21 lessons · 50 questions

Learn the EKG

From the conduction system to Sgarbossa: interactive lessons with labeled diagrams, live rhythm strips, and clinical pearls.

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50 strip-based questions across Beginner, Intermediate and Advanced, with an explanation after every answer.

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Fundamentals

What is an EKG?

An electrocardiogram records the heart's electrical activity from the body surface. Every bump on the tracing is a wave of depolarization or repolarization moving toward or away from an electrode.

4 min · 3 sections

The Conduction System

Electrical impulses follow a fixed highway from the SA node to the ventricular muscle. Knowing the order, and the intrinsic rate of each pacemaker, explains every escape rhythm and block.

5 min · 2 sections

EKG Paper & Measurements

Standard EKG paper runs at 25 mm/s with a gain of 10 mm/mV. Every measurement you make, from rate to intervals to ST elevation, comes from counting boxes.

4 min · 2 sections

Waves & Intervals

The P Wave

The P wave is atrial depolarization. Its presence, shape, and relationship to the QRS tell you whether the rhythm is sinus and whether the atria are enlarged.

4 min · 3 sections

The QRS Complex

The QRS represents ventricular depolarization. Its width tells you whether conduction used the normal His–Purkinje system (narrow) or spread muscle-to-muscle (wide).

5 min · 3 sections

The T Wave

The T wave is ventricular repolarization and the most sensitive (and least specific) part of the EKG. Ischemia, electrolytes, drugs, and conduction problems all change it.

4 min · 3 sections

Key Intervals: PR, QRS, QT

Three intervals carry most of the diagnostic weight. Memorize the normal ranges and you'll catch blocks, pre-excitation, and arrhythmia risk.

5 min · 2 sections

The ST Segment

The ST segment runs from the J point (end of QRS) to the start of the T wave. It should be flat at the level of the PR/TP baseline. Elevation or depression is the central question of ischemia.

5 min · 3 sections

Axis & Interpretation

Heart Rate Calculation

Three methods cover every situation: the 300 rule for regular rhythms, the 1500 rule for precision, and the 6-second method for irregular rhythms.

4 min · 3 sections

Axis Determination

The electrical axis is the average direction of ventricular depolarization in the frontal plane. Normal is −30° to +90°. Two leads, I and aVF, get you there in seconds.

5 min · 2 sections

A Systematic Approach

Read every EKG in the same order, every time. A system prevents the most common error: seeing one obvious finding and missing the second.

5 min · 2 sections

Rhythm Recognition

Sinus Rhythms

Sinus rhythms share upright P waves in II, one P per QRS, and a constant PR. They differ only in rate and regularity.

4 min · 2 sections

Atrial Rhythms

Atrial rhythms originate above the AV node and usually produce a narrow QRS. The baseline between beats is where the diagnosis hides.

6 min · 5 sections

Junctional Rhythms

When the AV junction paces the heart, the ventricles depolarize normally (narrow QRS) but the atria are activated backward (inverted or absent P waves).

3 min · 1 sections

Ventricular Rhythms

Impulses that arise in the ventricles bypass the His–Purkinje system, so the QRS is wide and bizarre with a discordant T wave.

6 min · 3 sections

Heart Blocks

AV Blocks (1st–3rd Degree)

AV blocks describe how reliably atrial impulses reach the ventricles. March out the P waves with calipers (or paper), then ask how each P relates to its QRS.

6 min · 4 sections

Bundle Branch Blocks

When one bundle branch is blocked, that ventricle is depolarized late, through muscle. The QRS widens (≥ 0.12 s) and V1 and V6 tell you which side.

5 min · 2 sections

Ischemia

ST Changes & STEMI Evolution

Myocardial infarction evolves on the EKG over minutes to weeks. Recognizing each stage tells you how old the infarct is and whether reperfusion is still possible.

6 min · 3 sections

ACS Localization

Contiguous leads look at the same wall of the heart, and each wall is fed by a predictable artery. Localize the STE, find the reciprocal changes, and you'll know the culprit.

5 min · 2 sections

Advanced

Electrolyte Abnormalities

Potassium and calcium change repolarization in predictable ways. Hyperkalemia is the one that kills: learn its progression cold.

5 min · 3 sections

Chamber Enlargement

Hypertrophied muscle generates more voltage and takes longer to repolarize. Voltage criteria are specific but not sensitive; echo is the gold standard.

4 min · 3 sections