LWW/JHTR JHTR-D-14-00197 May 14, 2015 21:10
J Head Trauma Rehabil
Copyright
c 2015 Wolters Kluwer Health, Inc. All rights reserved.
Cerebral Blood Flow During Treadmill
Exercise Is a Marker of Physiological
Postconcussion Syndrome in Female
Athletes
Mary Clausen, MS; David R. Pendergast, EdD; Barry Willer, PhD; John Leddy, MD
Objective: Some patients with postconcussion syndrome (PCS) have reduced exercise capacity that may reflect
altered central cardiorespiratory control. The purpose of this study was to evaluate control of cerebral blood flow
(CBF) during exercise in females with PCS. Setting: University Concussion Clinic. Participants: Nine female
Division 1 collegiate team athletes with PCS (23 ± 6 years) and a reference group of 13 healthy female recreational
aerobic athletes (21 ± 3 years). Design: A prospective experimental study. All PCS athletes were compared with
the reference group at the beginning of the study. Six of the PCS athletes were subsequently measured before
and after a subsymptom threshold aerobic exercise treatment program. Main Measures: Exercise treadmill test
during which blood pressure (BP), minute ventilation (V˙ E), end-tidal CO2 (PETCO2 ), and CBF velocity (CBFV, by
transcranial Doppler) were measured. Results: Participants with PCS had significantly lower V˙ E (by 18%) and greater
PETCO2 (5%) and CBFV (14%) versus the reference group at similar workloads in association with appearance of
symptoms and premature exercise cessation. Subthreshold exercise normalized V˙ E, PETCO2 , CBFV and exercise
tolerance. Before treatment, PCS had low CO2 sensitivity that blunted their exercise ventilation. CO2 sensitivity
and ventilation improved after exercise treatment. Conclusion: Some PCS patients have exercise intolerance due
to abnormal CBF regulation that may be the result of concussion-induced altered sensitivity to CO2 . Return of
normal CBF control and exercise tolerance may be physiological markers of recovery from concussion. Key words:
carbon dioxide sensitivity, cerebral blood flow, exercise, postconcussion syndrome, ventilation
A LTHOUGH THE MAJORITY OF PATIENTS
with concussion recover within 7 to 10 days,1 a
significant minority continues to experience symptoms
is considered to be present in athletes with symptoms
persisting several weeks to months.4 Some of these
patients have symptom exacerbation during exercise5
called postconcussion syndrome (PCS).1,2 Postcon- that could be related to physiological changes resulting
cussion syndrome is defined by the World Health from concussion, in this case, failure to appropriately
Organization as persistence of 3 or more of the follow- regulate cerebral blood flow (CBF).6 Dysregulation of
ing symptoms after head injury: headache, dizziness, resting CBF has been shown in patients soon after
fatigue, irritability, insomnia, concentration difficulty, concussion by MRI measures of global CBF7 and by
or memory difficulty.3 In sport-related concussion, PCS functional MRI in local brain regions acutely after
concussion 8 and in patients with PCS.9,10 To our
knowledge, dynamic CBF during exercise has not been
Author Affiliations: Department of Physiology, Center for Research and evaluated in concussed patients, which has implications
Education in Special Environments, State University of New York, for return to sport or military action after concussion.
