The Neurosurgical Intensive Care Unit of the Future* FREDERICK A. SIMEONE, M.D., GLENN FRAZER, B.A., JAMES R. DOWNES, B.A., AND PHILIP VINALL, B.A. Through the integration of state of the art technology it is possible to envisage a monitoring system which will record physical, metabolic, and electrophysiological cerebral activity in the neurosurgical patient. With somewhat more imagination one can conceive computerized integration and ultimately a servo feedback system which would attempt to restore these activities to normal. Although apparently grandiose, it is noteworthy that the hardware to construct this system is currently available. As yet a few physicochemical concepts must be delineated before programming is possible. The heart of this system is a medium sized computer, herein called NICU. NICU receives information from the patient which is interpreted and interrelated. Subsequently, according to parameters assigned by the programming physician, the computer attempts to alter and improve these physiological states in a preprogrammed order of priority. The order of priority is determined by the nature of the input information which arrives through three separate monitoring systems. The first monitoring system, with the lowest priority, is the flow oriented module (Fig. 23.1). It consists of an intracranial pressure (ICP) monitoring device and a series of gamma detecting probes capable of recording isotope washout curves from the brain. Future isotopes will likely be administered by the inhalation technique (11) or refined intravenous injection methods. Periodic flow samplings will be recorded and combined mathematically with simultaneous perfusion pressure (PP) determinations. PP is simply the mean ICP subtracted from mean systemic arterial pressure (SAP). As intracranial pressure rises the perfusion pressure becomes smaller in the absence of a proportionalized increase in systemic arterial pressure. Under these circumstances one might assume that cerebral blood flow (CBF) would drop as well. Faced with falling cerebral blood flow and rising intracranial pressure one might assume that an artificial elevation of systemic arterial pressure would restore flow. This is true up to a point. As systemic arterial pres-
* Supported in part by National Institutes of Neurological Diseases and Blindness Grant No. NS 11186. 422
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CHAPTER 23
423
CBF -----~---... CVR-----.
I
P P - - -.... COMPUTER
ICP =~===~---I
SAP - - - - - - - - - - -... FLOW-ORIENTED MODULE FIG.
23.1. The flow oriented module.
sure rises, however, secondary brain edema evolves and the intracranial pressure may become disproportionately elevated so that flow will drop. At this point there is likely significant brain edema, and further elevations in systemic arterial pressure must therefore be avoided. The computer can constantly record the perfusion pressure and calculate an ideal range of systemic arterial pressure at which flow is restored without elevating intracranial pressure to edema producing levels. On-line administration of pressor or other blood flow increasing mechanisms could attempt to reach this level. The technical capability for this treatment is now available, but the ideal ranges of perfusion pressure, especially in the face of edema, are unknown. Several laboratories are pursuing these considerations. The second system is the metabolism oriented module (Fig. 23.2). This system has the capability of overriding the flow oriented system because the principal function of cerebral blood flow is to maintain brain metabolism. The most sensitive indicator of the metabolic state of the brain is the rate of utilization of oxygen (CMR02). Continuous p02 measurements will be possible through improved polarographic oximeter catheters which can be inserted in any artery as well as in the jugular vein. Employing a continuously recording hemoglobin saturation spectrograph, the computer will calculate on-line the content of oxygen in the arterial blood and from this subtract the oxygen content of the jugular venous blood. The difference represents the amount of oxygen extracted by the brain during the measurement interval. The computer can then multiply this figure for oxygen extraction by the cerebral blood flow to generate the CMR02. Data now available indicate that the CMR02 can, in the acute stage, accurately indicate the metabolic state of the brain. CMR02values below 60 per cent of control values are associated with clinically disordered cerebral function (9). When the CMR02 acutely drops to 30 per cent of control values, the patient is almost invariably comatose. The system could be easily modified to include glucose metabolism, as well as pH determinations, but these are far less accurate in the management of the acutely ill patient.
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INTENSIVE CARE UNIT: FUTURE
CLINICAL NEUROSURGERY Art. Art.
p0 2 3 0/0
02 sat..
CBF
Art. 02 Content
Art.Hb
(Art. - Jug. Ven.) 02 Content
Jug. Ven.
P02j-
Jug. Ven. °/0 ~ Sat.
Jug. Ven. 02 Content
Jug. Ven. Hb
CMROZ
METABOLISM MODULE FIG.
