Autonomic and autoregulatory responses to stressors in young men and women
Clausen, Mary R.
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Although studies have examined the effects of physiological stressors, specifically exercise and posture changes, on autonomic and autoregulatory function no clear conclusions have been made particularly regarding the regulation of cerebral blood flow (CBF). Additionally post concussion syndrome (PCS), diagnosed as symptoms related to a concussion present weeks after the concussion occurred, has not been properly examined. The causation of PCS and the effectiveness of an exercise therapy program are in need of evaluation with appropriate physiological tests. Twenty-two normal subjects (13 women and 9 men) were recruited to determine the normal autonomic and autoregulatory responses to exercise and posture changes. The physiological tests used were a CO 2 sensitivity test using a rebreathing technique, a graded exercise test on a treadmill and a tilt table test consisting of supine, head-down tilt (HDT) and erect positions. Heart rate, blood pressure and cerebral blood flow velocity (CBFV) were measured throughout all three tests. Additionally, cardiac output, stroke volume and forearm blood flow were measured during the tilt table test, while ventilation was measured during the CO 2 sensitivity and exercise test. Two subjects diagnosed with PCS were also recruited to evaluate the usefulness of the three tests in determining the causation of PCS and the effectiveness of an exercise therapy program for treatment. During the exercise test it was found that in most subjects CBFV and thus cerebral blood flow (CBF) increased 30.84% as a function of oxygen consumption (VO 2 , expressed as a % of maximal VO 2 ) from 60.56 ± 15.87 cm/s up to 77.67 ± 15.69 cm/s at a specific VO 2 , after which it decreased 16.50% to 65.13 ± 15.95 cm/s. This decrease occurred shortly after the ventilatory threshold when ventilation began to increase exponentially and the pressure of end-tidal CO 2 (P ET CO 2 ) began to decrease (respiratory compensation for metabolic acidosis). The reduction in CBFV occurred in spite of the observation that mean arterial pressure increased continually throughout the exercise test as a function of VO 2 . During the tilt table test CBFV was significantly lower during the erect position when compared to supine and HDT, however no differences were found between supine and HDT. CBFV was also significantly higher in women than men throughout the entire test. PCS subjects were able to tolerate all three tests and differences were seen in measurements pre and post treatment with an exercise therapy program. These results suggest that CBF does not remain steady during exercise and is augmented by several physiological factors. The relative contribution of each factor to the regulation of CBF varies depending on the state the body is in, i.e. exercise, rest, orthostatic stress, etc. Differences in women and men in absolute CBFV and thus CBF may be present and warrants future research. The tests used gave reliable and valid indications of cerebral, cardiovascular and autonomic functions. They were tolerated by the PCS subjects and could be successfully used in future studies of the physiological mechanisms behind PCS and the effectiveness of an exercise therapy program. Key words : Autonomic regulation, cerebral blood flow, post concussion syndrome, exercise, posture