J. Neurol. Neurosurg. Psychiat., 1964, 27, 66

A critical evaluation of rheoencephalography incontrol subjects and in proven cases of

cerebrovascular diseaseCARLOS PEREZ-BORJA AND JOHN S. MEYER

From the Departments of Neurology, Detroit Receiving Hospital, Wayne State UniversityCollege of Medicine and the Wayne Center for Cerebrovascular Disease,

Harper Hospital, Detroit, Michigan, U.S.A.

When an alternating current passes through a partof the body, modulated changes of impedance occurdue to the pulsatile flow of blood, and the possibilityof correlating such changes with modifications of theperipheral circulation (Mann, 1937) stimulated thedevelopment of the technique. Several modificationsof the technique have been used for studies ofperipheral circulation (Nyboer, 1944; Holzer andPolzer, 1945) in various parts of the body. Theprinciples and application were lately summarizedin Nyboer's monograph (1959).Recording changes of electrical impedance with

electrodes applied to the head was first reported byPolzer and Schuhfried (1950). Several authors havepublished results using similar techniques, includingAuinger, Kaindl, and Neumayr (1953), Kaindl,Kraus, and Partan (1955), Oehninger, Stanham,Cortes, Fiandra, and Ferrari (1955), Spunda (1955),Serra and Urso (1956), Colonna and Ricciardi (1957),Garbini and Poppi (1957), Gentili, Garbini, andOrlandi (1957), Pratesi, Nuti, and Sciagra (1957),Orlandi, Garbini, and Gentili (1957), Gastaut,Rodler, Lechner, Bostem, and Naquet (1959),Kunert (1961), Lifshitz (1963), and others. Jenkner(1957) has termed this procedure 'rheoencephalo-graphy', and his work over a period of years hasculminated in the publication of a monograph(Jenkner, 1962) in which he has summarized hisobservations in over 10,000 clinical records.Development of a method for continuous record-

ing of cerebrovascular changes by a simple andharmless procedure, if reliable, would be of greatclinical usefulness. For this reason we have evaluatedrheoencephalography (R.E.G.) in a number ofproven cases of cerebrovascular disease and com-pared the records with those from a group of normalcontrols.

'This work was supported by grants from the Receiving HospitalResearch Corporation and the U.S. Public Health Service.

MATERIAL AND METHODS

The study involved two groups.The first group consistedof 19 control subjects who were free of neurologicaldisease and were 18 to 30 years of age. They were allapparently in good health and were selected from nursingpersonnel, medical students, and laboratory techniciansin our unit. The second group consisted of 28 patientswith proven cerebrovascular disease and neurologicaldeficit. In all 28 patients the entire cerebral circtulationwas studied by serial arteriograms from the aortic archto the brain after recording rheoencephalography. In oneadditional case, coarctation of the aortic arch distal tothe great vessels was demonstrated by aortography.The final classification of the 28 patients was:

Neurological Disorder

Unilateral occlusion of the internal carotid arteryBilateral occlusion of the internal carotid arteryUnilateral occlusion of the internal carotid artery plusarteriographically demonstrated pathology of thevertebro-basilar systemUnilateral stenosis of the internal carotid artery plusinvolvement of the vertebro-basilar systemBilateral stenosis of the internal carotid arteries plusvertebro-basilar involvementOcclusion or absence of one vertebral arteryStenosis of the vertebro-basilar system without carotidinvolvementDiffuse cerebral arteriosclerosis (small vessel disease)

No. of Cases

23

3

6

34

25

The ages of the patients ranged from 36 to 77 years,most of them being in the age group 50 to 60 years.

Rheoencephalographs were obtained with a PhysicalInstruments rheoencephalograph (model 450) whichuses a 2 mA current with an average input of 3V acrossthe head of the patient, and a frequency of 30 K.C.utilizing two simultaneous bridge circuits or the so-called'2 electrode system'. The electrodes were placed one on theforehead (midline) and the two others on the mastoidregions of either side. Care was taken in measuring theinter-electrode distance so that the electrodes were placedsymmetrically.

