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The effect of hypoxia on electrocortical activity in the cebus monkey
FXPER131ExTAL
The
Effect
PiEC'ROLO(;Y
of
25,
116-128
Hypoxia
(1969)
on Cebus
Electrocortical
Activity
in the
Monkey
Cebus monkeys were subjected to 8, 6, 4, and 2% inspired oxygen concentrations in nitrogen to create conditions of hypoxia without complete anoxia. cardiac arrest, asphyxia, c)r vascular occlusion of blood supply to the brain. The ECoG, EKG, and arterial blood oxygen saturations were examined before, dura 30-min period of hypoxia. The ECoG changes in the S7r ing, and after hypoxic group followed the course of events described below and were reversible: (a) The prehypoxic ECoG, recorded from the precentral and postcentral gyri, consisted of high amplitude 5-7 Hz activity with 15-20 Hz activity superimposed and interposed; (b) loss of the 15-20 Hz activity; (c) reduction in amplitude of 5-7 Hz activity; (d) appearance of short isoelectric periods ; (e) appearance oi spindle-burst activity ; ( f ) lengthening of isoelectric periods and disappearance of spindle bursts; and (g) complete loss of activity (isoelectric). The 4 and 6% hypoxic groups followed the same course of events described above except that the loss of 15-20 Hz occurred much more rapidly, the appearance of spindle-burst activity was extremely transient, and the isoelectric periods lengthened rapidly. In the 2% hypoxic group all ECoG events occurred precipitously, reaching the isoelectric phase within 7 min. Since these animals suffered cardiac arrest, only 9 min of hypoxia was possible before the recovery phase was instituted. The cardiac picture ( EKG) during the hypoxic interval showed evidence of right and left bundle branch block and arrhythmia. In some animals of the 4 and 2% hypoxic groups evidence of cardiac ischemia and arrest was noted. The percentage of oxygen saturation of blood in the 8% hypoxic group dropped to and leveled off at an average of approximately 48% in 3 min, approximately 36 and 18% in 1 min for the 6 and 4% hypoxic groups, respectively, and “off-scale” on the oximeters in l&l5 set in the 2% hypoxic
group. Introduction
Of all the tissues in the mammalian capable of withstanding oxygen want.
body, nervous tissue is the least Some somatic tissues can tolerate
1 Supported by Grant NBO6552-03 from the White’s address is: Division of Neurosurgery, Metropolitan General Hospital and Case L%:estern 116
National Institutes of Health. Dr. Department of Surgery, Cleveland Reserve University Medical School,
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total oxygen deprivation for a matter of hours whereas nervous tissue can withstand oxygen want for only a few minutes or widespread and permanent damage occurs ( 17). The literature contains a large number of experimental studies involving the effect of brain anoxia on the electroencephalogram (EEG). However, the majority of these reports used asphyxia, cardiac arrest, or occlusion of blood supply to the brain to produce the anoxic state (2, 13-15). Further, most of the investigations on the effect of anoxia on the electrical activity of the brain have been accomplished with the cat and dog. Even these data do not always agree. Naquet and Fernandes-Guardiola (1.2) stated that the cerebral cortical activity undergoes a rapid acceleration during early stages of anoxia. This is followed by a gradual depression of activity which linally becomes isoelectric. On the other hand, Dell and associates (5 ) reported that hypoxia first excites the reticular formation and is followed by cortical depression which they attributed to the release of reticular activity from cortical inhibitory control. We propose to examine the effect of severe hypoxia on cerebrocortica1 electrical activity (ECoG) without complete anoxia, cardiac arrest, asphyxia, or vascular occlusion. Methods
Subjects used in this study were 15 mature cebus monkeys (Cebtts alb$ons and Cebus apella) of both sexes. Average weight was 2 kg. Prior to the hypoxic procedure, the animals were anesthetized with sodium pentobarbital (Nembutal) 30 mg/kg, or a combination anesthetic consisting of 12 mg/kg Nembutal and 400 mg/kg ethyl carbamate (Urethan) intravenously. The animal was then intubated with a wire spiral pediatric cuffed endotrachial tube. The cuff was inflated to insure an intratracheal seal. The head was then mounted in the head holder of a Baltimore stereotaxic instrument. The scalp was opened, reflected, and the calvarium exposed. Silver-ball recording electrodes were placed epidurally on the precentral gyrus and postcentral gyrus (motor and sensory cortex). The skull was trephined over the central area of the gyri, the electrodes inserted and cemented to the skull with dental cement. At the same time, a femoral artery and vein were exposed and cannulated. The arterial and venous cannulae were connected to a T valve which, in turn, was coupled to a blood pressure transducer and the cuvette of a Waters oximeter for measuring continuous blood oxygen saturations. At low oxygen saturation (below 40%) the Waters oximeter error was unacceptable. Therefore, it was necessary to take arterial blood samplesand run the sampleson an American Optical oximeter and an ASTRUP for PaO, which was mathematically
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converted to oxygen saturation (6 ). The three blood oxygen saturations were then averaged and used only as an estimate of the hemoglobin saturation. The EKG leads were attached to fore- and hind legs. Routinely, the standard EKG runs utilizing leads I, II, and III were used to examine changes in the cardiac characteristic during the hypoxic period. Low concentrations of oxygen were delivered to the monkey from tanks filled with 8% oxygen and 92% nitrogen (error -+0.270), and 4% oxygen and 96% nitrogen (error +O.Z%). These tanks were connected to the flowmeters via a rubber bag reservoir on an anesthesia machine and then delivered to the animal via a Harvard piston pump animal respirator. The Howmeters allowed adjustment of oxygen percentages ranging between 2% and 8% in increments of 0.25%. A concentration of S% osygen in 92% nitrogen was delivered to the Harvard respirator at flows of 2 liters/min. The respirator pump was set to deliver 25 ml/inspiration at rates of 30 inspirations/min. This same procedure was carried out for each of the other inspired concentrations of gas (6,4, and 2% oxygen in nitrogen). A base ECoG run was taken at the normal blood osygen saturation of the animal and allowed to continuously record during the hypoxia period (30 min). Next, the animal was placed on room air or -COO/,oxygen for 5-15 min, particularly if the EKG showed ischemic changes. After the heart and blood pressure stablized and if there was no spontaneousrespiration, the animal was continued on a mixture of 40% oxygen in nitrogen or on room air through the Harvard respirator until spontaneous respiration began. The ECoG records were obtained at 5-min intervals during the recovery phase. When the ECoG approached normal a final record was taken. The monkey was then deinstrumented. all cannulae were removed, and all wounds closed. Each animal was given chloromycetin and bicillin intramuscularly and returned to a recovery cage. In the case of one of the animals which did not recover ECOG activity or spontaneous respiration (after an extended resuscitation effort), autopsy was performed immediately. Some (four j survived 3-18 hours after hypoxia and postmortems were performed the following day. The other ten monkeys recovered without incident. Results
The Hypoxic Elect~ocorticogua~~~. The cebus monkeys prehypoxic ECoG recorded from the precentral gyrus (motor area j contained predominantly two frequencies: high-amplitude (150 pv) 5-7 Hz activity on which was superimposed 15-20 H z activity of less than 50 pv. The postcentral gyrus recordings showed a somewhat slower picture, having predominantly 10-15 Hz activity with occasional periods of 5-7 Hz activity interspersed. These
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ELECTROCORTICAL
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FIG. 1. Sample tracings of the ECoG before, during, and after hypoxic interval. Top trace, percentral gyrus (sensory cortex) ; middle trace, postcentral gyrus (motor cortex) ; bottom trace, right hemisphere (sensory to motor cortex). 411 recordings were made bipolarly. Vertical columns : 8, 6, 4, and 2% Oz. Horizontal: A, prehypoxic ECoG; B, after 15 min hypoxia; C, after 30 min hypoxia; D, after oxygen saturation of blood and blood pressure had returned to prehypoxic levels; E, immediately before termination of experiment. The ECoG in the last column (the 2% hypoxia group) : A, prehypoxic ECoG; B, after 5 min hypoxia; C, after 8 min hypoxia; D, after restoration of heart action and 90% oxygen saturation; E, immediately before termination of experiment. Calibration : vertical, 100 gv ; horizontal, 1 sec.
