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ECG electrocardiogram learning guide basics interpretation 2025

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ECG electrocardiogram normal sinus rhythm waveform labeled diagram

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I now have comprehensive material from Harrison's, Medical Physiology, and Miller's Anesthesia, plus excellent images. Here is a complete ECG learning guide:

📟 ECG Learning Guide

Sources: Harrison's Principles of Internal Medicine 22e, Medical Physiology (Boron & Boulpaep), Miller's Anesthesia 10e

1. What is an ECG?

An electrocardiogram (ECG) is a surface recording of the electrical potential differences generated by the heart. It reflects every step of the cardiac conduction cycle — from the sinoatrial (SA) node firing to ventricular repolarization.

2. The ECG Paper & Calibration

ECG paper has two grids:

Box size	Time	Voltage
Small box (1 mm)	0.04 s	0.1 mV
Large box (5 mm)	0.20 s	0.5 mV

5 large boxes = 1 second.

Quick heart rate formula: Rate = 300 ÷ (number of large boxes in R-R interval)

1 box → 300 bpm | 2 → 150 | 3 → 100 | 4 → 75 | 5 → 60 | 6 → 50

3. The Normal ECG Waveform

The Wiggers diagram below shows exactly how the ECG waveform relates to mechanical events of the cardiac cycle:

Cardiac cycle diagram showing ECG waveforms correlated with ventricular pressure, aortic flow, ventricular volume, and heart sounds

Miller's Anesthesia — Electrical and mechanical events during a single cardiac cycle

The Waves Explained

Wave/Interval	Represents	Normal Duration
P wave	Atrial depolarization (SA node → atria)	< 0.12 s (3 small boxes)
PR interval	AV node conduction delay	0.12–0.20 s (3–5 small boxes)
QRS complex	Ventricular depolarization	< 0.12 s (3 small boxes)
ST segment	Ventricular plateau (early repolarization)	Isoelectric (flat)
T wave	Ventricular repolarization	Upright in I, II, V2–V6
QT interval	Total ventricular action potential	Varies with rate (QTc < 0.44 s)
U wave	Papillary muscle / slow repolarization	Small, same direction as T

Key physiology:

The P wave = SA node fires → impulse travels through atria via internodal tracts
The PR interval = deliberate AV node delay (gives ventricles time to fill)
The QRS = depolarization down the His bundle → left and right bundle branches → Purkinje fibers → ventricular myocardium
The T wave = ventricular repolarization (note: depolarization is Q→S; repolarization is T)

(Medical Physiology, Box 21-2)

4. The 12-Lead System

A standard ECG uses 10 electrodes to create 12 views of the heart.

12-lead ECG electrode placement showing precordial leads V1–V6 and limb leads RA, LA, RL, LL with resulting 12-lead tracing

Precordial (Chest) Lead Positions

Lead	Position
V1	4th intercostal space, right sternal border
V2	4th intercostal space, left sternal border
V3	Between V2 and V4
V4	5th intercostal space, midclavicular line
V5	Anterior axillary line (same level as V4)
V6	Midaxillary line (same level as V4)

Limb Leads & What They "See"

Lead Group	Leads	Wall of Heart
Inferior	II, III, aVF	Inferior wall (RCA territory)
Lateral	I, aVL, V5, V6	Lateral wall (LCx territory)
Anterior	V1–V4	Anterior wall (LAD territory)
Reciprocal	aVR	Global — expected to be negative

5. Systematic Interpretation — The 14-Step Approach

(Harrison's Principles of Internal Medicine 22e)

Always analyze in this order to avoid errors of omission:

Standardization / calibration (1 mV = 10 mm? Any artifacts or lead misplacement?)
Rhythm (regular/irregular? P before every QRS?)
Heart rate (use the 300-rule above)
PR interval (AV conduction — normal 0.12–0.20 s)
QRS interval (< 0.12 s = narrow = normal; ≥ 0.12 s = wide = bundle branch block)
QT / QTc interval (prolonged QT → torsades risk)
Mean QRS electrical axis (normal −30° to +90°)
P waves (size, shape, duration)
QRS voltages (high = LVH; low = pericardial effusion/obesity)
Precordial R-wave progression (should grow from V1→V5)
Abnormal Q waves (> 1 small box wide or > ¼ R height = pathological)
ST segments (elevation or depression)
T waves (inversion, hyperacute changes)
U waves (prominent in hypokalemia)

Always compare with previous ECGs — this is invaluable.