Electroencephalographic and rheoencephalographicrecordings were monitored with a Grass model 6 electro-

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encephalographic machine. Simultaneous recordings wereobtained as follows: right and left hemisphere R.E.G.curves, right and left central to occipital E.E.G. tracings.The E.K.G. was also recorded on the graph. In some casesa pressure cuff was wrapped around the head below thelevel of the electrode and was inflated above arterial pres-sure during the recordings.The R.E.G. curves were analysed as follows:

AMPLITUDE This was measured in ohms (based on thecalibration signal) from the baseline to the highest pointof the curve, which was usually the first peak.

ANGLE OF INCLINATION The angle of inclination of therising part of the curve (called 'angle' hereafter) wasmeasured directly in degrees, tracing the angle as definedby Jenkner (1962) from the point of origin of the curveto the steepest part of the ascending limb, referred to asthe horizontal baseline.

TIME Time was the moment at which the first peak wasreached (referred to from now on as 'time') as measuredin milliseconds from the Q wave of the simultaneouslyrecorded E.K.G. to the first peak of the R.E.G. wave.Records made at paper speed of 3 and 6 cm./sec. wereused for this purpose.

WAVE FORM Consideration was given to the number ofpeaks present in a single pulse, the regularity of the wave,and the inclination of the descending part of the curve.

Several manoeuvres were routinely performed duringthe recording, including carotid compression (one at atime), jugular compression (both simultaneously),hyperventilation, breath holding, 100% 02 respiration,and respiration of 5% CO2 in 2. Not all these manoeuvreswere achieved in all patients because of their clinicalstate or lack of cooperation.

A REST B HYII! >_~~L8

RESULTS

R.E.G. IN NORMAL CONTROLS The amplitude of theR.E.G. pulse ranged from 0 10 to 025 ohms; inmost cases, however, it was between 0-10 and 018with a mean value of 0-15. Four of the 19 controls(21 %) showed asymmetry of amplitude between therecords on the two sides of the head (5 to 25 %Y). Theamplitude of the waves varied from moment tomoment in the same record so that the mean wastaken of at least five consecutive waves. The ampli-tude sometimes showed significant changes in thesame record when there was cardiac arhythmia. Insuch cases the highest waves corresponded to the heartbeat which followed the longest period of asystole.In instances of premature ventricular contractions,the R.E.G. wave was very small or not present at all.The angle varied between 750 and 890 with a mean

of 810 and the steepest curves corresponded, usually,although not always, to the highest amplitude waves.

Although time showed a wide range of variationfrom 200 to 430 msec., most of the records showedtime from 250 to 350 msec. with a mean of 290 msec.

No time difference was found, at rest, between thesimultaneous records of the two sides in the sameperson.The wave form also varied considerably in normal

subjects. Curves were seen with one peak and straightregular lines, two peaks (Fig. 1) with sharp descend-ing curves, three peaks, and even curves withplateaux (Fig. 2) and other irregular aspects.When the pressure cuff about the head was inflated

above arterial pressure during the recordings,drastic changes occurred which consisted mainly in

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FIG. 1. Rheoencephalograms ina 22-year-old normal volunteer.In this as in all other figures thesamples in B, C, and D cor-

respond to the same selectionshown in A.

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A REST,: ~ ~ 007

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reduction of amplitude and angle of the curve tominimal values. Recovery occurred slowly to 10 to40% of previous amplitude in 80% of subjects. Ofthe remaining subjects, in 10% the R.E.G. pulserecovered to the pre-occlusion amplitude and in therest (10 %) there was actually an increase in amplitudeof 10% to 20%. The angle usually followed thechanges in amplitude. In some cases, similarchanges, but usuallyan increase in amplitude, was pro-duced with pressures below the systolicblood pressure.

