|Low frequency volume fluctuations; Rheoencephalography;
Pulse amplitude; Normalized spectrum analysis
|At the end of the eighteenth century it was demonstrated that
both intracranial cerebrospinal fluid (CSF) and cerebrovascular blood
circulation (CBS) could be functionally characterized as periodically
fluctuating systems not connected to either heart or respiratory
activities [1,2]. However, some decades later the question was arose
whether these volume fluctuations are inherent only to the cranium as
they appear in open skull situations. The reason for this question was the
use of new technology and observations on animal brain surfaces (with
cats and dogs) through hermetical transparent skull “windows”. These
studies indicated the absence of heart and respiratory pulsations but
other independent volume fluctuations inside a closed cranium [3,4].
At the same time, some investigators, based on their experimental data
received through data collected by other instrumentation, supported
the concept of the presence of slow liquid volume fluctuations inside an
intact craniospinal cavity [5-10].
|The next step in the study of slow fluctuations inside the craniospinal
cavity was made possible by developing special experimental techniques
based on advances in electronic hardware over 50-60 years. This
period could be regarded as the start of systematic investigations into
slow intracranial fluctuations. For this purpose direct high frequency
alternating currents were used. Based on direct current polarography
the methods for recording of oxygen and hydrogen availability (pO2
and H2) in brain tissue were constructed. Recordings of pO2 are
based on inserting insulated fine gold or platinum wire electrode with
small working surfaces directly into brain tissue. The second referent electrode with a comparatively larger surface was then placed on the
head’s skin surface. Both electrodes were connected with a direct
voltage source (0.3-0.8 V). The sending electrode was connected with
a minus charge and the referent electrode with a positive charge. The
value of current in this circuit will be proportional to pO2. Using
this method over a number of years considerable data was collected
regarding pO2 fluctuations in human and animal brains. These studies
used frequencies of 0.3–0.7 Hz and were conducted under different
physiological situations [11-15]. If +0.3 V voltage is applied to the same
platinum electrodes, it is possible to record pH2 as an indicator of local
CBF using hydrogen clearance methods.
|These recordings of pO2 represent slow fluctuation with related
frequencies, and are widely used for the quantitative measurement of
local CBF in animal experiments [16-18]. They are also used in clinical
observations during neurosurgical procedures . It is very important
to note that fluctuations of pO2 and pH2 in animal and human
recordings look quite similar.
|Recording of high frequency electrical impedance (15-100
kHz) between plate electrodes – (1.5–2 cm2 in area), placed on the
human head allows the researcher to observe slow fluctuations. These
fluctuation measurements are determined by the difference in the
electrical conductivities of blood, CSF and brain tissue [20-22]. This
method, called Reoencephalography (REG), allows the recording of
three kinds of fluctuations – connected with heart activity, respiration
and very slow fluctuations. The latter are called waves of the third order.
Over two decades considerable data concerning the physiological and
medical relationship of fluctuations, primarily related to pulsations
from heart activity, has been analyzed. A number of indexes of
REG pulse changes have been proposed to calculate the basis of the
cerebrovascular parameters [23,24]. However, for a few years, interest
in the REG method became non-existent. This was due to the fact
that the true meaning of these indices was accurate only for a narrow
range of physiological and clinical conditions. Respiratory waves and
waves of the third order didn’t attract attention from medical scientific
investigators due to the problems connected with accurate and timely
|Over 60 to 70 years other methodologies were developed which
could also record slow intracranial fluctuations. These include a group
of mini-invasive methods which could be applied to neurosurgical
patients – thermography, miniature intracranial pressure transducers,
implantation of gold and platinum wire electrodes (100 μ or less) for
recording brain oxygen levels and CBF by the hydrogen clearance
method [13-15]. All these methods make it possible to record slow
fluctuations of intracranial origin.
|Background of fluctuations
|The methodologies used in different research studies rest
conditions and under functional tests in healthy volunteers. In some
cases of pathology; as well as in animal experiments, this experimental
material provided the basis for certain conclusions about liquid volume
fluctuations inside the craniospinal cavity. These studies indicate that
intracranial slow fluctuations have multiple sources.
|Firstly, there are fluctuations reflecting control processes in the
blood circulatory system that are responsible for controlling arterial
pressure and activating periodical changes in arterial pressure. These
have variations in amplitude of (0.1-1.0 of stroke pressure) and
frequencies of 3-15 cycles per minute at rest conditions. There are also
slow arterial pressure fluctuations with irregular time durations [25,26].
