ECG criteria
- The mean atrial depolarization vector is usually directed inferiorly and to the left; excitation begins in the region of the sinus node and spreads across both atria toward the AV junction, forming the P wave.
- Ventricular depolarization (QRS) begins predominantly in the endocardial regions and propagates across multiple wavefronts via the His-Purkinje system; the normal mean QRS vector is typically directed to the left and inferiorly, frequently posteriorly, though its direction depends on the sequence of activation, anatomy, and conduction.
- If the vector is directed toward the positive electrode of a lead, a positive wave is recorded; if directed away from it, a negative wave is recorded.
- If the vector is perpendicular to the lead axis, the complex is isoelectric or biphasic.
- The electrical axis of the heart (EAH) is the predominant direction of the mean net QRS vector in the frontal plane, assessed primarily using limb leads (I, II, III, aVF).
- Normally, the mean T vector is generally directed similarly to the mean QRS vector, which is why the T and QRS waves are usually concordant; complete alignment of directions is not required.
- The transition zone in the precordial leads is typically V3–V4, where the amplitudes of R and S are approximately equal; its shift is evaluated alongside R-wave progression and other features.
What it looks like on ECG
A vector is not directly visible on the tracing—it is an abstraction used to explain waveform morphology. At any given moment of the cardiac cycle, millions of myocardial cells depolarize and repolarize, and their individual microvectors summate into a single net (resultant) vector for that moment. Each lead records the potential difference between its electrodes; in the dipole model, this approximately corresponds to the projection of the net cardiac vector onto the lead axis. The amplitude depends on the magnitude of the vector itself, not just the angle.
Sequence of vectors throughout the cycle:
| Phase | Vector | Wave/segment | Direction |
|---|---|---|---|
| Atrial depolarization | P vector | P wave | inferior, leftward (toward II, aVF) |
| Interventricular septum depolarization | initial QRS vector | Q wave (if present) | right to left |
| Ventricular wall depolarization | main QRS vector | R wave | leftward, inferior, frequently posterior (LV dominates in mass) |
| Ventricular repolarization | T vector | T wave | usually similar to QRS (concordant in most leads) |
In the limb leads (I, II, III, aVL, aVF, aVR), the QRS vector is projected onto the frontal plane, which is used to calculate the EAH. In the precordial leads (V1–V6), the same vector is projected onto the horizontal plane, yielding the transition zone and R-wave progression from V1 to V6. At the same time, the 12 leads provide complementary rather than entirely independent views of electrical activity: the six limb leads are mathematically related (Einthoven's triangle and augmented leads), while the six precordial leads record potentials relative to the Wilson central terminal. This is a working dipole approximation, not 12 literal independent "photographs" of a single unchanging vector.
Mechanism
At rest, a myocardial cell is polarized: negative on the inside, positive on the outside. The depolarization wave is a wavefront moving across the cell and tissue, reversing the charge. An electrode toward which the depolarization wavefront is moving sees a rising positive potential, while an electrode behind the wavefront sees a negative potential.
Ventricular activation does not proceed as a single transmural wave, but rather along multiple wavefronts via the His-Purkinje system: first the septum, then the apex, the free walls of both ventricles, and lastly the basal regions. Because the heart is a three-dimensional organ with varying wall thickness and mass (the LV is more massive than the RV), the net vector constantly changes in magnitude and direction, tracing a loop (the vectorcardiographic QRS loop). An ECG lead is a single projection axis of this loop.
Clinical application
- Calculation of EAH and diagnosis of left/right axis deviation
- Assessment of the transition zone and its shift (poor R-wave progression)
- Understanding why ventricular hypertrophy can increase and sometimes shift the mean QRS vector toward the increased myocardial mass—however, the axis direction and the presence of voltage/"strain" criteria are evaluated based on the totality of criteria rather than vector shift alone
- Explanation of ST-segment changes in MI: with transmural/epicardial injury, contiguous leads overlying the injury zone may exhibit ST-elevation, while reciprocal leads show reciprocal ST-depression; subendocardial ischemia is more frequently accompanied by ST-depression without elevation. Reciprocal changes are not present in every MI, and interpretation must always account for the clinical picture, contiguous leads, and ACS criteria
- The basis for bundle branch blocks: altered pathways of depolarization change the direction and velocity of the vector
Differential diagnosis
Vector theory is neither a diagnosis nor a finding, but an explanatory tool. Confusion usually arises elsewhere: students confuse the vector itself (the net electrical dipole of the myocardium) with the lead axis (the line along which the electrode "views" the heart). Wave polarity is determined by the angle between them and the magnitude of the vector, not by the vector alone.