Unilateral carotid compression produced homo-lateral decrease in amplitude of the R.E.G. curve on11 occasions (Fig. 1), bilateral depression on 10occasions (Fig. 2), and homolateral or bilateralincrease on three occasions. No significant changesresulted on four occasions. The changes usuallylasted only a few seconds and recovered quickly. Insome cases they were accompanied by the appearanceor increase (Fig. 2) of the second and third peaks andchanges in the wave forms to plateaux or roundedwaves. No homolateral or bilateral increase in timewas seen in normal subjects during carotid com-pression. In some instances, while compression ofone carotid artery produced little or no change,compression of the opposite artery sometimes pro-duced gross homolateral or contralateral depression.Compression of both jugular veins performed in

15 subjects produced bilateral decrease of amplitudeof the R.E.G. in seven (Fig. 2), increase in amplitudein four, and no change in four. Both decrease andincrease in amplitude was seen as a transient pheno-menon with quick recovery and in some there wasasymmetry between the two sides.

Hyperventilation produced depression of ampli-

B R. CAROTID COMPRESSION030

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FIG. 2. Rheoencephalogram inan 18-year-old normal volunteer.

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REG 1OSil EEG I IV.I SEC. I SEC.

tude and diminished angle in eight of 15 subjects(Fig. 1), increase of both amplitude and angle infour, and no changes in three. The changes oftenwere asymmetrical and in some cases the change wastransitory and the record appeared normal at the endof the test.Breath holding produced an increase in amplitude

and angle in seven of 15 subjects, increase of bothparameters in six (Fig. 2), and no changes in two.

Respiration of 100% 02 for three minutes pro-duced depression of amplitude and angle in nine of16 subjects (Fig. 1), increase in three, and nochanges in four.

Respiration of a mixture of 5% CO. in 02 forthree minutes performed on 14 subjects produceddepression of amplitude and angle in five cases,increase in four, and no change in five.

R.E.G. IN CEREBROVASCULAR DISEASE Analysis ofthe resting R.E.G. in the entire group of patientswith cerebrovascular disease was as follows: Theamplitude varied between 0.06 and 0.25 ohms, withthe majority (20 of 29) having an amplitude equal toor above the mean found in our normal material(0-15 ohms). Of the remainder, four were between01I0 and 0 15 ohms and only five had a loweramplitude than was found in the 'normal' group.Twelve showed asymmetry of amplitude rangingbetween 10 and 50% with only five patients showingan asymmetry greater than 25 %.

In 28 patients the angle varied between 750 and900 and the mean was 81.50. In only one case wasthere an angle below 750. In only two instances wereasynchronous first peaks found at rest.

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R.E.G. IN UNILATERAL OCCLUSIONS OF THE INTERNALCAROTID ARTERY Of two patients, one showed anasymmetric record with lower amplitude on theright, despite the fact that the occluded carotidartery was on the left side. The second patientshowed a high amplitude symmetrical record. Com-pression of either common carotid artery in the neckproduced bilateral depression. Since the internalcarotid artery was occluded but the external carotidartery was patent, it seems evident that the R.E.G.on the side of the occlusion was influenced by thecollateral circulation from the external carotidartery through the scalp. There was no difference intime between the two sides. According to the de-scriptions available in the literature (Jenkner, 1962),it was not possible to diagnose either of these twopatients on the basis of the rheoencephalogram.