Volume fluctuations are controlled by periodical pressure changes in
the craniosacral systemic hemodynamics. Local fluctuations inside the
craniospinal cavity are characterized by a high level of heterogeneity
and may be independent even in much localized brain tissue. Some
role in the origin of the global volume fluctuations may be due to
periodical CSF movements . This mechanism may be responsible
for maintaining the metabolic balance in the Central Nervous System
for all body organs. CSF’s role in organizing the function and structure
has only been investigated recently .
|Secondly, changes in central venous pressure (CVP), controlled by
intercostals and diaphragm respiratory muscles movements, and are
distinctly different below and above the diaphragm. Cardiac activity also
initiates pulsations in the jugular veins; and these controllers interact.
Thus the final pattern of fluctuations is a combination of these forces.
One more factor, which enters into these fluctuations, is the pattern of
pulsations in the cranial and lumbar regions of the spinal cavity itself . Respiratory fluctuations reflected in the central venous pressure
have different phases caudally and cranially from the diaphragm
movements . Similar to the jelly brain tissue macrostructure this
allows the spinal fluid to be freely displaced, [31,32] and may allow CSF
replacement in different craniospinal cavity compartments. Total liquid
volume fluctuations may be integrated at different cranial and spinal
cavity levels with overall liquid volume balance occurring by regional
|Thirdly, there are local periodical slow fluctuations. These have
been observed in human studies in patients (with gold wire electrodes
100 μ in diameter and a 1-1.5 mm open insulation tip, and with a
distance between them of 3–5 mm, implanted directly into brain
tissue). These electrodes allow the recording of slow fluctuations
of pO2 and blood volume in small local areas of brain tissue. Slow
fluctuations of electrical impedance fluctuations, which correspond
to blood volume changes, have also been recorded and displayed. It is
important to emphasize that manifestation of these fluctuations may
be represented not only by amplitude changes, but also by fluctuations
in the amplitude of pulsations which completely disappear for a few
seconds. This phenomenon, described some decades ago , has
been explained about 20-30 years later as the effect of fluctuations of
perivascular gaps and changes in volume of extracellular liquid [33-35].
These observations have been confirmed in animal experiments (rat,
rabbit, cat), using 30-40 μ electrodes. This research shows the similarity
of pO2 and electrical impedance slow fluctuations. The studies also
demonstrate the differences between fluctuations recorded with a 0.5-
2.0 mm distance between the electrodes . The use of a pH2 clearance
method, electrochemical generation of hydrogen in brain tissue of
narcotized rats, shows confirms similarity of slow fluctuations received
by electrical impedance and pH2 clearance methods Importantly,
the frequency ranges of slow fluctuations are in a narrow band for all
investigated mammalians – 0.1-0.4 Hz .
|Fourthly, the high level of localization of slow volume fluctuations
in brain tissue allows us to predict that this type of fluctuation represents
processes in brain tissue connected with the brain’s functional activity
and metabolism. Direct evidence of this is the appearance in some
experimental conditions with conscious cats (in a darkened room)
of the close correlation between cortex electrical activity (15-24 Hz),
electrical impedance and pO2 with frequencies of 10-15 cycles/min in
the visual cortex . Human observations demonstrate an increase
in the frequency of slow fluctuations as recorded by REG during
emotional excitation and evoked stress situations . The fluctuations
decreased with decreased brain activity as observed after brain injury
 and they ceased entirely during narcosis. It was also observed that
slow fluctuations did not change during significant vasodilation when
the vasodilation was not influenced by sympathetic stimulation or
inhibition. Fluctuations decreased during hypercapnia and increased
during oxygenation. [14,15,38,39].
|Fifthly, some biochemical indices also demonstrate slow fluctuation
activity. This was observed in experiments with unanesthetized cat and
rabbits. These experiments showed that cortical cytochrome oxidase
redox state (CYT) and cortical blood volume (CBV) normally oscillate
at 0.4 to 0.5 Hz. These continuous complex oscillations represent
fluctuations of the cortical metabolic rate. Their frequencies are varying
over time, and indicate interhemispheric synchronicity between
distances of 50 mm in 2 cortical regions . This data allows us to
conclude that localized slow fluctuations are connected with metabolic
changes in brain tissue; and is closely connected with the functional
activity of specific local brain regions. It is important to emphasize, that the resolution between neighboring brain regions may change
according to brain functional activity. This conclusion is illustrated
by data of cross-correlation analysis of pO2 fluctuations taken from
electrodes in associated areas of the brain cortex with distances of 2-3
mm between them, during a 2 minute mental functional test (Raven
matrix). Before test the coefficient of cross-correlation between slow
pO2 fluctuations is about 0.5– 0.7; during test after 1 min it increases
up to 0.7–0.9; after 2 min, at the end of test, is between 0.7–0.8 , and
2 min after finishing of the test this coefficient decreases back to the
original test value. Reaction to this test taken from surface (cortex) and
deep (white matter) electrodes are different .
