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Intraoperative Electroencephalographic Monitoring During Carotid Surgery with Routine Shunting
Intraoperative Electroencephalographic Monitoring During Carotid Surgery with Routine Shunting Mauricette Scheyvaerts, MD, Raymond Limet, MD, Likge, Belgium
Fifty-nine patients undergoing sixty-four carotid reconstructions with routine intraluminal shunting had intraoperative electroencephalographic monitoring. The onset of rhythm or amplitude disturbances was demonstrated in 14 patients during exposure of the carotid artery, and in 24 patients during initial carotid clamping. Disturbances were seen in 15 patients during the period of intraluminal shunting and increased momentarily during the second clamping period. During closure of the surgical wound, all abnormalities disappeared except in one patient who ultimately developed a neurologic deficit upon awakening. Although patients who maintained normal electroencephalographic readings had higher carotid stump pressure (59 mm) than those who did not (42 mm), individual values were scattered. Intraoperative monitoring during carotid surgery with routine shunting has little usefulness. (Ann Vasc Surg 1990;4:318-322) KEY WORDS:
Electroencephalographic monitoring; carotid endarterectomy.
monitoring for cerebral ischemia could detect ischemic zones, possibly leading to immediate correction of the situation. To accomplish this, the Mayo Clinic neurosurgicat team [3] has advocated permanent intraoperative electroencephalogram (EEG) monitoring arguing that "no one awakens with (a new neurologic) deficit that has not been foreseen on electroencephalograms." The goal of this study was to verify if intraoperative EEG monitoring is indeed a reliable and useful criterion of cerebral ischemia in patients undergoing carotid surgery with routine shunting.
There is little doubt as to the beneficial role of carotid endarterectomy in patients having ipsilateral transient ischemic attacks (TIAs). Controversy still exists, however, concerning asymptomatic carotid lesions. Some authors argue that little is known about the natural history of asymptomatic carotid lesions, and that there is no statistical proof as yet justifying the mortality and morbidity, even though small, associated with carotid surgery [1]. To determine the natural history of carotid lesions, however, would be a monumental undertaking, because, as strokes are rare events, long-term longitudinal studies would be necessary [2]. Awaiting irrefutable conclusions as to what is best, surgeons worldwide have strived to limit the mortality and morbidity of carotid operations by improving their operative strategies and indications. Intraoperative
MATERIAL AND METHODS
From the Service de Neurochirurgie and the Clinique Chirurgicale A, Hdpital de BaviOre, LiOge, Belgium. Reprint requests: Professor R. Limet, quai Mativa, 15, 4020 Likge, Belgium.
Sixty-four carotid endarterectomies were carried out under permanent EEG monitoring. A special eight lead head cap (Fig. 1) was placed on the patient before induction for preoperative EEG monitoring. The patient underwent narcosis with intravenous Pentothal, 4 mg/kg, and was maintained
318
EEG MONITORING
VOLUME 4 N o 4 - 1990
DURING
CAROTID
SURGERY
319
throughout this phase. (2) During the first crossclamping (mean = 5 min, 24 sec), the EEG recordings of two of the four patients with preexisting abnormalities worsened. Twenty-four patients demT~ T~ onstrated rhythm or amplitude abnormalities. Hemodynamics remained stable. EEG abnormalities G / C3- T3 " " ~ ' ~ ' - ' ~ ' ~ ' ~ ' ~ " ~ " ~ ' ¢ ~ " were localized in 16 cases. (3) After insertion of the shunt, no new abnormalities appeared. Most abnor~T3 - 0,, mal EEG recordings improved with nine returning to normal. Eight focal abnormalities persisted, compared with 16 during phase 2. (4) During the second Fig. 1. Position of EEG. cross-clamping period abnormalities increased or reappeared in 20 patients (11 were focal). (5) During wound closure, abnormalities disappeared progresanesthetized by 1%-2% Isoflurane