23.2. The metabolism oriented module.
The final system is the function oriented module (Fig. 23.3) which is not necessary in all cases, but, when used, can override or at least force a reassessment of the other two modules. It consists of a form fitting cap with multiple electrode points for recording the electroencephalogram (EEG). Recording electrodes are chosen according to the clinical problem. NICU's computer of average transients is capable of analyzing and displaying cerebral evoked potentials associated with stimuli given to the eyes, ears, and various parts of the musculoskeletal and integumentary systems. The magnitude of this response and the delay between the stimulus and response are recorded and stored. In order to detect an evoked response from a cutaneous or proprioceptive stimulus in an extremity, one must have a morphologically and metabolically intact central nervous system (CNS) (1). Significant reduction in amplitude or a greater delay in reception of this response is likely the first sign of altered function in the CNS. Although reliable evoked potential recordings from the brain are possible after a variety of stimuli to the body and special sense organs, quantitation of amplitude and duration delay remain to be determined (4). When these become available, the function oriented system can control treatment maneuvers in certain specific instances. This system might have an analogous implication in comatose patients that subtle changes in levels of consciousness have in more alert patients. The pattern of the evoked response may provide morphological information as well. In addition to EEG wave form and frequency (which can also be analyzed by NICU), detection
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424
EVOKED RESPONSE Computer of Average Transients Stimulus Site Amplitude Delay
EEG Waveform and Frequency Analysis
Metabolic Correlation
ECHO ENCEPHALOGRAPHY
ENG
CRT Display Computer Storage
Caloric Response Quantitation
-----FUNCTION MODULE FIG. 23.3. The function oriented module. This illustration also shows the echo transducer and the ENG electrodes which are attached to the skull cap.
of degrees of transtentorial herniation may be presumed (7). If, for instance, a patient's visual evoked responses are preserved but his responses from stimuli to the extremities are gradually lost, one might assume morphological interruption of the brain stem. This could be confirmed by accurately quantitated caloric responses with electronystagmographic (ENG) monitoring. With current evoked response techniques the system just mentioned is highly idealized. There is great variability in all aspects of evoked response recording. The effects of attention and habituation to stimuli are numerous. Yet this method would be used often in unconscious patients where these factors are diminished. The complete neurological examination would be the ultimate guide in the awake subject. More troublesome are the as yet mysterious peculiarities of the evoked response wave form and amplitude; e.g., the response may be greater at times in the face of depressed brain function. However, such monitoring might still be satisfactory in the assessment of changing states of brain activity (3). Patients undergoing major intracranial surgery might be subjected to a series of base line analyses so that they can serve as their own controls. In this instance NICU's parameters could be represented by percentages of control rather than presumed normal values. As described above, NICU's
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425
INTENSIVE CARE UNIT: FUTURE
CLINICAL NEUROSURGERY
activities are principally receptive and secondarily integrative or diagnostic. The interpretation of physiological data and their interrelation can suggest diagnostic possibilities which can be printed out on command. For instance, if cerebral blood flow and oxygen metabolism fall in the face of normal intracranial pressure and systemic arterial pressure, the most likely diagnosis is intra- or extracranial vascular obstruction. Localizing information in the comatose patient evolves when evoked potential data are combined with intracranial dynamics and metabolism. The computer's third function, in addition to reception of data and diagnosis, is actual on-line alterations of physiology to the patient's benefit. This is perhaps the most eerie aspect of this concept, because it now assumes the physician's role not only in diagnosis but also in treatment. Of course this phase of the program can be suppressed by the treating physician at will. As physiological data evolve, minute by minute control of cerebral dynamics may become important in the prevention of complications (Fig. 23.4). For instance if the computer determines that the reductions in flow, function, and metabolism are secondary to a fall in systemic arterial pressure, the feedback intravenous pressor drip might quickly restore function. Although this type of servo mechanism may not be necessary, it seems reasonable to assume that the future will bring far better methods of control of the patient's respiration. Since this is so important in the management of obtunded patients, we should devote some time to the automatic maintenance of respiratory function. Ordinarily a respirator is applied at a rate and depth which "seem" satisfactory to the attending physician or nurse. Periodic blood gas samples may lead to respirator adjustment. The delays in obtaining blood gas determinations, as well as their marked flucRespirator Module
Metabolism Module
1Flow Module L
~ Function Module -C01UTER
I /
Data Output Servo System Respirator lCP ~ DIAGNOSIS SAP
CBF
FIG.
Diagnostic Adjuncts Angiography CATT
Nleu 23.4 The integration of NICU's components.
