Biomedical Engineering Reference
In-Depth Information
a typical rat. Low-frequency fluctuations, as observed from the
distinct frequency bands, originated predominantly from the
cortical region spanning both hemispheres. The power of all
frequencies below 0.1 Hz increased during exsanguination in the
cerebral cortex.
4. Discussion
4.1. Baseline BOLD
Signal Decrease
with Hypotension
Exsanguination led to an enhancement in the magnitude of the
low frequency fluctuations (
Fig. 12.1e
) despite variation in the
baseline BOLD signal decrease. Two of the rats studied indicated
a very minimal (2%) decrease in the mean baseline BOLD sig-
nal intensity during exsanguination while five rats showed no sig-
nificant change. The small variation in the mean BOLD signal
change observed in two rats may be due to inherent variability in
autoregulatory limits in normal anesthetized rats
(37)
. A change
in hematocrit as a result of hypovolemia can also cause a minimal
decrease in the baseline BOLD signal. Variation in the increase
in arteriolar CBV with autoregulated CBF during hypotension
can influence the decrease in baseline BOLD signals at high fields
(38)
. However, the rate of MAP change determines the capac-
ity of cerebral vasculature to maintain mean CBF levels. Barzo
et al.
(39)
have observed that baseline CBF remains unchanged
if hypotension is induced at a rate less than 24 mmHg/min in ure-
thane anesthetized rats. The rate of decrease in MAP in our study
was less than 24 mmHg/min as exsanguination was carried out
by passive bleeding of the rat over a period of three minutes and
is very unlikely to have contributed to the decrease in the base-
line BOLD signal. As hypotension does not significantly affect the
BOLD signal response to neural activation
(40)
, exsanguination-
induced hypotension may not confound the hemodynamic fluc-
tuations if influenced by underlying neural activity.
Amplitude of the BOLD signal fluctuations decreased sub-
stantially after replacement of withdrawn blood. While the MAP
returned to the normal control levels, the amplitude of the BOLD
signal fluctuations did not completely decrease to control levels.
The hysteresis in the BOLD signal fluctuation amplitude cannot
be attributed to hypotension alone since alternate methods that
induce hypotension such as lower body negative pressure in anes-
thetized rats have shown to be relatively non-invasive where CBF
fluctuation amplitude completely returned to normal control lev-
els prior to hypotension
(41)
. Events such as plasma volume
refill can occur during exsanguination by transfer of extravascu-
lar fluid into the circulation that can transport brain extracellular
compounds into the blood stream
(42)
. It is possible that such