R.E.G. IN BILATERAL OCCLUSION OF THE INTERNALCAROTID ARTERY Of three patients, one showed alow amplitude R.E.G. record bilaterally which wascompatible with the arteriographic diagnosis. Asecond patient showed a high amplitude and sym-metrical record with synchronous peaks. Carotidcompression of either side produced bilateraldepression; since the external carotid arteries werepatent, this again suggests participation of the extra-cerebral circulation in the R.E.G. wave. There wasa great increase of external circulation in the arterio-grams, and by inspection, the arteries of the scalpwere tortuous and dilated. Breath holding in thispatient produced gross depression of the R.E.G.waves. The third patient showed a high amplitudebut asymmetric record, the left side having the higheramplitude. This patient had suffered a known

thrombosis of the right internal carotid artery severalyears previously and came to the hospital becauseof recent thrombosis of the left internal carotidartery. The external carotid arteries were patent.Carotid compression on either side produced greatdepression of the R.E.G., again indicating parti-cipation of the scalp circulation in the rheoencephalo-gram. The last two cases could not be diagnosed bythe R.E.G. trace.

R.E.G. IN UNILATERAL INTERNAL CAROTID OCCLUSIONWITH STENOSIS OF VERTEBRO-BASILAR SYSTEM Ofthree patients, one showed a 50% asymmetry ofamplitude with the higher amplitude on the left.Although there was no asynchrony of the peaks,this was considered to be diagnostic of occlusion ofthe right carotid artery and this proved to be thecase. However, the right carotid artery was known tohave been occluded some years earlier and the patientwas admitted because of a recent infarct in the leftcerebral hemisphere with stenosis of the left carotidsystem and right hemiplegia. The remaining twopatients showed medium voltage symmetrical,synchronous records from which it was not possibleto diagnose carotid occlusion (Fig. 3). In one of thelatter cases, carotid compression in the neck on theside opposite the occluded carotid artery producedunilateral depression of the rheoencephalogram.When the occluded vessel was compressed there wasbilateral decrease in amplitude.

Analysis of the data for the total series of patientswith unilateral or bilateral occlusion of the internalcarotid artery (eight cases) was as follows: In twocases the R.E.G. was suggestive of the correctdiagnosis; four were not diagnostic and were in-

A RESTA"A

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FIG. 3. Rheoencephalogram in apatient with occlusion of theright internal carotid artery.

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EKG

B C02

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distinguishable from normal records; the remainingtwo showed abnormal R.E.G. records over the hemi-sphere opposite the affected carotid artery. Carotidcompression on the affected side produced bilateraldepression in three out of six instances, homo-lateral depression in two, homolateral increase inone, and no change in one. Carotid compression onthe unaffected side produced homolateral depressionin one patient and bilateral depression in the other.Jugular compression produced bilateral depressionin all cases in which this manoeuvre was performed.

Hyperventilation also produced bilateral depres-sion in all patients tested. Breath holding causedbilateral depression in two of three patients andbilateral increase in R.E.G. amplitude in one.

Breathing 100% oxygen produced bilateral depres-sion in one of three patients and no changes in theother two. Breathing 5% CO2 in oxygen producedbilateral increase in four of five patients and nochange in one.

R.E.G. IN UNILATERAL STENOSIS OF THE INTERNALCAROTID ARTERY WITH VERTEBRO-BASILAR STENOSISOf six patients, three showed asymmetry with lowerR.E.G. amplitude on the side of the stenosedcarotid artery. In these cases the diagnosis could besuspected from the trace. In two of these patientsthe peaks were synchronous. In the third, besidesthe asymmetry, the affected side showed a plateauwave with three peaks, and by measurement, theaffected side showed a shorter time interval for thefirst peak but the second peak of this plateau wavewas synchronous with the first on the normal side.Compression of the affected carotid artery producedhomolateral increase in amplitude. Of the threeremaining patients, two showed records that wereconsidered normal, having medium voltage, sym-metrical and synchronous records. The third showedbilateral reduction of amplitude with irregular wavesbut synchronous peaks that were not of lateralizingvalue.

R.E.G. IN BILATERAL STENOSIS OF THE CAROTIDARTERIES WITH VERTEBRO-BASILAR STENOSIS Of threepatients, two had normal, symmetrical, medium orhigh amplitude records. In one of these, compressionof the right carotid artery produced bilateral increasein amplitude while compression of the left carotidartery caused bilateral depression.