Buffalo (Ms Clausen and Dr Pendergast); SUNY Buffalo, Orthopaedics,
University Sports Medicine, Buffalo, New York (Dr Leddy); and Cerebral autoregulation (CA) allows the brain to
Department of Psychiatry, SUNY Buffalo (Dr Willer). maintain an almost constant perfusion pressure in the
Supplemental digital content is available for this article. Direct URL citation face of varying levels of systemic blood pressure (BP),
appears in the printed text and is provided in the HTML and PDF versions protecting the brain from changes in sympathetic nerve
of this article on the journal’s Web site (www.headtraumarehab.com). activity, mean arterial blood pressure (MAP), and arte-
The authors declare no conflicts of interest. rial CO2 levels (PaCO2 ).11,12 Concussion has effects on
Corresponding Author: John Leddy, MD, SUNY Buffalo, Orthopaedics, systemic physiological systems.6 When compared with
University Sports Medicine, 160 Farber Hall, Buffalo, NY 14214 controls, for example, patients with concussion and mild
(leddy@buffalo.edu). TBI have altered heart rates (HRs)13 and HR variabil-
DOI: 10.1097/HTR.0000000000000145 ity when moving from supine to standing14 and during
1
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LWW/JHTR JHTR-D-14-00197 May 14, 2015 21:10
2 JOURNAL OF HEAD TRAUMA REHABILITATION
steady-state exercise,15 which reflects altered autonomic exercise in female patients with PCS. We focused on fe-
nervous system (ANS) function.14,16 Cerebral autoregu- males because they may be at greater risk of concussion
lation may also be disturbed after concussion such that and of developing PCS.24 Prior studies of the regulation
changes in systemic BP may induce excessive changes of CBF during exercise have been conducted on men
in cerebral perfusion pressure.17 We have shown that during cycle ergometry,11,12,20 but most exercise that
patients with PCS exercising on a treadmill had abnor- is symptom-producing in PCS is running and the test
mally increased BP at low exercise intensities associated proposed to evaluate the physiology of PCS patients
with the appearance or exacerbation of symptoms5,18 is a walk/run on a treadmill.25 We hypothesized that
and we hypothesized that this reflects a failure of female patients with PCS but not healthy patients would
CA, with resulting hyperperfusion of the brain during have elevated PaCO2 levels at low exercise intensities
exercise. and thus greater CBF leading to symptom exacerbation
One of the primary influences on CBF is the arterial and early exercise cessation. We hypothesized that the
CO2 tension (PaCO2 ). Depressed PaCO2 reduces CBF elevated PaCO2 would be the result of a pulmonary hy-
and increased PaCO2 increases CBF.19 For a given car- poventilation (ie, reduced V˙ E ) during exercise because
bon dioxide production (V˙ CO2 ), the PaCO2 is inversely of reduced CO2 sensitivity. Finally, we hypothesized
proportional to the pulmonary ventilation (V˙ E ): as V˙ E that a program of subsymptom threshold aerobic ex-
increases PaCO2 decreases and vice versa. Normally, as ercise treatment would normalize the physiology in
oxygen consumption (V˙ O2 ) and V˙ CO2 increase with ex- patients with PCS and restore their exercise tolerance.
ercise intensity, PaCO2 increases until the onset of ex-
cess blood lactic acid accumulation, at which point METHODS
there is a hyperventilation, that is, a respiratory com-
The study protocol was reviewed and approved by
pensation for the metabolic acidosis that reduces PaCO2
the Human Subjects Institutional Review Board at the
and CBF.11,12,20 The onset of this compensatory hy-
University at Buffalo and the participants signed an
perventilation is called the “Ventilatory Threshold.”21
informed consent form.
The PaCO2 where this hyperventilation begins varies
among individuals; that is, some are more sensitive
Subjects
to the effects of CO2 than others. This is known as
“CO2 sensitivity.”22 There is no reason to expect that Nine female athletes with PCS were recruited from the
patients with concussion would have a different V˙ CO2 University at Buffalo Concussion Clinic (see Table 1).
than healthy patients as the V˙ O2 for a given exercise in- All satisfied World Health Organization criteria for PCS
tensity is not different.18 It is, however, possible that they and were symptomatic for more than 6 weeks but less
have a different sensitivity to the effects of CO2 if con- than 12 weeks (mean duration of symptoms 9 weeks).
cussion damaged the control centers for the ANS and Sports included soccer, basketball, and volleyball. A ref-
V˙ E located in the brainstem. There is evidence that some erence group of 13 female age-matched, nonconcussed,
patients with concussion have injury to the brainstem.23 aerobically trained recreational athletes was recruited
If central damage altered V˙ E in patients with concussion, from the University community (see Table 1). They were
abnormal control of CBF could produce symptoms training 3 to 5 days per week (30-60 min/day) for at least
such as headache or dizziness that limit their exercise 2 years. They continued their regular training through-
tolerance. out the study; thus, on the basis of prior work,26,27 we
The purpose of this study was to examine the rela- assumed that their CO2 sensitivity would be stable and
tionships between V˙ E , PaCO2 , and CBF during treadmill so they were only tested at the beginning of the study.
TABLE 1 Physical characteristics
Height, ˙ 2 peak,
Vo PCSS
Group Age, y cm Weight, kg mL/kg/min severity score
Reference 21 ± 3 165 ± 9 60.95 ± 9.23 34.40 ± 4.51 5.5
PCS-Pre 23 ± 6 167 ± 3 62.23 ± 4.75 23.50 ± 2.82a 19a
PCS-Post 23 ± 6 167 ± 3 62.23 ± 4.75 40.65 ± 1.62b 6.5b
Abbreviations: PCS-Pre, concussed subjects before exercise treatment; PCS-Post, concussed subjects after exercise treatment; PCSS,
postconcussion symptom scale, symptom severity score.
a = significantly different from reference (<.001).
b = significantly different from PCS-Pre (<.001).