patterns of activity were considered “normal” for animals given iv injections of the combination anesthetic consisting of Nembutal and Urethan (Fig. 1). Little 15-20 Hz activity was observed in two of the 8% hypoxia group after injection of 30 mg/kg Nembutal. The predominant frequency in these animals was 10-15 Hz.
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Oxygen 8%. hfter S-10 min on an inspired mixture of 8% osygen in nitrogen, the ECoG showed a general reduction in amplitude, particularly in the 5-7 Hz frequencies, plus short isoelectric periods (0.4-0.6 set in duration). Also characteristic during this interval was the appearance of short ( l-SK) synchronous spindle-burst activity (30 Hz) usually confined to the precentral gyrus, spindle bursts infrequently appearing in the postcentral gyrus as well. After 20-30 min of hypoxia (8% j the ECoG contained longer isoelectric intervals (3-6 set ) with low-amplitude 10-I 5 Hz activity of l-2 set duration interspersed. At this time the spindle bursts mentioned above had disappeared. At the end of 30 min the animal was placed on 40% oxygen or room ail via the Harvard respirator. Recovery of prehypoxic ECoG occurred gradually, usually within an hour after being placed on lo% oxygen mixture or on the animal’s own respiration. Synchronous spindle-burst activity sometimes appeared within 15 min after blood oxygen saturations reached 80% or more, then disappeared completely ( Fig. 1 ). Oxygen 6%. The prehypoxic ECoG at 6% oxygen was similar to that reported for the 8% hypoxia group. Since a more acute drop of oxygen saturation of the blood occurred, usually within 1 min to levels of approximately 30%, the loss of 5-7 Hz frequencies was immediate and the appearance of spindle-burst activity was more transient than that observed in the 8% hypoxia group. The appearance of isoelectric periods of short duration (about 0.5 set) occurred within 2 min of hypoxia. These periods lengthened to 34 set as the hypoxic interval was extended. The isoelectric periods were separated by I-2-set periods of low-amplitude bursts of 20-30 Hz activity, sometimes resembling the spindle bursts described previously but having low amplitude and without synchronous characteristics (Fig. 1). The above sequence of events persisted to the end of the hypoxic period without further significant change. Recovery from hypoxia was characterized by the reappearance of the high-amplitude slower frequencies (5-7 H z ) and the gradual disappearance of the isoelectric periods. The faster frequencies slowed to 15-20 Hz and returned to their original amplitudes. Spindle-burst activity was not observed in this group of animals during recovery from hypoxia (Fig. 1). Oxygen 4%. The prehypoxic ECoG at 4% oxygen was characteristically similar to that reported previously for the S and 6% hypoxia groups. The acute decline in oxygen saturation of the blood was similar to that observed in the 6% hypoxia group. Further, the appearance of isoelectric periods occurred within 2 min of hypoxia and lengthened to 5-7-9~ intervals rapidly, usually within 5 min of hypoxia. The ECoG activity disappearec~
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rapidly and only extremely short bursts of low-amplitude 15-20 Hz activity was still evident after 10 min of hypoxia. Some slow wave activity (3-5 Hz) appeared sporadically throughout the hypoxic interval. Near the end of the 30-min period, the electrical activity was almost isoelectric. Only sporadic short intervals of low-amplitude 1.5-20 Hz activity appeared. This ECoG picture was accompanied by some cardiac changes and low blood pressure. Recovery from hypoxia was characterized by the appearance of synchronous low-voltage 15-20 Hz activity with short intervals of low-voltage 3-5 Hz activity. The isoelectric periods became shorter and more infrequent as the synchronous low-voltage 15-20 Hz activity became more prominent. Near termination and recovery the synchrony of the 15-20 Hz activity disappeared. The activity then became desynchronized and appeared similar to the prehypoxic activity. Two of the three animals subjected to 4% hypoxia succumbed several hours following recovery from hypoxia. The other one survived and recovered fully without incident (Fig. 1). Oxygen 2%. On1 y t wo monkeys were subjected to 2% hypoxia. Both animals exhibited a precipitous drop in blood oxygen saturation (within S-10 set) which caused rapid and dramatic cardiac changes accompanied by an acute drop in blood pressure within 9 min. Both these animals suffered cardiac arrest but heart action was restored by cardiac massage and placing the animals on 40% oxygen via the respirator. The blood pressure reached only 90 mm Hg in one animal, However, the ECoG never recovered any of the prehypoxic patterns, remaining relatively isoelectric. Spontaneous respiration was not induced in this animal. This experiment was terminated after several hours of attempted resuscitation. After heart action was restored on the second animal and the blood pressure climed to 190 mm Hg (systolic), efforts to improve the cardiac status and ECoG activity were successful and the blood oxygen saturations were better than 90%. The second animal survived for a few hours after recovery from 2% hypoxia. Hypoxic Electrocardiogram and Blood Pressure. Figure 2 illustrates the typical EKG cardiac cycle at normal oxygen saturations, blood pressure, and heart rate. Also illustrated are typical changes occurring in the EKG during the hypoxic interval. The following description of the EKG is based entirely on lead II records since they proved to be the most revealing. During the administration of 8% inspired oxygen concentrations, there appeared concurrently a reduction in amplitude of the “R” wave and a lengthening of the “S-T” interval. Next, the “S” wave deepened and a slight elevation of the “T” wave occurred. The EKG quickly reversed and returned to the typical EKG characteristics during recovery from 8% hypoxia. In the five monkeys subjected to
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FIG. 2. EKG tracings of the cchus monkeys’ cardiac cycle hefore hyrosia ( 1 ) ; during hypoxia showing right bundle branch block (2, 5) ; left bundle branch block (3) ; (6) just before cardiac arrest. a severely ischemic heart (4) ; fibrillation
So/o hypoxia, was 150/80; to I@/90 termination None of this
the average blood pressure before being subjected to hyposia at 20-30 min into the hypoxic period, the blood pressure fell (average) and just before deinstrumenting the animal and of the experiment, the blood pressure reached lSS/SS (Fig. 3 1. group succumbed.
