The first electrocardiograph was invented in 1895. That device looked a lot different from today’s machines, and there are some other contenders for the title of “first.”

Research:

• AlGhatrif, Majd, and Joseph Lindsay. “A brief review: history to understand fundamentals of electrocardiography.” Journal of community hospital internal medicine perspectives vol. 2,1 10.3402/jchimp.v2i1.14383. 30 Apr. 2012, doi:10.3402/jchimp.v2i1.14383
• Baldassarre, Antonio et al. “The Role of Electrocardiography in Occupational Medicine, from Einthoven's Invention to the Digital Era of Wearable Devices.” International journal of environmental research and public health vol. 17,14 4975. 10 Jul. 2020, doi:10.3390/ijerph17144975
• Browne, Sir Thomas. “Chap. IV: Of Bodies Electrical.” From Pseudodoxia Epidemica. 1672. https://penelope.uchicago.edu/pseudodoxia/pseudo24.html
• Case Western Reserve. “Cambridge Electrocardiograph, 1920.” https://artsci.case.edu/dittrick/online-exhibits/explore-the-artifacts/cambridge-electrocardiograph-1920/
• Fisch, Charles. “Centennial of the string galvanometer and the electrocardiogram.” Journal of the American College of Cardiology. Volume 36, Issue 6, 15 November 2000. https://www.sciencedirect.com/science/article/pii/S0735109700009761
• Friedman, Paul A. “The Electrocardiogram at 100 Years: History and Future.” Circulation. Volume 149, Number 6. https://doi.org/10.1161/CIRCULATIONAHA.123.065489.
• Fye, W. Bruce. “A History of the Origin, Evolution and Impact of Electrocardiography.” The American Journal of Cardiology. Vol. 73, No. 13. 5/15/1994.
• Goodrich, Joanna. “Forget Electrodes, the First EKG Machine Used Buckets of Saline Solution and Telephone Wire.” IEEE Spectrum. 1/5/2021. https://spectrum.ieee.org/forget-electrodes-the-first-ekg-machine-used-buckets-of-saline-solution-and-telephone-wire
• Howell, Joel D. “Early Perceptions of the Electrocardiogram: From Arrythmia to Infarction.” Bulletin of the History of Medicine, SPRING 1984, Vol. 58, No. 1. Via JSTOR. https://www.jstor.org/stable/44441681
• Jenkens, Dean and Dr Stephen Gerred. “A (not so) brief history of electrocardiography.” ECG Library. 2009. https://ecglibrary.com/ecghist.html
• Macfarlane PW, Kennedy J. Automated ECG Interpretation—A Brief History from High Expectations to Deepest Networks. Hearts. 2021; 2(4):433-448. https://doi.org/10.3390/hearts2040034
• Rautaharju, Pentti M. “Eyewitness to history: Landmarks in the development of computerized electrocardiography.” Journal of Electrocardiology 49 (2016) 1 – 6.
• Rivera-Ruiz, Moises et al. “Einthoven's string galvanometer: the first electrocardiograph.” Texas Heart Institute journal vol. 35,2 (2008): 174-8.
• Salam, Amar M. “The Invention of Electrocardiography Machine.” HeartViews. 2019 Nov 14;20(4):181–183. doi: 10.4103/HEARTVIEWS.HEARTVIEWS_102_19.
• Vincent, Rony. “From a laboratory to the wearables: a review on history and evolution of electrocardiogram.” Iberoamerican Journal of Medicine, vol. 4, núm. 4, pp. 248-255, 2022. https://www.redalyc.org/journal/6920/692072548011/html/