in the breathing sively in all patients except one whose EEG resystem. Muscle relaxation was administered by mained abnormal for 24 hours. This patient presuccinylcholine. All patients routinely had an in- sented a deficit upon awakening which resolved dwelling shunt placed once the internal carotid progressively during the first postoperative week artery was cleared. The phases during which the (RIND). All other patients had uneventful recoverEEG activity of the brain was recorded were: (1) ies. anesthetic induction and surgical dissection; (2) iniWe attempted to correlate EEG findings with the tial carotid clamping (average time = 5 min, 25 sec); contralateral carotid artery status. When the con(3) shunt period (average time = 10 min, 30 sec); (4) tralateral carotid was normal or had a nonsignificant second carotid clamping, after removal of shunt stenosis, 13 of 33 EEG readings were abnormal (average time -- 4 min, 36 sec); (5) closure of (39%). When the contralateral carotid artery was wound. significantly compromised, nine of 26 EEGs were A total of 64 operations were performed in 59 pa- abnormal (34%). Six of 16 patients with hemodytients. There were 15 women whose mean age was 65 namically significant contralaterat stenosis had new years (range: 57-73 years), three of whom underwent abnormal EEG findings whereas three of I0 patients bilateral operations, and 44 men whose mean age was with contralateral occlusion had EEG disorders. 65 years (range: 41-81 years), with two undergoing Therefore, there was no apparent relationship bebilateral operations. Ten patients were free of symp- tween EEG changes and contralateral carotid statoms at the time of operation. Of the 49 patients with tus. neurologic symptoms, 42 had had TIAs, and seven a We studied the relationship between EEG small stroke. Of the total of 59 patients, 33 had unilat- changes and carotid stump pressure, i.e. the carotid eral involvement with a normal or nonsignificantly back pressure, due to the contralateral carotid arstenotic contralateral carotid artery, while 26 patients tery, the vertebral arteries and other collateral (31 operations) had bilateral involvement including 10 branches. After carotid clamping, carotid arterial complete occlusions and 16 stenoses in the contralat- pressure was measured distal to the clamp on the eral carotid artery greater than 50% reduction of inter- internal carotid artery (stump pressure). Stump hal diameter. pressure was higher (59 mm Hg) in the group for which EEG remained normal during clamping than in the group for which EEG was disturbed (42 mm Hg). Using the Student t-test for unpaired data, the RESULTS difference between stump pressures in the two There were no deaths in this series. One patient groups is not significant (p = 0.074); since the had a neurologic deficit upon awakening which distribution of the variable is asymmetrical in the resolved progressively in four days (RIND). EEG two groups, the pressure data were converted into modifications were described as generalized or lo- their logarithms; at that time the Student t-test for calized, persistent or transient, and involved unpaired data is significant (p = 0.031). Individual rhythm, amplitude or both (Figs. 2, 3). The follow- values, however, varied greatly (Fig. 5). ing changes were observed during each of the above-defined phases (Fig. 4): (1) During the induction and dissection the four patients with preexistDISCUSSION ing preoperative abnormal EEG findings remained We were unable to find any relationship between in the same state. In 14 other cases, generalized (n = 9) or localized (n = 5), rhythm (n = 6), or EEG abnormalities and clinically detectable cereamplitude (n = 8) abnormalities were found. Large bral ischemic compromise in this series of patients. systemic blood pressure variations were observed In Imparato's experience [4], carotid clamping was
I
~4- C4
ANNALS OF VASCtJLAR SURGERY
EEG M O N I T O R I N G D U R I N G CAROTID S U R G E R Y
320
TITT. L.
I I . ~0. toO4
T|TT L.
11,10,1114
1" Shunt
1" 1St Ck~mplng
t . ~o'
(RI c a r o t ~ )
b
o,...