In the third patient, there was an asymmetrical(40%) record with higher amplitude on the left.Compression of the left carotid artery producedbilateral depression, while compression of the rightcarotid artery produced no change. In this case therewere atherosclerotic plaques of both internal carotidarteries with the left side being the more involved.

The right vertebral artery was occluded. The R.E.G.was considered misleading since it would suggestthat the severest lesion was on the less affected side.

Analysis of the data in the total series of unilateralor bilateral stenosis of the carotid artery showed thatof nine patients, the R.E.G. in three would be con-sidered diagnostic and in six not diagnostic. Carotidcompression on the affected side produced bilateraldepression in one of four cases, unilateral depressionin one, and bilateral increase in two.

Jugular compression produced bilateral depres-sion in four of seven patients, bilateral increase intwo, and unilateral increase (corresponding to theaffected side) in one.

Hyperventilation produced bilateral depressionin one of five patients, bilateral increase in two,unilateral increase in one, and no change in one.

Breath holding caused bilateral depression in twoof six patients, bilateral increase in two, and nochange in two. Breathing 100% oxygen producedbilateral increase in three of five patients, bilateraldepression in one, and no change in one. Breathing5% CO2 in oxygen caused bilateral increase in threeand no change in two.

R.E.G. IN VERTEBRAL ARTERY OCCLUSION Of fourpatients, two were considered to have normal R.E.G.records with symmetrical and synchronous, mediumamplitude waves. One showed an asymmetricalrecord with higher amplitude on the left (left vertebralartery occluded). In this case compression of theright carotid artery produced a slight homolateraldepression with great increase in amplitude on theleft. Compression of the left carotid artery produceda bilateral increase in amplitude. The fourth patientshowed an asymmetrical record with higher ampli-tude on the left. (The right vertebral artery wasoccluded.)

R.E.G. IN STENOSIS OF THE VERTEBRAL ARTERY Oftwo patients, one with right vertebral artery stenosisshowed a high amplitude symmetrical record. Whenthe right carotid artery was compressed, there washomolateral depression and with compression of theleft carotid artery there was no change. The secondpatient had bilateral stenosis of the vertebral arteries.This case showed an asymmetrical R.E.G. recordwith higher amplitude on the left and increase intime at the left. Right carotid compression producedbilateral depression and left carotid compressionproduced homolateral depression.

Analysis of the data on all the patients withocclusion or stenosis in the vertebro-basilar system(six cases) showed that three had R.E.G. recordsconsidered to be normal; one had R.E.G. depressionon the side of the occluded vertebral artery; one had

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R.E.G. depression on the side contralateral to theoccluded vertebral artery; and one record in a caseof bilateral vertebral stenosis was asymmetrical.

Carotid compression on the side of the affectedvertebral artery produced homolateral depression infour of seven instances, bilateral depression in one,contralateral depression in one, and bilateral increasein one. Carotid compression on the side opposite tothe affected vertebral artery caused homolateraldepression in one of four instances, bilateral depres-sion in one, contralateral increase in one, and nochange in one. Jugular compression producedbilateral increase in five out of six patients andbilateral depression in one. Hyperventilation pro-duced bilateral increase in two out of five patients,bilateral decrease in one, and no change in two.Breath holding caused no change in the threepatients tested. Breathing 100% 02 produced bi-lateral increase in two of four patients, bilateraldepression in one, and unilateral depression in one.Breathing 50% CO2 in oxygen produced bilateralincrease in four of five patients and bilateral depres-sion in the other one.

