ECG & Bio-potentials
Every heartbeat is orchestrated by a precisely timed wave of electrical activity. Specialised cells in the heart generate and propagate ionic currents that cause muscle fibres to contract in a coordinated sequence — first the atria (upper chambers), then the ventricles (lower chambers). These tiny voltage differences, on the order of millivolts, spread through the body's conductive tissues and reach the skin, where they can be detected by electrodes placed on the surface. Recording these signals over time produces an electrocardiogram (ECG, or EKG from the German Elektrokardiogramm). Bio-potentials are electrical potentials generated by living cells. They arise from the movement of ions — chiefly sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻) — across cell membranes through ion channels. When a cardiac cell is stimulated, it undergoes depolarisation: Na⁺ rushes in, making the inside of the cell temporarily positive relative to the outside. This reversal of the resting membrane potential (about −90 mV in cardiac muscle) triggers an action potential that propagates from cell to cell. The subsequent repolarisation, when K⁺ flows out restoring the negative interior, completes one electrical cycle. The ECG is one of the most powerful and non-invasive diagnostic tools in medicine. By examining the shape, duration, and timing of the characteristic waves in the ECG trace, clinicians can identify arrhythmias (irregular heart rhythms), myocardial infarctions (heart attacks), conduction defects, and electrolyte imbalances. Understanding the physics behind how these bio-potentials are generated and measured is fundamental to medical physics and bioelectronics.
- Resting Membrane Potential
- Action Potential and Depolarisation
- The Conduction System
- Volume Conduction and Electrode Detection
- Reading the ECG Trace: PQRST Waves
- Instrumentation and Signal Amplification
Resting Membrane Potential
At rest, cardiac muscle cells maintain a potential difference of approximately −90 mV across their membrane (inside negative). This is sustained by the Na⁺/K⁺-ATPase pump and selective ion permeability. Think of the cell membrane as a charged capacitor holding energy ready to be released.
Action Potential and Depolarisation
When stimulated, voltage-gated Na⁺ channels open rapidly, flooding the cell with positive charge. The membrane potential swings from −90 mV to about +30 mV — depolarisation. In cardiac cells, a prolonged plateau phase (caused by Ca²⁺ influx) keeps the cell contracted for ~200–300 ms, much longer than a nerve action potential, preventing tetanic (sustained) contraction.
The Conduction System
The electrical signal originates in the sinoatrial (SA) node — the heart's natural pacemaker — located in the right atrium. It spreads across both atria, reaches the atrioventricular (AV) node, then travels rapidly down the Bundle of His and Purkinje fibres to the ventricular muscle. This routing ensures atria contract before ventricles, optimising blood flow.
Volume Conduction and Electrode Detection
The body acts as a volume conductor — the ionic currents flowing in the heart create potential gradients throughout the torso. Electrodes on the skin detect these small voltages (typically 0.5–3 mV peak). A standard 12-lead ECG uses 10 electrodes (4 on limbs, 6 on the chest) to view the heart's electrical activity from 12 different geometrical perspectives (leads), each sensitive to a different component of the cardiac dipole vector.
Reading the ECG Trace: PQRST Waves
A normal ECG cycle contains named deflections: the P wave (atrial depolarisation), the QRS complex (rapid ventricular depolarisation, typically <120 ms), and the T wave (ventricular repolarisation). The PR interval (start of P to start of QRS, normally 120–200 ms) represents AV conduction time. The QT interval (start of QRS to end of T, normally <440 ms) reflects total ventricular electrical activity. A prolonged QT interval, for instance, indicates risk of dangerous arrhythmias.
Instrumentation and Signal Amplification
Because bio-potentials are so small, ECG amplifiers must have very high input impedance (>10 MΩ) so they don't load the circuit and alter the signal. Differential amplifiers are used to amplify the voltage difference between two electrodes while rejecting common-mode noise (e.g., 50/60 Hz mains interference picked up by both electrodes equally). This common-mode rejection ratio (CMRR) is typically >80 dB in clinical ECG machines.
The Key Equation
- V_{\text{lead}} — Lead voltage (mV): The voltage signal recorded by the ECG for a given lead configuration, typically in the range 0.1–3 mV.
- \phi_A — Potential at electrode A (mV): The electric potential measured at the positive electrode of the lead, arising from ionic currents in the body.
- \phi_B — Potential at electrode B (mV): The electric potential measured at the negative (reference) electrode of the lead. For Lead I, A is the left arm and B is the right arm.
Worked Example
Problem. An ECG trace shows that the R-R interval (the time between two successive R peaks, representing one complete cardiac cycle) measures 0.75 s. (a) Calculate the heart rate in beats per minute. (b) The QRS complex duration is measured as 80 ms. Is this within the normal range? (c) If the paper speed is 25 mm/s and the QT interval spans 11 small squares (each 1 mm wide), calculate the QT interval in milliseconds and comment on whether it is normal for this heart rate.
Answer. Heart rate = 80 bpm; QRS is normal; QTc ≈ 508 ms, which is prolonged.
Origins · 1 minWillem Einthoven and how this was discovered
Einthoven invented the string galvanometer in 1901, which was sensitive enough to record the tiny electrical signals from the heart with unprecedented accuracy. He systematically described and named the P, Q, R, S, and T waves of the ECG trace — nomenclature still used universally today. He also defined the three standard bipolar limb leads (Lead I, II, and III) and formulated Einthoven's triangle, a geometrical model relating the voltages recorded by these leads: V_I + V_III = V_II. He was awarded the Nobel Prize in Physiology or Medicine in 1924 for his discovery of the electrocardiogram. Einthoven's early string galvanometer weighed about 270 kg and required five people to operate it. Patients had to immerse their hands and one foot in buckets of saline solution to make electrical contact!
Related Concepts
- Action Potentials & Nerve Conduction — The ECG signal originates from cardiac action potentials; understanding ion channel dynamics and membrane depolarisation is essential to interpreting ECG waveforms.
- Biomedical Electrodes & Transducers — ECG signals are detected by electrodes that convert ionic current in tissue into electronic current; electrode-electrolyte interface physics determines signal quality.
- Differential Amplifiers & Signal Processing — ECG machines use differential amplifiers with high CMRR to extract millivolt bio-potential signals from background electrical noise.
- Medical Ultrasound — Like ECG, ultrasound is a non-invasive diagnostic tool in medical physics; comparing modalities deepens understanding of how different physical principles are applied clinically.