22) 200 A
F
00 au -
A 6 % l 4 HYPOXIA
20 -
Nl PRE HYPO
.5 -MIN
I5 HYPOXIA
34
40 02+ IOmnn
zET,”
TERM
FIG. 3. Changes in blood pressure during the hypoxic period and after recovery from hypoxia. PRE HYPO, prehypoxic blood pressure; MIN HYPOXIA, changes in blood pressure during a 30-min hypoxic period; 40 Oz + 10 min, blood pressure after 10 min on 40% inspired oxygen mixture; OWN RESP, after spontaneous respiration began ; TERM, at termination of experiment.
In the group of five monkeys subjected to 670 inspired oxygen concentrations, the EKG followed the same course of events described for the 8% hypoxia group above. However, in addition, a negative “Q” wave developed with marked reduction in amplitude of the “R” wave. There also occurred a marked “S-T” elevation with a prominent “T” wave.
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During recovery from the hypoxic state, the EKG characteristic returned to the prehypoxic state slower than the 8% hypoxia group. The prehypoxic blood pressure averaged 145/90 and during the hypoxic state fell to 1 lo/60 (average). The blood pressure returned to 170/85 during recovery from hypoxia and gradually fell to ISO/SO shortly before termination of the experiment (Fig. 3). One of these animals died shortly after termination of the experiment. The other four animals survived and recovered without incident. In the 4 and 2% hypoxia groups, there occurred a relatively rapid change in the EKG. These animals developed EKG characteristics indicative of right- and left-bundle blocks, ischemic myocardial changes, and finally ventricular fibrillation followed by cardiac arrest. Shortly after the appearance of EKG configuration indicating an ischemic heart, ventricular fibrillation occurred with arrest following shortly thereafter. Resuscitation was possible in three of the four monkeys exhibiting the above EKG characteristics ; one animal failed to respond to resuscitation (a 2% hypoxia animal). One animal of these two groups (a 4% hypoxia animal) showed a characteristic EKG indicative of severe bundle-branch block with extrasystoles, but did not suffer cardiac arrest. This monkey survived and recovered from hypoxia without neurological damage. The average blood pressure of the 4% hypoxia group before hypoxia was 160/90. This blood pressure fell to S5/55 and on recovery from hypoxia returned to 155/80 (average). Even though two of the three animals died some hours after hypoxia and after resuscitation, they were able to maintain relatively good levels of blood pressure. The two monkeys subjected to 2% inspired oxygen concentration became anoxic within 4 min. The EKG showed a drop in amplitude of the “R” wave, the “S-T” segment developed a deep notch, and the “T” wave rapidly increased in amplitude. Extra systoles developed and cardiac arrest occurred about 7 min after the 2% oxygen was administered. Neither of these animals survived, even though cardiac action was restored raising the blood pressure to 180/90 before surgical recovery of one animal was instituted. The prehypoxic blood pressure was 170/90. and after 8 min of hypoxia the blood pressure dropped to 60/50 as the arrhythmia developed and fell to zero pressure at cardiac arrest. After being placed on 40% oxygen concentration, cardiac massage restored heart action and showed an overshoot increase of 200/175, leveling off at 180/90 before closure of wounds. These two animals succumbed between 3 and 16 hr after the hypoxia period (Fig. 3). Blood Oxygen Saturation. The monkeys subjected to 870 hypoxia showed a gradual drop in blood oxygen saturation, reaching an average saturation of 48% at approximately 3 min. Figure 4 illustrates the rate of decline of oxygen saturations for each of the hypoxic groups. This
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level of blood oxygen saturation was maintained throughout the half-hour period of hypoxia. The ECoG changes described previously began appearing when the oxygen saturation dropped to about 75% and maximum changes occurred after the saturation dropped to about 50% and continued to the end of the hypoxic period. The blood pressure remained relatively constant throughout the hypoxic period, showing a slight increase as the blood oxygen saturation started to fall and was only down about 10 mm Hg after 30 min of hypoxia ( 170-I 50 mm Hg).
FIG.
group.
4. Rate of change in average blood See Fig. 3 for group designation.
oxygen
saturations
for
each
hypoxic
In the 6% hypoxia group, the blood oxygen saturation dropped more acutely. The prehypoxia level averaged 90% saturation and within 2 min dropped to an average of 36% saturation. This level remained unchanged throughout the 30-min hypoxic interval. The ECoG changes occurred more rapidly and were more severe when compared to the 8% hypoxia group. The blood pressure fell gradually from an average for the five animals of 150 mm Hg (systolic) to 110 mm Hg after 30 min of hypoxia, except for one animal which suffered cardiac arrhythmia and rapid drop in blood pressure. This animal recovered but died several hours after termination of the experiment. The 4% hypoxia group followed the same sequence of events as reported above for the 6% hypoxia group, except that the blood oxygen saturation dropped to an average of 18% at the end of the 30-min hypoxia interval. Two of these animals died 3- 16 hr after termination of the experiment. One monkey was observed to have cardiac arrhythmia and bundle-branch block
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about 10 min after starting the hypoxic period. Another animal -showed minimal EKG changes, was recovered from hypoxia but died several hours after termination of the experiment. The two monkeys subjected to 2% hypoxia suffered profound blood oxygen saturation and cardiac effects. The blood oxygen saturation dropped to zero or “off scale” on the oximeters within lo-15 sec. The heart failed rapidly, going into arrest in about 7 min, and resuscitation procedures were instituted at that time. The ECoG became isoelectric rapidly. One animal did not recover even though heart action was restored. The other animal recovered both his prehypoxic blood pressure, heart rate, and ECoG, but died several hours after closure of wounds. Au~u~s$~. Cause of death in all the animals in which postmortem examination was performed was due to cardiorespiratory failure. The hearts revealed regions of myocardial hyperemia and infarction on either side of the interventricular groove on the sternocostal surface. Some cardiac enlargement was evident. In two cases moderate lung edema was also noted. The brains of these animals were dissected and examined but no gross evidence of pathologic change was found. Other organ systems were also negative. Discussion
It is apparent from the data that electrocortical activity is relatively resistant to the effects of. hypoxia since extremely low inspired oxygen tensions must be used to produce significant changes. These changes are readily reversible if the hypoxic interval is minimized and blood pressure and cerebral blood flow can be adequately maintained. The course of electrocortical events occurring during hypoxia without anoxia in the cebus monkey is as follows : (a) disappearance of faster frequencies; (b) gradual reduction in amplitude of the slower frequencies; (c) appearance of isoelectric periods of varying lengths of time depending on the severity of hypoxia ; (d) app earance of spindle-burst activity coupled with intervals of failing electrical activity, and (e) loss of all activity (isoelectric). Recovery of the ECoG depends largely on two factors: the level of oxygen available; and the cardiac status and thus blood flow to the brain. If the above two phenomena occur simultaneously over a prolonged period, the ECoG is not recoverable and death of the animal ensues. Two observed phenomena, the occurrence of isoelectric periods and the appearance of spindle bursts, are difficult to explain. Van Harreveld (16) described the characteristics of EEG activity in the cat a:;er IO-30 min of asphyxia as consisting of spindles of 10-20 set duration separated by low