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CBF During Treadmill Exercise Is a Marker of Physiological PCS in Female Athletes 3
Design spite increasing exercise intensity and the respiratory
exchange ratio exceeded 1.1.30 Heart rate was continu-
All athletes with PCS were compared with the ref-
ally monitored using a Polar heart rate monitor (Polar
erence group at the beginning of the study. After this,
810i T61, Kempele, Finland). Blood pressure was taken
6 of the athletes with PCS were measured before and
during the last minute of each stage from the left arm us-
after a 12-week subsymptom threshold aerobic exercise
ing a sphygmomanometer (Tango, SunTech Medical In-
treatment program previously demonstrated to improve
struments, Morrisville, North Carolina). As an estimate
outcome in patients with PCS.5 Three athletes with PCS
of PaCO2 , end-tidal CO2 (PET CO2 )31 was measured by
did not complete the program because they were away
mass spectrometer and recorded by a BIOPAC system
from the university for an extended period. The refer-
(MP150CE, Goleta, California).
ence group was assessed only at the beginning of the
study as we assumed that their data would not change
CO2 sensitivity test
significantly as they kept their training constant during
the study period. The procedure used was a modified version of the re-
breathing method described by Read and Leigh.32 Par-
Physiological tests ticipants were seated in a chair with a curtain masking
the equipment, except for the mouthpiece they breathed
All participants completed a graded exercise test dur- from. They wore a nose clip throughout the test. They
ing which physiological variables were measured as well were instructed to relax and breathe into and out of the
as a standardized CO2 sensitivity test. Resting concus- mouthpiece as naturally as possible. The mouthpiece
sion symptoms were recorded in the reference group was connected to a spirometer (Ohio Medical Products,
and for subjects with PCS before and after exercise treat- Houston, Texas). The spirometer was filled to a volume
ment, prior to exercise testing, using the Post-concussion of 10 L with a gas mixture containing approximately 5%
Symptom Scale, a validated assessment instrument CO2 , 50% O2 , and 45% N2 . Participants breathed room
that includes 22 symptoms of concussion (headache, air for the first 4 minutes after which they rebreathed
dizziness, photophobia, etc) with sound psychomet- the gas mixture for 8 minutes until inhaled CO2 reached
ric properties and normative data for men and women 9.0% or the participant voluntarily stopped. Heart rate
(see Table 1).28 was continually recorded using a Polar heart rate mon-
itor. Blood pressure was taken every 2 minutes us-
Treadmill test ing a BioZ system sphygmomanometer (BioZ, Cardio
Dynamics, San Diego, California). PET CO2 was mea-
Participants performed a graded exercise test on
sured by mass spectrometer (Perkin-Elmer, Pomona,
a treadmill (Trackmaster, Full Vision Inc, Newton,
California) and recorded by a BIOPAC system.
Kansas). Participants walked initially at a speed of 3.0
mph at 0% grade. After 3 minutes, the grade was raised
Cerebral blood flow velocity
by 2.0% every 2 minutes until participants reached vol-
untary exhaustion. This protocol allowed participants Cerebral blood flow velocity (CBFV) was measured
to reach a “physiological steady state” for each intensity continuously during both the CO2 sensitivity and tread-
level.21 The participants in both groups were instructed mill tests. Cerebral blood flow velocity was measured bi-
to notify the administrator of increased or new concus- laterally from the middle cerebral artery (MCA) and con-
sion symptoms. When they reported exacerbation of tinually recorded using the Transcranial Doppler (TCD).
