Alicia sees a twelve-lead ECG and assumes twelve sensors are looking directly at twelve parts of the heart. Tricia points to the ten physical electrodes. Kai Kai asks the question that makes the whole recording intelligible: what is a lead, what is an electrode, and how does a changing electrical field become a wave on paper?
An electrocardiogram records changing differences in electrical potential at the body surface and displays those differences from defined viewing directions called leads. The heart’s depolarisation and repolarisation create spatial electrical vectors. Each lead projects those vectors onto its own axis, producing positive, negative or biphasic deflections. The familiar P wave, QRS complex and T wave are therefore views of organised electrical activity, not direct pictures of contraction or blood flow.
This article supports How the Heart Works. How the Heartbeat Works owns the underlying pacemaker and conduction mechanism. Here the reader job is measurement: how that electrical mechanism becomes a surface recording.
This is an educational explanation of normal ECG principles, not a guide to diagnosing chest pain, arrhythmia, infarction, electrolyte disturbance or an individual’s tracing.
1. An ECG is a voltage-difference recording
The heart’s electrical activity produces changing electric fields that spread through conductive body tissues. Surface electrodes detect potential at selected locations.
A lead is formed by comparing potentials in a defined way. The trace therefore represents a voltage difference as a function of time, not the absolute electrical potential of one heart cell.
The AHA/ACCF/HRS standardisation statement The Electrocardiogram and Its Technology emphasises that ECG interpretation depends on understanding how these representative waveforms are derived and displayed.
2. An electrode is a contact point; a lead is a view
The standard twelve-lead ECG uses ten physical electrodes: four associated with the limbs and six placed across the chest.
Those ten electrodes generate twelve conventional leads because the limb electrode signals are combined in several different ways. A lead is therefore a mathematical or electrical viewpoint, not one sticker.
This distinction prevents a common mistake. Twelve displayed traces do not imply twelve independent sensors attached to twelve internal cardiac structures.
3. The limb leads view the frontal plane
Leads I, II and III are bipolar limb leads. Lead I compares left arm with right arm, Lead II compares left leg with right arm, and Lead III compares left leg with left arm under the standard sign conventions.
The augmented limb leads aVR, aVL and aVF provide three additional frontal-plane viewpoints using one designated positive limb electrode referenced against a combination of the others.
Together, the six limb leads sample the heart’s electrical activity from multiple directions around the frontal plane. The NCBI text Electrocardiography describes these axes and their relation to cardiac vectors.
4. The chest leads view the horizontal plane
Leads V1 through V6 are precordial leads placed across the chest. They view cardiac electrical activity in the horizontal plane from right anterior toward left lateral positions.
Each chest electrode is compared with a composite reference, historically related to Wilson’s central terminal. The resulting leads show how ventricular depolarisation projects across the anterior chest.
The frontal and horizontal lead sets therefore provide complementary spatial information rather than twelve repeated copies of one signal.
5. A cardiac vector has magnitude and direction
At any instant, the combined electrical activity of many myocardial cells can be represented approximately as a resultant vector with direction and magnitude.
A lead records the projection of that vector onto the lead’s axis. A vector pointing strongly toward a lead’s positive direction produces a large positive deflection. A vector pointing away produces a negative deflection.
If the vector is nearly perpendicular to the lead axis, the net deflection can be small or biphasic because positive and negative components cancel more closely.
6. One electrical event can look different in different leads
The same ventricular depolarisation event is viewed from twelve directions. It can therefore appear strongly positive in one lead, negative in another and nearly isoelectric in a third.
These differences do not mean the heart performed twelve different electrical events. They are different projections of one three-dimensional process.
A shadow analogy is useful: one object can cast different outlines on walls facing different directions. The analogy captures projection but not the complex conductive geometry of the torso.
7. Depolarisation toward a positive electrode usually creates an upward deflection
For a depolarisation wavefront, movement toward the positive direction of a lead generally produces a positive deflection, while movement away produces a negative deflection.
The rule is a projection rule rather than a statement that positive charge physically piles up under one electrode. It describes how the extracellular potential field changes as organised activation spreads.
Repolarisation requires more careful sign reasoning because it represents recovery of previously depolarised tissue. A repolarisation wave moving away from a positive electrode can create a positive deflection.
8. The P wave represents atrial depolarisation
The sinoatrial node initiates the normal atrial activation sequence. Depolarisation spreads through right and left atrial myocardium, producing the P wave on the surface ECG.
The P wave is smaller than the QRS complex because the atrial muscle mass is smaller and the activation geometry differs.