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frequency activity varying from a few seconds to 1 min. He did not offer an explanation of the origin of the spindle activity other than stating that this activity was abnormal and originated from either the cortex itself or could he produced by some subcortical structures. Van Harreveld made no mention of isoelectric periods separating the spindle activity. Apparently, some activity was noted between subsequent spindle bursts. Naquet and Fernandes-Gunrdiola (12) offered an explanation of both the isoelectric period (they called it “refractory period”) and the spindleburst activity. The isoelectric periods may have been due to a gradual and massive decrease in afferent stimuli reaching the cortex. When extremely long isoelectric periods were found, then actual “deafferentiation” had occurred. They also thought that the spindle-hurst activity observed in their cats was due to an initial enhancement of the recruitment phenomena. When these spindles disappeared during late stages of anoxia, the cortex became “nonactive.” However, what caused the “nonactivation” was not explained. It is our hypothesis that two major mechanisms contribute to the changes observed in ECoG activity under conditions of severe hypoxia. These are as follows : (a ) Lack of available oxygen. Proper exchange of blood gases is not possible at low hemoglobin saturations causing the accumulation of anerobic metabolites which at first stimulate the energy output exhibited by the appearance of the spindle-burst activity. then gradually block the energy output, thus accounting for the isoelectric intervals (7). Further evidence for this theory has been reported by Van Liere (17). He reported that during hypoxia the brain lactic acid increases and a concentration of potassium occurs extracellularly in the brain, both factors decreasing brain metabolism. (b) Reduction of blood flow to the brain. It has been shown that when the cortical blood flow increases, the mean frequencies in electrocortical activity also increase. and zlicc e’crsa ( 1. 8). Both of the above mechanisms cause, finally. the development of brain edema and ischemia, which account for the isoelectric ECoG near the end of the hypoxic interval. This is reversible providing blood flow and blood osygen saturation can be raised within 5-10 min after the hypoxia period. If not, spontaneous respiration cannot be elicited, nor does the ECoG recover and the blood preesure remains at relatively low levels. Because of the importance of maintaining cardiac status, the EKG was monitored continuously before. during, and after instituting the hypoxic state. Lead II records were the most revealing and detailed analysis demonstrated the similarity of the EKG with that of man and other nonhuman primates (3, 10, 11). Figure 2 illustrates tracings of the EKG which were typically “normal” before hypoxia and the changes in the EKG during the period of hypoxia. From the detailed description of these
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changes in Results, one can recognize the configurations of right- and left-bundle branch blocks and the ischemic failing heart. In the failing heart, as the “T” waves became markedly elevated, fibrillation followed shortly thereafter, and finally total arrest occurred. In the monkeys in which this occurred, the hypoxic period was terminated by placing the animal on 40% oxygen via the Harvard respirator and rapid closedthoracic cardiac massage was instituted. In usually less than 2 min a ventricular beat was reestablished and shortly thereafter the EKG began returning. The blood pressure rose rapidly at first, then leveled off, and again rose slowly over a period of 30 min reaching values of 200/175. The above description is reminiscent of coronary vessel occlusion in man. The most controversial data collected in these experiments was that concerning oxygen saturation of the blood. Cole and Hawkins (4) and Linden and associates (9) compared most of the available techniques for determining oxygen saturation of blood. They compared their results with the Van Slyke method which is known to be the most accurate standard laboratory method for these determinations. The recent use of other “quick” methods employing “oximetry” (photometric) on one hand, and measuring oxygen tension on the other hand, necessitated determinations of their reliability. The above authors stated that all methods assessed except the Van Slyke technique were inaccurate, particularly below 40% oxygen saturation levels. Therefore, we consider our determinations of low oxygen saturation of blood reported in this paper as being only “relative estimates” and containing as much as 20% error in absolute values. However, by combining and averaging the oxygen saturations as determined by the three methods used in this investigation, a reduction in error was achieved. Even then, the reader should consider the oxygen saturations as approximations. References 1. BALD\.-MOVLINIEK. M., and D. H. INGVAK. 1968. EEG frequency content related to regional blood flow of cerebral cortex in cat. Exptl. Brain Res. 5: 55-60. 2. BARTLE~, H. S., and G. H. BISHOP. 1933. Factors determining the form of the electrical response from the optic cortex of the rabbit. Am. J. Pkysiol. 103: 173-184. 3. BURCH, G. E., and T. WINSOR. 1961. “A Primer of Electrocardiography.” Lea & Febiger, Philadelphia. 4. COLE, P. V., and L. H. HAWKINS. 1967. The measurement of the oxygen content of whole blood. Bio-Med. Ewgr. 2: 5663. 5. DELL, P., A. HUGELIN, and M. BONVALLET. 1961. Effects of hypoxia on the reticular and cortical diffuse systems, pp. 46-58. In “Cerebral Anoxia and the Electroencephalogram.” J. S. Meyer and H. Gastaut [eds.]. Thomas, Springfield. Illinois.
128 6. 7. 8.
9.
10. 11. II?.
13.
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AL.
DITTMEK, D. S. 1961. Blood and other body fluids, pp. 141-168. ZU “Biological Handbook.” Fed. .qm. Sot. Exptl. Biol., Washington, D. C. FREEMAN, J., and D. H. IKWAR. 1968. Elimination by hypoxia of cerebral blood flow. Autoregulation and EEG relationships. Erptl. Rvnbl Rcs. 5: 61-71. LEE, J. F., G. T. TI.~~AI.L, J. C. GREENFIELD, Jr<., and G. L. ODOM. 1966. Cerebral blood flow in the monkey. J. ,Vz~~/~.wr,q. 24 : 719-726. LIICDEN, R. J., J. R. LEDSOME, and J. NORMAX. 1965. Simple methods for the determination of the concentration of carbon dioxide and oxygen in blood. BYif. J. ~-lrlacsth. 34 : 77-88. MALINOW, BI. R. 1966. An electrocardiographic study of Jlacc~a wrzrlatfrz. Folia Prif~tafologg 4 : 5165. k~ALIKO\v, M. R., and C. \V. DELANSOY. 1966. The electrocard:ogram of Cpopitfrertts szigrr. I:olitr Priurtrfoiogg 4 : 66-73. NAQUET, R., and A. FERNAKDES-GUARDIOLA. 1961. Effects of various types of anoxia on spontaneous and evoked cerebral activity in the cat, pp. 72%. III “Cerebral Anosia and the Electroencc~)halogra~n.” J. S. Meyer and H. Gastaut [eds.]. Thomas, Springfield, Illinois. PROWDICZ-NEMINSKI, W. 1%‘. 1925. Zur Iienntnis der elektrischen unter der Innervations vorgange in den funktionellen Elementen und Geweben des tier&hen Organismus. Elektrocerebrogramm der Saiigetiere. =irrA. Gcs.
Physiol. 209 : 362482. 14. IS.
16.
17.
H. N., and -4. J. DERBYSHIRE. 1934. Electrical activity of the motor cortex during cerebral anemia. .irn. J. Phgsioi. 109 : 99. TEN CATE, J., and G. P. hl. HORSTEN. 1951. Effect of hyposaemia on the electrical activity of the cerebral cortex. Arfu I’llgsio[. Phclmrnrol. Nrrrl. 2: 2-12. VA~V HAKKEWLD, A. 1947. The electroencephalogram after prolonged brain asphyxiation. 1. A’rwop~~.vsio~. 10 : 361-370. VAX LIERE, E. J.. and J. C. STI~KSEY. 196.7. Effect of hypoxia on the nervow system, pp. 276-349. ITI “Hypoxia.” Llniv. of Chicago Press, Chicago.