symptoms, the test was terminated. The test was termi- The MCA was observed through the transtemporal win-
nated for reference subjects when the subject reported dow. The MCA was accurately identified by the depth of
symptom exacerbation or reached exhaustion (defined the sample volume, direction of flow with relation to the
as a rating of perceived exertion of ≥ 18).29 probe, and the relationship and depth of the M-mode
Expired gas was collected in Douglas bags for the color bands to each other. Doppler probes were held
last minute of each stage. The volume of the expired at a constant angle of insonation. Measurements were
gas was measured using a dry gas meter (Harvard Dry taken at a depth of 35 to 55 mm. The Doppler power
Gas Meter, Model # AH-50-6164, Harvard Apparatus was set between 80% and 100%, always using the lowest
Inc, Holliston, Massachusetts) and the temperature de- possible power. The Doppler probes were fixated in the
termined (Yellow Springs Instrument Co, Inc, Yellow appropriate position on the head using a secure head
Springs, Ohio). Gas concentrations were analyzed by set (see Figure 1). The same trained, experienced techni-
the mass spectrometer (Perkin Elmer 1100), which was cian recorded all TCD signals and used quality control
calibrated before each test. V˙ O2 and V˙ CO2 were calcu- measures in setting up the experiments throughout the
lated using standard procedures. V˙ O2 was considered study. TCD values from both sides of the head revealed
maximum if it plateaued (<150 ml/min increase) de- mean values of 72 ± 13 cm/s for the right side and
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4 JOURNAL OF HEAD TRAUMA REHABILITATION
bation or when their prescribed exercise duration was
reached, whichever came first. Participants returned for
treadmill tests every 3 weeks until their symptoms were
no longer exacerbated on the treadmill. Participants kept
daily reports of symptoms, which were reviewed weekly
for compliance with the program. Participants saw the
study physician for a repeat evaluation to ensure that
they were symptom free after completion of the final
treadmill test (to voluntary exhaustion). Physiologic res-
olution of PCS was defined as the ability to exercise to
voluntary exhaustion without exacerbation of concus-
sion symptoms.5,29
Statistical analysis
Figure 1. Representative example of the Transcranial Doppler
setup during treadmill testing in a patient with postconcussion All statistics were calculated using SigmaPlot 10.0 by
syndrome. Doppler probes are held at a constant angle of SigmaStat 3.5 (Systat Software Inc, Chicago, Illinois).
insonation during exercise using the secure head set. All descriptive data were expressed as means ± standard
deviation. A statistically significant result was defined
73 ± 11 cm/s for the left side. The correlation coeffi- as P ≤ .05. Data were first tested to see whether they
cient between sides was 0.93 with a slope of 1.04. displayed a normal distribution. If the data passed the
Inferring changes in CBF from CBFV requires the as- normality test, parametric statistics were used. If statis-
sumption that the diameter of the carotid blood vessels tically significant differences were identified, the Holm-
is constant.33 Although MCA diameters may be age- Sidak method was used for multiple comparisons to iso-
related, the subjects of this study were of similar age and late the groups that differed. If the data did not pass the
there is no evidence that the MCA diameter changes normality test, nonparametric statistics were used, that
in women during exercise.34 TCD has been shown in is, the Friedman repeated-measures ANOVA on ranks
many studies to be a valid indicator of CBF during was used to determine statistical significance. If the data
exercise12,35-44 and our reference group data (at similar were statistically significant, the Tukey test was used
V˙ O2 s) are in agreement with the CBFV values reported for multiple comparisons to isolate the groups that dif-
in these studies. fered (see the Supplemental Digital Content, available
The CO2 in the exhaled air was monitored at: http://links.lww.com/JHTR/A141).
and recorded continuously by a mass spectrometer
(MGA1100, Perkin-Elmer, Pomona, California) for de- RESULTS
termination of PET CO2 . Ventilation was determined by
Nine patients with PCS and 13 reference subjects
a flow meter. Data were recorded and analyzed using a
completed the exercise test at the beginning of the study.
BIOPAC system (MP150CE, Goleta, California). V˙ E was
Of the 9 patients with PCS, 6 of them performed an
plotted as a function of the corresponding PET CO2 . A
exercise treatment program and were retested after com-
regression was determined for normal breathing and that
pleting the program. Based on established criteria,45 all
influenced by the CO2 inhalation. The CO2 threshold
of the reference subjects reached their maximal V˙ O2
was the PET CO2 point at which the ventilation increased
and had reliable CBFV, V˙ E and PET CO2 data, in spite
above normal breathing. The slope of the V˙ E (L/min) in-
of reaching only 95% of their age predicted maximal
crease versus PET CO2 (mmHg) was defined as the CO2
HR. Twelve of the 13 reference subjects followed the
sensitivity.22
same pattern in their exercise V˙ E , PET CO2 , and CBFV.