R.E.G. IN DIFFUSE CEREBRAL ARTERIOSCLEROSIS Offive patients, four showed medium or high voltage,symmetrical and well-regulated records that werenot different from those found in normal controls.One patient showed a low-voltage, asymmetrical(10%) record with irregular wave form and threepeaks. Carotid compression produced homolateraldepression in two of nine instances, bilateral depres-sion in two, bilateral increase in three, and nochange in two. Jugular compression produced uni-lateral depression in one of four patients, no changein one, bilateral depression in one, and bilateralincrease in one. Hyperventilation produced bilateraldepression in two of three patients and no change inthe other. Breath holding produced bilateral depres-sion in two of four patients, bilateral increase in one,and no change in one. Breathing 100% 02 causedbilateral depression in three of five patients and nochange in two. Breathing 50% CO2 in oxygen pro-duced bilateral increase in three of five patients andno change in two.

R.E.G. IN COARCTATION OF THE AORTA In one patientwith coarctation of the aorta there was a high-amplitude symmetrical record with a synchronouspeak of 415 msec. Carotid compression producedhomolateral depression. Jugular compression pro-duced bilateral depression with a diminished timecourse. Hyperventilation resulted in a great increasein amplitude with a diminished time course. Breathholding caused bilateral depression with the appear-ance of plateau waves and increased time course.

Breathing 100% 02 caused bilateral depression andbreathing 5% CO2 in oxygen caused a bilateralincrease in the rheoencephalogram.

DISCUSSION

Review of our records made with normal controlsshows that it is difficult to define which R.E.G. recordsshould be considered abnormal. All parametersvaried within such a wide range that practically allthe recordings in patients with cerebrovasculardisease, with few exceptions, could be comparedwith one or more of the normal tracings. From ourdata it would seem advisable that only asymmetrygreater than 25% and asynchronous first peaksshould be classified as definitely abnormal.The results obtained with the different manoeuvres

used were also different from those described byothers in normals (Jenkner, 1962). Carotid compres-sion produced an increase in amplitude and angle ina number of subjects and no changes in others.Furthermore, in the same subject the changes pro-duced in the R.E.G. by compression of one sidecould be much greater than those produced bycompression of the contralateral artery, suggestingthat in normal individuals the cranial circulation(external as well as internal) can be considerablygreater on one side than on the other.

Jugular compression, hyperventilation, breathholding, 100% 02 breathing, and 5% CO2 breathingproduced paradoxical results in approximately 50%of cases, that is, an effect opposite to what has beendescribed as normal. Analysis of the records inpatients with cerebrovascular disease showed thatonly in seven of them (25 %) was the R.E.G.compatible with the clinical and arteriographicdiagnosis, while in four the R.E.G. was misleading,suggesting circulatory disorder on the side oppositeto the actual side of pathology. In the rest, R.E.G.records could not be differentiated from thoseobtained in normal cases despite the fact thatproven advanced cerebrovascular pathology waspresent.The special manoeuvres performed in the study of

cerebrovascular disease also were not helpful. Infact, they produced paradoxical results in manycases and did not follow the pattern that mighthave been anticipated if the R.E.G. depended oncerebral circulation alone. Compression of theoccluded carotid artery produced homolateral orbilateral depression in five of seven instances.

Several considerations have to be made in theinterpretation of these data. Technical factors maybe partly responsible. The two-electrode R.E.G.systems being used at present in the commercially

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available machines may not be the best method ofrecording cerebral changes in impedance throughthe intact skull. However, in our experience, trial ofa four-electrode system has not overcome thesetechnical problems. Much more important is theinescapable fact that the external carotid circulationparticipates considerably in the R.E.G. waves. Thisis evidenced by the fact that there is a diminutionin amplitude by as much as 40% and decline inangle of the R.E.G. wave when a pressure cuff isinflated on the head below the placement of theelectrodes. The recordings of normal or high-amplitude R.E.G. waves on the side of an occludedinternal carotid artery must mean that the externalcarotid circulation is being recorded. In the lattercase, compression of the carotid artery in the neckdiminishes the amplitude of the homolateral R.E.G.wave. The participation of the external circulation isalso present to some degree in normal individualsbut it is increased after internal carotid occlusion bythe collateral circulation through the scalp that hasbeen shown in some of the patients by arteriographicstudies and by visual inspection of the superficialtemporal arteries. It was of interest in these casesthat no difference in time course was found betweenthe two sides of the head. It is believed that thevenouis circulation of the head must have consider-able influence on the R.E.G. curve. This is wellillustrated in the cases in which inflation of the cuffabove venous pressure (but below the systolicpressure) produced considerable change in theR.E.G. waves, usually an increase in amplitude.The variability of the results of the different tests

