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University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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This study
examined the consistency between three indexes of cerebral blood flow
(CBF) obtained by using transcranial Doppler ultrasound in eight human
volunteers. Each subject undertook three sessions of graded exercise,
consisting of 6 min of rest, 6 min at 20% of maximal oxygen uptake
(
O2 max), 6 min at
40% O2 max, and 6 min of recovery. Values were obtained every 10 ms for the velocity
associated with the maximal frequency of the Doppler shift
(VP), the intensity-weighted mean velocity
(VIWM), and
total signal power (P). Beat-by-beat
averages for three indexes
(
P, IWM,
provided significantly different results for the percent changes in CBF
with exercise. At 20% of
O2 max,
P and
IWM showed
significant (P < 0.05) increases of
8 and 6%, respectively, whereas
showed a nonsignificant increase of 3%. At 40% of
O2 max,
P and
IWM showed
significant (P < 0.05) increases of 14 and 8%,
respectively, whereas
showed a nonsignificant increase of 4%. Our results suggest that the
increase in CBF with exercise that has been reported with transcranial Doppler ultrasound needs to be treated with caution, as much of the response could arise as an artifact from the increase in
amplitude and frequency of the arterial pressure waveform.
Doppler power; cerebral blood flow
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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MODERN IMAGING TECHNIQUES have revealed certain very localized increases in cerebral blood flow with dynamic exercise, for example, those to certain parts of the primary motor cortex (2, 24). However, there remain conflicting reports as to whether there are significant overall changes in cerebral blood flow during dynamic exercise. This lack of agreement is related, in part, to whether global (11, 12, 22, 26), cortical (6, 9, 10, 23), or larger regional changes (4, 5, 10, 13-15) in cerebral blood flow are being measured. It may also be related to the limitations associated with the various techniques used and the different intensities of exercise studied.
Techniques based on Doppler ultrasound have given fairly consistent
results, suggesting modest increases of 10-42% during light-to-moderate dynamic exercise [20-60% of maximal
oxygen uptake (O2 max)] (4, 5, 9,
10, 13-15, 19). However, all of these relate to beat averages of
the velocities associated with the maximal frequencies of the Doppler
shift. The use of this velocity as an index of cerebral blood flow
requires, first, that the maximal velocity is proportional to the mean
velocity of the blood flow in the vessel, and, second, that the
cross-sectional area of the blood vessel remains unchanged.
During exercise, both heart rate and the pulsatility of arterial blood
pressure increase markedly, and, consequently, the characteristics of
cerebral blood flow may change. In this case, it is not clear that
changes in the velocities associated with the maximal frequencies
of the Doppler shift necessarily reflect changes in flow.
The purpose of this study is to examine further the changes in the Doppler signal during light-to-moderate-intensity dynamic exercise, and, in particular, to examine the degree of consistency among different indexes of cerebral blood flow. Four variables obtained from the Doppler spectrum will be examined. The first is the velocity associated with the maximal frequency of the Doppler shift (VP). This velocity is associated with the blood that is moving fastest within the vessel. The second is the intensity-weighted mean velocity, based on the entire velocity spectrum (VIWM). This velocity represents a mean velocity averaged over the entire cross section of the blood vessel. The third variable is an index of cross-sectional area and is the total power of the reflected Doppler signal (P), which is a measure of the total number of ultrasound scatterers causing a Doppler shift (i.e., red blood cells). The fourth variable is another flow index, which tries to account for changes in cross-sectional area and is the product of P and VIWM (P · VIWM).
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Definitions
Let the ith element of a Doppler spectrum have an amplitude ai and a frequency shift of fi. Let the maximum frequency shift for which an amplitude is observed be at the mth element in the spectrum and be denoted by fm. Let
be the constant of proportionality between the frequency shift,
fi, and the
associated velocity, vi, such that
vi = fi. The
following functions on the Doppler spectrum may now be defined.
Glossary
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The Doppler spectra vary with blood flow throughout the cardiac cycle. Suppose there are n spectra within a cardiac cycle, and let the subscript j represent the jth Doppler spectrum within that cycle. The following mean values may be calculated for the cycle.