They had similar CO2 sensitivity and their data agreed
Exercise treatment program
with previous studies.11,12,35,40 One subject increased
Six participants with PCS underwent an aerobic exer- her V˙ E with V˙ CO2 such that her PaCO2 (PETCO2 ) was
cise treatment program that has previously been shown maintained in the normal range (nominally 40 mmHg)
to be safe and reliable.25 Participants performed aero- and thus her CBFV did not increase with submaximal
bic exercise using an HR monitor 20 minutes per day, exercise. Her data therefore were not included in the
5 to 6 days per week, at an intensity of 80% of the reference group analysis.
maximum HR achieved during the first treadmill test. A representative example of the TCD setup is pre-
They were required to have someone present during sented in Figure 1. A representative color Doppler CBFV
exercise for safety monitoring and were instructed to signal obtained during the treadmill test in a patient
terminate exercise at the first sign of symptom exacer- with PCS is depicted in Figure 2. Prior to subthreshold
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LWW/JHTR JHTR-D-14-00197 May 14, 2015 21:10
CBF During Treadmill Exercise Is a Marker of Physiological PCS in Female Athletes 5
TABLE 2 Physiological responses to
exercise testing
Maximum
Exercise exercise
Group Variable onset tolerance
Reference HR (b/min) 82 ± 12 194 ± 8
PCS-Pre HR (b/min) 117 ± 17a 144 ± 20a
PCS-Post HR (b/min) 115 ± 12a 173 ± 11b
Reference SBP (mmHg) 116 ± 8 163 ± 16
PCS-Pre SBP (mmHg) 120 ± 6 138 ± 6a
PCS-Post SBP (mmHg) 111 ± 11 160 ± 14b
Reference DBP (mmHg) 77 ± 9 75 ± 8
PCS-Pre DBP (mmHg) 79 ± 10 74 ± 12
Figure 2. Representative raw data tracing from the screen of PCS-Post DBP (mmHg) 78 ± 6 81 ± 5
the Transcranial Doppler unit during treadmill walking in a
patient with postconcussion syndrome. X-axes represent time. Abbreviations: b/min, beats per minute; DBP, diastolic blood pres-
Top panel y-axis: depth of the artery in mm. Bottom panel sure; HR, heart rate; PCS-Pre, patients with PCS before exercise
y-axis: blood flow speed in centimeters per second. treatment; PCS-Post, patients with PCS after exercise treatment;
SBP, systolic blood pressure.
a Significantly different from reference (<.001).
exercise treatment, patients with PCS had limited exer- b Significantly different from PCS-Pre (<.001).
cise tolerance because of symptom exacerbation dur-
ing the treadmill test. After exercise treatment, they Expired ventilation (V˙ E ) was significantly depressed
had normal exercise tolerance without symptom ex- and therefore PET CO2 was significantly elevated in the
acerbation on the treadmill. Normative data from the patients with PCS compared with the reference group at
Post-concussion Symptom Scale indicate that healthy all V˙ O2 s (see Figures 4 and 5, respectively). The CO2 sen-
females can report up to a symptom severity score of sitivity of patients with PCS was significantly depressed
9.28 Table 1 shows that prior to exercise treatment, pa- (1.35 ± 2.11 L/min/mmHg CO2 ) versus the reference
tients with PCS had an elevated level of resting symp- group (2.88 ± 0.60 L/min/mmHg CO2 ) before treat-
toms when compared with the reference group. Resting ment (see Figure 6).
symptoms in patients with PCS returned to reference
group levels after exercise treatment.
Before exercise treatment
Up until the point of exercise cessation, there were no
significant differences in the V˙ O2 values at all treadmill
settings between patients with PCS and the reference
group and between patients with PCS pre- and postex-
ercise treatment (data not shown). Patients with PCS
before treatment had greater HRs at exercise onset ver-
sus the reference group (see Table 2).
Patients with PCS stopped exercise on the treadmill
at a V˙ O2 of 23.50 ± 2.82 mL/kg/min (see Table 1), an
HR of 144 ± 20 beats per minute (see Table 2), and
a systolic BP (SBP) of 138 ± 6 mmHg, with a nor- Figure 3. Mean data for cerebral blood flow velocity are plot-
ted as a function of V˙ O2 for patients with postconcussion syn-
mal DBP response (see Table 2), well below their re-
drome (PCS) pre- (PCS-Pre, n = 6) and postexercise treatment
spective predicted maximal values. They stopped exer-
(PCS-Post, n = 6) as well as for reference subjects (Control,
cise because of significant exacerbation of symptoms of n = 12). ∗ A significant difference (P ≤ .05) between values for
headache and dizziness, which defined their symptom- PCS patients and reference subjects; + a significant difference
limited threshold.5 As can be seen in Figure 3, patients (P ≤ .05) between PCS patients post- compared with preexer-
with PCS had significantly higher CBFV at all V˙ O2 val- cise treatment. The coefficient of variation (CV) for the PCS
ues. Their maximal V˙ O2 achieved was significantly lower patients preexercise treatment ranged from 4% to 14%, for
at the point where symptoms increased (at their limit of postexercise treatment from 11% to 14%, and for reference
exercise tolerance) versus the reference group. subjects from 30% to 33%.
