospital Corporation of America, “Anesthesia monitoring review.” January 1984.
Returning to Work After Stroke Younger patients in professional-managerial positions are more likely to return to work following a stroke than are blue collar workers or farmers, according to a study in the January issue of the Journal of the American Medical Association. George Howard, MSPH, and colleagues from the Bowman Gray School of Medicine in WinstonSalem, NC, studied 379 patients living one year after a stroke. They determined that age, occupation, degree of disability, race, and the brain hemisphere affected were the most significant factors. “Relatively young patients (aged 55 years or younger) are the most likely to return to work following a stroke,” the researchers said. The odds are almost as good (76%) for those 55 to 65, but plummet to 14%for those more than 65, most of whom undoubtedly accept retirement.
1049
JUNE 1985, VOL 41, NO 6
Intraoperative Monitoring THEEEG MONITORCAN BE A WINDOW TO THE BRAIN
Margaret F. Fay, RN; Virginia R. Delyanis, RN
A
nesthesiology is a subtle, complex discipline, requiring early detection of significant events that could presage complications. Modern anesthesia advances allow the vast majority of patients to recover without untoward side effects. Nevertheless, a small but definitive risk of intraoperative complications remains. Today’s anesthesia team faces a wider patient age spectrum, more complex operative procedures, and newer and more potent narcotics and anesthetic agents than ever before. Each factor increases potential risks, including cardiac irregularities, adverse drug reactions, intraoperative strokes, and even death. According to an article in the Journal of the Hospital Corporation of America, While the practice of surgery is far safer than it ever has been before, 10,000 to 20,000 anesthesia-relateddeaths still occur every year in the United States. In 1979,300 anesthesia accident cases were settled in court for a total Margaret R Fay, RN, MA, is presia!ent and chief executive o&er of Five Pointr Communicntion COT, Minneapolir, and an OR day surgery staff nurse at SL Maiyk Hospital Minneapolis. She ir a graduate of SL Catherinek Hospital School of Nursing Omaha, and earned her MA in psychologyfromColumbh Pa@c University, and her BA in business from Columbia University, New York City. Virginia R. Lklyank, RN, is a director of clinical development and northwest regional sales manager for Neurologics, Inc. She earned her diploma from Cen&al Maine Medical Center School of Nursing, Lewiston, Me. She ir also a neumsurgical nurse consultant in Tacoma, Wash 1046
of $40 million in awards. A significant number of anesthesia accidents could be avoided if patients were more routinely monitored for EEG during surgery.’ Intraoperative monitoring, specifically the electroencephalogram, or EEG monitor, can reduce potential complications associated with anesthesia. It has been well documented that anesthesia risk can be minimized and complications reduced in direct proportion to the type, amount, and quality of information available to the anesthesia team. Information derived from the EEG is one of the best indicators of the physiological condition of the brain and its alteration by anesthetic agents. The central nervous system (CNS) has not routinely been monitored, even though the desired effects of anesthesia specifically impact it. But because the brain is the organ system most vulnerable to hypoxia and cerebral ischemia, measurement of functional CNS parameters should be a primary goal. Brain status is influenced by an extraordinary variety of interacting factors, including blood pressure, central venous pressure (CVP), intracranial pressure (ICP), cardiac output, temperature, blood concentratiodhernatocrit and viscosity, cerebral metabolic rate, pH, KO,, and PO,. Anesthetic agents further affect these already complex factors. The EEG provides a reliable and feasible technique for intraoperative neurologic assessment (seeFig 1). The EEG can be “a window to the brain,” and recent advances in medical technology have reduced its cost and equipment complexity. EEG monitoring is not only desir-
JUNE 1985, VOL 41, NO 6
AORN JOURNAL
Fig 1. The EEG provides intraoperative neurologic assessment.
able-it has become an essential adjunct to traditional monitoring systems.
Measurement Parameters
T
he newer, two-channel EEG monitor extracts key data from the EEG signalan irregular occilatory potential generated by the cerebral cortex. The data is visually translated through a series of LED (light emitting display) lights on the unit’s face. The processed information is described through two key measurement parameters: power and frequency. Each parameter has two channels, measuring right and left hemispheric status. The power measurement relates to detection of cerebral low blood flow or ischemia; frequency measurements relate to the approximate depth of anesthesia. Power is the amplitude or height of a wave squared. Frequency is the number of times a signal crosses a given baseline in a specified time period (cycles per second, or Hz-see Fig 2). EEG waves, defined by Greek letters, (see Table l), correlate with specific cycle parameters. Both power and frequency can be accurately measured and related to a patient’s neurologic status. For clarity, power can be thought of as the blood volume correlative, and frequency as the content within that blood volume. Because the brain’s repertoire of electrical behavior is limited, the brain will produce fastwave activity in response to small amounts of
an anesthetic agent. Ischemia abolishes the brain’s ability to produce this chemically evoked response. Loss of drug-induced fast-wave activity is displayed as a decline in the power scale (see Fig 3). This documented fact supercedes earlier theories that the presence of slow-wave activity is the crucial indicator of ischemia. A significant drop in power from an established baseline prompts immediate investigation of possible causes-was the patient’s position changed? Has there been a change in circulating volume? Is there carotid insufficiency? The frequency measurement-a modified baseline crossing technique-enables the user to determine if the patient’s activity is mainly slow, mainly fast, or mixed. Derivations in frequency
Traditional Terms Power = a2
Frequency Cycles Per Second (H,) Fig 2. Frequency is measured in cycles per second. 1047
AORN JOURNAL
JUNE 1985, VOL 41, NO 6
Table I
Probabk Causes
EEG Waves
(Power)
Delta: Slowest waves of the EEG. 0 to 4 cycles per second or hertz Theta: Faster waves. 5 to 7.5 cycles per second or hertz Alpha: Faster waves. 7.5 to 12 cycles per second or hertz Beta: Faster waves. 13 to 20 cycles per second or hertz are related to adequacy or inadequacy of oxygen, ventilation, carbon dioxide measurements, and/ or the effects of pharmacologic agents on the anesthetized patient. Like an electrocardiogram (EKG), or a single blood pressure reading, an EEG signal is not valuable in and of itself. An individual EEG wave provides no clinically useful information. A series of waves, however, shows a trend, a developing pattern, or a characteristic signature. These can help identify clinically meaningful data indicative of significant changes in the anesthetized patient’s condition. Signatures are a series of consistently repeated changes in power, frequency, or both. A signature changes with various anesthetic agents, but as a user becomes more knowledgeable and experienced with EEG monitoring, she can correlate signaturesto various clinical responses.It is dificult to describe an average signature, but the following examples are typical signatures of the average healthy anesthetized patient proceeding from an awake state through induction of anesthesia. In an awake state, the amplitude of abundant alpha activity can often be modulated simply by having a patient open or close his eyes. With the patient’s eyes closed, a prompt increase occurs in power and frequency. When the eyes are opened, power declines, but frequency changes little, because the amplitude of the alpha waves is attenuated. In a preanesthetic state, there is generally an abundance of low-voltage fast activity, with an integrated amplitude not much different than in an “eyes-open’’ alpha state. Preanesthetic medications abolish the alpha activity and replace 1048
I
Circulating Volume? By-Pass Pump Perfusion? Carotid Insuficiency? Patient Position?
