• Development and refinement of cardiac electrical measurement and mapping methods across the 1920s–30s, linking graphic heart-beat registration, surface potential distributions, action currents, and geometry to ECG-based diagnostics. [1] [14] [7] [4] [20] [8]

• Ischemia, anoxemia, and electrolyte modulation emerged as key determinants of cardiac electrical activity, with ischemic ECG changes, hypoxemia effects, potassium influence, and ballistocardiography linking physiology to pathology. [9] [6] [10] [11] [12]

• Conduction geometry and body context shaped electrical signals: heart position, body conductance, and electrode arrangement altered Q-R-S morphology and surface potentials, guiding interpretation and modeling. [15] [14] [4] [7]

• Clinical and population perspectives on ECG patterns and variability—normative four-lead ECGs, hypertension-associated changes, heart rate dynamics, and comparative physiology across subjects. [8] [19] [17] [18]