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6 JOURNAL OF HEAD TRAUMA REHABILITATION
Figure 6. Mean data ± standard deviation for CO2 sensitivity
Figure 4. Mean data for expired ventilation are plotted as a are shown for patients pre- (n = 6) and postexercise treatment
function of V˙ O2 for PCS patients pre- (PCS-Pre, n = 6) and (n = 6) as well as for reference subjects (Control, n = 12).
postexercise treatment (PCS-Post, n = 6) as well as for refer- ∗
A significant difference (P ≤ .05) between values for PCS
ence subjects (Control, n = 12). ∗ A significant difference (P ≤ patients and reference subjects; + a significant difference (P ≤
.05) between values for PCS patients and reference subjects; .05) between PCS patients post- compared with preexercise
+
a significant difference (P ≤ .05) between PCS patients pos- treatment.
texercise treatment compared with preexercise treatment. The
coefficient of variation (CV) for the PCS patients preexercise exhaustion without symptom exacerbation, achieving
treatment ranged from 19% to 24%, for postexercise treatment
85% of their age predicted maximum HR (173 ± 11
from 14% to 19%, and for reference subjects from 18% to 20%.
beats per minute), an SBP of 160 ± 14 mmHg, and
a V˙ O2 of 40.65 ± 1.62 mL/kg/min (see Table 1). It
should be noted that even after exercise treatment, pa-
tients with PCS achieved only 90% of their predicted
V˙ O2 max (based on HR data) and did not have the respi-
ratory compensation for metabolic acidosis seen in the
reference group (see Figure 4); thus, they did not have a
reduction in CBFV at their highest V˙ O2 (see Figure 3).
As can be seen in Figures 4 and 5, after exercise treat-
ment V˙ E increased significantly and PET CO2 decreased
significantly in patients with PCS versus pretreatment
values and were not different from the reference group
values. The increased V˙ E was associated with a signif-
icant increase in CO2 sensitivity versus before treat-
ment (see Figure 6). In addition, CBFV was significantly
Figure 5. Mean data for end-tidal CO2 (an estimate of arterial lower after when compared with before treatment (see
CO2 ) are plotted as a function of V˙ O2 for PCS patients pre- Figure 3).
(PCS-Pre, n = 6) and postexercise treatment (PCS-Post, n = 6)
as well as for reference subjects (Control, n = 12). ∗ A signifi-
cant difference (P ≤ .05) between values for PCS patients and DISCUSSION
reference subjects; + a significant difference (P ≤ .05) between This study shows that Division 1 female college ath-
PCS patients postexercise treatment compared with preexer- letes with PCS had a physiological disturbance affect-
cise treatment. The coefficient of variation (CV) for the PCS
ing control of CBF in association with limited exer-
patients preexercise treatment ranged from 14% to 15%, for
cise tolerance. When compared at equivalent workloads
postexercise treatment from 11% to 19%, and for reference ˙ 2 s) with a reference group of aerobically trained
subjects from 19% to 20%. (ie, VO
recreational female athletes, patients with PCS had ex-
acerbation of symptoms of headache and dizziness that
After exercise treatment
occurred well below their maximal exercise capacity. The
The HR and BP responses during the exercise test after physiological data show that patients with PCS had a rel-
subthreshold exercise treatment are shown in Table 2. ative hypoventilation (lower V˙ E ) that disproportionately
Patients with PCS after treatment still had greater HRs at increased their PaCO2 and CBFV, which suggests that
exercise onset versus the reference group. In agreement CBF was similarly increased. The relative hypoventila-
with a previous study,5 patients with PCS exercised to tion appears to have been a consequence of reduced
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LWW/JHTR JHTR-D-14-00197 May 14, 2015 21:10
CBF During Treadmill Exercise Is a Marker of Physiological PCS in Female Athletes 7
sensitivity to the effects of CO2 . We hypothesize that sitivity would have changed had it been measured 12
this reflects altered central (medullary) and/or peripheral weeks later.26,27 Furthermore, endurance training lowers
(carotid body) respiratory control that to our knowledge CO2 sensitivity in normal subjects. A 12-week program
has not been reported previously in patients with con- of subthreshold aerobic exercise treatment increased CO2
cussion or patients with PCS. CO2 sensitivity in patients sensitivity in our PCS patients, which is consistent with
with PCS normalized after a program of subthreshold an effect of concussion resolution rather than an effect
aerobic exercise treatment in association with restora- of physical training.