Fig 3. Decline in power with deep anesthesia.
Probabk Causes (Frequency)
I
a Adequate Ventilation? a Anesthetic Level?
Narcotic Effect? Hypothermia?
JI Fig 4. Potential causes of a drop in frequency.
the background with abundant low-voltage fast activity. This drug-induced fast activity can not be modulated by opening and closing the eyes. During induction with a rapidly acting barbiturate like thiopental, the EEG changes. Depending on how rapidly the barbiturate is infused, the signature may change from lowvoltage fast activity to high-voltage slow activity. With a light level of anesthesia, the amplitude of drug-induced fast activity increases. Normally, all volatile anesthetic agents create these low levels of activity. As the anesthetic level deepens, EEG frequency progressively declines, particularly when narcotic supplements are employed. Generally, there is a corresponding drop in blood
AORN JOURNAL
JUNE 1985, VOL 41. NO 6
pressure, a condition that creates a decline in power as well. As the EEG slows, cardiac depressant effects are more likely. Cardiac depression can create ischemia, which reduces EEG frequency. This condition would first be indicated by the drop in frequency, and eventually by a drop in power. Frequency declines as anesthetic saturations deepen. Aside from these “normal” signatures during the stages of anesthesia, signatures are also affected by hypothermia, hypoxia, and high dosage narcotics. Each has a corresponding characteristic. In hypothermia, there is a gradual reduction in EEG frequency as the temperature declines. With low flow or CNS hypoxia, a sudden persistent symetric or asymetric drop in power and/or frequency occurs. With high dosage narcotics, like fentanyl, the EEG slows. At the same time, a rhythmic pattern not associated with low blood flow occurs (Fig 4). Signatures, loss of consciousness, and enflurane also correlate. Two grouping of waves are noted with this agent. First delta and low theta waves are noted with a slight gap in high theta. The alpha and low beta are also observed. As anesthetic levels lighten, the gap narrows markedly. Halothane generates dominant, large amplitude activity in the alpha and low beta bands. High concentrations of the agent shift the dominant activity to sub-alpha and slow theta activity. Each signature is reflected graphically by the LED lights on the monitor’s face. As the user becomes accustomed to various LED patterns, her interpretative skills increase. EEG monitoring can also aid early detection of many surgical problems-increased depth of anesthesia, stroke, and general hemispheric low blood flow condition. For instance, in carotid endarterectomy, or similar situations in which circulation is compromised, EEG signatures show a sudden decrease in amplitude and with increased severity, a total loss of power and frequency. Once the problem is rectified, there is a dramatic return of both. Damage created by cerebral ischemia or CNS hypoxia can be detected and managed by manipulating critical variables such as depth of anesthesia, increased circulating volume, or
systolic blood pressure.
Conclusion n spite of its ability to serve as an effective “reporter” on changes in the brain’s EEG activity, the EEG cannot replace the professional practitioner. The staff must integrate the etiology of changes with knowledge of EEG activity, and relate this information to the host of independent variables that can affect a patient’s overall condition. As OR and recovery room nurses acquire the interpretive skills necessary to assess changes in the signatures or patterns associated with independent variables, the EEG monitor will be a critical adjunct to the nursing 0 diagnostic process.
I
Note 1. Journal of the Hospital Corporation of America, “Anesthesia monitoring review.” January 1984.
Returning to Work After Stroke Younger patients in professional-managerial positions are more likely to return to work following a stroke than are blue collar workers or farmers, according to a study in the January issue of the Journal of the American Medical Association. George Howard, MSPH, and colleagues from the Bowman Gray School of Medicine in WinstonSalem, NC, studied 379 patients living one year after a stroke. They determined that age, occupation, degree of disability, race, and the brain hemisphere affected were the most significant factors. “Relatively young patients (aged 55 years or younger) are the most likely to return to work following a stroke,” the researchers said. The odds are almost as good (76%) for those 55 to 65, but plummet to 14%for those more than 65, most of whom undoubtedly accept retirement.
1049