Electrocardiogram (ECG / EKG)

An ECG is a recording of the tiny electrical potential differences on the body's surface that reflect the heart's electrical activity. It was introduced by Einthoven in the early 1900s and has changed surprisingly little since. The key insight is that the heart does not depolarize all at once - different regions activate in a precise sequence, creating measurable voltage differences between body surface electrodes at different times.

1. The Cardiac Conduction System

Before reading any ECG, you need to know where the electricity comes from.

Cardiac conduction system showing SA node, AV node, Bundle of His, bundle branches, and Purkinje fibers

The normal conduction pathway (Goldman-Cecil Medicine):

SA node (sinoatrial node) - in the high lateral right atrium near the superior vena cava. It spontaneously depolarizes at the highest rate (~60-100/min) and is the dominant pacemaker.
Depolarization spreads across both atria via specialized tracts (Bachmann bundle carries it to the left atrium). This triggers atrial contraction.
AV node - the only normal electrical connection between atria and ventricles. It deliberately slows conduction, creating a critical delay that allows the atria to finish contracting before the ventricles begin.
Bundle of His - exits the AV node and bifurcates into right and left bundle branches (the left further divides into anterior and posterior fascicles).
Purkinje system - the finest branches. Conduction here is extremely fast, so both ventricles depolarize almost simultaneously despite their large mass.
The electrical signal finally reaches individual ventricular cardiomyocytes, triggering mechanical contraction.

"The AV node is an area of relatively slow conduction, and a delay between atrial and ventricular contraction normally occurs at this locus." - Miller's Anesthesia, 10e

2. The ECG Waveform

Each heartbeat produces a characteristic waveform with named components. Waves represent depolarization or repolarization; intervals include waves; segments are the flat regions between waves.

Normal ECG waveform from lead II showing P wave, QRS complex, T wave, PR interval, ST segment, and QT interval

The Waves, Segments, and Intervals

Component	What it represents	Normal value
P wave	Atrial depolarization	Duration 0.06-0.12 sec; amplitude 2-3 mm
PR interval	Time from atrial depolarization onset to ventricular depolarization onset (includes AV node delay)	120-200 ms (3-5 small boxes)
PR segment	Flat portion = AV node conduction time	Isoelectric (flat)
QRS complex	Ventricular depolarization (Q = initial downstroke, R = upstroke, S = final downstroke)	<120 ms (3 small boxes)
ST segment	Period between end of ventricular depolarization and start of repolarization	Isoelectric (flat, at baseline)
T wave	Ventricular repolarization	Upright in most leads
QT interval	Full ventricular depolarization + repolarization (QRS + ST + T wave)	Corrected QTc <440 ms
U wave	Seen after T wave, most prominent in V2-V3; possibly Purkinje repolarization	Small, may be absent
J point	Junction between end of QRS and start of ST segment	At baseline

Why is atrial repolarization not visible? It is buried within (and electrically masked by) the much larger QRS complex.

Why is QRS duration similar to P wave duration despite the ventricles being much larger? Because the His-Purkinje system conducts extremely fast, so the ventricles depolarize nearly as quickly as the atria.

3. The ECG Paper and Measurements

Normal ECG on grid paper showing measurements: 1 mm = 0.04 sec horizontally, 1 mm = 0.1 mV vertically, with all standard intervals labeled

Standard ECG paper (Goldman-Cecil Medicine):

Horizontal axis = time: 1 small box (1 mm) = 0.04 sec; 1 large box (5 mm) = 0.2 sec
Vertical axis = voltage: 1 small box (1 mm) = 0.1 mV; 10 mm = 1 mV
Recording speed: 25 mm/sec
Standard recording: 10 seconds per strip

Calculating Heart Rate

Regular rhythm: Count large boxes between two consecutive R waves. Divide 300 by that number. (e.g., 4 large boxes = 300/4 = 75 bpm)
Irregular rhythm: Count QRS complexes in a 10-second strip and multiply by 6.

4. The 12 Leads

A standard ECG has 12 leads, each giving a different "view" of the heart's electrical activity:

Limb leads (I, II, III): Look at the heart in the frontal plane
Augmented limb leads (aVR, aVL, aVF): Additional frontal plane views
Precordial (chest) leads (V1-V6): Look at the heart in the horizontal plane, progressing from right to left across the chest

Each lead records the same electrical events but from a different angle, so the same wave may appear positive in one lead and negative in another depending on whether the wave is moving toward or away from that electrode.

5. ECG and the Cardiac Cycle Together

The ECG perfectly coordinates with mechanical events:

Cardiac cycle diagram correlating ECG waves with ventricular pressure, aortic flow, ventricular volume, and heart sounds over time

P wave → atrial systole (atrial contraction, the "atrial kick")
End of R wave → mitral/tricuspid valves close (S1 heart sound)
QRS complex → ventricular depolarization → start of ventricular systole
ST segment → isovolumetric relaxation, then aortic/pulmonic valves open
T wave → ventricular repolarization → end of systole, start of diastole
After T wave → aortic/pulmonic valves close (S2 heart sound), ventricular filling begins

6. Autonomic Influence on the ECG

The autonomic nervous system modifies conduction and rate, which changes what you see on the ECG (Costanzo Physiology, 7e):

Effect	Sympathetic	Parasympathetic
Heart rate	Increases (faster SA nodal firing)	Decreases (slower SA nodal firing)
AV conduction	Faster (shorter PR interval)	Slower (longer PR interval, may cause heart block)
Mechanism	Increased I_Ca in AV node	Decreased I_Ca + increased I_K-ACh in AV node

7. Common ECG Abnormalities (Quick Reference)

Finding	Possible Cause
Prolonged PR (>200 ms)	1st-degree AV block, digoxin toxicity, hyperkalemia
Wide QRS (>120 ms)	Bundle branch block, hyperkalemia, ventricular rhythm
ST elevation	STEMI, pericarditis, Brugada syndrome, early repolarization
ST depression	Subendocardial ischemia, digoxin effect, strain
Prolonged QT	Hypokalemia, hypomagnesemia, drugs (antiarrhythmics, antipsychotics) - risk of torsades
Peaked T waves	Early hyperkalemia
Absent P waves with irregular rhythm	Atrial fibrillation
J wave (Osborn wave)	Hypothermia, hypercalcemia

Summary

The ECG translates the heart's electrical sequence into a readable tracing. P = atria depolarize, PR interval = AV node delay, QRS = ventricles depolarize, ST segment = electrical pause before repolarization, T = ventricles repolarize. Abnormalities in the shape, duration, or position of any of these components point directly to the underlying cardiac problem.

Sources: Costanzo Physiology 7e, Goldman-Cecil Medicine (International Ed.), Miller's Anesthesia 10e

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