Brain Research, 562 (1991) 13-16 © 1991 Elsevier Science Publishers B.V. All rights reserved. 0006-8993/91/$03.50 ADONIS 000689939117052Z

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Magnetic fields associated with anoxic depolarization in anesthetized rats Y." Takanashi 1, M. Chopp 1'2, S.R. Levine 1, J. Kim 1, J.E. Moran 1'2, N. Tepley 1'2, Q. Chen l, G.L. Barkley I and K.M.A. Welch 1,2 1Department of Neurology, Henry Ford Hospital, Detroit, MI 48202 (U.S.A.) and 2Deparment of Physics, Oakland University, Rochester, MI 48309 (U.S.A.) (Accepted 21 May 1991)

Key words: Anoxic depolarization; Asphyxia; Magnetoencephalography; Electrocorticography; Neuromagnetism

We have performed simultaneous measurements of the DC-magnetoencephalogram (DC-MEG) and DC-electrocorticogram (DC-ECoG) in rats (n = 6) subjected to 90 s of reversible anoxia. The onset of major shifts of electric and magnetic signals occurred at 52 -+ 18 (S.D.) and 68 -+ 14 (S.D.), respectively, and reached a peak at 83 - 27 and 102 -+ 19 (S.D.) s, respectively, after termination of mechanical ventilation. DC-ECoG signal deflections were always associated with DC-MEG deflections. The time of onset and peak signals in both DCMEG and DC-ECoG changes caused by asphyxia were highly correlated (r + 0.83, 0.94; P < 0.05, 0.001; respectively). Our observations suggest that the non-invasive technique of DC-MEG is reliable and may provide insight into the mechanisms of anoxic cerebral depolarization. INTRODUCTION Brain anoxia evokes direct current electrocorticogram ( D C - E C o G ) shifts from baseline activity 3"5"6. These shifts develop simultaneously in different areas of the cortical surface, and can be measured when conventional E E G has completel3, disappeared 3'5. D C shifts associated with brain anoxia may provide important information on the physiological state of cortical tissue 5. However, DCE C o G has not b e e n used diagnostically because of technical difficulties and the invasiveness of the procedure to patients. The ionic currents associated with D C shifts in cortex may generate detectable extracranial magnetic fields 6'8. However, to our knowledge, m e a s u r e m e n t s of the magnetic fields produced by anoxic depolarization have not been reported. Clearly, if such fields are detectable, noninvasive neuromagnetic field m e a s u r e m e n t s would facilitate the study of anoxic depolarization. We therefore performed non-invasive measurements, using a 7-channel superconducting m a g n e t o m e t e r (BTi model 607), of the D C - n e u r o m a g n e t i c field ( D C - M E G ) shifts in rats subjected to reversible anoxia, and compared the neuromagnetic data to simultaneous D C - E C o G measurements. O u r results indicate that neuromagnetic measurem e n t of anoxic depolarization in brain correlates with

the D C - E C o G m e a s u r e m e n t , and therefore may provide a non-invasive m e a s u r e m e n t of anoxic depolarization. MATERIALS AND METHODS Six adult Wistar rats (280-350 g) were studied. Animals were anesthetized intraperitoneally with sodium pentobarbital (40 mg/ kg). A femoral artery and vein were cannulated for blood gas monitoring and supplemental anesthetic infusion. Two small burr holes (1.5 mm, diameter) were made in the skull. One was placed 3 mm posterior and 3 mm lateral to the bregma on the right side, and used as an active electrode position. The other was placed 4 mm anterior and 3 mm lateral to the bregma on the left side. The cerebral cortex, under the anterior hole, was carefully electrically cauterized and used as a reference position 3. The electrodes were inserted subdurally and secured with bone wax. After the surgical procedure, rats were given atropine sulfate (0.5 mg/kg, i.p.) and intubated using a non-magnetic tube. The head was immobilized using a non-magnetic head-holder built in"our laboratory. The animals were paralyzed with an intravenous injection of d-tubocurarine (1 mg/kg) and mechanically ventilated on room air. Electrical AC and DC recordings were performed using 0.2-mm diameter Ag-AgC1 electrodes. The animals were grounded through an electrode placed on the nose. DC-MEG measurements (0-50 Hz) were performed in a magnetically shielded room to avoid signal contamination from fields due to power lines and moving vehicles. Since the animal can carry magnetic particles on fur or within the digestive tract, before initiating experiments the animal body was demagnetized using a videotape eraser. Nevertheless, magnetic noise pulses and large sudden movement artifacts were occasionally observed in the data (as shown in Fig. 2). These artifacts were easily discriminated from physiological phenomena2. The hexagonal array of 18-mm diameter magnetometer coils was