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426
RESPIRATOR
427
PROGRAM
Blood Gas Parameters- Max./Min. P02 PC02 Arterial pH End-tidal C02
Respirator Parameters- Max. Rate Depth
Servo
Control of Respirator
RESPIRATOR MODULE FIG.
23.5. The respirator module.
tuation, particularly in acutely ill patients, makes this concept already archaic. At present polarigraphic oximeter catheters are capable of measuring the partial pressure of oxygen in arterial blood. Similar catheters for measurement of the partial pressure of carbon dioxide are now available. These intra-arterial electrodes can send the computer a continuous record of blood gas status. Within limits predetermined by the physician, the computer can alter the rate of respiration until these blood gas values come within the desired range (Fig. 23.5). When a maximum respiratory rate is reached, the depth of respiration, within safe limits, can be increased appropriately. This mechanism would be of particular value in the reduction of elevated intracranial pressure through sustained hypocarbia (6). Our experience with these catheters has led to the discovery of surprising fluctuations of arterial oxygen tension, often unsuspected and often to dangerously low levels. A more comprehensive monitoring system devised by Brackett and associates (2) may prove especially useful in multiply injured patients. This unit also measures pulmonary air pressure, inspired air pressure, inspired and expired O2 and CO2 , cardiac output, and central venous pressure. From these data the following can be calculated: respiratory quotient, pulmonary O2 and CO2 consumption, lung compliance and resistance, pulmonary shunting, etc. At present neurosurgeons rely heavily on so-called "morphological" techniques, notably cerebral angiography, for the diagnosis of some of the above
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INTENSIVE CARE UNIT: FUTURE
CLINICAL NEUROSURGERY
mentioned problems. Actually nothing can replace the delineation of an intracranial shift in the failing patient. Transportation of an acutely ill patient to the x-ray department may discourage prompt angiography, and repeated contrast studies are rarely used. With a portable fluoroscopic screen, no bigger than the current x-ray cassette, bedside arteriograms can be easily stored on NICU's magnetic tape and played back at will for subsequent comparison. Far more innovative than anything mentioned so far has been the development of computerized axial transverse tomography (CATT) pioneered by EMI Limited (Hayes, Middlesex, England). Soon a portable unit will be available, perhaps using NICU hardware, which will aid immeasurably in the study of cerebral dynamics. For instance, the computer reports that intracranial pressure is rising and the flow is falling in the face of adequate systemic arterial pressure. Evoked responses from both infratentorial and supratentorial sense organs on one side are reduced whereas they are normal on the contralateral side. These rapidly occurring changes are diagnosed as either cerebral edema or an evolving intracerebral hematoma. The angiogram is of no help in this differential. A computerized axial tomogram is done, the numerical values for tissue density in the diseased portion of the involved hemisphere indicate coagulated blood, and appropriate action is taken. HYPOTHETICAL EXAMPLE OF THE TOTAL SYSTEM IN ACTION
A 47-year-old man is admitted with the only history that he was found on the street having a generalized convulsion. Clinical neurological examinationand routine laboratory studies are of no help. The spinal fluid shows no blood and the chemistries are normal. The seizures subside spontaneously as he is prepared for monitoring by NICU, and he gradually regains consciousness as control determinations are made. Shortly thereafter his seizures recur but they ultimately respond to treatment. 'I'wo hours later, however, he is still deeply comatose. Repeat studies are displayed by NICU (Fig. 23.6). They are in turn interpreted by NICU and diagnosis and suggestions are printed out (Fig. 23.7). This report prompts a review of medication orders and nursing notes which indicates that, because of a change in nursing shifts, a double dose of intravenous barbiturates was given. NICU is capable of suggesting such a diagnosis in a comatose patient even in the absence of control information. CONCLUSIONS
This brief digression into fantasy is meant to emphasize that presently available techniques, lacking only a method of integration, could significantly improve the care of our patients. No mention has been made of im-
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428
.. cu
PAfI&Nr STATUS REPORT
..-TlUf
ArT&"DI~G PKYStCIA~
9-6-1.
.'80 MRS
MRAIC&fUt
9-6-1.
1510 MRS
CURRENT VALUE
PREVIOUS VAl.UE
C8I' LEFTKI:M "IGKT
.8.19 .7.98
6O.aa
tCP
I.
ao
'I'"
61.13
'liT PRESS
120/80
140180
PERF PRESS
1a
80
UWn-VEIt) oa COIn'
a.1
6.0
OIRoe
1.0
3.6
8IOIC&D Rt:SP~S& &Y&S
70& 701
RIGHT LUT
MRS
vm.