The P wave does not directly record atrial contraction. Mechanical contraction follows electrical activation after excitation–contraction coupling.
9. The PR interval includes more than AV nodal delay
The PR interval is measured from the onset of the P wave to the onset of the QRS complex. It therefore spans atrial depolarisation plus conduction through the atrioventricular node, His bundle and proximal specialised conduction system before ventricular depolarisation becomes visible.
Calling the PR interval simply the AV-node delay is therefore incomplete. The AV node contributes importantly, but the interval includes several stages.
This is a general measurement lesson: an interval between two visible landmarks often contains more than one hidden process.
10. The QRS complex represents ventricular depolarisation
After traversing the atrioventricular node and His–Purkinje system, activation spreads rapidly through the ventricular myocardium. The aggregate surface signal creates the QRS complex.
The letters Q, R and S describe deflection sequence, not three separate anatomical structures. A Q wave is the first negative deflection before an R wave; R is a positive deflection; S is a negative deflection following R.
Different leads can show different combinations—qR, rS, QS or other patterns—because they view the same ventricular activation sequence from different directions.
11. Atrial repolarisation is usually hidden inside the QRS
Atrial cells repolarise after their depolarisation, but the resulting surface signal is relatively small.
Ventricular depolarisation occurs around the same period and produces the much larger QRS complex, which usually obscures atrial repolarisation.
The ECG therefore can contain overlapping biological events even when only one dominant waveform is visually obvious.
12. The ST segment occurs while much ventricular myocardium is depolarised
The ST segment begins after the QRS complex and precedes the T wave. During this interval much of the ventricular myocardium is in a relatively uniform depolarised state.
Because large regions are at similar electrical state, there is less net spatial voltage difference to produce a large surface deflection, so the segment lies near the isoelectric baseline under ordinary conditions.
The 2026 StatPearls edition of ST Segment describes this interval between ventricular depolarisation and repolarisation.
13. The T wave represents ventricular repolarisation
As ventricular myocytes recover toward their resting electrical state, the changing spatial pattern produces the T wave.
Repolarisation does not simply run backward along the exact path of depolarisation. Action-potential durations differ across myocardial regions, so recovery follows its own spatial sequence.
This is why a T wave can be positive in a lead where the main QRS is also positive even though repolarisation is conceptually the opposite cellular process.
14. The QT interval spans ventricular depolarisation through repolarisation
The QT interval begins at the start of the QRS complex and ends at the end of the T wave. It therefore covers the visible duration of ventricular electrical activation and recovery.
QT duration changes with heart rate, which is why corrected-QT formulas are often used clinically and in research. Different correction formulas make different assumptions.
The AHA standardisation statement notes that defining the end of the T wave and measuring global QT across leads can itself introduce methodological variation. Even a familiar interval has a measurement problem beneath it.
15. The ECG records electrical timing before mechanical timing
Ventricular depolarisation precedes the main rise in ventricular pressure because calcium handling and cross-bridge activation take time. Valve opening occurs later still, after pressure crosses the relevant arterial threshold.
A QRS complex therefore cannot be used as a direct timestamp for peak contraction or aortic ejection without accounting for electromechanical delay.
This is one reason simultaneous ECG, pressure and ultrasound recordings are valuable: they align different stages of one causal chain.
16. Electrical axis summarises the main frontal-plane direction
The mean QRS axis describes the average direction of ventricular depolarisation in the frontal plane.
It can be estimated by comparing the net QRS direction in selected limb leads. A lead aligned with the mean vector tends to show a larger positive deflection; a nearly perpendicular lead tends to be more isoelectric.
Axis is therefore a geometric summary of the recorded electrical field, not a physical arrow printed inside the heart.
17. Einthoven’s relationship is an electrical consistency rule
For the three standard bipolar limb leads, the idealised relationship Lead II = Lead I + Lead III follows from how the potential differences are defined.
If Lead I is LA − RA and Lead III is LL − LA, adding them gives LL − RA, which is Lead II.
This is not a physiological law about how much current each limb receives. It is an algebraic consequence of the lead definitions.
18. Standard calibration makes amplitude and time comparable
A conventional ECG is commonly displayed at 25 mm/s, so one small 1 mm horizontal box represents 0.04 s and one large 5 mm box represents 0.20 s.
A common voltage calibration is 10 mm per millivolt, so a 1 mV signal produces a 10 mm vertical deflection.
These are display conventions. Changing the paper speed or gain changes the visual size of the wave without changing the underlying cardiac event. Interpretation therefore must know the calibration.
