1843: Emil Du Bois-Reymond (German physiologist) 1st described “action potentials” of muscular contraction. Named different waves: O (stable equilibrium), P, Q, R, & S. Dr. Willis Hurst credits Willem Einthoven as 1st to use P, Q, R, S, & additional T wave.
1850: M. Hoffa demonstrated how to induce irregular ventricular contractions in canine hearts via electrical shock.
1867: 1st electrical measurement from heart.
1872: Alexander Muirhead attached wires to wrists of patient w/ fever to directly recorded & visualized cardiac activity (heartbeat) using Lipmann capillary electrometer by John Burdon Sanderson (British physiologist).
1887: Augustus Waller (English physiologist) published 1st human EKG; used capillary-electrometer fixed to projector; trace from heartbeat projected onto photographic plate; allowed real-time recording of heartbeat.
1893: Willem Einthoven introduced term “electrocardiogram”.
1895: Einthoven described how he used galvanometer to visualize electrical heart activity.
1901: Einthoven invented string galvanometer – more sensitive than Waller’s capillary electrometer; connected electrodes to pt & showed electrical difference b/t 2 electrodes on string galvanometer (Einthoven’s lead; 1st clinical instrument to record EKG).
1905: Einthoven recorded 1st “telecardiogram” from hospital to his laboratory 1.5 km away.
1906: Einthoven assigned letters P, Q, R, S, & T to deflections (or 1895 – names waves) & described EKG features cardiovascular disorders; published 1st article demonstrating abnormal EKGs (LBBB, RBBB, LAE, RAE, Uw, QRS notching, ventricular extrasystoles, ventricular bigeminy, atrial flutter, total AV block).
1912: Einthoven invents Einthoven triangle.
1924: Einthoven awarded Nobel Prize in Medicine. Goldberger adds augmented & precordial leads (standard EKG).
1944: Wilson Central Terminal: the “invariant” reference
1946: Heart vector & leads 
1953: Developed lead system from torso phantom measurements 
1956: Frank Lead System: based on 3D-Dipole
1963: Body surface potential mapping resulting pattern showed not always dipole
Modern: Basic principles still in use; advances in instrumentation (cumbersome laboratory device to compact electronic systems that included computerized EKG interpretation).
Present: In contrast to data & limited anterior view of heart from traditional 12-lead EKG, 80-lead EKG uses 80 leads on patient’s front & back to analyze 360° spatial view of heart; new technology may allow for more rapid & accurate STEMI detection, thereby increasing speed of care.
1. H.C. Burger & J.B. Milaan, Br Heart J 1946 8(3);157-1761.
2. Richard McFee & Franklin D. Johnston, Circ Vol III.