tion of V˙ E , PET CO2 , CBFV, and exercise tolerance to Cerebral artery resistance is very sensitive to changes
reference group levels. These data suggest that concus- in PaCO2 .21 Breathing 7% CO2 can increase CBF ap-
sion can alter the physiological response to arterial CO2 . proximately 2-fold in both men19 and women,50 which
Reduced CO2 sensitivity resulted in a blunted ventila- is similar to what we observed in this study during exer-
tory response to the increase in V˙ CO2 (CO2 production) cise. CO2 sensitivity is a measure of the level of ventila-
during exercise. This elevated the PaCO2 that in turn ele- tion for a given arterial CO2 tension (PaCO2 ). Some
vated exercise CBFV, which suggests that CBF increased. people are very sensitive; that is, they breathe a lot
The elevated CBFV was out of proportion to exercise in- at a particular PaCO2 . Others, however, hypoventilate
tensity (V˙ O2 ) and produced symptoms of headache and at the same PaCO2 ; that is, they have low CO2 sensi-
dizziness that limited exercise tolerance. Subthreshold tivity and tolerate higher levels of PaCO2 chronically
aerobic exercise treatment normalized the CO2 physi- (hypercapnia).51 Reduced CO2 response, hypoventila-
ology and exercise capacity of our patients with PCS. tion, increased PET CO2 , and ensuing hypercapnia in
Our data suggest that return of normal CO2 sensitivity response to exercise have been observed in trained un-
and of exercise tolerance can be objective markers of derwater swimmers52 and in nonimmersed divers.53 The
recovery from concussion. locus of altered CO2 sensitivity after concussion is not
Some athletes do not breathe as much as do others known. One possibility is that concussion damages the
in response to increasing CO2 levels; that is, they have brainstem, or communication with the brainstem, thus
reduced CO2 sensitivity. In addition, their ventilatory interfering with the action of the central chemorecep-
response to the increased CO2 production during ex- tors that are highly sensitive to changes in PaCO2 .54 We
ercise is also blunted.46 The subjects with PCS in the have identified brainstem white matter abnormalities by
present study had low CO2 sensitivity and a blunted diffusion tensor imaging in patients with PCS.23 Others
ventilatory response to the increased V˙ CO2 during ex- have identified abnormal regional cerebrovascular re-
ercise. We propose that the low CO2 sensitivity in our sponsiveness during provocative CO2 challenge in pa-
athletes with PCS prior to subthreshold exercise training tients with PCS not present in healthy controls.55 Thus,
was not a consequence of their training status. Subjects altered CO2 physiology may be a biological marker of
with PCS in this study played anaerobic sports such as concussion.
soccer, basketball, and volleyball. These types of athletes The physiological responses of the reference group
do not have reduced CO2 sensitivity.47 Rather, reduced followed the pattern typically observed during treadmill
CO2 sensitivity has been reported only in endurance- exercise.21,30 Many recent studies using TCD show that
trained athletes.46,48 In addition, even if the subjects in CBF increases gradually during exercise. 36-38,40-44,56-66
this study had low CO2 sensitivity to begin with, inac- Ogoh et al,40 for example, found that both CBFV and
tivity eliminates the depressed CO2 sensitivity acquired MAP increased by similar degrees during exercise, which
during active training27,47,49 and our subjects with PCS is consistent with our results at low and medium levels
were inactive for an average of 9 weeks prior to ex- of exercise. At greater workloads beyond the ventilatory
ercise intervention. Therefore, even if they had expe- threshold, however, this relationship changed. Cerebral
rienced some level of a training effect to lower CO2 blood flow velocity significantly decreased following the
sensitivity, it would likely have been lost during their hyperventilatory reduction in PaCO2 despite increasing
prolonged period of inactivity. Thus, it is reasonable to MAP. This suggests that PaCO2 is the primary driving
conclude that the low CO2 sensitivity prior to subthresh- force to CBFV during exercise. Not everyone has, how-
old training in the athletes with PCS was the result of ever, shown this relationship during exercise. Cerebral
concussion. blood flow velocity has been reported to increase with
Could the effects on CO2 sensitivity we observed increasing MAP without changes in PaCO2 .64 This may
with subthreshold exercise training in the PCS subjects be the case at low exercise levels below the ventilatory
have been observed with a similar training program in threshold but appears not to be at the higher inten-
nonconcussed subjects? We think this is unlikely. The sities where hyperventilation normally reduces PaCO2
reference sample was composed of aerobically trained and CBFV.