Clinical significance and management
A physician needs an understanding of the vector model not for a standalone conclusion, but as a foundation for interpreting virtually all ECG findings: EAH, hypertrophy, blocks, and infarct zones. There is no specific "treatment" or management strategy for this concept—it is the theoretical basis of ECG interpretation. In practice, a physician applies it by comparing the polarity and amplitude of waves across different leads to reconstruct the direction of the net electrical event in the heart.
Common interpretation errors
- Confusing the depolarization vector with the repolarization vector: these are distinct processes, and normally the net T vector is generally similar to QRS, but its direction and axis do not have to coincide with QRS
- Forgetting that a lead records the projection of a vector rather than its true magnitude directly—the waveform amplitude in a single lead does not equal the "strength" of the heart and also depends on the volume conductor and electrode placement
- Applying vector logic to a single lead without correlating it with the other 11
- Believing that the QRS vector is a single instant rather than a trajectory (loop) that changes over the duration of the QRS complex
- Visualizing ventricular activation as a single "endocardium → epicardium" wave rather than the result of multiple excitation wavefronts through the His-Purkinje system
1. What does an ECG lead record relative to the cardiac vector?
The lead records the potential difference between electrodes; in the dipole model, this approximately corresponds to the projection of the vector onto the lead axis, and amplitude also depends on the vector magnitude.
2. What does the electrical axis of the heart (EAH) correspond to?
The electrical axis of the heart (EAH) is assessed as the predominant direction of the averaged QRS vector projected onto the frontal plane via limb leads.
3. How will the complex in a lead change if the depolarization vector is perpendicular to its axis?
The projection of the vector onto an axis perpendicular to its direction is close to zero, which is why the complex is isoelectric or biphasic.
Frequently asked questions
Why does a doctor need to know about vectors if it is not a diagnosis?
The vector model explains why a wave is positive in one lead and negative in another. Without it, it is difficult to understand the logic of the EAH, hypertrophy, and reciprocal changes in myocardial infarction.
How does a vector differ from a lead axis?
The cardiac vector is the resultant vector of the electrical activity (dipole moment) of the myocardium at a given moment; the lead axis is the direction along which the potential difference is recorded, approximately corresponding to the projection of this vector. The polarity and amplitude of the wave depend on the angle between the vector and the lead axis, as well as on the magnitude of the vector itself.
Why is the QRS vector called a loop rather than a line?
During the QRS complex, which typically lasts about 60–100 ms with a narrow QRS, the direction and magnitude of the net vector continuously change—from the initial septal vector to the final vector of the basal regions—forming a vector loop whose shape and degree of closure are variable.
How is the vector related to the EAH?
The EAH is the predominant direction of the mean resultant QRS vector in the frontal plane, assessed mainly from wave amplitudes in the limb leads.
Why does the T wave normally often have the same polarity as the QRS?
Repolarization generally spreads from the epicardium to the endocardium, i.e., in the direction opposite to the typical spread of depolarization. Due to the reverse polarity of repolarization, the resultant T vector is usually similar to QRS; therefore, T is concordant with QRS in most leads, although their axes do not have to completely coincide.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
- Video lessons and theory for every topic
- Simulator case studies under every lesson
- Unlimited ECG simulator practice
- Study guide: "All ECGs in 25 minutes"
ECG Strip Simulator
ECG, radiology, histology, pathomorphology, pharmacology — thousands of flashcards with detailed explanations.
- Real ECG strips, not textbook drawings
- Instant answers and explanations
- Streaks and progress tracking in your dashboard
Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger A. Clinical Electrocardiography: A Simplified Approach
- Hampton J. The ECG Made Easy
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.