TITT.L t l , ~ . H H I 4
TITT. L
P~.c~ ~
IlLWJII~
r~ _ c~ ~ '-~mn,,,,~'e w ~ ~ ' ~ '
? 2nd clm'nplng (Rt carotid)
+ I'N-
C
1" Declamplng (Rt carotl~)
.1o"
.1o"
d
Fig. 2. Example of major EEG modifications: (a) During initial clamping period activity slowed down and then amplitude decreased on clamped side. First rhythm, then voltage change on clamped side. (b) After insertion of shunt EEG rhythm disturbances improved after ten seconds. Amplitude later returned to normal. (c) After second clamping rhythm disorders appeared immediately on ipsilateral side to carotid clamping. Most low-voltage waves were observed after I min, 20 sec. (d) Rhythm disorders improved 10 sec after declamping. Restoration of rhythm and amplitude disorder was complete after 20 sec.
associated with a 7.6% incidence of clinical deficits which became immediately apparent when patients were operated under local anesthesia. In patients operated under general anesthesia with permanent EEG monitoring, up to 19% of patients had abnormal EEG recordings. General anesthesia and consequent hemodynamic variations can therefore be responsible for certain EEG abnormalities. Similarly, in Byer's series [5], nine of 47 patients (19%) presented EEG disturbances. In Sundt's larger series [6], EEG modifications were detected in 451 of 1145 cases (40%). Our figure of 39% is therefore close to the average found in the literature. In Sundt's series, there were 321 cases of focal abnormalities when the carotid artery was clamped. Of these 321 cases, readings returned to normal in 319 patients once an indwelling infernal shunt was placed. In the two remaining cases, embolization due to insertion of the shunt was the suspected cause of EEG changes. There were 130 cases where EEG changes occurred independently of carotid clamping. Of these, 120 changes were " m i n o r " and were due either to the depth of anesthesia or to preoperative abnormalities. In ten cases, the abnor-
mality persisted, and in all 10, the patient presented with a new neurologic deficit upon awakening, transient in one case, and permanent in nine. Sundt [6] therefore concluded that there are no new postoperative deficits without prior EEG changes. Unfortunately, foreknowledge of these changes does not allow alteration of the course of events: EEG disturbances are helpless witnesses. Although it has been widely demonstrated that most patients can safely undergo carotid surgery without a shunt, the findings in this short series showed that EEG abnormalities caused by initial clamping disappeared after insertion of an indwelling shunt. Internal shunts therefore seem to afford protective benefits. One may argue that, even though undeniable, this protection is certainly not indispensable. Once cerebral flow has been restored after declamping, EEG patterns may be expected to return to normal. However in support of the use of shunts, it might be recalled that intraoperative cerebral ischemia as reflected by EEG modifications may lead to subclinical deterioration of cerebral function [7] even though perceptible clinical deficits are not demonstrable upon awakening.
EEG M O N I T O R I N G D U R I N G CAROTID S URGER Y
VOLUME 4 N o 4 - 1990
Correlation between stump pressure and EEG changes
Minor changes HEN.
321
R. 1~,a4,It4
I(,~
Norn"~lEEG
Abnormal EEG
110- ~I
]-i
C3~¥3
Fig. 3. Example of minor EEG modifications independent of clamping, and related to hemodynamic variations due to anesthesia. Relative hypotension (120/60 mmHg) induced generalized slowdown of rhythm compared with normal values seen in highertension (170/90, 180/90 mmHg)
The relationship between encephalogram and back (or stump) pressure was not statistically significant unless the pressure data were transformed into their logarithms. The large dispersion of points in Figure 5 shows the variability of collateral development in each individual case. It must be underscored, as well, that stump pressure does not perfectly reflect distal cerebral perfusion. Theoretically effective cerebral perfusion pressure is equal to the arterial minus venous pressure [8]. In our study, however, jugular venous pressure was not measured. Moreover, stump pressure varies according to the technique of anesthesia used [9]. In any case, an overall trend toward a relationship between stump pressure and EEG disturbances was strongly suggested in this series in contrast to results in other series [10]. EEG changes are correlated with cerebral blood
Progressionof the "abnormalities"of the pedoperatfveEEG
Induction Direction
1st Cross-clamping
Shunt
2rid Cros.~chzmping
Closure
I i
1
I
/ ...,r
Fig. 4. Changes in number of EEG abnormalities found in the various phases of the operation.
