❤️ ECG & the Heartbeat — Electrical Waves of the Heart

The ECG voltage at each point in the beat cycle is computed as a sum of Gaussian functions: V(t) = sum of A_i times exp(-(t - mu_i)^2 / (2 * sigma_i^2)), where each Gaussian models one wave component (P, Q, R, S, T) with its own amplitude A_i, timing mu_i, and width sigma_i. The beat period is T = 60 / HR seconds, and the phase within each beat is p = t mod T, normalised to 0..1. Ventricular fibrillation is approximated by a sum of incommensurate sine waves at frequencies around 20-100 Hz, replacing the structured Gaussian model.

A standard 12-lead clinical ECG is recorded at 25 mm/s paper speed and 10 mm/mV calibration, so each large 5 mm square equals 0.2 seconds, allowing cardiologists to measure intervals (PR, QRS, QT) and calculate heart rate by dividing 300 by the number of large squares between R peaks. ECGs are used to diagnose myocardial infarction (by ST-segment changes), arrhythmias, conduction blocks, electrolyte imbalances, and drug toxicity within minutes of recording.

Yes, this is a common misconception. A trained endurance athlete may have a resting rate of 35-50 bpm (bradycardia by textbook definition) because their stroke volume is large and each beat pumps more blood; this is a sign of cardiovascular fitness, not disease. Conversely, a normal rate of 70 bpm in a sedentary person may reflect a lower cardiac output per beat. Context, fitness level, and the presence of symptoms always matter far more than the number alone.

Willem Einthoven, a Dutch physiologist, developed the string galvanometer ECG between 1901 and 1903 and published the first systematic ECG recordings of cardiac arrhythmias. He defined the P, Q, R, S, and T wave nomenclature still used today. Einthoven received the Nobel Prize in Physiology or Medicine in 1924 for this discovery. Earlier work by Augustus Waller in 1887 demonstrated that the heart's electrical activity could be recorded from the body surface, but Einthoven's instrument was sensitive and practical enough for clinical use.

The ECG is closely related to the cardiac action potential at the cellular level, where sodium, calcium, and potassium ion channels open and close in sequence to produce the depolarisation and repolarisation currents. The mechanical contraction of the heart (producing the pulse and blood pressure waveform) follows the electrical signal with a short delay. Electromyography (EMG) uses similar surface electrode techniques to record muscle action potentials, and electroencephalography (EEG) records analogous electrical waves from the brain.

Modern smartwatches and fitness bands incorporate single-lead ECG electrodes that can detect atrial fibrillation in real time by analysing R-R interval irregularity. Implantable cardiac monitors can record continuously for up to three years. In biomedical engineering, deep-learning models trained on millions of ECG recordings can now identify conditions such as hypertrophic cardiomyopathy and low ejection fraction from a resting ECG with accuracy approaching that of trained cardiologists.

Researchers are developing high-density multi-electrode mapping systems that can reconstruct three-dimensional electrical activation maps of the whole heart in real time, enabling more precise ablation therapy for complex arrhythmias. Optogenetics is being explored to control cardiac rhythm with light rather than electrical shocks. Computational cardiac models that couple ion-channel dynamics, tissue mechanics, and body-surface ECG prediction are being used to personalise treatment planning and predict which patients are at risk of sudden cardiac death before an event occurs.