recreational athletes who continued their regular train- Our patients with PCS stopped exercise because of
ing program and thus it is unlikely that their CO2 sen- the development of symptoms at a lower SBP than the
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8 JOURNAL OF HEAD TRAUMA REHABILITATION
reference group but at levels that are typically within movement artifact. As shown in Figure 2, however, the
the autoregulatory range.21 Despite this, CBFV was signals we obtained were of high quality and an expe-
significantly elevated at all V˙ O2 s versus controls within rienced technician performed all of the measurements,
the range of blood pressures where CA would normally thus removing any concern about operator dependence.
prevent increased CBF,21 leading to speculation that CA We did not measure CA directly or CBFV in cerebral
was not intact in patients with PCS. Laboratory67 and arteries other than the MCA. The CBF response to exer-
clinical studies68 have shown abnormal CA in moderate cise may represent an increase in brain activation42 from
to severe TBI that could be related to the reported alter- increased metabolites in neurons or as a result of neural
ation in ANS activity in patients with brain injury.14,69 control.41,42,57 Linkis et al,57 for example, reported that
Our results are consistent with these reports as patients during exercise of different body parts CBFV increased
with PCS demonstrated altered HR responses versus the in both women and men in the artery supplying the
reference group. The response of patients with PCS to cortical representation of the muscles used during the
exercise treatment is consistent with a normalization of exercise. Jorgensen et al41 found that CBFV increased
ANS function since HRs during exercise returned to the in the MCA but not the anterior cerebral artery in cy-
reference group levels, which has been demonstrated cling women and men. We did not measure flow in
in patients having recovered from moderate to severe the anterior cerebral artery so it is possible that the in-
TBI.70 creased activation (ie, cerebral metabolism) in the cor-
What could explain the resolution of abnormal con- tical representation of the muscles being used during
cussion physiology with subthreshold exercise treat- exercise was responsible for the increased CBFV. Even
ment? Exercise training is well known to increase if that were the case, it does not invalidate the integrated
parasympathetic activity at rest71 and may have restored physiological response we observed in our patients
ANS balance in patients with PCS. Progressive stepwise with PCS.
aerobic training may improve CA by conditioning the
brain to gradually adapt to repetitive mild elevations of
CONCLUSION
systolic BP.36 With respect to ventilation, it has been
shown that a controlled progressive breathing training This study shows that some patients with PCS have
program can increase CO2 sensitivity in persons with abnormal physiological function that limits their exer-
low CO2 sensitivity to begin with.22 The progressive cise tolerance and that subthreshold aerobic exercise
subthreshold exercise program performed by the pa- treatment can improve the physiology and restore ex-
tients with PCS may have improved CO2 sensitivity ercise tolerance to normal. A key physiological mecha-
by simulating such a breathing pattern. nism appears to be a reduced sensitivity to the effects
Limitations of the study include the use of a refer- of arterial CO2 in patients after concussion that blunts
ence group as opposed to employing an identical train- exercise ventilation, increasing arterial CO2 levels, and
ing regimen in the healthy subjects and measuring CO2 therefore CBFV out of proportion to exercise intensity
sensitivity afterward in them. The PCS patients were (V˙ O2 ). We believe that this disproportionate CBFV re-
elite athletes from various team sports while the refer- sponse produces symptoms such as headache and dizzi-
ence group included trained but recreational athletes. ness that limit exercise tolerance after concussion. These
In future studies, a control group composed of seden- data are important for clinicians. They support the hy-
tary subjects or subjects with fitness levels matched to pothesis that patients with PCS with exercise intolerance
the athletes during their PCS would add to the under- have a physiological source for their symptoms that may
standing of the relationships among CO2 sensitivity, be amenable to individualized subthreshold aerobic ex-
V˙ E, V˙ CO2 , PaCO2 , and CBFV. Even though the differ- ercise treatment or to other approaches that improve
ences between the PCS and the reference groups were autonomic function, such as ventilatory biofeedback,72 .
significant, because of the potential heterogeneity of re- Of perhaps greater significance, the data suggest that re-
sponses from PCS patients, further studies with more turn of normal control of exercise CBF and of exercise
subjects would be desirable to confirm these results. Al- tolerance could be objective physiological markers of
though TCD is widely used in similar studies and has recovery from concussion, which has implications for
been shown to be valid and reliable by many authors, establishing prognosis and preventing premature return
there are potential errors due to probe placement and to sport, activity, or military duty.
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