Glossary
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Subjects
The study involved eight young adults (3 women, 5 men). The study requirements were fully explained to all participants, with each giving informed consent before participation in the study. The research was approved by the Central Oxford Research Ethics Committee. Participants were not taking any medication, all were nonsmokers, and none had any history of cardiovascular, cerebrovascular, or respiratory disease.Protocols
The subjects visited the laboratory on two occasions. The first visit served as an opporunity to obtain preliminary data, and the second to perform the submaximal exercise experiments.The first visit included a brief medical history including age, height,
weight, and resting arterial blood pressure. This was followed by two
cycle exercise (ramp) tests to determine for each subject the workloads
that were equivalent to 20 (WLI) and 40% (WLII) of
O2 max for use in the
subsequent submaximal exercise experiments. The techniques employed for
measuring oxygen consumption have been described elsewhere (18). The
particular percentages of
O2 max were chosen
because previous studies using transcranial Doppler ultrasound have
suggested that the largest increases in cerebral blood flow may occur
in this intensity domain of exercise (4).
The two ramplike tests were administered 30 min apart. The exercise tests were preceded by a 6-min period at rest, during which baseline data were collected. Then, the subjects were instructed to begin pedaling (60-80 rpm), and the workload was increased automatically each minute in increments of 15-25 W to elicit a test of 8- to 12-min duration before maximum exercise capacity was reached.
The second laboratory visit was held within a few (2-7) days after the first and included three sessions of submaximal exercise on the cycle ergometer. Each submaximal exercise session lasted 24 min and consisted of a 6-min period of rest, then 6 min at WLI, 6 min at WLII, and finally a 6-min period of recovery. In two of the submaximal exercise sessions, subjects breathed through a mouthpiece, whereas in the third session no mouthpiece was used and a catheter was taped to one of the subject's nostrils to sample the end-tidal PCO2 (PETCO2) and PO2 (PETO2). Each session of exercise was separated by a 30-min period of rest to ensure full recovery from the previous test.
Apparatus and Technique
A Mijnhardt KEM-3 electromagnetically braked cycle ergometer (CardioKinetics) set in the constant-power mode was used for the exercise testing (8). To record stable measurements from the Doppler system, it was necessary to ensure immobilization of the head and upper body during the exercise tests. Therefore, the experiments were conducted with subjects sitting behind the cycle ergometer, in a semisupine position on an exercise bench (York DB5 folding dumbbell bench) that was fastened to the cycle ergometer. This provided a firm headrest for the subject while exercising. The subjects' feet were fastened tightly to the pedals by using standard bicycle toe-clips and straps. Subjects were asked to maintain a pedaling frequency of between 60 and 80 rpm.Heart rate was monitored from an electrocardiogram by using electrodes attached in a modified V-5 configuration. A noninvasive measurement of finger arterial pressure was taken throughout the exercise test protocols (Ohmeda 2300, Finapres). The Finapres data were sampled every 10 ms. These data, along with the occurrence of each QRS complex from the electrocardiogram, were logged to a computer and saved for later analysis.
End-tidal gases (PETCO2 and PETO2) were measured in all experiments. Gas was sampled at a rate of 80 ml/min and analyzed by mass spectrometry (Airspec MGA 3000) for fractional concentrations of O2, CO2, N2, and Ar. A computer sampled the experimental variables every 20 ms.
Backscattered Doppler signals from the right middle cerebral artery were measured by using a 2-MHz pulsed Doppler ultrasound system (PCDop842, SciMed). The Doppler system was adapted by the manufacturer to make the Doppler signals (maximum and intensity-weighted mean Doppler frequency shifts and P) available as analog signals. These were updated each time a new spectrum was calculated every 10 ms. Our data-acquisition system sampled those signals every 10 ms.
