{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"institution":[{"name":"bioRxiv"}],"indexed":{"date-parts":[[2026,8,2]],"date-time":"2026-08-02T17:44:33Z","timestamp":1785692673500,"version":"3.56.0"},"posted":{"date-parts":[[2026,7,30]]},"group-title":"Bioengineering","reference-count":29,"publisher":"openRxiv","license":[{"start":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T00:00:00Z","timestamp":1785369600000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100000070","name":"National Institute of Biomedical Imaging and Bioengineering","doi-asserted-by":"crossref","award":["R01EB035129"],"award-info":[{"award-number":["R01EB035129"]}],"id":[{"id":"10.13039\/100000070","id-type":"DOI","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"accepted":{"date-parts":[[2026,7,30]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                <jats:sec>\n                  <jats:title>Background<\/jats:title>\n                  <jats:p>Transcranial direct current stimulation (tDCS) devices adjust output voltage to maintain the target current despite varying impedance. Pulsatile blood flow produces beat-synchronous changes in tissue impedance.<\/jats:p>\n                <\/jats:sec>\n                <jats:sec>\n                  <jats:title>Objective<\/jats:title>\n                  <jats:p>To determine whether impedance-derived heart rate (IHR), heart rate variability (IHRV), and respiration (IDR) can be estimated from tDCS output voltage without additional physiological sensors.<\/jats:p>\n                <\/jats:sec>\n                <jats:sec>\n                  <jats:title>Methods<\/jats:title>\n                  <jats:p>A custom analog front-end acquires the tDCS output voltage across its full dynamic DC range and superimposed AC fluctuations with high precision. Beats detected from the AC-coupled signal yielded normal-to-normal intervals for HR, HRV, and interval-derived respiration. Accuracy was quantified as mean absolute error (MAE) in 10 healthy laboratory participants against ECG and respiration-monitor references, and against chest-strap RR intervals in 19 at-home sessions from 10 participants with mild-to-moderate depression.<\/jats:p>\n                <\/jats:sec>\n                <jats:sec>\n                  <jats:title>Results<\/jats:title>\n                  <jats:p>\n                    Laboratory MAEs versus ECG were 0.57 bpm for HR, 9.40 ms for SDNN, and 18.90 ms for RMSSD (r = 0.995, 0.859, and 0.778; N = 10); respiratory-rate MAE was 1.36 breaths\/min (r = 0.852; N = 6). Across 1-5 mA of tDCS, the cardiac voltage \u0394V\n                    <jats:sub>cardiac<\/jats:sub>\n                    (t) amplitude scaled linearly with current (slope, 0.073 mV\/mA; p &lt; 0.001). The pulsatile impedance \u0394Z\n                    <jats:sub>cardiac<\/jats:sub>\n                    (t) = (\u0394V\n                    <jats:sub>cardiac<\/jats:sub>\n                    (t)\/I\n                    <jats:sub>applied<\/jats:sub>\n                    ) amplitude averaged 0.080 \u00b1 0.029 \u03a9 (mean \u00b1 SD) across 50 participant-current observations, with no significant dependence on intensity (slope, \u22120.002 \u03a9\/mA; p = 0.086). At-home MAEs were 1.43 bpm for HR, 8.86 ms for SDNN, and 24.92 ms for RMSSD (r = 0.995, 0.767, and 0.680; 19 sessions).\n                  <\/jats:p>\n                <\/jats:sec>\n                <jats:sec>\n                  <jats:title>Conclusions<\/jats:title>\n                  <jats:p>tDCS output voltage contains a recoverable cardiac-synchronous signal arising from pulsatile impedance, enabling HR, HRV, and respiratory monitoring without additional physiological sensors.<\/jats:p>\n                <\/jats:sec>","DOI":"10.64898\/2026.07.29.741585","type":"posted-content","created":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T14:35:16Z","timestamp":1785422116000},"source":"Crossref","is-referenced-by-count":0,"title":["Impedance-Derived Heart Rate and Heart Rate Variability from tDCS Output Voltage: Sensorless Physiological Monitoring During Electrical Neuromodulation"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0009-0002-3171-4674","authenticated-orcid":false,"given":"Yahia","family":"Abdalla","sequence":"first","affiliation":[{"name":"Department of Biomedical Engineering, The City College of New York, CUNY, New York, NY"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-1149-6943","authenticated-orcid":false,"given":"Benjamin","family":"Babaev","sequence":"additional","affiliation":[{"name":"Department of Biomedical Engineering, The City College of New York, CUNY, New York, NY"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kevin","family":"Walsh","sequence":"additional","affiliation":[{"name":"Department of Biomedical Engineering, The City College of New York, CUNY, New York, NY"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Catarina","family":"Ferraz","sequence":"additional","affiliation":[{"name":"Department of Biomedical Engineering, The City College of New York, CUNY, New York, NY"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4429-9713","authenticated-orcid":false,"given":"Leigh","family":"Charvet","sequence":"additional","affiliation":[{"name":"Department of Neurology, New York University Grossman School of Medicine, New York, NY"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6226-5627","authenticated-orcid":false,"given":"Giuseppina","family":"Pilloni","sequence":"additional","affiliation":[{"name":"Department of Neurology, New York University Grossman School of Medicine, New York, NY"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0493-8333","authenticated-orcid":false,"given":"Marom","family":"Bikson","sequence":"additional","affiliation":[{"name":"Department of Biomedical Engineering, The City College of New York, CUNY, New York, 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