|Categories of fluctuations
|Collected data and their analysis shows that slow fluctuations should
be divided into three categories from the point of view of initiation:
|1) The most global slow wave activity inside the crania – spinal
cavity is initiated by changes of indices in the central hemodynamics:
systemic arterial and venous pressure, which have periodic components
connected with control processes of the blood circulatory system. Some
modulations of volume/pressure relations inside crania - spinal cavities
may cause periodical global CSF articulatory movements of the skull
bones and as a result, changes in the skull dynamics and its internal
|2) Changes of blood volume internally were evoked by vascular
volume changes initiated by periodical processes of the heart and
respiratory activities. They basically have regional peculiarities,
determined by pressure/flow indices, which are different in magisterial
arteries and veins, supplying and removing blood from different regions
in cranial and spinal cavities.
|The one and two models have now been investigated many times,
as has the analysis of REG pulsations used in practice, but the third
model is still unclear. This modeling was of interest to our investigators,
mainly due to the absence of knowledge until recent times
|Aim of paper
|Aim of this paper focuses on the study of slow fluctuations
connected brain functioning, which reflects the fluctuations of
brain blood volume and CSF and consequently CSF circulation.
Recently, REG methodology was applied to healthy persons at rest
conditions, during different physiological functional tests and in
some cases of pathology. Experiments with awake rabbits have also
been conducted. For quantitative analysis of intracranial slow volume
fluctuations computerized analysis of spectral analysis of fragments
of REG continuous recordings were used to evaluate their spectrum
characteristics in ranges of 0 – 0.4 Hz (5–15 cycles per minute), provided
under different conditions. For further evaluation of the data received,
Transcrsnial Dopplerography (TCD), and respiratory chest movements
(Resp.) were also simultaneously recorded.
|There have not been to date direct methods to quantitatively record
periodical changes in fluctuations of the liquid media inside closed
crania-spinal cavities. In principle, it may be acceptable for these
purposes to use MRI methodology. However, the application of MRI
is accompanied by some problems (technical analysis subjectivity,
absolute subject/patient immobilization, the high price of equipment,
difficulties with receiving an accurate data signal for analyze by spectral
methods). Therefore for the purpose of this present study it was
necessary to select a methodology, which was direct and could provide
comparative data dynamically, and also be applied multiple times
safely to the same patient. Additionally, it should be relatively easy to
use for complex investigations. Rheoencephalography - REG method,
in its most modern version was chosen as the most acceptable. REG
methodology provides comparatively quantitative spectrum analysis,
due to its calibration capabilities, using as a standard unit value of
amplitude the pulse spectral line
|The REG method is characterized by a number of important and
useful properties. As has been shown by special investigations [14,42],
the REG method allows the monitoring of changes in blood and CSF
volumes in the brain, where distribution of an electrical field is initiated
by electrodes placed on the skin surface of the head.. By varying the
electrode placement measurement configuration of the head could be
changed. Therefore, REG allows the investigation of different regions of
the cranial cavity. REG does not create any biological influence by the
electrical current applied and therefore it is possible to provide multiple
observations of the same subject.
|In this study we have used a new REG modification –
Multifrequency (MultiREG), which permits simultaneously recording
on three frequencies: 16, 100 and 200 kHz - manufactured by “MISTAR”
(Russian Federation). This unit provides information concerning the
water content of brain tissue and additional information reflecting
intracranial water volume changes. In this study the MultiREG goal was
to find the optimal conditions to receive valid spectral diagrams and to
establish any factors which could be involved in changing its pattern. All
investigations were conducted with fronto-mastoid electrode position
for both hemispheres to evaluate spectrum hemispheric asymmetry.
|REG and TCD coupling
|For purpose of evaluation of CSF mobility, a fragment of recording
was selected for spectrum analysis, using the MultiREG, TCD in
basement of MCA by “MultiDop” (DWL, Germany) and chest
respiratory movements by a specially constructed chest band. By such
selection of MultiREG electrodes and TCD probe positions the current
distribution inside the cranium includes the entire brain region and
separately each hemisphere, which is supplied by blood through the
MCA. This allows us to receive comparable data for different subjects
and different physiological conditions and to compare the MFREG
fluctuations with linear blood velocity in the basement of the MCA.
