SPECTRAL CHARACTERISTICS OF SPONTANEOUS
OSCILLATIONS IN CEREBRAL HAEMODYNAMICS
ARE POSTURE DEPENDENT
Ilias Tachtsidis# , Clare E. Elwell# , Chuen-Wai Lee# , Terence S. Leung# ,
Martin Smith* , David T. Delpy#
1. INTRODUCTION
Autonomic reflexes are responsible for adjusting the cardiovascular system in
response to gravitational displacement of blood during changes in posture1 . In human
physiology, a posture change from supine to standing results in venous pooling in the
lower limbs and pelvic area (equivalent to the loss of 500ml of blood from the systemic
circulation) and an increased filtration from capillaries into interstitial space2 . In turn,
these effects cause a transient dip in effective circulating blood volume and potentially a
small reduction in cerebral and peripheral oxygen delivery. This effect is not prolonged in
a healthy individual since the decrease in arterial pressure triggers an immediate response
from the baroreceptor-mediated sympathetic mechanisms.
Near-infrared spectroscopy (NIRS) is an optical technique that provides
information on tissue oxygenation and haemodynamics on a continuous, direct, and noninvasive basis. The technique relies on the relative transparency of human tissue to light
in the near-infrared region and on the oxygenation dependent absorption changes in tissue
caused by chromophores mainly oxy - and deoxy - haemoglobin (O2 Hb and HHb). By
measuring changes in light absorption at different wavelengths, tissue oxygenation can be
monitored continuously.
Previous studies3,4 using NIRS have described spontaneous oscillations in the
cerebral circulation of normal human volunteers, which may have a regulatory role. The
aim of this study was to investigate the posture dependence of the magnitude of the low
frequency oscillations in cerebral oxy haemoglobin concentration and blood pressure.
The dependence of baseline values of cerebral tissue oxygenation and blood pressure
with changes in posture was also studied.
#
Department of Medical Physics & Bioengineering, University College London, London, WC1E 6JA, U.K.
www.medphys.ucl.ac.uk, email: iliastac@medphys.ucl.ac.uk.
*
Neuroanaesthesia Department, The National Hosp ital for Neurology and Neurosurgery, London, WC1N 3BG,
U.K
1
2
2. METHODS
2.1 Experimental methods
10 healthy volunteers of mean age 24±6 years took part in this study (the local
ethics committee approved the protocol for the study, and all subjects gave informed
consent for participation).
A continuous wave near-infrared spectrometer (NIRS), which incorporates
spatially resolved spectroscopy and a sampling rate of 6Hz (NIRO 300, Hamamatsu
Photonics KK) was used to measure the absolute cerebral tissue oxygenation index
(TOI)5 and changes in O2 Hb and HHb over the frontal cortex. The probe was placed on
the forehead (taking care to avoid the midline sinuses) and was shielded from ambient
light by using bandages and black cloth; the experiments took place in a darkened room.
An optode spacing of 5 cm was used and the differential pathlength factor6 was assumed
to be 6.26.
A Portapres® system (TNO Institute of Applied Physics, Biomedical
Instrumentation) was used to continuously measure non-invasive blood pressure from the
finger. The measurement is performed on two (adjacent) fingers alternately at selectable
intervals to prevent the build up of metabolites in one finger. The data was collected to a
PC via a serial link at 100 Hz sampling rate, then a full beat-to-beat analysis was
performed with the manufacturers Beatscope software in order to derive mean blood
pressure (MBP), diastolic and systolic blood pressure (DBP and SBP). The DBP signal,
after extraction and interpolation from the blood pressure signal was resampled to 6 Hz.
Each volunteer underwent three postural changes. For the first 10 minutes the
volunteer was placed in the supine position, then the subject was required to stand for
another five minutes and then to sit down on a chair for a further 10 minutes.
2.2 Power spectral analysis
Power spectral density (PSD) provides information on the power of frequencies
within a signal as a function of frequency. A non-parametric FFT (fast Fourier transform)
algorithm was used on the ∆[O2 Hb] signal as measured by the NIRO 300 and the DBP
signal as derived from the blood pressure waveform obtained from the Portapres®. The
Welch technique with a Hanning window of 256 sample points (~ 42 second sliding
window) and an overlap of 128 points was performed. The signals to be analysed were
detrended to remove baseline shifts. The ∆[O2 Hb] data was selected for this PSD analysis
since previous studies have shown that oscillations are most prominent in this signal3, 4 .
The low frequency (LF) comp onent, defined as 0.04-0.15Hz, was focused upon
in this study, as it can be indicative of sympathetic stimulation of the circulation7, 8. The
spectral power for this band was calculated from the area under the PSD curve using the
trapezoidal method.
3. RESULTS
3.1 TOI and MBP
The mean TOI and MBP for all 10 subjects were calculated for a 30 second
interval near the beginning of each posture to evaluate a baseline. Paired t-tests showed
no significant differences between these baseline signals recorded at each posture.
