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# How ECG waves are built: the vectors of the QRS, T and P

- Mechanism
- Exam priority: medium

At each instant the heart's current has one main direction and size, an arrow. Each lead records how much of that arrow points its way.

Updated Nov 13, 2026

Also called: Vectorial analysis, Instantaneous mean vector, Vectorcardiogram

**See it in 3D:** [Interventricular septum](https://durgical.com/en/anatomy/interventricular-septum/) · [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) · [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/) · [Bundle branches](https://durgical.com/en/anatomy/right-bundle-branch/) · [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/)

## Key points

- Projection rule: an arrow parallel to a lead's axis gives the largest deflection; at 90° to it, almost nothing (an isoelectric or tiny biphasic complex).
- First QRS vector: the septum depolarizes left to right, giving the small septal q in I, aVL, V5–V6 and the small r in V1.
- Main QRS vector: the thick left ventricle dominates, pointing left, down and back: tall R in II and V5–V6, deep S in V1.
- Last vectors: posterobasal LV and the upper septum point up and back, giving small terminal S waves.
- Normal T vector lies within about 45–60° of the mean QRS vector, so T has the same sign as the main QRS in most leads.
- Mean P vector ≈ +60° (right atrium first, then left): upright P in II, inverted P in aVR.

## What it is

Vector analysis treats the heart's electrical activity at each moment as a single arrow (vector): its direction is the net direction of current flow and its length the voltage. Projecting that arrow onto each lead's axis predicts the size and sign of the deflection in that lead. Following the arrow through the beat explains the normal shape of the P, QRS and T in every lead.

## Key facts

- Vector length is set by the mass of muscle depolarizing at that instant; this is why hypertrophy raises QRS voltage.
- A lead perpendicular to the mean QRS vector shows the most equiphasic complex; this is the quick way to estimate the axis.
- When conduction is abnormal (bundle branch block), depolarization and repolarization travel the same way, so T becomes opposite to the QRS (secondary T change).
- In a sinus rhythm the P loop is small because atrial muscle mass is small (P ≤ 0.25 mV).

## How it works, step by step

- Current flows from the depolarized (negative) region toward the still-resting (positive) region; the vector points toward the resting muscle.
- ≈ 0.01 s into the QRS the left septal surface fires first, so the vector points right and forward: small q in left leads, small r in V1.
- Apex and the walls of both ventricles follow; the left ventricle is about three times thicker, so the vector swings strongly left and down.
- This large leftward, downward vector makes the R wave and sets the mean electrical axis (normally ≈ +60°).
- The last areas to fire are the posterior base of the LV and the outflow regions: a small vector up and back, the terminal S.
- Repolarization starts at the epicardium and apex, which have shorter action potentials, so its vector points the same way as the main QRS vector.
- The atria fire from the sinus node down and to the left: the P vector. Atrial repolarization (Ta wave) points the opposite way and is usually hidden in the QRS.
- Joining the tips of all instantaneous vectors gives a loop, the vectorcardiogram: a small P loop, a large QRS loop and a medium T loop.

### Electrocardiogram: a normal beat in lead II

**HR** 75 /min **RR** 0.80 s **PR** 0.16 s **QRS** 0.09 s **QT** 0.37 s **QTc** 0.41 s

Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.

- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part

Tap the QRS and T: each wave is the projection of the heart's arrow onto lead II at that moment.

### Action potential

**Rest** −90 mV **Threshold** −70 mV **Peak** +25 mV **Duration** ≈ 280 ms

Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.

- Mainly Na⁺
- Mainly K⁺
- Mainly Ca²⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period

The ventricular plateau and repolarization that set the timing of the T wave.

## Successive ventricular vectors and the deflections they give

| Moment of the QRS | Vector direction | Lead I / V6 | Lead V1 |
| --- | --- | --- | --- |
| Early (septum) | Right and forward | Small q | Small r |
| Middle (free walls, apex) | Left, down, back | Tall R | Deep S |
| Late (posterior base) | Up and back | Small s | End of S |
| T wave (repolarization) | Close to the mean QRS | Upright T | T often flat or inverted (normal in V1) |

## Clinical notes

- Loss of the septal q in I and V6 suggests the septum is no longer activated from left to right, as in left bundle branch block.
- A new T wave pointing far from the QRS vector (wide QRS–T angle) is a sign of primary repolarization change, such as ischemia or hypertrophy with strain.

## Memory aid

Shadow on a ruler: the heart's arrow casts a shadow on each lead; long shadow, big wave; no shadow, flat wave.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does a normal lead V6 begin its QRS with a small negative q wave?

The septum depolarizes first from left to right, so the earliest vector points away from the left-sided V6 and gives a brief negative deflection.

## The mean QRS vector at one instant points at +150°. What will lead I record at that moment?

A negative deflection, since +150° projects onto the negative half of lead I (0°). The voltage will be fairly large because the arrow is far from perpendicular to lead I.

## Why is the normal T wave concordant with the QRS, but discordant in bundle branch block?

Normally repolarization runs opposite to depolarization (shorter epicardial action potentials), so the signs cancel. In block, slow cell-to-cell conduction makes repolarization follow the activation order, so T opposes the QRS.

## Why is the atrial repolarization wave rarely seen on a normal ECG?

It is small (little muscle) and occurs at the same time as the much larger QRS, which hides it.

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## Connections

## See it in 3D 5

The structures of Durgical's 3D anatomy where it happens.

- [Interventricular septum](https://durgical.com/en/anatomy/interventricular-septum/)
- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)
- [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/)
- [Bundle branches](https://durgical.com/en/anatomy/right-bundle-branch/)
- [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/)

## Related topics 6

- [The cardiac electrical axis](https://durgical.com/en/physiology/the-cardiac-electrical-axis/)
- [ECG basics: waves, intervals and leads](https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/)
- [ECG leads: Einthoven's triangle, augmented and chest leads](https://durgical.com/en/physiology/ecg-leads-einthoven-s-triangle-augmented-and-chest-leads/)
- [Bundle branch and fascicular blocks](https://durgical.com/en/physiology/bundle-branch-and-fascicular-blocks/)
- [The cardiac conduction system](https://durgical.com/en/physiology/the-cardiac-conduction-system/)
- [The ventricular action potential](https://durgical.com/en/physiology/the-ventricular-action-potential/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Electrocardiogram: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003868.htm) (MedlinePlus, US National Library of Medicine, 2025); [Hypertensive Heart Disease](https://www.ncbi.nlm.nih.gov/books/NBK539800/) (StatPearls (NCBI Bookshelf), 2025).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/how-ecg-waves-are-built-the-vectors-of-the-qrs-t-and-p/