SIMPSON,
The
Effect
PiEC'ROLO(;Y
of
25,
116-128
Hypoxia
(1969)
on Cebus
Electrocortical
Activity
in the
Monkey
Cebus monkeys were subjected to 8, 6, 4, and 2% inspired oxygen concentrations in nitrogen to create conditions of hypoxia without complete anoxia. cardiac arrest, asphyxia, c)r vascular occlusion of blood supply to the brain. The ECoG, EKG, and arterial blood oxygen saturations were examined before, dura 30-min period of hypoxia. The ECoG changes in the S7r ing, and after hypoxic group followed the course of events described below and were reversible: (a) The prehypoxic ECoG, recorded from the precentral and postcentral gyri, consisted of high amplitude 5-7 Hz activity with 15-20 Hz activity superimposed and interposed; (b) loss of the 15-20 Hz activity; (c) reduction in amplitude of 5-7 Hz activity; (d) appearance of short isoelectric periods ; (e) appearance oi spindle-burst activity ; ( f ) lengthening of isoelectric periods and disappearance of spindle bursts; and (g) complete loss of activity (isoelectric). The 4 and 6% hypoxic groups followed the same course of events described above except that the loss of 15-20 Hz occurred much more rapidly, the appearance of spindle-burst activity was extremely transient, and the isoelectric periods lengthened rapidly. In the 2% hypoxic group all ECoG events occurred precipitously, reaching the isoelectric phase within 7 min. Since these animals suffered cardiac arrest, only 9 min of hypoxia was possible before the recovery phase was instituted. The cardiac picture ( EKG) during the hypoxic interval showed evidence of right and left bundle branch block and arrhythmia. In some animals of the 4 and 2% hypoxic groups evidence of cardiac ischemia and arrest was noted. The percentage of oxygen saturation of blood in the 8% hypoxic group dropped to and leveled off at an average of approximately 48% in 3 min, approximately 36 and 18% in 1 min for the 6 and 4% hypoxic groups, respectively, and “off-scale” on the oximeters in l&l5 set in the 2% hypoxic
group. Introduction
Of all the tissues in the mammalian capable of withstanding oxygen want.
body, nervous tissue is the least Some somatic tissues can tolerate
1 Supported by Grant NBO6552-03 from the White’s address is: Division of Neurosurgery, Metropolitan General Hospital and Case L%:estern 116
National Institutes of Health. Dr. Department of Surgery, Cleveland Reserve University Medical School,
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ELECTROCORTICAL
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total oxygen deprivation for a matter of hours whereas nervous tissue can withstand oxygen want for only a few minutes or widespread and permanent damage occurs ( 17). The literature contains a large number of experimental studies involving the effect of brain anoxia on the electroencephalogram (EEG). However, the majority of these reports used asphyxia, cardiac arrest, or occlusion of blood supply to the brain to produce the anoxic state (2, 13-15). Further, most of the investigations on the effect of anoxia on the electrical activity of the brain have been accomplished with the cat and dog. Even these data do not always agree. Naquet and Fernandes-Guardiola (1.2) stated that the cerebral cortical activity undergoes a rapid acceleration during early stages of anoxia. This is followed by a gradual depression of activity which linally becomes isoelectric. On the other hand, Dell and associates (5 ) reported that hypoxia first excites the reticular formation and is followed by cortical depression which they attributed to the release of reticular activity from cortical inhibitory control. We propose to examine the effect of severe hypoxia on cerebrocortica1 electrical activity (ECoG) without complete anoxia, cardiac arrest, asphyxia, or vascular occlusion. Methods
Subjects used in this study were 15 mature cebus monkeys (Cebtts alb$ons and Cebus apella) of both sexes. Average weight was 2 kg. Prior to the hypoxic procedure, the animals were anesthetized with sodium pentobarbital (Nembutal) 30 mg/kg, or a combination anesthetic consisting of 12 mg/kg Nembutal and 400 mg/kg ethyl carbamate (Urethan) intravenously. The animal was then intubated with a wire spiral pediatric cuffed endotrachial tube. The cuff was inflated to insure an intratracheal seal. The head was then mounted in the head holder of a Baltimore stereotaxic instrument. The scalp was opened, reflected, and the calvarium exposed. Silver-ball recording electrodes were placed epidurally on the precentral gyrus and postcentral gyrus (motor and sensory cortex). The skull was trephined over the central area of the gyri, the electrodes inserted and cemented to the skull with dental cement. At the same time, a femoral artery and vein were exposed and cannulated. The arterial and venous cannulae were connected to a T valve which, in turn, was coupled to a blood pressure transducer and the cuvette of a Waters oximeter for measuring continuous blood oxygen saturations. At low oxygen saturation (below 40%) the Waters oximeter error was unacceptable. Therefore, it was necessary to take arterial blood samplesand run the sampleson an American Optical oximeter and an ASTRUP for PaO, which was mathematically
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converted to oxygen saturation (6 ). The three blood oxygen saturations were then averaged and used only as an estimate of the hemoglobin saturation. The EKG leads were attached to fore- and hind legs. Routinely, the standard EKG runs utilizing leads I, II, and III were used to examine changes in the cardiac characteristic during the hypoxic period. Low concentrations of oxygen were delivered to the monkey from tanks filled with 8% oxygen and 92% nitrogen (error -+0.270), and 4% oxygen and 96% nitrogen (error +O.Z%). These tanks were connected to the flowmeters via a rubber bag reservoir on an anesthesia machine and then delivered to the animal via a Harvard piston pump animal respirator. The Howmeters allowed adjustment of oxygen percentages ranging between 2% and 8% in increments of 0.25%. A concentration of S% osygen in 92% nitrogen was delivered to the Harvard respirator at flows of 2 liters/min. The respirator pump was set to deliver 25 ml/inspiration at rates of 30 inspirations/min. This same procedure was carried out for each of the other inspired concentrations of gas (6,4, and 2% oxygen in nitrogen). A base ECoG run was taken at the normal blood osygen saturation of the animal and allowed to continuously record during the hypoxia period (30 min). Next, the animal was placed on room air or -COO/,oxygen for 5-15 min, particularly if the EKG showed ischemic changes. After the heart and blood pressure stablized and if there was no spontaneousrespiration, the animal was continued on a mixture of 40% oxygen in nitrogen or on room air through the Harvard respirator until spontaneous respiration began. The ECoG records were obtained at 5-min intervals during the recovery phase. When the ECoG approached normal a final record was taken. The monkey was then deinstrumented. all cannulae were removed, and all wounds closed. Each animal was given chloromycetin and bicillin intramuscularly and returned to a recovery cage. In the case of one of the animals which did not recover ECOG activity or spontaneous respiration (after an extended resuscitation effort), autopsy was performed immediately. Some (four j survived 3-18 hours after hypoxia and postmortems were performed the following day. The other ten monkeys recovered without incident. Results
The Hypoxic Elect~ocorticogua~~~. The cebus monkeys prehypoxic ECoG recorded from the precentral gyrus (motor area j contained predominantly two frequencies: high-amplitude (150 pv) 5-7 Hz activity on which was superimposed 15-20 H z activity of less than 50 pv. The postcentral gyrus recordings showed a somewhat slower picture, having predominantly 10-15 Hz activity with occasional periods of 5-7 Hz activity interspersed. These
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FIG. 1. Sample tracings of the ECoG before, during, and after hypoxic interval. Top trace, percentral gyrus (sensory cortex) ; middle trace, postcentral gyrus (motor cortex) ; bottom trace, right hemisphere (sensory to motor cortex). 411 recordings were made bipolarly. Vertical columns : 8, 6, 4, and 2% Oz. Horizontal: A, prehypoxic ECoG; B, after 15 min hypoxia; C, after 30 min hypoxia; D, after oxygen saturation of blood and blood pressure had returned to prehypoxic levels; E, immediately before termination of experiment. The ECoG in the last column (the 2% hypoxia group) : A, prehypoxic ECoG; B, after 5 min hypoxia; C, after 8 min hypoxia; D, after restoration of heart action and 90% oxygen saturation; E, immediately before termination of experiment. Calibration : vertical, 100 gv ; horizontal, 1 sec.