To obtain useful measurements of VIWM and P, it is necessary that the whole of the vessel be insonated. The procedure for ensuring this was as follows. The middle cerebral artery was identified by an insonation pathway through the right temporal window above the zygomatic arch (1, 20). The insonation depth (the distance from the probe to the start of the Doppler-sample volume) was set initially at a depth of 5.0-5.5 cm, and then a short search procedure began (by varying the angle and position of the probe) to identify a window that provided Doppler spectra from the middle cerebral artery. The sample depth was then increased in small increments of 0.7-0.8 mm until the quality of the Doppler spectra from the middle cerebral artery became poor (usually, at ~5.5-6.0 cm). At this point, the sample depth was decreased, in small increments (again in small steps of 0.7-0.8 mm), to a depth of 4.5 cm. At each depth, a short search was performed by making small adjustments to the angle and the position of the probe to assess the relative magnitude of P, along with the quality of the Doppler spectra. The sample was then returned to the depth at which the Doppler power signal was maximized and, at that depth, the angle and position of insonation were adjusted to provide the maximum Doppler power signal (this always was associated with the highest quality Doppler spectra). The center of this position was identified with a marker directly on the skin, the Doppler probe was removed, and a headband device (Müller and Moll Fixation, Nicolet Instruments) was strapped snugly around the subject's head. The Doppler probe was securely positioned in this headband device to maintain the optimal insonation position and angle.
Analysis
Visualization of profiles for VP, VIWM, and P over the cardiac cycle. Profiles for VP, VIWM, and P were determined over the cardiac cycle by using data from the last minute of each of the four different 6-min periods (i.e., rest, WLI, WLII, and recovery). To achieve this, the 10-ms data for VP, VIWM, and P for each subject for each test were ensemble averaged by using the peak value for VP within each cardiac cycle as the central data point around which 140 other data points were aligned. Thus the averaging procedure included 70 10-ms samples before, and 70 10-ms samples after, the occurrence of each peak value for VP within a given cardiac cycle. The results from the three repetitions of the exercise protocol in each subject were used to calculate overall averages for each subject. Finally, the averages for each subject were combined to calculate overall profiles for VP, VIWM, and P for the entire group.
Averaging of data for statistical analysis.
The data that related to middle cerebral artery blood flow comprise one
observation for VP,
VIWM, and
P every 10 ms.
P · VIWM was calculated every 10 ms. To give averages for
VP,
VIWM,
P, and
P · VIWM
over longer periods of time, the variables were first averaged over
each heartbeat to give the beat-by-beat average values,
P,
IWM,
, and
. For
the statistical analysis, these beat-by-beat data were then averaged to
give a 3-min value for rest (+3 to +6 min) and 1-min values for each of
WLI (+11 to +12 min), WLII (+17 to +18 min), and recovery (+23 to +24 min).
P,
IWM, , and
. The
beat-by-beat data during the 3-min period immediately before the onset
of exercise were used as the baseline (100%) values in this process.
Statistics.
Changes in P,
IWM,
, and
were
assessed statistically by using ANOVA with period (rest, WLI, WLII,
recovery) as a fixed factor and subjects as a random factor. Post hoc
comparisons were made by using t-tests
with the appropriate Bonferroni correction. Additionally, Student's
paired t-tests were used to compare
percent changes in P
with percent changes in
IWM during
exercise. The overall level of statistical significance was taken as
P < 0.05.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Preliminary Observations
The average age, height, and weight of the eight subjects were 22.3 ± 2.4 (SD) yr, 169.8 ± 8.1 cm, and 63.8 ± 8.2 kg, respectively. All were normotensive with average values for systolic, diastolic, and mean arterial blood pressure of 104.1 ± 4.8, 67.4 ± 5.1, and 79.6 ± 4.3 mmHg, respectively. Average values determined for WLI and WLII are given in Table 1.
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Average values for PETCO2 and PETO2 at each stage of the protocol are given in Table 1. Compared with resting values, PETCO2 was significantly increased at WLI and WLII. The relationship between arterial PCO2 and PETCO2 is not constant going from rest to exercise; PETCO2 underestimates arterial PCO2 at rest and overestimates it during exercise. Increases in PETCO2 on the order of magnitude reported in this study are broadly consistent with there being no underlying change in arterial PCO2 (21).
The average insonation depth for the subjects was 4.92 ± 0.26 cm.
Initial resting values for
P and
IWM were
58.7 ± 5.4 and 37.5 ± 3.4 cm/s, respectively.
Submaximal Exercise
General. Table 1 shows the values for work rate, heart rate, blood pressure, PETCO2, and PETO2 for control, WLI, WLII, and recovery. The 3-min period of rest immediately before the start of exercise was used for obtaining control values. The responses during the two levels of exercise and the subsequent recovery period were calculated over the last minute of each stage.
Changes in
VP,
VIWM, and
P profiles throughout the cardiac cycle.