Vt'tI.
RIGHT LEn
701 701
Viti. tilL
ARMS RIGHT LUT
701 701
v~
701 701
WNl.
•••
•••
51
183
151
III lal
31 251 551
37 •
51
V~
V. .
LEGS
alGHT LUT
BIG
• G AMPLITUDE "IlSQ
c. .-1
I-Ia
a51
.a-Io
.-0
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L~D
P'''4 ilH
100 3~
liESt' urI: Nat:SP "01-
FIG.
V!fa.
7 ••• 80 &00
WNt.
W~
lal
w..
...
100 35
.,&4
~O
23.6. Hypothetical NICU display.
pedance plethysmography (8), measurement of cerebral circulatory transit time (13) and brain blood volume with nondiffusible isotopes (10), differential supratentorial and infratentorial pressure measurements, spinal fluid metabolite measurements (particularly in the diagnosis of anaerobic brain metabolism) (5), refined constant echoencephalographic midline and ventricle measurement, and a variety of diagnostic techniques currently under study. Space does not permit the discussion of Ray's ingenious multipurpose brain probe which measures spontaneous and evoked electrical activity,
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429
INTENSIVE CARE UNIT: FUTURE
NlCU ATTENDING PHYSICIAN
MTIENT 1110 MRS
9-6-74
SI NCE Ta LAST
san &5
OF MIASURDCKNTS TAKEN AT 1 sao HRS BLOOD
ILOW HAS DICRSASED 801. 3)1.
INTRACRANIAl. PRESSURE HAS DECREASED
SYSTIIIIC ARTERIAL PRESSURE HAS DECREASED liiS A!tD PERFUSION
PRESSURE HAS D&CRBAS.lD 101.
or
N~
BRAIN FUNCTION.
ABOVE VALUES ARE WITHI!f LOVER LIMITS ARTERIOVE~OUS OXYGEN EXTRACTIO. OF
BlAIN HAS DBCRBASED TO e.l VaLl THUS CAUSING METABOLIC RATE OF QXYSD TO DECREASE FROM 3.6 TO 1.0.
1MIS IS COMPATIBLE WITH COMA. ~OKED
RESPONSES THROUGHOUT ENTIRE BRAIN ARE SYMMETRICALLY
DEPRESSED.
&£8 SHOVS REDUCTION IN OVERALL VOLTAGE WITH
INCREASE IN MEAN nBQUEIfCY TO OR MIDLINE SKIFT.
ao
CPS.
A~
ECHO SHOWS NO VENTRICULAR
BLOOD GASES CAN 8E MAINTAINED BY AN INCREASE
. . RESPIRATOR RATE TO 80 AND NO INCREASE IN RESPIRATOR VOLlME. DlA8NOSIS,
MILD REDUCTION OF BLOOD FLOV AND INTRACRANIAL PRESSURE
ASSOCIATED WITH A GREAT REDUCTION OF CNROI AND DECREASED EVOKED ~TIVITY
WITH NO MIDLINE SHIFT. SUPPRESSED KEG VOLTAGE. AND THE
PRESENCE OF FAST ACTIVITY INDICATE DRUG EVF£CT.
HAS PATIENT
RECEIVED BARBITURATES7
SUGGEST'
IF PATIENT HAS R&CBIV&D BRAIN DEPRESSING DRUGS. THESE
*Y BE MAINTAUIED OR DISCONTINUED AS CLINICALLY WARRANTED.
THIS
COULO 8E ASCERTAINED BY PERIODIC REDUCTION OF DRUG DOSAGE.
IF
PATIENT HAS NOT RECEIVED BRAI. DEPRESSING DRUGS. SEARCH FOR TOXIC OR MStABOLIC ENCEPHALOPATHY.
FIG.
23.7. Hypothetical NICU diagnostic printout with suggestions.
oxygen tension, local blood flow, blood brain barrier to tracer substances, and tissue impedance (12). Unfortunately this device requires violation of brain tissue and it may be too localized for general neurological monitoring. N or have we considered the newer treatment modalities suitable for servo control such as the so-called "protection" of brain metabolism with hypothermia or barbiturates, improvement of cerebral blood flow through brain capillary beds with charged particles, hyperoxygenation of blood, etc. The future development of the neurosurgical intensive care unit will depend, in small part, on the development of a few bits of information necessary to complete these concepts, and in large part on social priorities in the financial support of medical care.
