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Background: Heart rate variability (HRV) is a popular tool to quantify autonomic function. However, this typically requires an expensive 3-12 lead electrocardiogram (ECG) and BioAmp system. This investigation sought to determine the validity and reliability of an OpenBCI cyton biosensing board (open source) for accurately quantifying HRV. New method: A cyton board with a 3-lead ECG was employed to acquire heart rate waveform data, which was processed to obtain HRV within both time-and frequency-domains. The concurrent validity was compared to a simultaneous recording from an industry-standard 3-lead ECG (ADInstruments) (n ¼ 15). The reliability of the cyton board was compared between three days within a 7-day timespan (n ¼ 10). Upright quiet-stance short-term HRV metrics were quantified in time-and frequency-domains. Results: The two devices displayed excellent limits of agreements (all log mean differences AE0.4) and very high between-device variable associations (all r 2 > 0.98). Between the three time points in the same subjects, no differences were noted within time-(all p > 0.71) or frequency-domains (all p > 0.88) across testing points. Finally, all HRV metrics exhibited excellent levels of reliability through high Cronbach's Alpha (all !0.916) and intraclass correlation coefficients (all !0.930); and small standard error of the measurement (all 0.7) and typical error of the measurement (all 0.1) metrics.Comparison with existing methods: The cyton board with 3-lead ECG was compared with an industry-standard ADInstruments ECG during HRV assessments. There were no significant differences between devices with respect to time-and frequency-domains. The cyton board displayed high-levels of between-day reliability and provided values harmonious to previous ECG literature highlighting the applicability for longitudinal studies. Conclusion: With proper background knowledge regarding ECG principles and a small degree of set-up complexity, an open source cyton board can be created and employed to perform multimodal HRV assessments at a fraction of the cost (~4%) of an industry-standard ECG setup.

Objective Systemic hypoxia occurs in COVID-19 infection; however, it is unknown if cerebral hypoxia occurs in convalescent individuals. We have evidence from other conditions associated with central nervous system inflammation that hypoxia may occur in the brain. If so, hypoxia could reduce the quality of life and brain function. This study was undertaken to assess if brain hypoxia occurs in individuals after recovery from acute COVID-19 infection and if this hypoxia is associated with neurocognitive impairment and reduced quality of life. Methods Using frequency-domain near-infrared spectroscopy (fdNIRS), we measured cerebral tissue oxygen saturation (StO2) (a measure of hypoxia) in participants who had contracted COVID-19 at least 8 weeks prior to the study visit and healthy controls. We also conducted neuropsychological assessments and health-related quality of life assessments, fatigue, and depression. Results Fifty-six percent of the post-COVID-19 participants self-reported having persistent symptoms (from a list of 18), with the most reported symptom being fatigue and brain fog. There was a gradation in the decrease of oxyhemoglobin between controls, and normoxic and hypoxic post-COVID-19 groups (31.7 ± 8.3 μM, 27.8 ± 7.0 μM and 21.1 ± 7.2 μM, respectively, p = 0.028, p = 0.005, and p = 0.081). We detected that 24% of convalescent individuals’ post-COVID-19 infection had reduced StO2 in the brain and that this relates to reduced neurological function and quality of life. Interpretation We believe that the hypoxia reported here will have health consequences for these individuals, and this is reflected in the correlation of hypoxia with greater symptomology. With the fdNIRS technology, combined with neuropsychological assessment, we may be able to identify individuals at risk of hypoxia-related symptomology and target individuals that are likely to respond to treatments aimed at improving cerebral oxygenation.