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Human Physiology II · Systems Physiology

The Electrocardiogram (ECG/EKG)

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Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

The electrocardiogram (ECG/EKG) records the heart's electrical activity from skin electrodes. The reflects atrial depolarization, the ventricular depolarization, and the ventricular repolarization; the measures conduction from atria to ventricles, and the reflects the ventricular plateau. Leads are arranged around the heart ( plus augmented and chest leads) so each lead views the heart from a different angle. Departures from normal rhythm — too fast, too slow, uncoordinated, or blocked — are recognized as arrhythmias.

Why this matters

The ECG is a rapid, noninvasive first-line tool for evaluating rhythm, rate, conduction, and signs of ischemia. Arrhythmias are recognized by pattern: tachycardia is a heart rate above ~100 beats/min, bradycardia below ~60 beats/min; fibrillation is rapid, disorganized electrical activity (atrial fibrillation produces an irregularly irregular rhythm; ventricular fibrillation is a chaotic rhythm that stops effective pumping); and heart blocks are slowed or interrupted AV conduction, graded from first degree (prolonged PR) through second degree (intermittent conduction) to third degree (complete block, atria and ventricles beat independently). Diagnostic criteria, normal ranges, and protocols vary by institution and jurisdiction; urgent symptoms require immediate evaluation by qualified clinicians or local emergency services. These notes support education and do not replace clinical instruction or supervision.

The college version

1. Waves, Intervals, and Segments

The standard tracing consists of deflections (waves) and flat stretches (segments), with intervals spanning combinations.

  • P wave: atrial depolarization (the atria contract just after).
  • QRS complex: ventricular depolarization (a sharp, high-amplitude complex; it also masks the smaller atrial repolarization).
  • T wave: ventricular repolarization.
  • PR interval: from the start of the P wave to the start of the QRS; it represents conduction from the atria through the AV node to the ventricles (normally about 0.12–0.20 s).
  • ST segment: from the end of the QRS (the J point) to the start of the T wave; normally flat (isoelectric), reflecting the ventricular plateau when all cells are depolarized.

2. What the ECG Actually Measures

The ECG measures the net electrical field produced by the heart's depolarizing and repolarizing cells at the body surface — not the mechanical contraction. A wave points "up" when depolarization spreads toward a positive electrode, and "down" when it spreads away.

3. Lead Arrangements and Einthoven's Triangle

The standard leads begin with the three bipolar limb leads (I, II, III), which form Einthoven's triangle, an equilateral triangle conceptually drawn around the heart. Lead I compares the left arm (+) to the right arm (−), Lead II the left leg (+) to the right arm (−), and Lead III the left leg (+) to the left arm (−). By Einthoven's law, Lead II = Lead I + Lead III. A full 12-lead ECG adds three augmented unipolar limb leads (aVR, aVL, aVF) and six precordial (chest) leads (V1–V6) that view the heart from the front and sides.

How it works

  1. Electrodes are placed on the limbs and chest in standard positions.
  2. Each lead records the heart's electrical field from its own angle.
  3. The P wave appears as the SA impulse depolarizes the atria.
  4. After the PR interval (AV conduction delay), the QRS appears as the ventricles depolarize.
  5. The ST segment is flat while the ventricles remain depolarized (plateau).
  6. The T wave appears as the ventricles repolarize.
  7. The rhythm, rate, intervals, and waveform shapes are compared against normal ranges.

Common confusions

Do not confuseWithDifference
The P waveThe T waveP = atrial depolarization (small, before the QRS); T = ventricular repolarization (after the QRS)
The PR intervalThe ST segmentPR interval spans atrial-to-ventricular conduction; ST segment is the flat ventricular plateau
DepolarizationRepolarizationDepolarization makes the inside less negative and triggers contraction; repolarization restores the resting state
Atrial fibrillationVentricular fibrillationAtrial fibrillation still allows some ventricular pumping; ventricular fibrillation produces no effective pumping and is an emergency

Memory aids

"PQRST: Please Quietly Read The Strip." P wave → QRS complex → T wave, in that order, every beat. Use the sequence "P (atria fire) → QRS (ventricles fire) → T (ventricles recover)."

Quick review

Topic Recap

The ECG records the heart's electrical activity from multiple angles. The P wave, QRS complex, and T wave mark atrial depolarization, ventricular depolarization, and ventricular repolarization; the PR interval gauges AV conduction and the ST segment the ventricular plateau. Einthoven's triangle and the 12-lead system provide spatial views, and departures from normal rhythm define the basic arrhythmias.