(breath holding, jugular compression, hyperventi-lation, etc.), both in normal subjects and in patientswith cerebrovascular disease, also enhances the im-pression that there are several factors, besides thecerebral arterial circulation, that participate in theproduction of the R.E.G. waves. Until these can bebetter understood, R.E.G. recording appears to havelittle clinical usefulness in the diagnosis of cerebro-vascular disease.

SUMMARY

With techniques available commercially R.E.G.swere made on 19 control subjects and 28 patientswith proven cerebrovascular disease. After analysisof these data it did not appear that the R.E.G. is atechnique which provides reliable or consistentclinical information of use in the diagnosis of cere-brovascular disease. It seems evident that a signi-ficant proportion of the pulse wave is derived fromextracranial blood flow. Despite the ease of itsapplication and its harmlessness, more basic infor-mation is required to define the complex factorsinfluencing the R.E.G. waves as recorded from theintact scalp. Only then may its clinical usefulnessbecome apparent. At present, rheoencephalographyis of no obvious diagnostic value and may actuallybe misleading in the diagnosis and localization ofcerebral vascular disease.

REFERENCES

Auinger, W., Kaindl, F., and Neumayr, A. (1953). Z. Kreisl.-Forsch.,42, 104.

Colonna, L., and Ricciardi, G. (1957). Minerva med., 48, 3210.Garbini, C., and Poppi, A. (1957). Ibid., 48, 3212.Gastaut, H., Rodler, R., Lechner, H., Bostem, F., and Naquet, R.

(1959). Path. et Biol., 7, 1859.Gentili, C., Garbini, G. C., and Orlandi, G. (1957). Minerva med., 48,

3216.Holzer, W., and Polzer, K. (1945). Anz. Akad. Wiss. Wien, Mathem-

Naturw. Ki., 82, 17.Jenkner, F. L. (1957). Wien. klin. Wschr., 69, 619.

(1962). Rheoencephalography. A method for the continuousregistration of cerebrovascular changes. Thomas, Springfield,Illinois.

Kaindi, F., Kraus, H., and Partan, J. (1955). Zbl. Neurochir., 15, 6.Kunert, W. (1961). Nervenarzt, 32, 34.Lifshitz, K. (1963). J. nerv. ment. Dis., 136, 388.Mann, H. (1937). Proc. Soc. exp. Biol. (N. Y.), 36, 670.Nyboer, J. (1944). In Medical Physics, edited by 0. Glasser et al.,

Vol. 1. p. 340. Year Book Publishers, Chicago.(1959). Electrical Impedance Plethysmography. The electricalresistive measure of the blood pulse volume, peripheral andcentral blood flow. Thomas, Springfield, Illinois.

Oehninger, C., Stanham, J., Cortes, R., Fiandra, O., and Ferrari, M.(1955). An. Fac. Med. Montevideo, 40, 205.

Orlandi, G., Garbini, G. C., and Gentili, C. (1957). Minerva med., 48,3225.

Polzer, K., and Schuhfried, F. (1950). Wien. Z. Nervenheilk., 3, 295.Pratesi, F., Nuti, A., and Sciagra, A. (1957). Minerva med., 48, 3223.Serra, P., and Urso, L. (1956). Minerva anest., 22, 86.Spunda. C, (1955). Wien. klin. Wschr.. 67, 788.

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