3
3.2 Power spectral results
Figure 1 shows the oscillatory changes in ∆[O2 Hb] and the corresponding power
spectral analysis recorded in each posture for one volunteer, typical of the whole group. It
can be seen that the changes in the power of the LF component in the PSD are reflected
by the increase in amplitude of the 0.1Hz oscillations seen in the raw signal.
Power spectral analysis of cerebral ∆[O2 Hb] and DBP from all subjects were
carried out. The results of the LF component PSD of the ∆[O2 Hb] and DBP data for each
volunteer are shown in Figures 2 and 3. Paired t tests showed a significant increase in the
magnitude of the ∆[O2 Hb] oscillations as subjects changed postures from supine to
standing (p = 0.012), and between the supine and sitting postures (p = 0.002). A paired t
test also showed a significant increase (p < 0.0005) in the magnitude of the DBP
oscillations data as subjects changed postures from supine to standing.
4. DISCUSSION
This study has shown that the spectral characteristics of spontaneous oscillations
in cerebral oxyhaemoglobin concentration are posture dependent in normal volunteers;
the magnitude of oscillations in diastolic blood pressure also showed posture dependence.
There were no significant differences in bas eline values of tissue oxygenation index or
mean blood pressure between different postures.
Spontaneous oscillations have been demonstrated in various physiological
signals in humans including arterial blood pressure and cerebral blood flow velocity9 . It
has been suggested in previous studies that LF oscillations in cerebral haemodynamics
(as well as other signals) are representative of the periodic fluctuations present in the
microcirculation, a phenomenon known as vasomotion3 . The presence of synchronous
fluctuations in signals from different areas of the body suggests the origin of a common
regulatory mechanism. LF oscillations in heart rate variability have been shown to
increase in strength from supine to 90° head up tilt 7 . The reasons underlying these
changes have been thought to be a result of the baroreflex, which suggests the oscillations
could be a marker of sympathetic activity7,8 . The posture dependent ol w frequency
fluctuations (~0.1Hz) in cerebral ∆[O2 Hb] described in this study may be indicative of
the changes in sympathetic activity associated with changes in posture. Clinical tests of
patients with orthostatic hypotension due to sympathetic failure involve tilting the patient
through different postures. We believe that the posture dependence of the low frequency
oscillations described here may provide information about regulatory control in different
groups of autonomic failure patients. A clinical trial is currently underway to investigate
the use of this analysis tool in the study of patients with orthostatic hypotension due to
sympathetic failure.
Although we have concentrated our analysis around the defined low frequency
band, the power spectrum of the ∆[O2 Hb] and DPB signal shows oscillations in a variety
of frequency bands including very low frequency (VLF) (0.02-0.04Hz). The
physiological explanation of this component is much less defined and the existence of a
specific physiological process attributable to this frequency range might be questioned.
Furthermore the non-harmonic component which does not have coherent properties and
which is affected by algorithms of baseline or trend removal is commonly accepted as the
major constituent of VLF10 . Previous studies have shown the presence of cardiac and
4
respiratory related oscillations in NIRS signals, both of which are outside the LF range
considered here 3 .
The haemodynamic status of the resting brain, characterised in part by the low
frequency oscillations described in this paper, is an important factor when considering
measurements of the haemodynamic response of the functionally activated brain 3 .
Measurements of the haemodynamic response to functional activation performed using
fMRI require the subject to be supine. Measurements of the haemodynamic response to
functional activation using NIRS or EEG are generally performed with the patient
upright. Our data shows a significant difference, in the resting brain, between the
magnitude of these low frequency oscillations in ∆[O2 Hb] with different postures. This
posture dependence should therefore be considered when comparing haemodynamic
response data collected using different modalities.
ACKNOWLEDGMENTS
The authors would like to thank all the volunteers participated in this study and
Hamamatsu Photonics KK for providing the NIRO 300 spectrometer. This work was
supported by the EPSRC/MRC, grant No GR/N14248/01 (IT) and the Wellcome Trust
grant No GR/062558 (TSL).
LF
Figure 1. Raw ∆[O2 Hb] data and the corresponding power spectrum density (PSD)
analysis for the low frequency (LF) component for one volunteer with posture changes.
5
LF Power in Cerebral ∆[O2 Hb] with Changes in Postures
(µmolar of O2 Hb)^2 / Hz
1.4
1.2
Subject 1
Subject 2
1.0
Subject 3
Subject 4
Subject 5
0.8
Subject 6
Subject 7
Subject 8
0.6
0.4
Subject 9
Subject 10
0.2
0.0
Supine
Standing
Sitting on Chair
Figure 2. LF power of cerebral ∆[O2 Hb] for every volunteer, with changes in postures.
LF Power in DBP with Changes in Postures
45
DBP (mmHg)^2 / Hz
40
Subject 1
Subject 2
Subject 3
35
30
Subject 4
Subject 5
Subject 6
25
20
Subject 7
Subject 8
Subject 9
Subject 10
15
10
5
0
Supine
Standing
Sitting on Chair
Figure 3. LF power of DBP for every volunteer, with changes in postures.
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