Knowledge Check

  1. Which ECG event represents ventricular depolarization?
  2. What does the PR interval measure, and what does its prolongation suggest?
  3. Why does the same heartbeat look different in Lead II versus aVR?
  4. Distinguish tachycardia from bradycardia by heart rate.
  5. How does ventricular fibrillation differ from a heart block?

Answers and Rationales

  1. The QRS complex represents ventricular depolarization — the high-amplitude deflection that masks atrial repolarization.
  2. The PR interval measures conduction from the atria through the AV node to the ventricles; prolongation suggests slowed AV conduction (heart block).
  3. Each lead views the heart's electrical vector from a different angle, so the same event projects differently — Lead II points roughly along the heart's axis (large upright QRS), while aVR points oppositely (inverted).
  4. Tachycardia is a rate above ~100 beats/min; bradycardia is below ~60 beats/min.
  5. Ventricular fibrillation is rapid, disorganized electrical activity with no effective pumping; a heart block is slowed or interrupted AV conduction (not necessarily rapid or disorganized).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The ECG is like a crowd at a sports stadium doing "the wave." Electrodes placed around the body are like cameras filming from different seats; each camera sees the wave from its own angle. The P wave is the small ripple when the top section stands up (atria), the big QRS spike is the whole lower bowl jumping up (ventricles), and the T wave is everyone sitting back down (repolarization).

Comparison: it is like listening to a heartbeat through many microphones placed at different spots — each microphone hears the same beat slightly differently, and putting them together tells you where the beat started and how it traveled.

Where it stops being exact: the ECG records only electricity, not the actual muscle squeeze. A normal-looking ECG can still accompany poor pumping (and vice versa), and fine electrical details require precise lead placement and interpretation that go far beyond the "wave" analogy.

Simple Example

Imagine dropping a pebble in a pond. A sensor upstream of the ripple reads a positive blip as the wave approaches and a negative blip as it passes away. Each ECG lead is just such a sensor, and the shape of the blip tells you which way — and how strongly — the electrical wave moved past it.

Worked example

How a deflection forms:

  1. Resting (polarized) cells have a negative interior. The ECG records no deflection when all cells are in the same state — a flat baseline.
  2. When a region depolarizes, its surface becomes negative relative to still-polarized regions, creating a dipole (a tiny "battery" with a negative and positive end).
  3. The electrodes sense this dipole. If the wave of depolarization travels toward the positive electrode, the trace deflects upward; if away, downward.
  4. When the whole region is depolarized (or repolarized), there is no potential difference, and the trace returns to baseline.
  5. Because depolarization moves from the SA node across the atria, then down the septum and through the ventricles, the sequence produces the P–QRS–T pattern, and each lead sees a slightly different projection of the same events.

This is why the same heartbeat looks different in Lead II (a large upright QRS) versus aVR (an inverted QRS): each lead views the same electrical vector from a different angle.

Key takeaways

  • High yield: The P wave = atrial depolarization; QRS = ventricular depolarization; T wave = ventricular repolarization.
  • High yield: The PR interval reflects AV conduction; a prolonged PR interval indicates delayed or blocked conduction (heart block).
  • The ST segment is normally isoelectric; displacement can signal ischemia or infarction (though interpretation requires clinical context).
  • Lead II = Lead I + Lead III (Einthoven's law).
  • The ECG records electrical activity only — a normal ECG does not guarantee normal pumping.
  • Atrial repolarization is usually hidden within the QRS complex.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Name and describe the normal ECG waves, intervals, and segments (P wave, QRS complex, T wave, PR interval, ST segment).
  • Explain how the surface ECG records the electrical activity of the heart and what each deflection represents.
  • Describe Einthoven's triangle and the standard limb, augmented, and precordial leads.
  • Distinguish basic arrhythmias (tachycardia, bradycardia, fibrillation, heart blocks) by their electrical features.

Key vocabulary

P wave
Atrial depolarization
QRS complex
Ventricular depolarization
T wave
Ventricular repolarization
PR interval
Time from atrial to ventricular activation
ST segment
Flat stretch during the ventricular plateau
Einthoven's triangle
The three bipolar limb leads arranged as an equilateral triangle
Standard (12-lead) ECG
3 limb + 3 augmented + 6 precordial leads

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