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CLINICAL NEUROSURGERY
430
431
REFERENCES 1. Alajouanine, T., Scherrer, J., Barbizet, J., Calvet, J., and Verley, R. Potentiels evoques corticaux chez des sujets atteints de troubles somesthesiques. Rev. Neurol. (Paris), 98: 757-762, 1958. 2. Brackett, C. E., Overman, J., and Peters R. Monitors, ventilators, and the neurosurgeon. Clin. Neurosurg., 18: 166-186, 1971. 3. Brazier, M. A. B. The application of computers to electroencephalography. In Computers in Biomedical Research, edited by R. W. Stacy, and B. D. Waxman, Vol. 1, Ch. 12. Academic Press, New York, 1965. 4. Brazier, M. A. B., and Casby, J. U. An application of the MIT digital electronic correlator to a problem in EEG. Electroencephalogr. Clin. Neurophysiol., 3: 375-391, 1951. 5. Davies, P. W., and Brink, S. Direct measurement of brain oxygen concentrations with a platinum electrode. Fed. Proc., 1: 19, 1942. 6. Harper, A. M. The interrelationship between arterial PC0 2 and blood pressure in the regulation of blood flow through the cerebral cortex. Acta N eurol. Scand., 41: [sup pI. 14], 94-103, 1965. 7. Ingvar, D. H., and Soderberg, V. Cort.ical blood flow related to EEG patterns evoked by stimulation of the brain stem. Acta Physiol. Scand., 42: 130-143, 1958. 8. Kedrov, A. A., and Naumenko, A. I. Some specific regulations of the cerebral circulation. Fiziol. Zh. SSSR, 27: 431-438, 1941. 9. Meyer, J. S., Sakamoto, K., and Akiyama, M. Monitoring cerebral blood flow, metabolism, and EEG. Electroencephalogr. Clin. Neurophysiol., 23: 497-508, 1967. 10. Nylin, G., Silfverski6ld, B. P., Lofstedt, S., Regnstrom, 0., and Hedlund, S. Studies on cerebral blood flow in man, using radioactive-labelled erythrocytes. Brain, 83: 293-335, 1960. 11. Obrist, W. D., Thompson, H. K., Jr., King, C. H., and Wang, H. S. Determination of regional cerebral blood flow by inhalation of 133-Xenon. Circ. Res., 20: 124-135, 1967. 12. Ray, C. D. New instrumentation for in vivo determinations of brain function. Clin. Neurosurg., 18: 121-154, 1971. 13. Rowan, J. 0., Cross, J. N., Tedeschi, G. M., and Jennett, W. B. Limitations of circulation time in the diagnosis of intracranial disease. J. Neurol. Neurosurg. Psychiatry, 33: 739-744, 1970.
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INTENSIVE CARE UNIT: FUTURE
* Supported in part by National Institutes of Neurological Diseases and Blindness Grant No. NS 11186. 422
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CHAPTER 23
423
CBF -----~---... CVR-----.
I
P P - - -.... COMPUTER
ICP =~===~---I
SAP - - - - - - - - - - -... FLOW-ORIENTED MODULE FIG.
23.1. The flow oriented module.
sure rises, however, secondary brain edema evolves and the intracranial pressure may become disproportionately elevated so that flow will drop. At this point there is likely significant brain edema, and further elevations in systemic arterial pressure must therefore be avoided. The computer can constantly record the perfusion pressure and calculate an ideal range of systemic arterial pressure at which flow is restored without elevating intracranial pressure to edema producing levels. On-line administration of pressor or other blood flow increasing mechanisms could attempt to reach this level. The technical capability for this treatment is now available, but the ideal ranges of perfusion pressure, especially in the face of edema, are unknown. Several laboratories are pursuing these considerations. The second system is the metabolism oriented module (Fig. 23.2). This system has the capability of overriding the flow oriented system because the principal function of cerebral blood flow is to maintain brain metabolism. The most sensitive indicator of the metabolic state of the brain is the rate of utilization of oxygen (CMR02). Continuous p02 measurements will be possible through improved polarographic oximeter catheters which can be inserted in any artery as well as in the jugular vein. Employing a continuously recording hemoglobin saturation spectrograph, the computer will calculate on-line the content of oxygen in the arterial blood and from this subtract the oxygen content of the jugular venous blood. The difference represents the amount of oxygen extracted by the brain during the measurement interval. The computer can then multiply this figure for oxygen extraction by the cerebral blood flow to generate the CMR02. Data now available indicate that the CMR02 can, in the acute stage, accurately indicate the metabolic state of the brain. CMR02values below 60 per cent of control values are associated with clinically disordered cerebral function (9). When the CMR02 acutely drops to 30 per cent of control values, the patient is almost invariably comatose. The system could be easily modified to include glucose metabolism, as well as pH determinations, but these are far less accurate in the management of the acutely ill patient.