6. Heart Rate & Rhythm

Normal Sinus Rhythm vs Atrial Fibrillation

Comparison of normal sinus rhythm (regular, visible P waves) vs atrial fibrillation (irregularly irregular, no P waves, chaotic baseline)

Normal Sinus Rhythm: Regular R-R intervals, P wave before every QRS, rate 60–100 bpm.

Atrial Fibrillation: Irregularly irregular rhythm, no discernible P waves, fibrillatory baseline, variable R-R intervals.

7. AV Blocks (Heart Blocks)

Comparison of first, second, and third degree AV heart blocks showing PR interval prolongation, dropped beats, and complete AV dissociation

Block	ECG Finding	Clinical Significance
1st degree	PR interval > 0.20 s (5 small boxes), every P conducts	Usually benign
2nd degree Mobitz I (Wenckebach)	PR progressively lengthens until a QRS is dropped	AV node disease; often reversible
2nd degree Mobitz II	Constant PR, then sudden dropped QRS	His-Purkinje disease; needs pacing
3rd degree (complete)	P waves and QRS complexes completely dissociated; separate independent rates	Medical emergency; pacemaker required

8. ST Changes — Ischemia & Infarction

STEMI (ST-Elevation MI)

12-lead ECG showing acute anterior STEMI with ST elevation V1–V6, most prominent in V2–V4

STEMI criteria (from Tintinalli's Emergency Medicine):

≥ 1 mm ST elevation in ≥ 2 contiguous limb leads, OR
≥ 2 mm ST elevation in ≥ 2 contiguous precordial leads

Localization by leads:

Leads with STE	Infarct Location	Culprit Artery
V1–V4	Anterior	LAD
V1–V6 + I, aVL	Anterolateral	Proximal LAD
II, III, aVF	Inferior	RCA (or LCx)
I, aVL, V5–V6	Lateral	LCx
Posterior (tall R in V1–V2, ST depression)	Posterior	RCA/LCx

Reciprocal changes (ST depression in opposite leads) strongly support true STEMI.

9. Key Intervals at a Glance

Normal ECG Intervals
┌────────────────────────────────────────────────────┐
│  PR interval:    0.12 – 0.20 s                     │
│  QRS duration:   < 0.12 s                          │
│  QT interval:    0.35 – 0.44 s (rate-dependent)    │
│  QTc (Bazett):   < 0.44 s (men) / < 0.46 s (women)│
│  Normal axis:    −30° to +90°                      │
│  Heart rate:     60 – 100 bpm                      │
└────────────────────────────────────────────────────┘

10. Common Patterns Summary

Pattern	Key ECG Feature
Normal sinus rhythm	Regular, P→QRS→T, rate 60–100
Sinus bradycardia	Same as above, rate < 60
Sinus tachycardia	Same as above, rate > 100
Atrial fibrillation	Irregularly irregular, no P waves
Atrial flutter	Regular sawtooth P waves at ~300/min, 2:1 or 4:1 block
SVT (AVNRT)	Narrow QRS tachycardia, P waves hidden in/after QRS
VT	Wide QRS tachycardia (≥ 0.12 s), AV dissociation
VF	Chaotic, no identifiable complexes — cardiac arrest
LBBB	Wide QRS, dominant S in V1, broad notched R in V5–V6
RBBB	Wide QRS, RSR' (rabbit ears) in V1
LVH	Sokolow-Lyon: S(V1) + R(V5 or V6) > 35 mm
Hyperkalaemia	Peaked T waves → wide QRS → sine wave pattern
Long QT	QTc > 440 ms — risk of torsades de pointes
Pericarditis	Diffuse saddle-shaped ST elevation, PR depression

Learning Resources

Life in the Fast Lane ECG Library — free, case-based, comprehensive
ECG Wave-Maven (Harvard BIDMC) — 1000+ practice ECGs with answers
ACLS Medical Training ECG Basics — beginner-friendly overview
Harrison's Principles of Internal Medicine 22e, Chapter 247 — The Electrocardiogram
Medical Physiology (Boron & Boulpaep), Chapter 21 — Box 21-2

Start with rhythm and rate, then work through the 14 steps systematically. Pattern recognition comes with practice — aim to read at least 5–10 ECGs daily using a free online library like ECG Wave-Maven.

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