During the HV trial, all subjects were able to keep pace with the established breathing rate using the metronome. Healthy subjects demonstrated significant %vMCA decreases (P < 0.001) during each 20-s HV ranging from −28.2% ± 17.1% to −40.4% ± 7.9% (x― = −35.8% ± 12.2%). mTBI subjects also showed a significant decrease after each repetition. Decreases in the mTBI group ranged from −36.3% ± 11.2% to −42.5% ± 11.3% (x― = −40.2% ± 10.3%). %vMCA differed statistically between groups after the recovery period of the second repetition (−10.2% ± 11.4% vs −21.2% ± 6.4%, P = 0.016). %vMCA changes throughout the hypocapnic trials are summarized in Table 2 and Figure 2B.
Similar to the hypercapnic trial, the lowest vMCA values throughout each repetition of HV occurred approximately 5 s after returning to normal breathing. Some of the subjects experienced light-headedness that resolved quickly with commencement of normal breathing. This is a normal reaction in some people, and precautions were taken to avoid injury if syncope occurred (i.e., chair with armrests, personnel). No syncope events occurred during testing for any subjects.
This study examined the effects of mTBI on the cerebrovascular responses to hypercapnia and hypocapnia and is similar in nature to previous research presented by Tegeler et al. (41) in which cerebrovascular abnormalities in athletes were observed after mTBI even after neurocognitive function was restored. Using transcranial Doppler ultrasonography, our study incorporated a valid, reliable testing protocol to observe and document differences between healthy and mTBI-injured athletes while still following the current recommended return-to-play guidelines (28). With the surging interest into mTBI in sport, this study provides additional information and further insight into cerebrovascular pathophysiology after mTBI. The primary finding in our study was that under resting conditions, vMCA was not different between groups, but when challenged with a physiological stress (i.e., BH, HV), the mTBI group demonstrated impairment in CVR after mTBI.
A previous study suggested that the degree of uncoupling between the autonomic and cardiovascular systems was dependent upon severity of the initial trauma (18). Furthermore, athletes suffering mTBI typically become neuropsychologically asymptomatic within 2–14 d (22). In a study by Gall et al. (14), self-reported signs and symptoms of junior hockey players suffering an mTBI, without missing any playing time, returned to baseline values in approximately 2 d. These hockey players showed no significant differences in resting HR but elicited abnormal HR variability in response to submaximal exercise. It was proposed that the severity of neurological damage occurring with mTBI was insufficient to induce any significantly different cardiovascular responses at rest.
Conceptually, the concussed athletes in our current study elicited a similar cerebrovascular response pattern to hyper- and hypocapnia as the cardiovascular abnormalities in the study by Gall et al. (14). Resting vMCA values showed no significant difference between groups and fell within normal values established previously (4,34). However, once physiological stress (i.e., BH/HV) was introduced, significant differences between groups were observed at various times throughout the duration of testing. There were indications of cerebrovascular abnormality throughout the testing protocol of this study. For example and although not statistically significant, a trend was observed where the mTBI subjects’ CBFV failed to return to resting levels after successive hypercapnic challenges, whereas healthy subjects reached baseline levels. Second, there were significant differences between groups after the hypocapnic trials.
Because athletes are typically in better physiological condition than normal or diseased populations, recovery time after exercise is believed to be shorter than what is required in other untrained populations. This, in theory, should allow high-level athletes to better tolerate physiological stressors such as BH and HV. As proposed by Gall et al. (14), a single physiological stressor such as BH or HV would not provide enough physiological stress to induce any abnormal responses. In previously published research, vMCA values have been shown to return to resting values 20 to 40 s after BH in normal, healthy subjects (32,37). Thus, recovery time provided in this study should be adequate in allowing return to resting levels. Normalization of vMCA has been shown to be a slower process after HV than after BH (32,37). Our results are similar in that healthy subjects returned to near-baseline levels after BH but remained decreased after each HV.