patterns of activity were considered “normal” for animals given iv injections of the combination anesthetic consisting of Nembutal and Urethan (Fig. 1). Little 15-20 Hz activity was observed in two of the 8% hypoxia group after injection of 30 mg/kg Nembutal. The predominant frequency in these animals was 10-15 Hz.
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Oxygen 8%. hfter S-10 min on an inspired mixture of 8% osygen in nitrogen, the ECoG showed a general reduction in amplitude, particularly in the 5-7 Hz frequencies, plus short isoelectric periods (0.4-0.6 set in duration). Also characteristic during this interval was the appearance of short ( l-SK) synchronous spindle-burst activity (30 Hz) usually confined to the precentral gyrus, spindle bursts infrequently appearing in the postcentral gyrus as well. After 20-30 min of hypoxia (8% j the ECoG contained longer isoelectric intervals (3-6 set ) with low-amplitude 10-I 5 Hz activity of l-2 set duration interspersed. At this time the spindle bursts mentioned above had disappeared. At the end of 30 min the animal was placed on 40% oxygen or room ail via the Harvard respirator. Recovery of prehypoxic ECoG occurred gradually, usually within an hour after being placed on lo% oxygen mixture or on the animal’s own respiration. Synchronous spindle-burst activity sometimes appeared within 15 min after blood oxygen saturations reached 80% or more, then disappeared completely ( Fig. 1 ). Oxygen 6%. The prehypoxic ECoG at 6% oxygen was similar to that reported for the 8% hypoxia group. Since a more acute drop of oxygen saturation of the blood occurred, usually within 1 min to levels of approximately 30%, the loss of 5-7 Hz frequencies was immediate and the appearance of spindle-burst activity was more transient than that observed in the 8% hypoxia group. The appearance of isoelectric periods of short duration (about 0.5 set) occurred within 2 min of hypoxia. These periods lengthened to 34 set as the hypoxic interval was extended. The isoelectric periods were separated by I-2-set periods of low-amplitude bursts of 20-30 Hz activity, sometimes resembling the spindle bursts described previously but having low amplitude and without synchronous characteristics (Fig. 1). The above sequence of events persisted to the end of the hypoxic period without further significant change. Recovery from hypoxia was characterized by the reappearance of the high-amplitude slower frequencies (5-7 H z ) and the gradual disappearance of the isoelectric periods. The faster frequencies slowed to 15-20 Hz and returned to their original amplitudes. Spindle-burst activity was not observed in this group of animals during recovery from hypoxia (Fig. 1). Oxygen 4%. The prehypoxic ECoG at 4% oxygen was characteristically similar to that reported previously for the S and 6% hypoxia groups. The acute decline in oxygen saturation of the blood was similar to that observed in the 6% hypoxia group. Further, the appearance of isoelectric periods occurred within 2 min of hypoxia and lengthened to 5-7-9~ intervals rapidly, usually within 5 min of hypoxia. The ECoG activity disappearec~
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rapidly and only extremely short bursts of low-amplitude 15-20 Hz activity was still evident after 10 min of hypoxia. Some slow wave activity (3-5 Hz) appeared sporadically throughout the hypoxic interval. Near the end of the 30-min period, the electrical activity was almost isoelectric. Only sporadic short intervals of low-amplitude 1.5-20 Hz activity appeared. This ECoG picture was accompanied by some cardiac changes and low blood pressure. Recovery from hypoxia was characterized by the appearance of synchronous low-voltage 15-20 Hz activity with short intervals of low-voltage 3-5 Hz activity. The isoelectric periods became shorter and more infrequent as the synchronous low-voltage 15-20 Hz activity became more prominent. Near termination and recovery the synchrony of the 15-20 Hz activity disappeared. The activity then became desynchronized and appeared similar to the prehypoxic activity. Two of the three animals subjected to 4% hypoxia succumbed several hours following recovery from hypoxia. The other one survived and recovered fully without incident (Fig. 1). Oxygen 2%. On1 y t wo monkeys were subjected to 2% hypoxia. Both animals exhibited a precipitous drop in blood oxygen saturation (within S-10 set) which caused rapid and dramatic cardiac changes accompanied by an acute drop in blood pressure within 9 min. Both these animals suffered cardiac arrest but heart action was restored by cardiac massage and placing the animals on 40% oxygen via the respirator. The blood pressure reached only 90 mm Hg in one animal, However, the ECoG never recovered any of the prehypoxic patterns, remaining relatively isoelectric. Spontaneous respiration was not induced in this animal. This experiment was terminated after several hours of attempted resuscitation. After heart action was restored on the second animal and the blood pressure climed to 190 mm Hg (systolic), efforts to improve the cardiac status and ECoG activity were successful and the blood oxygen saturations were better than 90%. The second animal survived for a few hours after recovery from 2% hypoxia. Hypoxic Electrocardiogram and Blood Pressure. Figure 2 illustrates the typical EKG cardiac cycle at normal oxygen saturations, blood pressure, and heart rate. Also illustrated are typical changes occurring in the EKG during the hypoxic interval. The following description of the EKG is based entirely on lead II records since they proved to be the most revealing. During the administration of 8% inspired oxygen concentrations, there appeared concurrently a reduction in amplitude of the “R” wave and a lengthening of the “S-T” interval. Next, the “S” wave deepened and a slight elevation of the “T” wave occurred. The EKG quickly reversed and returned to the typical EKG characteristics during recovery from 8% hypoxia. In the five monkeys subjected to
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FIG. 2. EKG tracings of the cchus monkeys’ cardiac cycle hefore hyrosia ( 1 ) ; during hypoxia showing right bundle branch block (2, 5) ; left bundle branch block (3) ; (6) just before cardiac arrest. a severely ischemic heart (4) ; fibrillation
So/o hypoxia, was 150/80; to I@/90 termination None of this
the average blood pressure before being subjected to hyposia at 20-30 min into the hypoxic period, the blood pressure fell (average) and just before deinstrumenting the animal and of the experiment, the blood pressure reached lSS/SS (Fig. 3 1. group succumbed.