! //~
• firnit of autoregulatton
_+_(~L~ r
2olooFig. 5. Internal carotid artery back pressure values in individuals with normal/abnormal EEG.
flow better than with stump pressure. By measuring cerebral blood flow intraoperatively, Sundt and co-workers [3] were able to demonstrate that EEG abnormalities appeared when flow rates decreased to 30% of normal values. This critical flow rate is approximately 15 ml/100 g/min. In this series of 64 operations, we observed one transient neurologic deficit. A larger number of cases and neurologic complications would be necessary in order to draw any practical conclusions. It must be emphasized, however, that the number of EEG abnormalities appearing during clamping and up until closure, is much larger than the number of clinical complications. Even if one excludes generalized abnormalities, which are mostly due to hemodynamic or anesthetic factors, the number of focal events is high. Intraoperative EEG monitoring is therefore a sensitive yet not very specific method. Persistence of EEG disturbances until closure of the wound is completed is more specific and should call the surgeon's attention to the possibility of a technical problem. Only carotid thrombotic occlusion is correctable. If the neurologic accident is due to distal embolization or intracerebral hemorrhage, there is very little the surgeon can do. Although EEG monitoring is undoubtedly invaluable for surgeons using selective shunting, it is much less so for those who shunt routinely. Since intraoperative EEG monitoring implies the mobilization of an extra number of specialized medical personnel and technicians, prolongs operative time, and increases the cost of operation, we have abandoned this method after 59 patients because of limited costeffectiveness. As the only complication amenable to correction by extracranial surgery is intraoperative thrombotic occlusion of the internal carotid artery,
322
EEG MONITORING DURING CAROTID S U R G E R Y
we presently use intraoperative B-mode sonography as the sole method of assessing operative results.
4. 5.
CONCLUSION Intraoperative EEG monitoring is a sensitive but poorly specific method for detecting cerebral ischemia. Of importance to surgeons before deciding to insert indwelling shunt, it is of little value when shunts are used routinely.
REFERENCES
6.
7.
8.
1. WARLOW C. Carotid endarterectomy: does it work? Stroke 1984;15:1068-1076. 2. ROEDERER GO, LANGLOIS YE, JAGER KA, PRIMOZICH JF, BEACH KW, PHILLIPS D J, STRANDNESS DE. The natural history of carotid arterial disease in asymptomatic patients with cervical bruits. Stroke 1984;15:605613. 3. SUNDT TM, SHARBROUGH FW, PIEPGRAS DG, KEARNS TP, MESSICK JM, O ' F A L L O N WM. Correla-
mmm
9.
10.
ANNALS OF VASCULAR SURGERY
tion of cerebral blood flow and electroencephalographic changes during carotid endarterectomy. Mayo Clin Proc 1981 ;56:533-543. IMPARATO AM, RAMIREZ A, RILES T, MINTZER R. Cerebral protection in carotid surgery. Arch Surg 1982; 117:1073-1078. BYER JA, H E N Z E L JH, DEXTER JD. Correlation of intraoperative electroencephalography with neurologic deficit after carotid endarterectomy. South Med J 1979;72: 956-958. SUNDT TM, SHARBROUGH FW, ANDERSON RE, MICHENFELDER JD. Cerebral blood flow measurements and electroencephalograms during carotid endarterectomy. J Neurosurg 1974;41: 310-320. BRINKMAN SD, BRAUN P, GANJI S, MORRELL RM, JACOBS LA. Neuropsychological performance one week after carotid endarterectomy reflects intra-operative ischemia. Stroke 1984;15:497-503. ARCHIE JP, FELDTMAN RW. Determinants of cerebral perfusion pressure during carotid endarterectomy. Arch Surg 1982;117:319-322. McKAY RD, SUNDT TM, MICHENFELDER JD, GRONERT GA, MESSICK JM, SHARBROUGH FW, PIEPGRAS DG. Internal carotid artery stump pressure and cerebral blood flow during carotid endarterectomy. Anesthesiology 1976;45:390-399. BEEBE HG, PEARSON JM, COATSWORTH JJ. Comparison of carotid artery stump pressure and EEG monitoring in carotid endarterectomy. Am Surg 1978;4:655-660.