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INTENSIVE CARE UNIT: FUTURE
CLINICAL NEUROSURGERY Art. Art.
p0 2 3 0/0
02 sat..
CBF
Art. 02 Content
Art.Hb
(Art. - Jug. Ven.) 02 Content
Jug. Ven.
P02j-
Jug. Ven. °/0 ~ Sat.
Jug. Ven. 02 Content
Jug. Ven. Hb
CMROZ
METABOLISM MODULE FIG.
23.2. The metabolism oriented module.
The final system is the function oriented module (Fig. 23.3) which is not necessary in all cases, but, when used, can override or at least force a reassessment of the other two modules. It consists of a form fitting cap with multiple electrode points for recording the electroencephalogram (EEG). Recording electrodes are chosen according to the clinical problem. NICU's computer of average transients is capable of analyzing and displaying cerebral evoked potentials associated with stimuli given to the eyes, ears, and various parts of the musculoskeletal and integumentary systems. The magnitude of this response and the delay between the stimulus and response are recorded and stored. In order to detect an evoked response from a cutaneous or proprioceptive stimulus in an extremity, one must have a morphologically and metabolically intact central nervous system (CNS) (1). Significant reduction in amplitude or a greater delay in reception of this response is likely the first sign of altered function in the CNS. Although reliable evoked potential recordings from the brain are possible after a variety of stimuli to the body and special sense organs, quantitation of amplitude and duration delay remain to be determined (4). When these become available, the function oriented system can control treatment maneuvers in certain specific instances. This system might have an analogous implication in comatose patients that subtle changes in levels of consciousness have in more alert patients. The pattern of the evoked response may provide morphological information as well. In addition to EEG wave form and frequency (which can also be analyzed by NICU), detection
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/22/CN_suppl_1/422/4099436 by University of California, Santa Cruz user on 22 December 2019
424
EVOKED RESPONSE Computer of Average Transients Stimulus Site Amplitude Delay
EEG Waveform and Frequency Analysis
Metabolic Correlation
ECHO ENCEPHALOGRAPHY
ENG
CRT Display Computer Storage
Caloric Response Quantitation
-----FUNCTION MODULE FIG. 23.3. The function oriented module. This illustration also shows the echo transducer and the ENG electrodes which are attached to the skull cap.
of degrees of transtentorial herniation may be presumed (7). If, for instance, a patient's visual evoked responses are preserved but his responses from stimuli to the extremities are gradually lost, one might assume morphological interruption of the brain stem. This could be confirmed by accurately quantitated caloric responses with electronystagmographic (ENG) monitoring. With current evoked response techniques the system just mentioned is highly idealized. There is great variability in all aspects of evoked response recording. The effects of attention and habituation to stimuli are numerous. Yet this method would be used often in unconscious patients where these factors are diminished. The complete neurological examination would be the ultimate guide in the awake subject. More troublesome are the as yet mysterious peculiarities of the evoked response wave form and amplitude; e.g., the response may be greater at times in the face of depressed brain function. However, such monitoring might still be satisfactory in the assessment of changing states of brain activity (3). Patients undergoing major intracranial surgery might be subjected to a series of base line analyses so that they can serve as their own controls. In this instance NICU's parameters could be represented by percentages of control rather than presumed normal values. As described above, NICU's
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425
INTENSIVE CARE UNIT: FUTURE
CLINICAL NEUROSURGERY
activities are principally receptive and secondarily integrative or diagnostic. The interpretation of physiological data and their interrelation can suggest diagnostic possibilities which can be printed out on command. For instance, if cerebral blood flow and oxygen metabolism fall in the face of normal intracranial pressure and systemic arterial pressure, the most likely diagnosis is intra- or extracranial vascular obstruction. Localizing information in the comatose patient evolves when evoked potential data are combined with intracranial dynamics and metabolism. The computer's third function, in addition to reception of data and diagnosis, is actual on-line alterations of physiology to the patient's benefit. This is perhaps the most eerie aspect of this concept, because it now assumes the physician's role not only in diagnosis but also in treatment. Of course this phase of the program can be suppressed by the treating physician at will. As physiological data evolve, minute by minute control of cerebral dynamics may become important in the prevention of complications (Fig. 23.4). For instance if the computer determines that the reductions in flow, function, and metabolism are secondary to a fall in systemic arterial pressure, the feedback intravenous pressor drip might quickly restore function. Although this type of servo mechanism may not be necessary, it seems reasonable to assume that the future will bring far better methods of control of the patient's respiration. Since this is so important in the management of obtunded patients, we should devote some time to the automatic maintenance of respiratory function. Ordinarily a respirator is applied at a rate and depth which "seem" satisfactory to the attending physician or nurse. Periodic blood gas samples may lead to respirator adjustment. The delays in obtaining blood gas determinations, as well as their marked flucRespirator Module
Metabolism Module
1Flow Module L
~ Function Module -C01UTER
I /
Data Output Servo System Respirator lCP ~ DIAGNOSIS SAP
CBF
FIG.