19. Digital ECGs still depend on sampling and filtering
Modern ECG systems convert analogue body-surface signals into digital samples. Sampling rate must be high enough to preserve the relevant waveform information.
Filters reduce baseline wander, mains interference and high-frequency noise, but overly aggressive filtering can alter waveform morphology.
The displayed trace is therefore the result of acquisition and processing. Digital appearance does not make the measurement independent of signal-processing choices.
20. Electrode placement is part of the measurement definition
Moving an electrode changes the spatial potential being sampled and therefore changes the lead’s effective view.
Chest-lead placement is especially important because V1 through V6 are intended to occupy defined anatomical positions. Misplacement can alter wave amplitudes and patterns.
The 2026 StatPearls Electrocardiogram review notes that lead misplacement can create misleading tracings. Reproducibility therefore begins with reproducible geometry.
21. Motion and muscle activity create artifacts
Skeletal muscle produces electrical activity, movement changes the skin-electrode interface and mains electricity can introduce interference. These signals can contaminate the much smaller cardiac surface voltages.
An artifact can resemble rapid rhythm, baseline shift or irregular deflection even though the cardiac generator did not produce it.
Good interpretation therefore begins by asking whether the recorded feature is physiological or measurement-related before assigning a cardiac mechanism.
22. An ECG can be normal while a mechanical variable differs
Imagine two model hearts with similar electrical activation timing but different end-diastolic volumes. Their ECGs can look similar while stroke volume differs because filling and mechanics are downstream of the electrical signal.
Likewise, coronary flow, valve competence and chamber pressure are not measured directly by a surface ECG.
This does not make ECG weak. It makes it specific. A measurement becomes powerful when it is asked the question it was designed to answer.
23. Worked problem: why one QRS is positive and another negative
Suppose the mean ventricular depolarisation vector points leftward and downward. A lead with its positive axis pointing in a similar direction records a positive QRS. A lead whose positive axis points roughly opposite records a negative QRS.
The myocardium did not depolarise twice in opposite directions. Two lead axes projected the same spatial event differently.
The exercise is the key to reading multi-lead ECGs without treating each trace as a separate heartbeat.
24. Worked problem: display scale versus physiology
An ECG recorded at 50 mm/s spreads the same electrical event across twice as much horizontal paper as one displayed at 25 mm/s. A 100 ms QRS occupies 5 mm at 50 mm/s but 2.5 mm at 25 mm/s.
The event duration did not change. The display scale changed.
This is why measurements should be derived from calibrated units rather than from visual width alone.
25. Worked problem: electrical timing versus pumping
Model A and Model B both have a cycle length of 800 ms on ECG, corresponding to 75 beats per minute. A ejects 70 mL per beat; B ejects 50 mL.
Their outputs are 5.25 and 3.75 L/min despite identical electrical rate. The ECG timing alone cannot determine the difference.
To explain it, we need mechanical evidence such as filling, chamber volume, valve behaviour or pressure.
26. The ECG mechanism in one causal chain
The sinoatrial node initiates atrial depolarisation, producing the P wave across the appropriate lead projections. Conduction slows through the atrioventricular node and passes into the His–Purkinje system during the PR interval. Rapid ventricular depolarisation produces the QRS complex while atrial repolarisation is largely hidden. The ventricles remain broadly depolarised during the near-isoelectric ST segment. Spatially organised ventricular repolarisation produces the T wave. Ten electrodes supply the signals used to construct twelve standard lead views, each displaying a different projection of the same electrical field.
Alicia stops counting leads as sensors. Tricia draws lead axes instead of twelve little cameras. Kai Kai aligns the ECG with the pressure and echo timelines and marks the electromechanical delay. The trace has become a measurement of electrical sequence rather than a mysterious heartbeat picture.
The deeper lesson is that a surface signal becomes meaningful only when its geometry and physics are known. ECG interpretation starts with what was actually measured: voltage differences projected through defined leads over time.
Evidence trail and connected reading
For a current overview, see the 2026 StatPearls edition of Electrocardiogram. For lead geometry and classical electrocardiographic principles, see NCBI Clinical Methods: Electrocardiography. For technology and measurement standards, see the AHA/ACCF/HRS statement The Electrocardiogram and Its Technology.
Return to the parent: How the Heart Works. Continue to How the Heartbeat Works for the biological generator, How Autonomic Control of the Heart Works for rate and conduction regulation, and How Echocardiography Works for the mechanical imaging counterpart.