Correspondence: M. Chopp, Department of Neurology, Henry Ford Hospital, 2799 West Grand Blvd., Detroit, MI 48202, U.S.A. Fax: (1) (313) 8761318.

vcrsc sense, i.e. positive voltage -- downward dcHcction, ncgativc voltage - upward deflection. Asphyxia was transiently induced (90 s) by discontinuing mcchanical ventilation. DC-MEG. AC-ECoG and DC-ECoG were obtained simultaneously in all 6 animals for 6 min prior to asphyxia to obtain baseline values, and for 24 min during and after asphyxia. Blood gas measurements were obtained immediately prior to MEG recording, 60 s after initiating asphyxia, and 5. 10 and I5 min after termination of asphyxia. RESULTS

Table I summarizes values of arterial blood gases and pH obtained during the course of the experiment. A significant decrease

in pH and ~0,

phyxia (P < 0.001); pC0,

was detected

during

during as-

asphyxia

also signifi-

cantly increased (P CC 0.01). These values pre-asphyxic values at 5 min after resuming

returned to respiration,

except for ~0, values, which did not return phyxic values within 15 min.

to pre-as-

Fig. 2 shows a typical set of magnetic and electrical recordings prior to, during and after asphyxia. DC-MEG and DC-ECoG were relatively constant prior to asphyxiation. Noise pulses in the DC-MEG were sporadically observed. After termination of mechanical ventilation, AC-ECoG rapidly disappeared at 37 * 6 s (range 27-48 s). AC-ECoG reappeared at 6 + 4 s (range 2-14 s) after resuming respiration. In 4 rats DC-ECoG showed subtle positive (n = 3) or negative (n = 1) voltage deflections immediately after. termination of mechanical ventilation. A large positive voltage deflection, of mean amplitude 4.3 2 2.0 mV, was clearly detected in all rats. The mean onset latency of large positive voltage deflections from termination of ventilation was 52 5 18 s (range 28-74 s). These deflections reached a peak at 83 + 27 s (range 49-134 s) after termination of ventilation. After resuming mechanical ventilation, the large positive voltage deflections in DC-ECoG detected during anoxia returned to the previous baseline levels in 2 rats and showed a tendency to shift to a negative potential in 4 rats (see Fig. 2).

1

1 cm Fig. 1. Experimental arrangement of 7-channel magnetometer coils. The center coil (channel 1) is positioned over the skull as shown in the upper figure. The lateral view of the sensor coil arrangement is shown in the lower figure. The distance between sensor coil and head is within 20-25 mm.

positioned above the rat head as shown in Fig. 1. The central coil was positioned to measure the vertical field over the skull, 20-25 mm from the brain. Peripheral coils were centered at 21.5-mm radii around this point and inclined 7 ’ to the vertical. DC-ECoG was amplified with a battery-operated DC-preamplifier (Grass P18) and the DC signals were recorded by an EEG machine (Nihonkoden Inc; Japan) (DC-75 Hz). AC-ECoG was amplified with the EEG machine (l-75 Hz). All electric and magnetic signal channel data were digitized by a HP 694A multiprogrammer and stored on hard disc for later analysis and graphical display. Graphical display of DC-ECoG voltages are presented in an in-

TABLE I Blood gases and pH during the course of the experiment

Values are mean * SD. Total sample numbers + 6 Pre-asphyxia _~...

..~

~.._~.

After asphyxia 5 min

10 min

15 min

7.33 0.06

7.37 0.05

7.37 0.06

PH

mean S.D.

PC02

mean S.D.

37.7 9.1

48.4** 14.0

39.0 6.7

37.2 7.0

PO,

mean S.D.

82.4 8.4

30.4* 15.4

74.1* 9.8

75.1* 10.7

* P

Magnetic fields associated with anoxic depolarization in anesthetized rats.

We have performed simultaneous measurements of the DC-magnetoencephalogram (DC-MEG) and DC-electrocorticogram (DC-ECoG) in rats (n = 6) subjected to 9...
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