|Animal experiments were deemed necessary for receiving additional
data at two directions to investigate how common results of spectral
humans and animals are. Results of such investigations are opening
the possibilities to study with animal such particular conditions, which
is may be unacceptably in human investigation, for example a few
minutes hypoxic condition or to test effect to cerebrovascular control mechanisms of some drugs. For investigations healthy animal – rabbits
have been selected, due to their relative placid nature compared with
other laboratory mammalians and to use awake rabbit without any
premedication. MFREG electrodes (2 mm x 3 mm) will be placed
bilaterally to chemically depilated skin on animal head and softly fixed
by rubber bandage. It was possibly to obtain in gently restrained awake
rabbits comparatively long (2-5 minutes) a good quality fragments of
recordings at rest and under of some functional tests with different
respiratory gas mixtures – pure oxygen, hypoxic (7-8% oxygen), carbon
dioxide (7-8% CO2 in air). The study was performed in accordance
with the Declaration of Helsinki, with institutional ethics committee
approval, and all subjects provided written informed consent.
All experimental procedures conformed to recommendations of
Physiological Section of Russian National Committee on Bioethics and
were approved by local Institutional Animal Care Committee.
|Data outputs from all instrumentation were connected with PC
“Windows XP” via an analog-digital transformer “PowerLab-5” (AD
Instruments, Australia). The PowerLab software provides spectral
analysis of all recordings in a wide range of frequency bands. Two
spectrum fragments were used: one included frequencies 0.0–2.0 Hz;
together with slow fluctuations, pulse and respiratory fluctuations as
well as fluctuations which were related to the subject’s central arterial
pressure. This fragment was used for calculating the value of separate low
frequency spectral components by comparing with the REG pulse as a
standard signal, and which was designated as 1.0. In rabbit experiments
the most stabile and pronounced are respiratory chest movements and
their amplitude was designated as 1.0. The second fragment includes
frequencies which reflect slow fluctuations of intracranial liquid volume
changes. For low frequencies these are limed by fluctuations of the
central arterial pressure, which are 0.02–0.15 Hz. For high frequencies
these are limited by chest respiratory movements, which are 0.12–0.8.
Because frequencies of central arterial pressure and respiratory chest
movements limits these fluctuations may vary. In some cases low limits
of fluctuations of intracranial origin may be superimposed by slow
arterial pressure fluctuations and upper limits may by superimposed by
respiratory waves. Therefore, for clarity we compared spectral diagrams
fluctuations of intracranial origin with spectrum diagrams of arterial
pressure and the respiratory spectrum diagram, calculated at the
same frequency ranges 0.1–0.4 Hz (Figure 1) In animal experiments
the low frequency component may be superimposed with arterial
pressure fluctuation only, because the respiratory rate in rabbits is too
high 1.0–1.8 Hz. There are no noninvasive methods to record changes
of arterial pressure in rabbits, but indirectly these fluctuations are
reflected on ECG recordings (Figure 2), because the basis line and heart
rate are connected with changes of arterial pressure. It could give some
expression, concerning this kind of slow fluctuations.
|From Figure 1 and 2 the quality of spectral diagram may be different
and the interval between components depends on frequency of
quantification. On Figure 1 it was 1024 points to 1 inch of recording, on
Figure 2 – for 1 inch - 124.000 points. Of course there are extreme limits
of quantification and for every particular case it is necessary to select
two values – length of fragment of recording and rate of quantification
depends on the physiologic process under investigation.
|Selection of fragment of recordings for analysis.
|Significant problem is the selection of fragment of recording
for investigations. The fragment should be long enough to reflect
accurately the details of the process under investigation. As is have shown on Figure 3 – the length of the fragment for this present study
– to investigate peculiarities of slow fluctuation of intracranial origin,
and should be about 3 minutes. The fragment should be long enough
to reflect accurately the details of the process under investigation. As
is have shown on Figure 3 – the length of the fragment for this present
study – to investigate peculiarities of slow fluctuation of intracranial
origin, and should be about 3 minutes.
|Selection of rate of quantization
|The role of frequency of quantification depends on quality of the
spectral diagram. With the decrease of frequency of quantization the
resolution of the analyzed display of the spectral diagram also decreases.