Group averages for the profiles of VP,
VIWM, and
P throughout the cardiac cycle, during
the last minute of each of the four 6-min periods, are presented in
Fig. 1. From these data two observations can be made. First, compared with rest and recovery, the data for
VP and
VIWM during WLI
and WLII exhibit an increase in the magnitude of the variation in
velocity between the systolic and diastolic parts of the cardiac cycle.
Second, turning to the Doppler power signal, a reduction in
P is observed during systole in each of the four conditions. This reduction in
P appears markedly greater during WLI
and WLII than during rest or recovery. During WLI and WLII, some
individual differences were observed: although the decrease in
P appeared quite large (i.e., >15%)
in some subjects, it appeared much less (i.e., <5%) in others.
However, in all subjects and for all conditions, the decreases in
P appeared to be consistent, reproducible events.
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Beat-averaged Doppler power signal during exercise.
The group mean for the 3-min average at rest and the 1-min averages for
WLI, WLII, and recovery are given in Table 2 and presented in Fig. 2. The power signal appears to
decrease with increasing workload. These variations in the power signal
were statistically significant (ANOVA), with the value for
at WLII significantly below
those during rest, WLI, and recovery.
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Changes in
P,
IWM, and
during exercise.
The group means for the 3-min averages at rest and the 1-min averages
for WLI, WLII, and recovery are presented as absolute velocities and
normalized responses in Table 2 and are illustrated in Fig. 2. The
values for P and
IWM appear to increase with increasing workloads, and this was confirmed by
statistical analysis (ANOVA). Compared with rest,
P increased by 7.7 and 14.1% at WLI and WLII, respectively, whereas IWM
increased by 5.5 and 8.2% at WLI and WLII, respectively. In contrast,
the changes in
are much smaller and nonsignificant.
P appear to be higher
than those for
IWM. When
the differences between
P and
IWM were
assessed statistically, they were found to be significant, during both
WLI (difference of 2.24%, 95% confidence interval = 0.77-3.70%,
P < 0.01, paired t-test) and WLII (difference of
5.97%, 95% confidence interval = 4.09-7.85%,
P < 0.001, paired
t-test).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The purpose of this study was to examine the percent change in cerebral
blood flow from rest to exercise by using a number of different indexes
of cerebral blood flow derived from transcranial Doppler ultrasound
data. Each of the indexes of cerebral blood flow provided different
results for the percent change in cerebral blood flow between rest and
exercise. This lack of consistency indicates that Doppler indexes of
cerebral blood flow cannot necessarily be relied on as indicative of
changes in cerebral blood flow when rest is compared with exercise. In
particular, the modest increase in
P observed in this
and other studies may not reflect any real underlying increase in
cerebral blood flow with exercise.
Differences Between
P
and IWM
P and
IWM suggest
that the velocity flow profile within the vessel may be changing with
exercise. Blood flowing in the middle cerebral artery has entered the
vessel from around a bend from a branch point on the Circle of Willis.
Such structures can cause quite complicated velocity profiles within the vessel. In addition, the flow within the vessel is not steady but
varies because of the variation in arterial pressure throughout the
cardiac cycle. In such a flow there may be both some flow reversal
and/or flow separation for all or part of the cycle (16, 25). At the
onset of exercise, the cardiac cycle becomes shorter, and the magnitude
of the variation in arterial pressure within the cardiac cycle becomes
more extreme. These changes in pressure could certainly alter the
velocity profiles within the vessel directly. In addition, because the
walls of the vessel are not rigid, the variations in pressure could
also affect both geometry and vessel size, and these changes could also
have effects on the velocity profile. With the shortening of the
cardiac cycle and the increase in pulse pressure, it is possible that
the degree of any flow reversal and/or flow separation could increase.
If the overall mean flow is maintained, then there may be consequential increases in velocity elsewhere in the flow profile. Under conditions where the velocity flow profile is changing,
IWM, which is derived from the entire velocity spectrum, may well provide a more
accurate reflection of any change in overall flow than would
P.