22) 200 A
F
00 au -
A 6 % l 4 HYPOXIA
20 -
Nl PRE HYPO
.5 -MIN
I5 HYPOXIA
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40 02+ IOmnn
zET,”
TERM
FIG. 3. Changes in blood pressure during the hypoxic period and after recovery from hypoxia. PRE HYPO, prehypoxic blood pressure; MIN HYPOXIA, changes in blood pressure during a 30-min hypoxic period; 40 Oz + 10 min, blood pressure after 10 min on 40% inspired oxygen mixture; OWN RESP, after spontaneous respiration began ; TERM, at termination of experiment.
In the group of five monkeys subjected to 670 inspired oxygen concentrations, the EKG followed the same course of events described for the 8% hypoxia group above. However, in addition, a negative “Q” wave developed with marked reduction in amplitude of the “R” wave. There also occurred a marked “S-T” elevation with a prominent “T” wave.
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During recovery from the hypoxic state, the EKG characteristic returned to the prehypoxic state slower than the 8% hypoxia group. The prehypoxic blood pressure averaged 145/90 and during the hypoxic state fell to 1 lo/60 (average). The blood pressure returned to 170/85 during recovery from hypoxia and gradually fell to ISO/SO shortly before termination of the experiment (Fig. 3). One of these animals died shortly after termination of the experiment. The other four animals survived and recovered without incident. In the 4 and 2% hypoxia groups, there occurred a relatively rapid change in the EKG. These animals developed EKG characteristics indicative of right- and left-bundle blocks, ischemic myocardial changes, and finally ventricular fibrillation followed by cardiac arrest. Shortly after the appearance of EKG configuration indicating an ischemic heart, ventricular fibrillation occurred with arrest following shortly thereafter. Resuscitation was possible in three of the four monkeys exhibiting the above EKG characteristics ; one animal failed to respond to resuscitation (a 2% hypoxia animal). One animal of these two groups (a 4% hypoxia animal) showed a characteristic EKG indicative of severe bundle-branch block with extrasystoles, but did not suffer cardiac arrest. This monkey survived and recovered from hypoxia without neurological damage. The average blood pressure of the 4% hypoxia group before hypoxia was 160/90. This blood pressure fell to S5/55 and on recovery from hypoxia returned to 155/80 (average). Even though two of the three animals died some hours after hypoxia and after resuscitation, they were able to maintain relatively good levels of blood pressure. The two monkeys subjected to 2% inspired oxygen concentration became anoxic within 4 min. The EKG showed a drop in amplitude of the “R” wave, the “S-T” segment developed a deep notch, and the “T” wave rapidly increased in amplitude. Extra systoles developed and cardiac arrest occurred about 7 min after the 2% oxygen was administered. Neither of these animals survived, even though cardiac action was restored raising the blood pressure to 180/90 before surgical recovery of one animal was instituted. The prehypoxic blood pressure was 170/90. and after 8 min of hypoxia the blood pressure dropped to 60/50 as the arrhythmia developed and fell to zero pressure at cardiac arrest. After being placed on 40% oxygen concentration, cardiac massage restored heart action and showed an overshoot increase of 200/175, leveling off at 180/90 before closure of wounds. These two animals succumbed between 3 and 16 hr after the hypoxia period (Fig. 3). Blood Oxygen Saturation. The monkeys subjected to 870 hypoxia showed a gradual drop in blood oxygen saturation, reaching an average saturation of 48% at approximately 3 min. Figure 4 illustrates the rate of decline of oxygen saturations for each of the hypoxic groups. This
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level of blood oxygen saturation was maintained throughout the half-hour period of hypoxia. The ECoG changes described previously began appearing when the oxygen saturation dropped to about 75% and maximum changes occurred after the saturation dropped to about 50% and continued to the end of the hypoxic period. The blood pressure remained relatively constant throughout the hypoxic period, showing a slight increase as the blood oxygen saturation started to fall and was only down about 10 mm Hg after 30 min of hypoxia ( 170-I 50 mm Hg).
FIG.
group.
4. Rate of change in average blood See Fig. 3 for group designation.
oxygen
saturations
for
each
hypoxic
In the 6% hypoxia group, the blood oxygen saturation dropped more acutely. The prehypoxia level averaged 90% saturation and within 2 min dropped to an average of 36% saturation. This level remained unchanged throughout the 30-min hypoxic interval. The ECoG changes occurred more rapidly and were more severe when compared to the 8% hypoxia group. The blood pressure fell gradually from an average for the five animals of 150 mm Hg (systolic) to 110 mm Hg after 30 min of hypoxia, except for one animal which suffered cardiac arrhythmia and rapid drop in blood pressure. This animal recovered but died several hours after termination of the experiment. The 4% hypoxia group followed the same sequence of events as reported above for the 6% hypoxia group, except that the blood oxygen saturation dropped to an average of 18% at the end of the 30-min hypoxia interval. Two of these animals died 3- 16 hr after termination of the experiment. One monkey was observed to have cardiac arrhythmia and bundle-branch block
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about 10 min after starting the hypoxic period. Another animal -showed minimal EKG changes, was recovered from hypoxia but died several hours after termination of the experiment. The two monkeys subjected to 2% hypoxia suffered profound blood oxygen saturation and cardiac effects. The blood oxygen saturation dropped to zero or “off scale” on the oximeters within lo-15 sec. The heart failed rapidly, going into arrest in about 7 min, and resuscitation procedures were instituted at that time. The ECoG became isoelectric rapidly. One animal did not recover even though heart action was restored. The other animal recovered both his prehypoxic blood pressure, heart rate, and ECoG, but died several hours after closure of wounds. Au~u~s$~. Cause of death in all the animals in which postmortem examination was performed was due to cardiorespiratory failure. The hearts revealed regions of myocardial hyperemia and infarction on either side of the interventricular groove on the sternocostal surface. Some cardiac enlargement was evident. In two cases moderate lung edema was also noted. The brains of these animals were dissected and examined but no gross evidence of pathologic change was found. Other organ systems were also negative. Discussion
It is apparent from the data that electrocortical activity is relatively resistant to the effects of. hypoxia since extremely low inspired oxygen tensions must be used to produce significant changes. These changes are readily reversible if the hypoxic interval is minimized and blood pressure and cerebral blood flow can be adequately maintained. The course of electrocortical events occurring during hypoxia without anoxia in the cebus monkey is as follows : (a) disappearance of faster frequencies; (b) gradual reduction in amplitude of the slower frequencies; (c) appearance of isoelectric periods of varying lengths of time depending on the severity of hypoxia ; (d) app earance of spindle-burst activity coupled with intervals of failing electrical activity, and (e) loss of all activity (isoelectric). Recovery of the ECoG depends largely on two factors: the level of oxygen available; and the cardiac status and thus blood flow to the brain. If the above two phenomena occur simultaneously over a prolonged period, the ECoG is not recoverable and death of the animal ensues. Two observed phenomena, the occurrence of isoelectric periods and the appearance of spindle bursts, are difficult to explain. Van Harreveld (16) described the characteristics of EEG activity in the cat a:;er IO-30 min of asphyxia as consisting of spindles of 10-20 set duration separated by low