With decrease of quantification some details of spectral diagram are
missed, but fortunately up to some limits of frequency decrease it is
possible to see a general pattern in the spectral diagram. However, with
further decrease of frequency it is lost (Figure 4). This shows that the
lower limit of frequency of quantification for this particular subject is
about 32 k, and when the quality of the spectrum diagram is severely
depredated. The data quantification, presented in Figure 3 and 4, show
that the results of spectrum analysis and, therefore, its informational
meaning, critically depends on the optimal selection of the length of
analyzed recording fragment and the frequency of quantification for
every particular purpose of investigations. The present study length
of the investigated fragment has been selected at 160-180 seconds and
quantification 124 k. This limits the length of the functional tests used
in our investigations. Their duration should definitely be longer than 3
minutes and during all investigations the subject under investigations
should be at rest and passive. In some cases, depending on the particular
purpose of the investigation and its particular condition, it is possible to use a shorter fragment and less quantification, but the possibility of loss
of information should be taken into consideration.
|Materials and procedure of investigations
|Investigations were conducted on 27 healthy volunteers of both sexes
with age ranges of 17-25 years. The data were collected at rest, during
short time tests (apnea, hyperventilation, Stookey test) and long period
test - tilting head down 17o up to 30 min., as well as in experiments on
12 rabbits. Investigations were conducted with 27 healthy volunteers
of both sexes with age ranges of 17-25 years. Data was collected at rest
and for long periods (tilting head down at 17o) for up to 30 minutes.
12 rabbits were used for the animal experiments. Specific functional
tests were selected for the best quality spectrum analyses, as it requires
fragments of recordings at steady state conditions. The subject’s
system under investigation normally adapts to changed physiological
conditions in about 3 minutes. Observations how, that spectral diagrams
are very changeable and it is no reasons to apply statistical methods for
their analysis. Indeed, spectral diagram in frequency interval 0-0.3 Hz
include number of peaks of spectral lines but their position is vary. If
they will be averages, individuality of peaks will disappear. Investigation
with 27 persons show, that for all of them spectral diagram is similar,
but positions of particular peaks may vary. This permits to conclude
that low frequency spectral diagram is similar for healthy persons, but
not the same. It is necessary in future to develop quantitative methods
for evaluation of informational meaning of low spectrum peaks. It
is possibly to provide by two ways – to look for correlation between
particular the special lines and other physiological indices and to look
for correlations of particular spectral peaks on recordings, provided
on different frequencies of REG recording under different conditions,
for example, functional tests. However, investigations with functional
tests, used at this study takes to provide about 3 min recordings just
before test – for evaluation of spectrum diagram at rest conditions, then
recordings during the functional test, also about 3 min and the same
fragment of recording for post test period. The recordings are necessary
to evaluate influence of functional tests.
|Based on our results from the present investigations and previous
research investigations of intracranial liquid volume changes,
fluctuations are best studied and measured by high frequency (15-
200 kHz) impedance methodology. This method more accurately
measures the changes in the liquid media in the cranial cavity when
electrical current is passed between electrodes placed on the human
head. By varying the electrode position it is possible to change the
current distribution inside the cranium and therefore the cranial region
to be investigated. Taking this into account in all human impedance
recordings, the bi–fronto-mastoid MFREG electrode positions were
selected. We reviewed the results of spectral analysis in the frequency
ranges of 0–0.3 Hz, with 180 second fragments of MFREG recordings.
This frequency band included slow fluctuations in both human and
animal subjects . Recordings fragments should be selected without
artifacts and recorded on a 16, 100 and 200 kHz current frequency.
The middle frequency – 100 kHz is commonly used in impedance
investigations and comparing it with data received at 16 and 200 kHz
it is possible to investigate current distribution between intracellular
and extracellular spaces. The analyzed displays are very close. Heart
pulsation and chest respiratory movements are represented in both
spectral diagrams, but due to different heart and respiratory rates
they display differently. However, the narrow spectrum components of humans and animals look very similar. This gives support to
suggestions that spectrum components of humans and animals, which
are not connected with heart activity and respiration, have a similar
physiological background. The last observation is important not only
from a physiology point of view, which indicates the similarity of
metabolic processes in the human and animal brain, but that these
waves represent the liquid volume slow fluctuations of intracranial
origin. They also offer the opportunity to study these fluctuations
using data collected non-invasively from human and animal
investigations. This significantly expands the possibilities for studying
this physiological phenomenon, because it provides data from either
human or animal observations. This type of study can use different
physiological conditions and record determined physiological indices,
some of which it is possible to observe with humans, but others only in
|The study of the structure of the spectral diagrams at rest and
under different physiological conditions indicates that we deal with two
types of investigations. One is based on the comparison of spectrum
components of MFREG with the spectrum components of other
physiological processes of similar frequency also at rest conditions.