Diagnostic Adjuncts Angiography CATT
Nleu 23.4 The integration of NICU's components.
Downloaded from https://academic.oup.com/neurosurgery/article-abstract/22/CN_suppl_1/422/4099436 by University of California, Santa Cruz user on 22 December 2019
426
RESPIRATOR
427
PROGRAM
Blood Gas Parameters- Max./Min. P02 PC02 Arterial pH End-tidal C02
Respirator Parameters- Max. Rate Depth
Servo
Control of Respirator
RESPIRATOR MODULE FIG.
23.5. The respirator module.
tuation, particularly in acutely ill patients, makes this concept already archaic. At present polarigraphic oximeter catheters are capable of measuring the partial pressure of oxygen in arterial blood. Similar catheters for measurement of the partial pressure of carbon dioxide are now available. These intra-arterial electrodes can send the computer a continuous record of blood gas status. Within limits predetermined by the physician, the computer can alter the rate of respiration until these blood gas values come within the desired range (Fig. 23.5). When a maximum respiratory rate is reached, the depth of respiration, within safe limits, can be increased appropriately. This mechanism would be of particular value in the reduction of elevated intracranial pressure through sustained hypocarbia (6). Our experience with these catheters has led to the discovery of surprising fluctuations of arterial oxygen tension, often unsuspected and often to dangerously low levels. A more comprehensive monitoring system devised by Brackett and associates (2) may prove especially useful in multiply injured patients. This unit also measures pulmonary air pressure, inspired air pressure, inspired and expired O2 and CO2 , cardiac output, and central venous pressure. From these data the following can be calculated: respiratory quotient, pulmonary O2 and CO2 consumption, lung compliance and resistance, pulmonary shunting, etc. At present neurosurgeons rely heavily on so-called "morphological" techniques, notably cerebral angiography, for the diagnosis of some of the above
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INTENSIVE CARE UNIT: FUTURE
CLINICAL NEUROSURGERY
mentioned problems. Actually nothing can replace the delineation of an intracranial shift in the failing patient. Transportation of an acutely ill patient to the x-ray department may discourage prompt angiography, and repeated contrast studies are rarely used. With a portable fluoroscopic screen, no bigger than the current x-ray cassette, bedside arteriograms can be easily stored on NICU's magnetic tape and played back at will for subsequent comparison. Far more innovative than anything mentioned so far has been the development of computerized axial transverse tomography (CATT) pioneered by EMI Limited (Hayes, Middlesex, England). Soon a portable unit will be available, perhaps using NICU hardware, which will aid immeasurably in the study of cerebral dynamics. For instance, the computer reports that intracranial pressure is rising and the flow is falling in the face of adequate systemic arterial pressure. Evoked responses from both infratentorial and supratentorial sense organs on one side are reduced whereas they are normal on the contralateral side. These rapidly occurring changes are diagnosed as either cerebral edema or an evolving intracerebral hematoma. The angiogram is of no help in this differential. A computerized axial tomogram is done, the numerical values for tissue density in the diseased portion of the involved hemisphere indicate coagulated blood, and appropriate action is taken. HYPOTHETICAL EXAMPLE OF THE TOTAL SYSTEM IN ACTION
A 47-year-old man is admitted with the only history that he was found on the street having a generalized convulsion. Clinical neurological examinationand routine laboratory studies are of no help. The spinal fluid shows no blood and the chemistries are normal. The seizures subside spontaneously as he is prepared for monitoring by NICU, and he gradually regains consciousness as control determinations are made. Shortly thereafter his seizures recur but they ultimately respond to treatment. 'I'wo hours later, however, he is still deeply comatose. Repeat studies are displayed by NICU (Fig. 23.6). They are in turn interpreted by NICU and diagnosis and suggestions are printed out (Fig. 23.7). This report prompts a review of medication orders and nursing notes which indicates that, because of a change in nursing shifts, a double dose of intravenous barbiturates was given. NICU is capable of suggesting such a diagnosis in a comatose patient even in the absence of control information. CONCLUSIONS
This brief digression into fantasy is meant to emphasize that presently available techniques, lacking only a method of integration, could significantly improve the care of our patients. No mention has been made of im-
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428
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pedance plethysmography (8), measurement of cerebral circulatory transit time (13) and brain blood volume with nondiffusible isotopes (10), differential supratentorial and infratentorial pressure measurements, spinal fluid metabolite measurements (particularly in the diagnosis of anaerobic brain metabolism) (5), refined constant echoencephalographic midline and ventricle measurement, and a variety of diagnostic techniques currently under study. Space does not permit the discussion of Ray's ingenious multipurpose brain probe which measures spontaneous and evoked electrical activity,
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429
INTENSIVE CARE UNIT: FUTURE
NlCU ATTENDING PHYSICIAN
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FIG.