Fifty-nine patients undergoing sixty-four carotid reconstructions with routine intraluminal shunting had intraoperative electroencephalographic monitoring. The onset of rhythm or amplitude disturbances was demonstrated in 14 patients during exposure of the carotid artery, and in 24 patients during initial carotid clamping. Disturbances were seen in 15 patients during the period of intraluminal shunting and increased momentarily during the second clamping period. During closure of the surgical wound, all abnormalities disappeared except in one patient who ultimately developed a neurologic deficit upon awakening. Although patients who maintained normal electroencephalographic readings had higher carotid stump pressure (59 mm) than those who did not (42 mm), individual values were scattered. Intraoperative monitoring during carotid surgery with routine shunting has little usefulness. (Ann Vasc Surg 1990;4:318-322) KEY WORDS:
Electroencephalographic monitoring; carotid endarterectomy.
monitoring for cerebral ischemia could detect ischemic zones, possibly leading to immediate correction of the situation. To accomplish this, the Mayo Clinic neurosurgicat team [3] has advocated permanent intraoperative electroencephalogram (EEG) monitoring arguing that "no one awakens with (a new neurologic) deficit that has not been foreseen on electroencephalograms." The goal of this study was to verify if intraoperative EEG monitoring is indeed a reliable and useful criterion of cerebral ischemia in patients undergoing carotid surgery with routine shunting.
There is little doubt as to the beneficial role of carotid endarterectomy in patients having ipsilateral transient ischemic attacks (TIAs). Controversy still exists, however, concerning asymptomatic carotid lesions. Some authors argue that little is known about the natural history of asymptomatic carotid lesions, and that there is no statistical proof as yet justifying the mortality and morbidity, even though small, associated with carotid surgery [1]. To determine the natural history of carotid lesions, however, would be a monumental undertaking, because, as strokes are rare events, long-term longitudinal studies would be necessary [2]. Awaiting irrefutable conclusions as to what is best, surgeons worldwide have strived to limit the mortality and morbidity of carotid operations by improving their operative strategies and indications. Intraoperative
MATERIAL AND METHODS
From the Service de Neurochirurgie and the Clinique Chirurgicale A, Hdpital de BaviOre, LiOge, Belgium. Reprint requests: Professor R. Limet, quai Mativa, 15, 4020 Likge, Belgium.
Sixty-four carotid endarterectomies were carried out under permanent EEG monitoring. A special eight lead head cap (Fig. 1) was placed on the patient before induction for preoperative EEG monitoring. The patient underwent narcosis with intravenous Pentothal, 4 mg/kg, and was maintained
318
EEG MONITORING
VOLUME 4 N o 4 - 1990
DURING
CAROTID
SURGERY
319
throughout this phase. (2) During the first crossclamping (mean = 5 min, 24 sec), the EEG recordings of two of the four patients with preexisting abnormalities worsened. Twenty-four patients demT~ T~ onstrated rhythm or amplitude abnormalities. Hemodynamics remained stable. EEG abnormalities G / C3- T3 " " ~ ' ~ ' - ' ~ ' ~ ' ~ ' ~ " ~ " ~ ' ¢ ~ " were localized in 16 cases. (3) After insertion of the shunt, no new abnormalities appeared. Most abnor~T3 - 0,, mal EEG recordings improved with nine returning to normal. Eight focal abnormalities persisted, compared with 16 during phase 2. (4) During the second Fig. 1. Position of EEG. cross-clamping period abnormalities increased or reappeared in 20 patients (11 were focal). (5) During wound closure, abnormalities disappeared progresanesthetized by 1%-2% Isoflurane in the breathing sively in all patients except one whose EEG resystem. Muscle relaxation was administered by mained abnormal for 24 hours. This patient