The second is based on comparing spectral diagrams of recordings,
taken under changed of physiological conditions, which could be help
in understanding the background slow volume fluctuations inside the
cranium. It is more convenient to collect data from human observations
in one case, in the other animal experiments, based on the above
mentioned fact, concerning the similarity of low frequency spectrums
in human and animals (rabbits), which allow us to predict that these
slow fluctuations have a common origin in mammalians with a brain
metabolism close to that of humans.
|The comparing of spectrum diagrams of slow fluctuations with
indices in human persons at rest for about 180 seconds fragments of
simultaneous recordings of MFREG, (recorded on three frequencies –
16, 100 and 200kHz), together with TCD and respiratory fluctuations
are presented as spectrum diagrams in Figure 5: Firstly, it is necessary
to mention that the MFREG spectrum, taken from the left and right
hemispheres, generally look different and some lateral peculiarities
are clear. For example, the general pattern of the spectral diagram of
the left and right hemispheres looks dissimilar. Some details are of the
spectrums and their relations to the spectrum diagrams of TCDG and
respiration, obtained on different REG frequencies (this is have shown
by arrows at Figure 5). Some additional groups of spectral lines appear
to come from only one hemisphere. There are also other differences
in the displayed analysis, which belong to only one hemisphere.
Comparing the low frequency spectrum of the MFREG recordings at
different frequencies sees this.
|The current distribution between the electrodes, fixed on the human
head is different due to the frequency and electrical conductivity of the
liquids and tissue contained in the cranium. It is possible to see this in
Figure 5, by comparing the MFREG spectrums recorded on different
frequencies. This shows that there is left/right hemispheric asymmetry
of slow fluctuations, as was observed earlier with hemispheric
asymmetry of pulse CSF mobility and Cranial Compliance . The
similarities and differences of the peaks of dynamics in physiological
processes depend on a number of factors. It also indicates once more
that the crania-spinal space, filled with liquids, has some structural
peculiarities responsible for the difference of hemispheric CSF mobility, hemispheric cranial compliance and as a consequence of all of this –
the hemispheric difference in the spectral diagram of liquid volume
|For groups of peaks - red arrows - the most pronounced low
frequency peak belongs to TCD, which accompanies the MFREG
wave. The second peaks of the MFREG are in most cases larger than
the TCD peak and look different at both different frequencies in each
hemisphere. This indicates that the value of the second peak of MFREG
depends on some other physiologic process.
|Peaks of spectrum belong basically to MFREG and may vary,
but not significantly. The limits of variations of MFREG peaks at rest
conditions are 0.3–0.7, which were compared to the value of heart
pulse of MFREG, measured on a corresponding frequency and taken
as 1.0. Comparison of Spectral components of MFREG, recorded
using different frequencies, show that they have some differences, most
pronounced if compared to MFREG spectrums taken on 16 and 200
kHz and MFREG on 100 kHz. Where hydration of the brain may take
place, both 16 and 200 kHz REG spectrums will be demonstrated, but
only one REG spectrum on the 100 kHz frequency is shown in our
|Variation of spectral diagram
|Respiratory changes are dominant on the spectral diagram and
MFREG spectrum diagram at higher than 0.3 Hz and usually correspond
to the spectrum diagram of respiration (see green arrows on Figure 5).
Therefore, it is possible to say that below 0.1 Hz and higher than 0.3
Hz REG spectrum diagram generally depend on extra cranial factors.
Between these limits physiologic processes of intracranial origin, as
reflected by MFREG spectrum analysis, determine liquid volume
fluctuations. Significant information could be received from analysis
of changes in Spectral MFREG, recorded on three frequencies. TCD
and respiration component changes during functional tests, which are
directed to the cerebrovascular and CSF systems, also provide valuable
information. However, take into account that to achieve a good quality
low frequency spectrum diagram, as in Figure 3 and 4, it is necessary to
have for analysis about 180 seconds of continuous recording, practically
without interference. Significant changes in spectrum diagrams can be
observed by comparing data, received with aging subject (Figure 6)
there are some common spectral peaks for all investigated subjects, but
the maximal value of the spectrum diagram is different. With age the
general value of peaks on the spectral analysis diagram decrease. This
may be connected with a decrease in activity of some brain metabolic
|The most significant for the study of low frequency slow fluctuations
is to evaluate their changes under different physiologic conditions. For
this purpose functional physiological tests are used. However, in brain
circulatory physiology a short period of time - about 30 seconds in
duration of a functional test - We used Stookey and Valsalva tests, apnea
and hyperventilation. However, from the point of view of methodology
for low frequency spectrum analysis it is necessary to take for analysis
fragments of recordings of about 3 minutes to evaluate the majority of
peculiarities of the spectrum diagram in frequency ranges of – 0.3 Hz.