Changes in Doppler Power
The fall in power observed during systole was an unexpected finding. This phenomenon became more pronounced during exercise, causing to fall in exercise compared
with rest. Extracranial vessels increase in diameter during systole
(7), and, if intracranial vessels behave similarly, an increase in
power would be predicted. One possibility is that the change in power
reflects a genuine decrease in vessel cross-sectional area. However,
there are other possibilities. First, it is possible that systole
induces some relative movement between the Doppler probe and vessel so
that some of the vessel is lost. This might become worse with the
increased arterial pulsation and possible head movement that
accompanies exercise. However, with an ultrasound beam of ~1 cm in
diameter, a sample thickness of ~1 cm, and a vessel of ~0.3 cm in
diameter, it is difficult to envisage how such relative motion would
occur consistently across the subjects. A second possibility is that flow reversal or flow separation occurs across some of the vessel during the increased blood flow in systole. This would reduce the
number of forward-moving red cells within the sample during systole and
thus lead to a reduction in signal power. The possible causes of any
such flow reversal and/or flow separation have been discussed above. A
third possibility is that the reduction in signal power is related to
some signal-processing artifact, although we have no evidence to
support this.
Comparisons With Previous Studies
Our results forP
show a modest increase with submaximal exercise that is consistent with
other previous reports using transcranial Doppler ultrasound (4, 5, 9,
10, 13-15, 19). In contrast to these reports, studies measuring
global changes in cerebral blood flow, using the nitrous
oxide-inhalation technique (based on the Fick principle) (11, 12, 22,
26) or the 133Xe-inhalation
technique (3), generally show no changes in cerebral blood flow during
dynamic exercise. However, studies using the 133Xe-clearance technique to
determine larger regional (i.e., cortical) variations in cerebral blood
flow have reported modest increases (i.e., 25-31%) in cerebral
blood flow during dynamic exercise of light to moderate intensity
(i.e., 50% of
O2 max) (6, 9, 10, 17, 23).
Some studies have reported results of the simultaneous use of different techniques. Jørgensen et al. (10) reported similar magnitudes (i.e., 20-30%) for the changes in cerebral blood flow during moderate dynamic exercise as assessed by Doppler ultrasound and the 133Xe-clearance technique. However, Madsen et al. (14) reported a 22% increase in cerebral blood flow velocity by using Doppler ultrasound but found no change in global average cerebral blood flow by using 133Xe combined with the Kety-Schmidt technique (11). Overall, our results suggest that the observation of an increase in cerebral blood flow with exercise that has been reported with transcranial Doppler ultrasound needs to be treated with caution. One possibility is that much of the response could arise as an artifact from the increased amplitude and frequency of the arterial pressure waveform and its consequent effects on the flow profile of the arterial flow in the middle cerebral artery.
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ACKNOWLEDGEMENTS |
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We acknowledge the assistance provided by Robert Bowyer with computing (Matlab), the skilled technical assistance from David O'Connor, and the volunteers for their participation in the study.
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FOOTNOTES |
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This study was supported by the Wellcome Trust. M. J. Poulin was supported by a Heart and Stroke Foundation of Ontario (Canada) postdoctoral research fellowship (Grant F3555).
1 The Doppler ultrasound system used in the present study produced a Doppler spectrum every 10 ms, and the term "instantaneous" refers to the value obtained for one of these spectra.
Address for reprint requests and other correspondence: P. A Robbins, Univ. Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK (E-mail: peter.robbins{at}physiol.ox.ac.uk).
Received 17 September 1997; accepted in final form 16 December 1998.
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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C. H. E. Imray, S. D. Myers, K. T. S. Pattinson, A. R. Bradwell, C. W. Chan, S. Harris, P. Collins, A. D. Wright, and the Birmingham Medical Research Expeditionary Soci Effect of exercise on cerebral perfusion in humans at high altitude J Appl Physiol, August 1, 2005; 99(2): 699 - 706. [Abstract] [Full Text] [PDF]
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C. A. Giller, A. M. Giller, C. R. Cooper, and M. R. Hatab Evaluation of the cerebral hemodynamic response to rhythmic handgrip J Appl Physiol, June 1, 2000; 88(6): 2205 - 2213. [Abstract] [Full Text] [PDF]
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K. Ide, A. Horn, and N. H. Secher Cerebral metabolic response to submaximal exercise J Appl Physiol, November 1, 1999; 87(5): 1604 - 1608. [Abstract] [Full Text] [PDF]
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and 
at rest,
during submaximal exercise, and in the recovery after exercise
), average
VIWM
(
),
and average middle cerebral artery flow index
,
). Error bars, 1 SD.