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frequency activity varying from a few seconds to 1 min. He did not offer an explanation of the origin of the spindle activity other than stating that this activity was abnormal and originated from either the cortex itself or could he produced by some subcortical structures. Van Harreveld made no mention of isoelectric periods separating the spindle activity. Apparently, some activity was noted between subsequent spindle bursts. Naquet and Fernandes-Gunrdiola (12) offered an explanation of both the isoelectric period (they called it “refractory period”) and the spindleburst activity. The isoelectric periods may have been due to a gradual and massive decrease in afferent stimuli reaching the cortex. When extremely long isoelectric periods were found, then actual “deafferentiation” had occurred. They also thought that the spindle-hurst activity observed in their cats was due to an initial enhancement of the recruitment phenomena. When these spindles disappeared during late stages of anoxia, the cortex became “nonactive.” However, what caused the “nonactivation” was not explained. It is our hypothesis that two major mechanisms contribute to the changes observed in ECoG activity under conditions of severe hypoxia. These are as follows : (a ) Lack of available oxygen. Proper exchange of blood gases is not possible at low hemoglobin saturations causing the accumulation of anerobic metabolites which at first stimulate the energy output exhibited by the appearance of the spindle-burst activity. then gradually block the energy output, thus accounting for the isoelectric intervals (7). Further evidence for this theory has been reported by Van Liere (17). He reported that during hypoxia the brain lactic acid increases and a concentration of potassium occurs extracellularly in the brain, both factors decreasing brain metabolism. (b) Reduction of blood flow to the brain. It has been shown that when the cortical blood flow increases, the mean frequencies in electrocortical activity also increase. and zlicc e’crsa ( 1. 8). Both of the above mechanisms cause, finally. the development of brain edema and ischemia, which account for the isoelectric ECoG near the end of the hypoxic interval. This is reversible providing blood flow and blood osygen saturation can be raised within 5-10 min after the hypoxia period. If not, spontaneous respiration cannot be elicited, nor does the ECoG recover and the blood preesure remains at relatively low levels. Because of the importance of maintaining cardiac status, the EKG was monitored continuously before. during, and after instituting the hypoxic state. Lead II records were the most revealing and detailed analysis demonstrated the similarity of the EKG with that of man and other nonhuman primates (3, 10, 11). Figure 2 illustrates tracings of the EKG which were typically “normal” before hypoxia and the changes in the EKG during the period of hypoxia. From the detailed description of these
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changes in Results, one can recognize the configurations of right- and left-bundle branch blocks and the ischemic failing heart. In the failing heart, as the “T” waves became markedly elevated, fibrillation followed shortly thereafter, and finally total arrest occurred. In the monkeys in which this occurred, the hypoxic period was terminated by placing the animal on 40% oxygen via the Harvard respirator and rapid closedthoracic cardiac massage was instituted. In usually less than 2 min a ventricular beat was reestablished and shortly thereafter the EKG began returning. The blood pressure rose rapidly at first, then leveled off, and again rose slowly over a period of 30 min reaching values of 200/175. The above description is reminiscent of coronary vessel occlusion in man. The most controversial data collected in these experiments was that concerning oxygen saturation of the blood. Cole and Hawkins (4) and Linden and associates (9) compared most of the available techniques for determining oxygen saturation of blood. They compared their results with the Van Slyke method which is known to be the most accurate standard laboratory method for these determinations. The recent use of other “quick” methods employing “oximetry” (photometric) on one hand, and measuring oxygen tension on the other hand, necessitated determinations of their reliability. The above authors stated that all methods assessed except the Van Slyke technique were inaccurate, particularly below 40% oxygen saturation levels. Therefore, we consider our determinations of low oxygen saturation of blood reported in this paper as being only “relative estimates” and containing as much as 20% error in absolute values. However, by combining and averaging the oxygen saturations as determined by the three methods used in this investigation, a reduction in error was achieved. Even then, the reader should consider the oxygen saturations as approximations. References 1. BALD\.-MOVLINIEK. M., and D. H. INGVAK. 1968. EEG frequency content related to regional blood flow of cerebral cortex in cat. Exptl. Brain Res. 5: 55-60. 2. BARTLE~, H. S., and G. H. BISHOP. 1933. Factors determining the form of the electrical response from the optic cortex of the rabbit. Am. J. Pkysiol. 103: 173-184. 3. BURCH, G. E., and T. WINSOR. 1961. “A Primer of Electrocardiography.” Lea & Febiger, Philadelphia. 4. COLE, P. V., and L. H. HAWKINS. 1967. The measurement of the oxygen content of whole blood. Bio-Med. Ewgr. 2: 5663. 5. DELL, P., A. HUGELIN, and M. BONVALLET. 1961. Effects of hypoxia on the reticular and cortical diffuse systems, pp. 46-58. In “Cerebral Anoxia and the Electroencephalogram.” J. S. Meyer and H. Gastaut [eds.]. Thomas, Springfield. Illinois.
128 6. 7. 8.
9.
10. 11. II?.
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DITTMEK, D. S. 1961. Blood and other body fluids, pp. 141-168. ZU “Biological Handbook.” Fed. .qm. Sot. Exptl. Biol., Washington, D. C. FREEMAN, J., and D. H. IKWAR. 1968. Elimination by hypoxia of cerebral blood flow. Autoregulation and EEG relationships. Erptl. Rvnbl Rcs. 5: 61-71. LEE, J. F., G. T. TI.~~AI.L, J. C. GREENFIELD, Jr<., and G. L. ODOM. 1966. Cerebral blood flow in the monkey. J. ,Vz~~/~.wr,q. 24 : 719-726. LIICDEN, R. J., J. R. LEDSOME, and J. NORMAX. 1965. Simple methods for the determination of the concentration of carbon dioxide and oxygen in blood. BYif. J. ~-lrlacsth. 34 : 77-88. MALINOW, BI. R. 1966. An electrocardiographic study of Jlacc~a wrzrlatfrz. Folia Prif~tafologg 4 : 5165. k~ALIKO\v, M. R., and C. \V. DELANSOY. 1966. The electrocard:ogram of Cpopitfrertts szigrr. I:olitr Priurtrfoiogg 4 : 66-73. NAQUET, R., and A. FERNAKDES-GUARDIOLA. 1961. Effects of various types of anoxia on spontaneous and evoked cerebral activity in the cat, pp. 72%. III “Cerebral Anosia and the Electroencc~)halogra~n.” J. S. Meyer and H. Gastaut [eds.]. Thomas, Springfield, Illinois. PROWDICZ-NEMINSKI, W. 1%‘. 1925. Zur Iienntnis der elektrischen unter der Innervations vorgange in den funktionellen Elementen und Geweben des tier&hen Organismus. Elektrocerebrogramm der Saiigetiere. =irrA. Gcs.
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H. N., and -4. J. DERBYSHIRE. 1934. Electrical activity of the motor cortex during cerebral anemia. .irn. J. Phgsioi. 109 : 99. TEN CATE, J., and G. P. hl. HORSTEN. 1951. Effect of hyposaemia on the electrical activity of the cerebral cortex. Arfu I’llgsio[. Phclmrnrol. Nrrrl. 2: 2-12. VA~V HAKKEWLD, A. 1947. The electroencephalogram after prolonged brain asphyxiation. 1. A’rwop~~.vsio~. 10 : 361-370. VAX LIERE, E. J.. and J. C. STI~KSEY. 196.7. Effect of hypoxia on the nervow system, pp. 276-349. ITI “Hypoxia.” Llniv. of Chicago Press, Chicago.
SIMPSON,