Most of aforementioned functional tests are not valid for long term use,
and the orthostatic test with head down tilt could only be applied for
a period of number of minutes (results are shown on Figure 7.) It is
possible to see on Figure 7, which after 7 min. from the beginning of
tilting the amplitude of spectral lines increases at 16 kHz, but decreases at 100 and 200 kHz. This indicates that some redistribution between
blood flow control inside and outside of skull has taken place.
|Comparative changes of MFREG and TCD
|The changes of TCD spectral diagrams on Figure 7 also change
but not corresponding to the MFREG changes. This data shows that
the increase of blood pressure in the upper Cava vein evokes changes
in the spectrum diagram, reflecting liquid volume fluctuations inside
the cranium. It requires the specific study of identifying features to
determine the physiological meaning of each spectral peak on the REG
analysis diagram, though we are now realizing the importance of this
|The possibilities for long-term functional tests in experiments
with animals are much more feasible. It is possible to apply hypoxic
(about 5-7 % of oxygen in area), hypercapnic (7-10 % CO2 in area)
and hyperoxic (about 100% O2) functional tests. These tests have been
used for evaluating changes at low frequencies evoked by functional
tests to provide similar measurements in healthy adult rabbits (2.5–
3.0 kg) under non-invasive conditions. For MFREG recordings two
plate electrodes (2 x 4 mm) were fixed bi-occipital on the skin of the
animal’s head (Fur had been chemically removed) Together with two
frequencies of MFREG, the rate of respiration and ECG was measured.
The overall condition of the animal under these conditions showed that
the low frequency spectrum of MFREG, demonstrated above, is similar
to that of a human. However, the differences between data obtained at
different REG frequencies is not so pronounced in rabbits as in humans,
perhaps due to the comparatively thin skull bones of the investigated
animals. Therefore, experimental measurements were limited only to
data recorded using 16 and 100 kHz frequencies.
|The data presented in Figure 8 shows significant changes in
response of spectrum diagrams to changes of the respiratory gas
mixture, This is due to ability to measure long-term changes especially
comparatively long were hypoxic and hypercapnic tests. It was shown
that after a period of adaptation to the applied functional tests (6-7 min
after their start), some definite changes in the spectrum diagrams of 180
second duration REG recording could be observed. After pure oxygen
inhalation the line values, as represented in the spectral diagram,
basically decreased between 20-50 %, particularly at frequencies of
0.15–0.25 Hz. Inhalation of hypoxic gas mixture is accompanied
by some increase of line values (30-35%), displayed in the spectral
diagram by a decrease in the number of peaks in the analyzed spectrum
interval. During inhalation of 7.5 % of CO2 the area value of spectral
lines increased from 20-45 %, especially in frequencies of 0–0.2 Hz. The
data showed that inhalation of the above described gas mixtures evoked
definite changes in the low frequency spectrum, different for each of the
tested gas mixtures. The reason for these changes in the low frequency
spectrum diagram may be based on the influences of respiratory gases
on brain tissue metabolism and also direct influences on the control
mechanism of the brain circulatory supply. Physiological data support
both of these.
|Discussion and Conclusion.
|All the above data and diagrams definitely show that the use of
spectral analysis of slow liquid volume fluctuations is a powerful method
for the quantitative measurement of fluctuating physiologic processes
with changes in ranges of wave amplitude and frequency. Fluctuations
at different frequencies, mainly of low frequencies below that of the
heart activity and respiratory chest movements, and not of an electrical
origin, accompany numerous physiological processes; particularly in the functional circulatory systems and in the relationships between
them. The phenomenon of slow fluctuations has been known for more
than a century. Until the present time, studies of slow fluctuations were
mainly conducted occasionally, and didn’t form an informational index
in physiology or medicine. At that time only one branch of medicine
– Osteopathic Medicine - was interested in slow fluctuations of an
intracranial origin. However, the absence of instrumentation for the
quantitative evaluation of slow fluctuations only allowed evaluation by
osteopathic manual methods. A mathematical method for this purpose,
the Furrier analysis, was then developed.
|In the 21st century, fast and simple computer methods for the
calculation of the spectral diagram have been developed. The data
presented above is one of the first investigations of intracranial liquid
volume fluctuations. Of course, it was impossible to give a detailed
explanation of all the analyzed features in the low frequency spectrum.