23.7. Hypothetical NICU diagnostic printout with suggestions.
oxygen tension, local blood flow, blood brain barrier to tracer substances, and tissue impedance (12). Unfortunately this device requires violation of brain tissue and it may be too localized for general neurological monitoring. N or have we considered the newer treatment modalities suitable for servo control such as the so-called "protection" of brain metabolism with hypothermia or barbiturates, improvement of cerebral blood flow through brain capillary beds with charged particles, hyperoxygenation of blood, etc. The future development of the neurosurgical intensive care unit will depend, in small part, on the development of a few bits of information necessary to complete these concepts, and in large part on social priorities in the financial support of medical care.
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CLINICAL NEUROSURGERY
430
431
REFERENCES 1. Alajouanine, T., Scherrer, J., Barbizet, J., Calvet, J., and Verley, R. Potentiels evoques corticaux chez des sujets atteints de troubles somesthesiques. Rev. Neurol. (Paris), 98: 757-762, 1958. 2. Brackett, C. E., Overman, J., and Peters R. Monitors, ventilators, and the neurosurgeon. Clin. Neurosurg., 18: 166-186, 1971. 3. Brazier, M. A. B. The application of computers to electroencephalography. In Computers in Biomedical Research, edited by R. W. Stacy, and B. D. Waxman, Vol. 1, Ch. 12. Academic Press, New York, 1965. 4. Brazier, M. A. B., and Casby, J. U. An application of the MIT digital electronic correlator to a problem in EEG. Electroencephalogr. Clin. Neurophysiol., 3: 375-391, 1951. 5. Davies, P. W., and Brink, S. Direct measurement of brain oxygen concentrations with a platinum electrode. Fed. Proc., 1: 19, 1942. 6. Harper, A. M. The interrelationship between arterial PC0 2 and blood pressure in the regulation of blood flow through the cerebral cortex. Acta N eurol. Scand., 41: [sup pI. 14], 94-103, 1965. 7. Ingvar, D. H., and Soderberg, V. Cort.ical blood flow related to EEG patterns evoked by stimulation of the brain stem. Acta Physiol. Scand., 42: 130-143, 1958. 8. Kedrov, A. A., and Naumenko, A. I. Some specific regulations of the cerebral circulation. Fiziol. Zh. SSSR, 27: 431-438, 1941. 9. Meyer, J. S., Sakamoto, K., and Akiyama, M. Monitoring cerebral blood flow, metabolism, and EEG. Electroencephalogr. Clin. Neurophysiol., 23: 497-508, 1967. 10. Nylin, G., Silfverski6ld, B. P., Lofstedt, S., Regnstrom, 0., and Hedlund, S. Studies on cerebral blood flow in man, using radioactive-labelled erythrocytes. Brain, 83: 293-335, 1960. 11. Obrist, W. D., Thompson, H. K., Jr., King, C. H., and Wang, H. S. Determination of regional cerebral blood flow by inhalation of 133-Xenon. Circ. Res., 20: 124-135, 1967. 12. Ray, C. D. New instrumentation for in vivo determinations of brain function. Clin. Neurosurg., 18: 121-154, 1971. 13. Rowan, J. 0., Cross, J. N., Tedeschi, G. M., and Jennett, W. B. Limitations of circulation time in the diagnosis of intracranial disease. J. Neurol. Neurosurg. Psychiatry, 33: 739-744, 1970.
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INTENSIVE CARE UNIT: FUTURE
