Heart Rate Variability Is Associated with Motor Outcome 3-Months after Stroke
Introduction
Among the 795,000 individuals who sustain a stroke annually in the United States, almost 85% exhibit motor impairments in 1 limb immediately after stroke.1 These motor impairments are associated with a significant loss of long-term independence.2, 3 For example, motor impairments in the affected upper extremity and lower extremity not only limit individuals from independently performing daily tasks such as dressing or bathing but also restrict the ability to return to work or prestroke roles. This loss of independence is costly, with the cost of rehabilitation projected to be 1.29 billion dollars by 2050.4 Thus, there is a major public health need to minimize long-term dependency after stroke and to reduce associated personal and societal costs. Accurate predictors of the future motor outcome will allow clinicians to establish realistic and attainable rehabilitation goals, provide targeted interventions to enhance long-term independence, and reduce the length of inpatient stay and the cost of stroke rehabilitation. Although initial motor impairment is the best predictor to date,5, 6 it is still difficult to predict the long-term motor outcome in majority of individuals with severe motor impairments after stroke.7 An exploration of the physiological mechanisms associated with the motor outcome may further elucidate our understanding of the prediction of long-term motor outcome after stroke.
Heart rate variability (HRV), or the temporal variations between consecutive heartbeats, is one such physiological parameter, which may be associated with the future motor outcome after stroke. The cortical regions that control motor function also modulate vagus nerve activity,8 a cranial nerve that controls the autonomic functions of the heart. Vagal activity can be quantified using many analyses of HRV, such as the standard deviation (SD) between the consecutive heartbeats (SDNN).9 Fluctuations in HRV mediated by the vagal activity may be influenced by central or peripheral nervous system disorders.10 Furthermore, HRV has been studied to investigate the physiological changes and prognostic factors associated with many pathological conditions including stroke.10 When stroke damages the cortical pathways controlling the upper and lower extremities, concurrent reduction in vagal activity control reduces HRV.11 Thus, HRV is a plausible proxy marker for the integrity of cortical pathways related to the motor impairments of the affected upper and lower extremities, and stroke survivors with high HRV generally require less assistance to complete daily tasks.12
There are few specific data about the degree to which HRV measured shortly after stroke is related to the future motor outcome. The primary aim of this paper was to determine whether HRV acquired upon admission to inpatient rehabilitation is associated with motor outcome 3 months after stroke. We hypothesized that individuals with higher HRV at acute inpatient rehabilitation admission would exhibit greater movement in the affected upper and lower extremities after 3 months. The secondary aim of this paper was to determine whether HRV shows a stronger association with the motor outcome 3 months after stroke in individuals with severe initial motor impairments.
Section snippets
Methods
We recruited a convenience sample of 13 patients with acute stroke with a mean age of 61 years (SD = 12) from an acute inpatient rehabilitation hospital. Participants were included if they (1) were between the ages of 18 and 90 years of age; (2) had experienced a single episode of stroke, which was confirmed with magnetic resonance imaging scans; (3) had unilateral motor weakness characterized by 3 or less on Medical Research Council Score of major muscle groups of upper and lower extremities
Results
Our sample included 54% males with a mean age of 61 years (SD = 12). Table 1 shows the clinical characteristics, SDNN, FMUE, and FMLE scores of all the participants. Both HRV at admission (SDNN) (r = .70, P = .01) and initial upper extremity impairments (FMUE baseline) (r = .84, P = .005) showed a strong positive association with affected upper extremity at 3 months (FMUE at 3 months) (Figs 1, A,B). HRV at admission (SDNN) showed a stronger positive association (r = .60, P = .03) than did
Discussion
Our findings suggest that HRV is positively associated with motor outcome 3 months after stroke. This is consistent with previous studies, where individuals with lower HRV require greater assistance in daily tasks 60 days after stroke.12 It is likely that patients with lower HRV do not gain adequate movement in the affected upper and lower extremities to perform daily tasks 3 months after stroke and therefore require greater assistance.
Traditionally, little attempt has been paid to the
Acknowledgments
We would like to acknowledge Ms. Kara Kenton and Laura Waterstram, who were both affiliated with the Department of Occupational Therapy, University of Pittsburgh, for assisting in the recruitment and testing of the participants. We would also like to thank the study participants and their families.
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