A second consideration to keep in mind regarding the outcome of our study is related to the average number of days after injury. Most studies show that the majority of athletes are asymptomatic and ready to return to play after 7–10 d (14). The physiological changes between the onset of injury and full recovery are not well documented, and this study provides some preliminary information. We showed that by day 4.5 ± 1.1 (range = 3–7 d) after injury, abnormal physiological changes still exist. Moreover, we were still able to establish a valid, easily administered protocol for monitoring the physiological consequences of sport-induced mTBI that was consistent with the guidelines proposed by the CISG (28). This is important from a physiological and administrative perspective as it suggests that the guidelines proposed by the CISG provide a safe approach for return to play.
Although the results of this study are interesting and provide further insight into the pathophysiology of mTBI, there are many methodological considerations that should be addressed. First, percentage changes were primarily used in discussing the differences between groups. As described in previous research (1,8,21,31), the use of percentage change was implemented for two reasons: 1) using percentage change allows for the reduction of individual variability that is not related to the methodology of the study, and 2) using these values allows for comparison with previous and future studies (31).
Although the use of the BH method for inducing hypercapnia was easily administered, it proved to be effective in changing PaCO2 levels. However, we recognized that it differs from more commonly used steady-state methods. Longer time courses (90–300 s) of hypercapnia are typically used to allow vMCA to reach steady state in response to changes in PaCO2 (3). In this study, two limitations of a repeated BH method were evident. BHs lasting only 20 s fail to achieve steady-state status, and continuous PETCO2 data are absent throughout the maneuver because of the use of expired gas analysis. However, the methods implemented in this study provide short-term transient changes and address the notion of Gall et al. (14) that mTBI-induced abnormalities may not be present at rest and are only exacerbated in response to sufficient physiological stress. By performing repeated BH as well as HVs, the subjects in the study were exposed to aggressive testing protocols that more closely simulate intermittent activity associated with many types of sporting activities, which has implications with regards to current return-to-play decision guidelines. Although steady-state vMCA may not have been established in response to BH and HV, our observed changes in vMCA were noted using a repeated method and should be considered in future research.
As stated in the “Methods” section, only the right MCA was monitored in this study (with the exception of two subjects whose left MCA was monitored). In more severe traumatic brain injury, it is well known that significant differences in CBFV and CVR may exist between the ipsilateral and contralateral sides of the brain affected by injury (33,35). In these types of injuries, Glasgow Coma Scale scores are commonly ≤8, whereas in mTBI, scores are defined as being between 13 and 15. However, as stated earlier, a large number of people suffering mTBI will be self-reportedly asymptomatic within 2 d (14). mTBI has been commonly called physiologic without anatomical disruption of the brain, and when considering the lack of neurological signs associated with mTBI, it may be plausible that regional differences do not exist as in more severe TBI. The results of this study should not be discounted because of the lack of bilateral monitoring, but future mTBI research should monitor CBFV bilaterally to account for any differences that may be present.
In summary, CVR may be altered after mTBI. As supported previously by Gall et al. (13,14), the fact that responses are unchanged at rest but are abnormal once subjected to physiological stress seems to indicate that impairment in CVR is present after mTBI. This provides further support that a physiologically challenging protocol, like the one devised here, is needed to confirm whether athletes are fully recovered physiologically before returning to play. Further research into the pathophysiology of mTBI and its relationship to systemic physiological function is warranted before firm conclusions can be drawn about whether a protocol such as this is a feasible tool in managing mTBI. Future controlled studies using multimodal integrative physiological techniques (i.e., transcranial Doppler, near-infrared spectroscopy, blood pressure) on consecutive days immediately after the injury are needed to increase our understanding of the mechanism(s) of recovery from mTBI. This will provide an in-depth look into the cerebrovascular pathophysiology of mTBI as well as the relationship between mTBI and the CVR mechanism.
This study was supported by grants from the Canadian Institutes of Health Research (J.P.N.) and the Saskatchewan Health Research Foundation (J.P.N.).
The authors would like to thank Blaine Sautner, head coach of the University of Regina Cougars hockey team, and his players for their continued support of this research as well as all of the participants who were involved.
The authors report no conflict of interest.
The authors acknowledge that the results of this study do not constitute endorsement by the American College of Sports Medicine.
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