{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,14]],"date-time":"2026-04-14T05:15:32Z","timestamp":1776143732647,"version":"3.50.1"},"reference-count":28,"publisher":"MDPI AG","issue":"10","license":[{"start":{"date-parts":[[2015,9,25]],"date-time":"2015-09-25T00:00:00Z","timestamp":1443139200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>There are a limited number of studies on heat stress dynamics during exercise using the photoplethysmogram (PPG) and its second derivative (APG). However, we investigate the most suitable index from short PPG signal recordings for heat stress assessment. The APG waveform consists of a, b, c and d waves in systole and an e wave in diastole. Our preliminary results indicate that the use of the energy of aa area, derived from PPG signals measured from emergency responders in tropical conditions, is promising in determining the heat stress level using 20-s recordings. After examining 14 time domain features using leave-one-out cross-validation, we found that the aa energy extracted from PPG signals is the most informative feature for classifying heat-stressed subjects, with an overall accuracy of 79%. Moreover, the combination of the aa energy with the traditional heart rate variability index of heat stress (i.e., the square root of the mean of the squares of the successive aa intervals) improved the heat stress detection to an overall accuracy of 83%.<\/jats:p>","DOI":"10.3390\/s151024716","type":"journal-article","created":{"date-parts":[[2015,9,28]],"date-time":"2015-09-28T03:02:55Z","timestamp":1443409375000},"page":"24716-24734","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":19,"title":["On Time Domain Analysis of Photoplethysmogram Signals for Monitoring Heat Stress"],"prefix":"10.3390","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1831-0202","authenticated-orcid":false,"given":"Mohamed","family":"Elgendi","sequence":"first","affiliation":[{"name":"Electrical and Computer Engineering in Medicine Group, University of British Columbia and BC Children's Hospital, Vancouver, BC V6H 3N1, Canada"},{"name":"Department of Computing Science, University of Alberta, Edmonton, AB T6G 2E8, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Rich","family":"Fletcher","sequence":"additional","affiliation":[{"name":"Media Lab, Massachusetts Institute of Technology, Boston, MA 02139, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ian","family":"Norton","sequence":"additional","affiliation":[{"name":"National Critical Care and Trauma Response Centre, Darwin, NT 0810, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Matt","family":"Brearley","sequence":"additional","affiliation":[{"name":"National Critical Care and Trauma Response Centre, Darwin, NT 0810, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0945-2674","authenticated-orcid":false,"given":"Derek","family":"Abbott","sequence":"additional","affiliation":[{"name":"School of Electrical and Electronic Engineering, University of Adelaide, Adelaide, SA 5005, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Nigel","family":"Lovell","sequence":"additional","affiliation":[{"name":"Graduate School of Biomedical Engineering, UNSW, Sydney, NSW 2052, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Dale","family":"Schuurmans","sequence":"additional","affiliation":[{"name":"Department of Computing Science, University of Alberta, Edmonton, AB T6G 2E8, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2015,9,25]]},"reference":[{"key":"ref_1","unstructured":"Ezzati, M., Lopez, A., Rodgers, A., and Murray, C.J.L. (2004). Comparative Quantification of Health Risks: Global and Regional Burden of Disease due to Selected Major Risk Factors, World Health Organization. Technical Report."},{"key":"ref_2","first-page":"329","article-title":"National Athletic Trainers\u2019 Association position statement: Exertional heat illnesses","volume":"37","author":"Binkley","year":"2002","journal-title":"J. Athl. Train."},{"key":"ref_3","first-page":"333","article-title":"Validity of devices that assess body temperature during outdoor exercise in the heat","volume":"42","author":"Casa","year":"2007","journal-title":"J. Athl. Train."},{"key":"ref_4","first-page":"915","article-title":"Heart rate variability during high ambient heat exposure","volume":"77","author":"Cotterrell","year":"2006","journal-title":"Aviat. Space Environ. Med."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"1692","DOI":"10.1152\/japplphysiol.01124.2005","article-title":"Neural control and mechanisms of eccrine sweating during heat stress and exercise","volume":"100","author":"Shibasaki","year":"2006","journal-title":"J. Appl. Physiol."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"1801","DOI":"10.1172\/JCI105484","article-title":"Reductions in cardiac output, central blood volume, and stroke volume with thermal stress in normal men during exercise","volume":"45","author":"Rowell","year":"1966","journal-title":"J. Clin. Investig."},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Elgendi, M., Norton, I., Brearley, M., Abbott, D., and Schuurmans, D. (2013). Systolic peak detection in acceleration photoplethysmograms measured from emergency responders in tropical conditions. PLoS ONE, 8.","DOI":"10.1371\/journal.pone.0076585"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"282","DOI":"10.3390\/electronics3020282","article-title":"Wearable photoplethysmographic sensors\u2014Past and present","volume":"3","author":"Tamura","year":"2014","journal-title":"Electronics"},{"key":"ref_9","first-page":"42","article-title":"Classification of wave contour by first and second derivative of plethysmogram","volume":"7","author":"Seki","year":"1977","journal-title":"Pulse Wave"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"215","DOI":"10.2174\/157340312803217184","article-title":"standard terminologies for photoplethysmogram signals","volume":"8","author":"Elgendi","year":"2012","journal-title":"Curr. Cardiol. Rev."