Heart rate variability as a biomarker of functional outcomes in persons with acquired brain injury: Systematic review and meta-analysis

Abstract

This review aimed to quantify correlations between heart rate variability (HRV) and functional outcomes after acquired brain injury (ABI). We conducted a literature search from inception to January 2020 via electronic databases, using search terms with HRV, ABI, and functional outcomes. Meta-analyses included 16 studies with 906 persons with ABI. Results demonstrated significant associations: Low frequency (LF) (r = -0.28) and SDNN (r = -0.33) with neurological function; LF (r = -0.33), High frequency (HF) (r = -0.22), SDNN (r = -0.22), and RMSSD (r = -0.23) with emotional function; and LF (r = 0.34), HF (r = 0.41 to 0.43), SDNN (r = 0.43 to 0.51), and RMSSD (r = 0.46) with behavioral function. Results indicate that higher HRV is related to better neurological, emotional, and behavioral functions after ABI. In addition, persons with stroke showed lower HF (SMD = -0.50) and SDNN (SMD = -0.75) than healthy controls. The findings support the use of HRV as a biomarker to facilitate precise monitoring of post-ABI functions.

Keywords: Biomarker; Brain injuries; Functional outcomes; Heart rate variability; Meta-analysis; Outcome assessment (health care); Stroke.

Figure 1.. Flow diagram of the study selection process

This figure explains the study selection process, from the literature search to the full-text screening, to identify the included studies satisfying the inclusion and exclusion criteria.

Figure 2.. Correlations between HRV indices and neurological functions at the acute stage.

This figure includes forest plots of meta-analyses showing correlations between HRV indices and neurological functions after acquired brain injuries at the acute stage. We used Pearson’s correlation coefficients to quantify the effect sizes of correlations. We generated the plots according to the frequently used HRV indices in the included studies. Studies measuring neurological functions included the National Institutes of Health Stroke Scale and the Scandinavian Stroke Scale. Negative correlations in the plots indicate that higher HRV is correlated with better neurological function. HRV: Heart rate variability, NIHSS: National Institutes of Health Stroke Scale, SSS: Scandinavian Stroke Scale. Note: Negative correlations indicate that higher HRV is related to better neurological function.

Figure 3.. Correlations between HRV indices and emotional functions

This figure includes forest plots of meta-analyses showing correlations between HRV indices and emotional functions after acquired brain injuries. We used Pearson’s correlation coefficients to quantify the effect sizes of correlations. We generated the plots according to the frequently used HRV indices in the included studies. Studies measuring emotional functions included the Beck Anxiety Inventory, Toronto Alexithymia Scale, Hamilton Depression Rating Scale, and the Beck Depression Inventory. Out of the emotional assessments used in the included studies, studies frequently measured depression and anxiety. Negative correlations in the plots indicate that higher HRV is correlated with better emotional function. HRV: Heart rate variability, BAI: Beck Anxiety Inventory, TAS: Toronto Alexithymia Scale, HDS: Hamilton Depression Rating Scale, BDI: Beck Depression Inventory. Note: Negative correlations indicate that higher HRV is related to better emotional function. Sung et al (2016a): Sung, Chen, et al (2016). Sung et al (2016b): Sung, Lee, et al (2016)

Figure 4.. Correlations between HRV indices and behavioral functions

This figure includes forest plots of meta-analyses showing correlations between HRV indices and behavioral functions after acquired brain injuries. We used Pearson’s correlation coefficients to quantify the effect sizes of correlations. We generated the plots according to the frequently used HRV indices in the included studies. Studies measuring behavioral functions included the Functional Independence Measure, Motor Assessment Scale, Barthel Index, Lower Extremity of Fugl-Meyer Assessment, 6-Minute Walk Test, and Manual Muscle Testing. The Functional Independence Measure and Motor Assessment Scale were the two most common measures used in the included studies. Positive correlations in the plots indicate that higher HRV is correlated with better behavioral function. HRV: Heart rate variability, FIM: Functional Independence Measure, MAS: Motor Assessment Scale, BI: Barthel Index, FM-LE: Lower Extremity of Fugl-Meyer Assessment, 6MWT: 6-Minute Walk Test, MMT: Manual Muscle Testing. Note: Positive correlations indicate that higher HRV is related to better behavioral function.

Figure 5.. Comparisons of HRV indices between people with acquired brain injuries and healthy controls

This figure includes forest plots of meta-analyses comparing differences in HRV indices between people with acquired brain injuries and healthy controls. We used means and standard deviations to quantify the effect sizes of differences between the two comparison groups. We generated the plots according to the frequently used HRV indices in the included studies. Higher effect sizes indicate higher HRV.