Chapter 3 – Electrical Events During One Cardiac Cycle

Goal: From P wave → T wave, understand the full electrical sequence, which vector forms first, and what ECG part represents it.

Chapter Objectives

Which electrical event happens first in the cardiac cycle
Which vector is created at each step
Which ECG component (wave/segment) represents that event
Complete logic from P → PR → QRS → ST → T

Electrical first Mechanical later ECG = vectors Direction + strength + speed

1) What “One Cardiac Cycle” Means (Electrical View)

One cardiac cycle means one complete electrical sequence inside the heart: SA node firing → atrial depolarization → AV delay → ventricular depolarization → ventricular repolarization.

High-yield rule:
Electrical events happen first → mechanical contraction comes after a short delay.
The ECG displays electrical activity only, not contraction force.

Why does electrical happen before mechanical?

Because contraction requires calcium movement and cross-bridge cycling, which starts after cells depolarize. So ECG is the “trigger,” not the “pump output.”

2) First Electrical Event – Atrial Depolarization (P Wave)

Start point: SA node

The SA node in the upper right atrium initiates the impulse. From there, depolarization spreads across the right atrium and then the left atrium.

Vector direction (concept):

Overall spread is generally downward and leftward
Atria are thin → vector magnitude is small
No Purkinje-like rapid network in atria → speed is moderate

ECG representation:
P wave = atrial depolarization

P wave is small because the atrial muscle mass is small.

Common confusion: “Does P wave include SA node firing?”

SA node firing starts the process, but the ECG wave you see mainly reflects the spread of depolarization through atrial myocardium, not the tiny SA node itself.

3) Second Electrical Event – AV Nodal Delay (PR Segment)

After atrial depolarization, the impulse reaches the AV node. The AV node conducts slowly, creating a physiological delay.

Why is AV conduction slow?

AV node tissue is small
Cells conduct slowly
Depolarization is largely calcium-dependent (slow upstroke)

Result:

Delay is about ~0.1 sec (conceptually)
Electrical signal is small and not strongly detected on surface ECG

ECG representation:
PR segment = isoelectric line (electrical silence)

It’s not “no electricity in the heart.” It means no large net moving vector is being seen by the lead at that time.

PR segment vs PR interval (must-know difference)

PR segment = end of P wave to start of QRS (mostly AV nodal delay; flat line)
PR interval = start of P wave to start of QRS (atrial depolarization + AV conduction time)

4) Third Electrical Event – Onset of Ventricular Depolarization (Septal Depolarization → Q Wave)

From the AV node the impulse travels into the His bundle and then into the bundle branches. The first ventricular region to depolarize is usually the interventricular septum.

Septal activation direction (core concept):
Septum tends to depolarize from left → right because the left bundle activates the septum early.

Vector characteristics:

Small vector (septum is smaller than full ventricular mass)
Direction described as rightward (and often slightly upward)
Fast spread due to Purkinje delivery

ECG representation:
Q wave = septal depolarization

A small initial negative deflection may appear in leads that view that septal vector as moving away from their positive electrode.

Important: Not every lead shows a Q wave. It depends on the lead’s viewpoint (vector alignment).

5) Fourth Electrical Event – Major Ventricular Depolarization (R Wave)

After the septum, depolarization spreads through the major bulk of both ventricles: typically inner (endocardium) → outer (epicardium).

Left vs Right ventricle:

Left ventricle is much thicker → produces a stronger vector
Right ventricle contributes a smaller vector

Net result: vectors add up into a dominant direction that is commonly described as downward + leftward.

ECG representation:
R wave = major ventricular depolarization

This is the biggest wave because ventricular mass is the biggest.

Why is the R wave usually tall?

Large muscle mass = large magnitude vector
Rapid Purkinje delivery = quick spread = steep (sharp) deflection
Many leads are aligned to “see” this net vector well

6) Fifth Electrical Event – Basal Ventricular Depolarization (S Wave)

The final part of ventricular depolarization reaches the basal regions (near the top/base of the heart). Because this is the “last portion,” the net vector direction changes.

Vector characteristics:

Small vector (last bit of tissue)
Direction commonly described as upward + rightward
Fast conduction continues

ECG representation:
S wave = late/basal ventricular depolarization

Often appears as a negative deflection after R if that late vector moves away from the lead’s positive electrode.

7) QRS Complex = Complete Ventricular Depolarization

Wave	Main Electrical Meaning
Q	Septal depolarization (early ventricular activation)
R	Major ventricular depolarization (bulk of ventricular muscle)
S	Late/basal ventricular depolarization (final activation)

One line: QRS = depolarization spreading through the entire ventricles

8) ST Segment – Ventricles Fully Depolarized (Flat Line)

After QRS ends, the septum + ventricular walls + base are all depolarized. When all ventricular cells are in a similar electrical state, there is no net moving vector.

Electrical condition:

“Electrically silent” on surface ECG (no net vector)
Cells are largely in the plateau phase

ECG representation:
ST segment = isoelectric line

Flat ST does not mean “the heart stopped.” It means “no net vector is moving.”

9) Ventricular Repolarization – T Wave

Golden physiology rule:
The last area to depolarize tends to repolarize first

Sequence idea:

Depolarization often goes: inside → outside
Repolarization often goes: outside → inside

Many courses explain this by differences in action potential duration across layers (epicardial vs endocardial) and recovery timing.

ECG representation:
T wave = ventricular repolarization

Even though repolarization involves “negative” electrical recovery, the T wave can still be upright in many leads because of vector direction + polarity logic.

Why can the T wave be upright even though it’s repolarization?

Because the ECG deflection depends on vector direction + polarity. Repolarization is not simply “a negative wave.” If the net repolarization vector aligns in a way that produces a positive projection on that lead, the T wave appears upright.

Why Don’t We See Atrial Repolarization?

Atrial repolarization does occur, but it is usually not visible because it happens around the time ventricular depolarization occurs.

Reason:

Ventricular depolarization (QRS) has a much larger signal (ventricles are thick).
The atrial repolarization signal is small and gets “masked” by the QRS complex.

Chapter 3 Summary (Golden Table)

ECG Component	Electrical Event	What’s happening (simple)
P wave	Atrial depolarization	Impulse spreads across both atria
PR segment	AV nodal delay	Slow conduction; no big net vector on the surface
QRS complex	Ventricular depolarization	Septum → bulk ventricles → late/basal regions
ST segment	Ventricles fully depolarized	Uniform electrical state; net vector is ~0
T wave	Ventricular repolarization	Recovery phase; vector logic determines T direction

Visited 20 times, 1 visit(s) today