},{"key":"ref_11","first-page":"207","article-title":"Clinical usefulness of the second derivative of a plethysmogram (acceleration plethysmogram)","volume":"23","author":"Takazawa","year":"1993","journal-title":"Cardiology"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"14","DOI":"10.2174\/157340312801215782","article-title":"On the analysis of fingertip photoplethysmogram signals","volume":"8","author":"Elgendi","year":"2012","journal-title":"Curr. Cardiol. Rev."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"775","DOI":"10.1536\/ihj.39.775","article-title":"Correlation between wave components of the second derivative of plethysmogram and arterial distensibility","volume":"39","author":"Imanaga","year":"1998","journal-title":"Jpn. Heart J."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"365","DOI":"10.1161\/01.HYP.32.2.365","article-title":"Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform","volume":"32","author":"Takazawa","year":"1998","journal-title":"Hypertension"},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Taniguchi, K., Nishikawa, A., Nakagoe, H., Sugino, T., Sekimoto, M., Okada, K., Takiguchi, S., Monden, M., and Miyazaki, F. (2007, January 26\u201329). Evaluating the surgeon\u2019s stress when using surgical assistant robots. Proceedings of the 16th IEEE International Symposium on Robot and Human interactive Communication (RO-MAN 2007), Jeju Island, Korea.","DOI":"10.1109\/ROMAN.2007.4415210"},{"key":"ref_16","unstructured":"Elgendi, M., Jonkman, M., and de Boer, F. (2009, January 9\u201311). Measurement of a-a intervals at rest in the second derivative plethysmogram. Proceedings of the IEEE 2009 International Symposium on Bioelectronics and Bioinformatics, RMIT University, Melbourne, Australia."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Elgendi, M., Jonkman, M., and de Boer, F. (2010, January 26\u201328). Applying the APG to measure heart rate variability. Proceedings of the 2nd International Conference on Computer and Automation Engineering, Singapore.","DOI":"10.1109\/ICCAE.2010.5452042"},{"key":"ref_18","unstructured":"Elgendi, M., Jonkman, M., and de Boer, F. (2010, January 20\u201323). Heart rate variability measurement using the second derivative photoplethysmogram. Proceedings of the 3rd International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS2010), Valencia, Spain."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Fred, A., Filipe, J., and Gamboa, H. (2011). Biomedical Engineering Systems and Technologies, Springer. Communications in Computer and Information Science.","DOI":"10.1007\/978-3-642-18472-7"},{"key":"ref_20","unstructured":"Matsuyama, A. (2009). ECG and APG Signal Analysis during Exercise in a Hot Environment. [Ph.D. Thesis, Charles Darwin University]."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"125","DOI":"10.1016\/j.cmpb.2014.08.001","article-title":"Detection of c, d, and e waves in the acceleration photoplethysmogram","volume":"117","author":"Elgendi","year":"2014","journal-title":"Comput. Methods Programs Biomed."},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Elgendi, M., Norton, I., Brearley, M., Abbott, D., and Schuurmans, D. (2014). Detection of a and b waves in the acceleration photoplethysmogram. BioMed. Eng. Online, 13.","DOI":"10.1186\/1475-925X-13-139"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"139","DOI":"10.1017\/S1049023X12001847","article-title":"Physiological responses of medical team members to a simulated emergency in tropical field conditions","volume":"28","author":"Brearley","year":"2013","journal-title":"Prehosp. Disaster Med."},{"key":"ref_24","first-page":"3","article-title":"Teoria statistica delle classi e calcolo delle probabilit\u00e0","volume":"8","author":"Bonferroni","year":"1936","journal-title":"Pubbl. Ist. Super. Sci. Econ. Commer. Firenze"},{"key":"ref_25","first-page":"65","article-title":"A simple sequentially rejective multiple test procedure","volume":"6","author":"Holm","year":"1979","journal-title":"Scand. J. Stat."},{"key":"ref_26","unstructured":"Chen, C., Liaw, A., and Breiman, L. (2004). Using Random Forest to Learn Imbalanced Data, University of California. Technical Report."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"468","DOI":"10.1097\/00005768-199803000-00020","article-title":"Telemetry pill measurement of core temperature in humans during active heating and cooling","volume":"30","author":"Hoyt","year":"1998","journal-title":"Med. Sci. Sports Exerc."},{"key":"ref_28","first-page":"333","article-title":"Validity of devices that assess body temperature during outdoor exercise in the heat","volume":"42","author":"Casa","year":"2007","journal-title":"J. Athl. Train."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/15\/10\/24716\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T20:49:15Z","timestamp":1760215755000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/15\/10\/24716"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2015,9,25]]},"references-count":28,"journal-issue":{"issue":"10","published-online":{"date-parts":[[2015,10]]}},"alternative-id":["s151024716"],"URL":"https:\/\/doi.org\/10.3390\/s151024716","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2015,9,25]]}}}