Why Heartbeats Matter: Physics Behind ECG Waves

The Basics: Why Your Heart’s Electricity Matters

An ECG, also known as an EKG, records the electrical signals produced by the heart as it contracts and relaxes. But have you ever wondered how a few sticky pads and wires translate your heartbeat into a series of waves on a screen? The answer lies in electric fields, waveforms, and conductivity — the fundamental elements of physics. Each bump, spike, interval, and complex on the graph represents a certain electrical event in the cardiac cycle, from the pacemaker cells initiating a wave of depolarisation to the relaxation of the heart muscles. This is just one example of how physics and medicine come together—using principles of electricity to help doctors diagnose various heart conditions.

A Brief History of the ECG: From Capillary Tubes to Clinics

The electrocardiogram has been a cornerstone of medical diagnostics for over a century, but its roots trace back even further. In the late 1800s, physiologist Augustus Waller published the first human "electrogram," using a capillary electrometer to record the heart’s electrical activity. This technology was revolutionised by Dutch scientist Willem Einthoven in the early 1900s, who accounted for inertia and friction inside the capillaries, along with the existing ability to detect voltage changes produced by the heart’s electrical pulses. Over time, ECG machines became more accessible, and with advancements in electronics, performing an ECG test is now quicker and simpler than ever.

Decoding the Rhythm: Electrical Conduction and the ECG Trace

Every heartbeat begins with an electrical impulse generated by the sinoatrial node, often called the heart’s natural pacemaker. This impulse travels through the atria, reaches the atrioventricular node—where the signal is delayed—and then spreads through the bundle of His and Purkinje fibres, causing the heart muscle to contract in a coordinated rhythm. As these electrical signals move through the heart, they create shifting electric fields that radiate through the surrounding tissues owing to the contraction of the different chambers. Electrodes placed on the skin detect differences in electric potential caused by this wave of depolarisation and repolarisation. The ECG machine captures these changes as a continuous voltage-time graph—producing the P wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T wave (ventricular repolarization).

At the Cellular Level: How Ion Movement Creates Heart Rhythms

The electrical signals recorded by an ECG arise from differences in ion concentrations inside and outside heart cells. Inside cardiac muscle cells (intracellular fluid), potassium ions dominate, while outside (extracellular fluid), sodium and calcium ions are more abundant. This creates a voltage difference across the cell membrane known as the resting membrane potential. When the heart’s pacemaker triggers depolarisation, sodium ions rush into the cells, reversing this voltage difference and making the inside of the cell more positive. As this wave of depolarisation moves through different parts of the heart in a precise sequence, it creates shifting electrical fields that surface electrodes detect. The ECG graph captures these changes as waves, which can be used to interpret the regularity and rhythm of the heartbeat.

The Impact of ECGs Today

Each year, over 300 million ECGs are performed worldwide, underscoring their pivotal role in modern medicine. Studies have shown that early ECG detection can significantly reduce mortality rates in heart attack patients by preventing complications. The principles of electricity, ion movements, and electrical fields that make this technology possible are only achievable thanks to the combined work of both biologists and physicists—their knowledge and collaboration have created this amazing fusion of science that saves lives every day.