However, it is possible to conclude that the frequency ranges for slow
fluctuations of intracranial origin are limited generally to 0–0.3 Hz. It
is important, for representatives of mammalian species – rabbits – that
frequency limits for slow fluctuations of an intracranial origin are nearly
the same as for humans. It is now possible to study slow fluctuations,
using experiments with mammalian animals.
|It is also now possible to determine a number of factors, which
could be responsible for the intracranial liquid slow fluctuations. It is necessary first of all to remember that intracranial liquids are a passive
media with no internal forces for motion. The liquids could however
move when external forces are applied. From this viewpoint, for
intracranial liquid volume slow fluctuations a major source of force is
the cerebrovascular system; particularly different local levels of changes
to vascular tone due to changes in the functional activity of brain
tissue, change in brain metabolic processes, and changes in the general
circulatory and respiratory systems. All these factors are reflected in the
spectral diagram of intracranial liquid volume slow fluctuations. The
most interesting aspect of these studies was to establish correlations
between the amplitude of particular spectral lines and some indices
of brain metabolism. Our recorded data indicates that intracranial
low frequency volume fluctuates in ranges of approximately 0–0.3
Hz and reflects general brain metabolic processes with some of their
peculiarities. Spectral indices of liquid volume fluctuations and their
spectral diagrams may also reflect their quantitative features, which
may be significant for neurophysiology and clinical applications.
|This research data indicates that it is possible to observe a
correlation between brain volume slow fluctuations and its cognitive
function. Support for this statement could be investigations of brain
blood volume fluctuations of astronauts during acceleration testing
, which demonstrated an increase of amplitude in slow fluctuations
related to emotional load. Comparison of spectral diagrams for persons
with age dependent decreases in cognitive function in middle age (45-
55 years) and elderly (after 75 years with symptoms of dementia) with
brain circulatory insufficiently , show that cognitive dysfunction of
some of these subjects compared with healthy people as determined
by psychophysiological “Prognosis” method  is a general decrease
in amplitude and position of some groups of spectral lines. This data
demonstrates that slow brain volume fluctuations closely connected
with a change of brain function is both a metabolic activity and a
|This current research shows that intracranial slow volume
fluctuations in frequency ranges from 0.0 to 0.3 Hz reflect complicated
physiological processes in brain tissue and changes in the brain
circulatory system. The spectrum diagrams display a quantitative
measurement of these processes. It is important to take into account
that the quantitative representation of slow periodic fluctuations of an
intracranial origin is a prospective approach for the study of the control
processes responsible for the circulatory-metabolic support of healthy
brain function. The complex character of the low frequency spectrum,
reflect intracranial processes that include a number of single control
links and structural components and a comparison of low frequency
spectrums under different experimental conditions and models of
pathology. This methodology has two important advantages. The first is
the possibility to compare human and animal physiological models. This
is significant, because some issues are only possible to study in animal
experiments, while others – require human observation and common
aspects for comparing results. This is now possible with low frequency
spectral analysis. The second possibility is to study simultaneously the
physiological control mechanisms characterized by particular slow
fluctuations and at the same time associated biochemical processes
fluctuations. It is necessary to provide the specialized computer
analyses for identification of spectral lines, which is different for
different physiologic processes, but these results look very encouraging.
One possible way may be to base this on a comparison of low frequency
spectrums of intracranial origin at rest, with the same subject, with a
spectrum of other fluctuating processes taken in different experimental
|Thus, spectrum analysis opens the way for a new noninvasive
methodology for investigating the complicated physiological processes;
responsible for the brain functioning and the mechanisms that control
brain metabolic supply. This could clarify treatments aimed towards
healing brain dysfunction with the possibility that the spectrum
approach used in this study of low frequency intracranial fluctuations.
It looks real, that these changes may provide definitive results, indicate
initial functional changes of serious pathology. Described above
spectrums of slow fluctuations are similar to that of the applications
of light spectroscopy to chemistry and physics. So, that is one of
directions of wide spectral methodology, already used at the some
branches of natural sciences. This technology may also demonstrate
new perspectives for application to modern biophysical questions,
based on the discovery of presently unknown control systems not only
inside cranium, but in other areas of the body.
|Supported by RFFI Grant 14-03-00612
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