presuccinylcholine. All patients routinely had an in- sented a deficit upon awakening which resolved dwelling shunt placed once the internal carotid progressively during the first postoperative week artery was cleared. The phases during which the (RIND). All other patients had uneventful recoverEEG activity of the brain was recorded were: (1) ies. anesthetic induction and surgical dissection; (2) iniWe attempted to correlate EEG findings with the tial carotid clamping (average time = 5 min, 25 sec); contralateral carotid artery status. When the con(3) shunt period (average time = 10 min, 30 sec); (4) tralateral carotid was normal or had a nonsignificant second carotid clamping, after removal of shunt stenosis, 13 of 33 EEG readings were abnormal (average time -- 4 min, 36 sec); (5) closure of (39%). When the contralateral carotid artery was wound. significantly compromised, nine of 26 EEGs were A total of 64 operations were performed in 59 pa- abnormal (34%). Six of 16 patients with hemodytients. There were 15 women whose mean age was 65 namically significant contralaterat stenosis had new years (range: 57-73 years), three of whom underwent abnormal EEG findings whereas three of I0 patients bilateral operations, and 44 men whose mean age was with contralateral occlusion had EEG disorders. 65 years (range: 41-81 years), with two undergoing Therefore, there was no apparent relationship bebilateral operations. Ten patients were free of symp- tween EEG changes and contralateral carotid statoms at the time of operation. Of the 49 patients with tus. neurologic symptoms, 42 had had TIAs, and seven a We studied the relationship between EEG small stroke. Of the total of 59 patients, 33 had unilat- changes and carotid stump pressure, i.e. the carotid eral involvement with a normal or nonsignificantly back pressure, due to the contralateral carotid arstenotic contralateral carotid artery, while 26 patients tery, the vertebral arteries and other collateral (31 operations) had bilateral involvement including 10 branches. After carotid clamping, carotid arterial complete occlusions and 16 stenoses in the contralat- pressure was measured distal to the clamp on the eral carotid artery greater than 50% reduction of inter- internal carotid artery (stump pressure). Stump hal diameter. pressure was higher (59 mm Hg) in the group for which EEG remained normal during clamping than in the group for which EEG was disturbed (42 mm Hg). Using the Student t-test for unpaired data, the RESULTS difference between stump pressures in the two There were no deaths in this series. One patient groups is not significant (p = 0.074); since the had a neurologic deficit upon awakening which distribution of the variable is asymmetrical in the resolved progressively in four days (RIND). EEG two groups, the pressure data were converted into modifications were described as generalized or lo- their logarithms; at that time the Student t-test for calized, persistent or transient, and involved unpaired data is significant (p = 0.031). Individual rhythm, amplitude or both (Figs. 2, 3). The follow- values, however, varied greatly (Fig. 5). ing changes were observed during each of the above-defined phases (Fig. 4): (1) During the induction and dissection the four patients with preexistDISCUSSION ing preoperative abnormal EEG findings remained We were unable to find any relationship between in the same state. In 14 other cases, generalized (n = 9) or localized (n = 5), rhythm (n = 6), or EEG abnormalities and clinically detectable cereamplitude (n = 8) abnormalities were found. Large bral ischemic compromise in this series of patients. systemic blood pressure variations were observed In Imparato's experience [4], carotid clamping was
I
~4- C4
ANNALS OF VASCtJLAR SURGERY
EEG M O N I T O R I N G D U R I N G CAROTID S U R G E R Y
320
TITT. L.
I I . ~0. toO4
T|TT L.
11,10,1114
1" Shunt
1" 1St Ck~mplng
t . ~o'
(RI c a r o t ~ )
b
o,...
