Ecg intruption with pictures

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Loading Image

Reading File

Loading Image

Reading File

Loading Image

Reading File

Loading Image

Now I have all the key images and content. Let me compile a comprehensive, well-illustrated ECG interpretation guide.

ECG Interpretation - A Complete Visual Guide

Sources: Harrison's Principles of Internal Medicine 22E (2025) & Ganong's Review of Medical Physiology

1. What is an ECG?

An electrocardiogram (ECG) records the electrical activity of the heart from 12 different viewpoints (leads). Each lead "looks" at the heart from a different angle, which is why the same beat looks different in each lead. The standard 12-lead ECG includes:

Limb leads: I, II, III (bipolar) and aVR, aVL, aVF (augmented unipolar)
Precordial leads: V1-V6 (chest leads, shown in the diagram below)

2. Lead Placement & Normal ECG Appearance

The precordial leads (V1-V6) are placed across the chest wall. Each electrode sees a different face of the heart, and the resulting waveform shape changes progressively from V1 to V6:

Normal ECG - lead positions and corresponding waveforms across a human torso

Normal ECG tracings showing each lead position and the corresponding waveform. Note how the QRS morphology transitions from a small r-wave in V1 to a dominant R-wave in V5-V6 (the "R-wave progression").

3. The Cardiac Axis (Einthoven's Triangle)

The mean QRS axis describes the overall direction of ventricular electrical spread in the frontal plane. Normal axis is -30° to +100°.

Einthoven's triangle and cardiac axis calculation

Left: Einthoven's triangle with leads I, II, III. Center: How the mean QRS vector is calculated by measuring R-wave minus negative deflection amplitudes in each lead. Right: Reference axes (hexaxial system) for axis determination.

Axis	Value	Common Causes
Normal	-30° to +100°	-
Left axis deviation	More negative than -30°	LVH, left anterior fascicular block, inferior MI
Right axis deviation	More positive than +90-100°	RVH, lateral MI, left posterior fascicular block

4. ECG Waveforms: P-QRS-T

Each cardiac cycle produces a characteristic pattern of deflections:

Component	Represents	Normal Duration
P wave	Atrial depolarization	<120 ms, upright in II, negative in aVR
PR interval	AV conduction time	120-200 ms
QRS complex	Ventricular depolarization	<120 ms
ST segment	Early ventricular repolarization	At baseline (isoelectric)
T wave	Ventricular repolarization	Concordant with QRS polarity
QT interval	Total ventricular systole	Corrected QTc <450 ms (men), <460 ms (women)
U wave	Late repolarization	Small, same polarity as T wave

Ventricular Depolarization - Two Phases

Ventricular depolarization phases with QRS formation in V1 and V6

Phase 1: Septal depolarization goes left-to-right (small r in V1, small q in V6). Phase 2: Left ventricular depolarization dominates, with the vector pointing left/posterior (deep S in V1, tall R in V6).

5. A Real Normal ECG

The ECG below is from a healthy male with sinus rhythm at 75 bpm, PR interval 160 ms, QRS duration 80 ms, QTc ~390 ms, and axis ~+70°:

Normal 12-lead ECG from a healthy adult male

Key features: Regular P waves before every QRS, narrow QRS, normal R-wave progression V1→V6, flat ST segments, upright T waves in lateral leads. - Harrison's Principles of Internal Medicine 22E, Figure 247-7

6. Systematic Approach to ECG Reading (Harrison's 14-step method)

Every ECG should be analyzed in this order:

Calibration/standardization and technical quality
Rhythm
Heart rate
PR interval / AV conduction
QRS interval (duration)
QT/QTc interval
Mean QRS electrical axis
P waves (morphology)
QRS voltages
Precordial R-wave progression
Abnormal Q waves
ST segments
T waves
U waves

7. Atrial Abnormalities (P Wave Changes)

The P wave is the first clue to atrial pathology:

Right and left atrial overload P-wave patterns

Right atrial overload (P-pulmonale): Tall, peaked P waves (≥2.5 mm) - the two atria are shown here with their depolarization vectors
Left atrial abnormality (P-mitrale): Broad (≥120 ms), notched P wave in limb leads; biphasic P in V1 with prominent negative terminal component

8. Ventricular Hypertrophy

LVH and RVH QRS patterns compared to normal in V1 and V6

Left Ventricular Hypertrophy (LVH):

Tall R wave in V5-V6, deep S wave in V1 (Sokolow-Lyon: SV1 + RV5 ≥35 mm)
ST depression + T-wave inversion in lateral leads ("strain pattern")

Right Ventricular Hypertrophy (RVH):

Tall R wave in V1 (R ≥ S), right axis deviation
T-wave inversions in right precordial leads (V1-V3)
qR or rSR' pattern in V1

9. Ischemia and Infarction (The Most Important ECG Application)

Currents of Injury

Subendocardial vs transmural ischemia ST vector diagrams

Subendocardial ischemia (A): ST vector points inward → ST depression in overlying leads
Transmural/epicardial ischemia (B): ST vector points outward → ST elevation in overlying leads

ST Elevation MI - Localizing the Territory

Severe anterior STEMI with ST elevations and Q waves in precordial leads

Anterior STEMI: Diffuse ST elevations across V1-V6, with deep S waves and prominent T-wave changes. This represents LAD territory occlusion.

Territory	Leads with Changes	Artery
Anterior	V1-V6, I, aVL	LAD
Inferior	II, III, aVF	RCA or LCx
Lateral	I, aVL, V5-V6	LCx
Posterior	Reciprocal ST↓ in V1-V3	RCA or LCx
Right ventricle	V1, V3R-V4R	RCA (proximal)

ECG Evolution in STEMI

Hyperacute T waves (minutes) - tall, peaked T waves
ST elevation (minutes to hours) - "tombstone" pattern
Q waves form (hours to days) - pathological if >40 ms wide, >25% of R wave
T-wave inversion (hours to weeks) - symmetric, deep
Q waves persist (permanent marker of old infarct)

10. Bundle Branch Blocks

Right Bundle Branch Block (RBBB): Wide QRS (≥120 ms), rSR' ("rabbit ears") in V1, wide S wave in V6, I
Left Bundle Branch Block (LBBB): Wide QRS, broad monophasic R in I, aVL, V5-V6; QS or rS in V1. Important: LBBB makes ST changes unreliable for ischemia diagnosis.
Secondary T-wave changes (T wave opposite to terminal QRS deflection) are expected with bundle branch blocks - they are not independently meaningful.

11. Quick Summary Table

ECG Finding	Think of...
Short PR (<120 ms)	WPW, AV junctional rhythm
Long PR (>200 ms)	First-degree AV block
Wide QRS (>120 ms)	BBB, hyperkalemia, WPW, pacemaker
ST elevation	STEMI, pericarditis, Brugada, early repolarization
ST depression	Subendocardial ischemia, digoxin, hypokalemia
Peaked P waves	Right atrial overload (e.g., cor pulmonale)
Broad notched P	Left atrial abnormality (e.g., mitral stenosis)
Long QTc	Hypokalemia, drugs (amiodarone, sotalol, TCAs)
Prominent U waves	Hypokalemia
Electrical alternans	Pericardial effusion / tamponade
Delta wave + short PR	Wolff-Parkinson-White

Sources: Harrison's Principles of Internal Medicine 22E (2025), Chapter 247 (Electrocardiography) | Ganong's Review of Medical Physiology 26E, Chapter 29

This is a shared conversation. Sign in to Orris to start your own chat.