TITT.L t l , ~ . H H I 4
TITT. L
P~.c~ ~
IlLWJII~
r~ _ c~ ~ '-~mn,,,,~'e w ~ ~ ' ~ '
? 2nd clm'nplng (Rt carotid)
+ I'N-
C
1" Declamplng (Rt carotl~)
.1o"
.1o"
d
Fig. 2. Example of major EEG modifications: (a) During initial clamping period activity slowed down and then amplitude decreased on clamped side. First rhythm, then voltage change on clamped side. (b) After insertion of shunt EEG rhythm disturbances improved after ten seconds. Amplitude later returned to normal. (c) After second clamping rhythm disorders appeared immediately on ipsilateral side to carotid clamping. Most low-voltage waves were observed after I min, 20 sec. (d) Rhythm disorders improved 10 sec after declamping. Restoration of rhythm and amplitude disorder was complete after 20 sec.
associated with a 7.6% incidence of clinical deficits which became immediately apparent when patients were operated under local anesthesia. In patients operated under general anesthesia with permanent EEG monitoring, up to 19% of patients had abnormal EEG recordings. General anesthesia and consequent hemodynamic variations can therefore be responsible for certain EEG abnormalities. Similarly, in Byer's series [5], nine of 47 patients (19%) presented EEG disturbances. In Sundt's larger series [6], EEG modifications were detected in 451 of 1145 cases (40%). Our figure of 39% is therefore close to the average found in the literature. In Sundt's series, there were 321 cases of focal abnormalities when the carotid artery was clamped. Of these 321 cases, readings returned to normal in 319 patients once an indwelling infernal shunt was placed. In the two remaining cases, embolization due to insertion of the shunt was the suspected cause of EEG changes. There were 130 cases where EEG changes occurred independently of carotid clamping. Of these, 120 changes were " m i n o r " and were due either to the depth of anesthesia or to preoperative abnormalities. In ten cases, the abnor-
mality persisted, and in all 10, the patient presented with a new neurologic deficit upon awakening, transient in one case, and permanent in nine. Sundt [6] therefore concluded that there are no new postoperative deficits without prior EEG changes. Unfortunately, foreknowledge of these changes does not allow alteration of the course of events: EEG disturbances are helpless witnesses. Although it has been widely demonstrated that most patients can safely undergo carotid surgery without a shunt, the findings in this short series showed that EEG abnormalities caused by initial clamping disappeared after insertion of an indwelling shunt. Internal shunts therefore seem to afford protective benefits. One may argue that, even though undeniable, this protection is certainly not indispensable. Once cerebral flow has been restored after declamping, EEG patterns may be expected to return to normal. However in support of the use of shunts, it might be recalled that intraoperative cerebral ischemia as reflected by EEG modifications may lead to subclinical deterioration of cerebral function [7] even though perceptible clinical deficits are not demonstrable upon awakening.
EEG M O N I T O R I N G D U R I N G CAROTID S URGER Y
VOLUME 4 N o 4 - 1990
Correlation between stump pressure and EEG changes
Minor changes HEN.
321
R. 1~,a4,It4
I(,~
Norn"~lEEG
Abnormal EEG
110- ~I
]-i
C3~¥3
Fig. 3. Example of minor EEG modifications independent of clamping, and related to hemodynamic variations due to anesthesia. Relative hypotension (120/60 mmHg) induced generalized slowdown of rhythm compared with normal values seen in highertension (170/90, 180/90 mmHg)
The relationship between encephalogram and back (or stump) pressure was not statistically significant unless the pressure data were transformed into their logarithms. The large dispersion of points in Figure 5 shows the variability of collateral development in each individual case. It must be underscored, as well, that stump pressure does not perfectly reflect distal cerebral perfusion. Theoretically effective cerebral perfusion pressure is equal to the arterial minus venous pressure [8]. In our study, however, jugular venous pressure was not measured. Moreover, stump pressure varies according to the technique of anesthesia used [9]. In any case, an overall trend toward a relationship between stump pressure and EEG disturbances was strongly suggested in this series in contrast to results in other series [10]. EEG changes are correlated with cerebral blood
Progressionof the "abnormalities"of the pedoperatfveEEG
Induction Direction
1st Cross-clamping
Shunt
2rid Cros.~chzmping
Closure
I i
1
I
/ ...,r
Fig. 4. Changes in number of EEG abnormalities found in the various phases of the operation.
! //~
• firnit of autoregulatton
_+_(~L~ r
2olooFig. 5. Internal carotid artery back pressure values in individuals with normal/abnormal EEG.
flow better than with stump pressure. By measuring cerebral blood flow intraoperatively, Sundt and co-workers [3] were able to demonstrate that EEG abnormalities appeared when flow rates decreased to 30% of normal values. This critical flow rate is approximately 15 ml/100 g/min. In this series of 64 operations, we observed one transient neurologic deficit. A larger number of cases and neurologic complications would be necessary in order to draw any practical conclusions. It must be emphasized, however, that the number of EEG abnormalities appearing during clamping and up until closure, is much larger than the number of clinical complications. Even if one excludes generalized abnormalities, which are mostly due to hemodynamic or anesthetic factors, the number of focal events is high. Intraoperative EEG monitoring is therefore a sensitive yet not very specific method. Persistence of EEG disturbances until closure of the wound is completed is more specific and should call the surgeon's attention to the possibility of a technical problem. Only carotid thrombotic occlusion is correctable. If the neurologic accident is due to distal embolization or intracerebral hemorrhage, there is very little the surgeon can do. Although EEG monitoring is undoubtedly invaluable for surgeons using selective shunting, it is much less so for those who shunt routinely. Since intraoperative EEG monitoring implies the mobilization of an extra number of specialized medical personnel and technicians, prolongs operative time, and increases the cost of operation, we have abandoned this method after 59 patients because of limited costeffectiveness. As the only complication amenable to correction by extracranial surgery is intraoperative thrombotic occlusion of the internal carotid artery,
322
EEG MONITORING DURING CAROTID S U R G E R Y
we presently use intraoperative B-mode sonography as the sole method of assessing operative results.
4. 5.
CONCLUSION Intraoperative EEG monitoring is a sensitive but poorly specific method for detecting cerebral ischemia. Of importance to surgeons before deciding to insert indwelling shunt, it is of little value when shunts are used routinely.
REFERENCES
6.
7.
8.
1. WARLOW C. Carotid endarterectomy: does it work? Stroke 1984;15:1068-1076. 2. ROEDERER GO, LANGLOIS YE, JAGER KA, PRIMOZICH JF, BEACH KW, PHILLIPS D J, STRANDNESS DE. The natural history of carotid arterial disease in asymptomatic patients with cervical bruits. Stroke 1984;15:605613. 3. SUNDT TM, SHARBROUGH FW, PIEPGRAS DG, KEARNS TP, MESSICK JM, O ' F A L L O N WM. Correla-
mmm
9.
10.
ANNALS OF VASCULAR SURGERY
tion of cerebral blood flow and electroencephalographic changes during carotid endarterectomy. Mayo Clin Proc 1981 ;56:533-543. IMPARATO AM, RAMIREZ A, RILES T, MINTZER R. Cerebral protection in carotid surgery. Arch Surg 1982; 117:1073-1078. BYER JA, H E N Z E L JH, DEXTER JD. Correlation of intraoperative electroencephalography with neurologic deficit after carotid endarterectomy. South Med J 1979;72: 956-958. SUNDT TM, SHARBROUGH FW, ANDERSON RE, MICHENFELDER JD. Cerebral blood flow measurements and electroencephalograms during carotid endarterectomy. J Neurosurg 1974;41: 310-320. BRINKMAN SD, BRAUN P, GANJI S, MORRELL RM, JACOBS LA. Neuropsychological performance one week after carotid endarterectomy reflects intra-operative ischemia. Stroke 1984;15:497-503. ARCHIE JP, FELDTMAN RW. Determinants of cerebral perfusion pressure during carotid endarterectomy. Arch Surg 1982;117:319-322. McKAY RD, SUNDT TM, MICHENFELDER JD, GRONERT GA, MESSICK JM, SHARBROUGH FW, PIEPGRAS DG. Internal carotid artery stump pressure and cerebral blood flow during carotid endarterectomy. Anesthesiology 1976;45:390-399. BEEBE HG, PEARSON JM, COATSWORTH JJ. Comparison of carotid artery stump pressure and EEG monitoring in carotid endarterectomy. Am Surg 1978;4:655-660.